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Updated: Wednesday, July 27, 2022

dhcpd.conf (5)


dhcpd.conf - dhcpd configuration file


Please see following description for synopsis


dhcpd.conf(5)                 File Formats Manual                dhcpd.conf(5)

       dhcpd.conf - dhcpd configuration file

       The  dhcpd.conf  file contains configuration information for dhcpd, the
       Internet Systems Consortium DHCP Server.

       The dhcpd.conf file is a free-form ASCII text file.  It  is  parsed  by
       the  recursive-descent  parser  built into dhcpd.  The file may contain
       extra tabs and newlines for formatting purposes.  Keywords in the  file
       are  case-insensitive.  Comments may be placed anywhere within the file
       (except within quotes).  Comments begin with the # character and end at
       the end of the line.

       The file essentially consists of a list of statements.  Statements fall
       into two broad categories - parameters and declarations.

       Parameter statements either say how to do something (e.g., how  long  a
       lease  to  offer),  whether to do something (e.g., should dhcpd provide
       addresses to unknown clients), or what parameters  to  provide  to  the
       client (e.g., use gateway

       Declarations  are  used  to  describe  the  topology of the network, to
       describe clients on the network,  to  provide  addresses  that  can  be
       assigned  to  clients,  or to apply a group of parameters to a group of
       declarations.  In any group of parameters and declarations, all parame-
       ters  must  be  specified before any declarations which depend on those
       parameters may be specified.

       Declarations about network topology include the shared-network and  the
       subnet  declarations.   If  clients  on  a  subnet  are  to be assigned
       addresses dynamically, a range declaration must appear within the  sub-
       net  declaration.   For  clients with statically assigned addresses, or
       for installations where only known clients will be  served,  each  such
       client  must  have a host declaration.  If parameters are to be applied
       to a group of declarations which are not related strictly on a per-sub-
       net basis, the group declaration can be used.

       For  every  subnet  which will be served, and for every subnet to which
       the dhcp server is connected, there must  be  one  subnet  declaration,
       which  tells  dhcpd how to recognize that an address is on that subnet.
       A subnet declaration is required for each subnet even if  no  addresses
       will be dynamically allocated on that subnet.

       Some  installations  have  physical  networks on which more than one IP
       subnet operates.  For example, if there is a site-wide requirement that
       8-bit  subnet  masks  be  used, but a department with a single physical
       ethernet network expands to the point where it has more than 254 nodes,
       it may be necessary to run two 8-bit subnets on the same ethernet until
       such time as a new physical network can be added.  In  this  case,  the
       subnet  declarations  for  these  two  networks  must  be enclosed in a
       shared-network declaration.

       Note that even when the shared-network declaration is absent, an  empty
       one  is  created  by  the  server to contain the subnet (and any scoped
       parameters included in the subnet).  For practical purposes, this means
       that  "stateless"  DHCP  clients,  which are not tied to addresses (and
       therefore subnets) will receive  the  same  configuration  as  stateful

       Some  sites  may  have  departments which have clients on more than one
       subnet, but it may be desirable to offer those clients a uniform set of
       parameters  which  are  different than what would be offered to clients
       from other departments on the same subnet.  For clients which  will  be
       declared  explicitly  with host declarations, these declarations can be
       enclosed in a group declaration along with  the  parameters  which  are
       common to that department.  For clients whose addresses will be dynami-
       cally assigned, class declarations and conditional declarations may  be
       used  to  group  parameter  assignments based on information the client

       When a client is to be booted, its boot parameters  are  determined  by
       consulting that client's host declaration (if any), and then consulting
       any class declarations matching the client, followed by the pool,  sub-
       net  and shared-network declarations for the IP address assigned to the
       client.  Each of these declarations itself  appears  within  a  lexical
       scope,  and  all  declarations at less specific lexical scopes are also
       consulted for client option declarations.  Scopes are never  considered
       twice,  and  if  parameters  are  declared  in more than one scope, the
       parameter declared in the most specific scope is the one that is used.

       When dhcpd tries to find a host declaration  for  a  client,  it  first
       looks for a host declaration which has a fixed-address declaration that
       lists an IP address that is valid for the subnet or shared  network  on
       which  the  client  is  booting.  If it doesn't find any such entry, it
       tries to find an entry which has no fixed-address declaration.

       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       group {
         group-specific parameters...
         host zappo.test.isc.org {
           host-specific parameters...
         host beppo.test.isc.org {
           host-specific parameters...
         host harpo.test.isc.org {
           host-specific parameters...

                                      Figure 1

       Notice that at the beginning of the file, there's a  place  for  global
       parameters.  These might be things like the organization's domain name,
       the addresses of the name servers (if they are  common  to  the  entire
       organization), and so on.  So, for example:

            option domain-name "isc.org";
            option domain-name-servers ns1.isc.org, ns2.isc.org;

                                      Figure 2

       As  you  can see in Figure 2, you can specify host addresses in parame-
       ters using their domain names rather than their numeric  IP  addresses.
       If  a given hostname resolves to more than one IP address (for example,
       if that host has two ethernet interfaces), then  where  possible,  both
       addresses are supplied to the client.

       The  most obvious reason for having subnet-specific parameters as shown
       in Figure 1 is that each subnet, of necessity, has its own router.   So
       for the first subnet, for example, there should be something like:

            option routers;

       Note  that  the  address  here  is  specified numerically.  This is not
       required - if you have a different domain name for  each  interface  on
       your  router, it's perfectly legitimate to use the domain name for that
       interface instead of the numeric address.  However, in many cases there
       may  be only one domain name for all of a router's IP addresses, and it
       would not be appropriate to use that name here.

       In Figure 1 there is also a  group  statement,  which  provides  common
       parameters  for  a set of three hosts - zappo, beppo and harpo.  As you
       can see, these hosts are all in the test.isc.org domain,  so  it  might
       make  sense  for a group-specific parameter to override the domain name
       supplied to these hosts:

            option domain-name "test.isc.org";

       Also, given the domain they're in, these are  probably  test  machines.
       If we wanted to test the DHCP leasing mechanism, we might set the lease
       timeout somewhat shorter than the default:

            max-lease-time 120;
            default-lease-time 120;

       You may have noticed that while some parameters start with  the  option
       keyword, some do not.  Parameters starting with the option keyword cor-
       respond to actual DHCP options, while parameters that do not start with
       the  option  keyword  either  control  the  behavior of the DHCP server
       (e.g., how long a lease dhcpd will give out), or specify client parame-
       ters  that  are not optional in the DHCP protocol (for example, server-
       name and filename).

       In Figure 1, each  host  had  host-specific  parameters.   These  could
       include  such  things  as  the  hostname  option, the name of a file to
       upload (the filename parameter) and the  address  of  the  server  from
       which  to upload the file (the next-server parameter).  In general, any
       parameter can appear anywhere that parameters are allowed, and will  be
       applied according to the scope in which the parameter appears.

       Imagine that you have a site with a lot of NCD X-Terminals.  These ter-
       minals come in a variety of models, and you want to  specify  the  boot
       files  for each model.  One way to do this would be to have host decla-
       rations for each server and group them by model:

       group {
         filename "Xncd19r";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
         host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
         host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }

       group {
         filename "Xncd19c";
         next-server ncd-booter;

         host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
         host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }

       group {
         filename "XncdHMX";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
         host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
         host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }

       The pool and pool6 declarations can  be  used  to  specify  a  pool  of
       addresses  that  will  be  treated  differently  than  another  pool of
       addresses, even on the same network segment or  subnet.   For  example,
       you  may  want to provide a large set of addresses that can be assigned
       to DHCP clients that are registered to your DHCP server, while  provid-
       ing  a  smaller set of addresses, possibly with short lease times, that
       are available for unknown clients.  If you have a firewall, you may  be
       able to arrange for addresses from one pool to be allowed access to the
       Internet, while addresses in another pool  are  not,  thus  encouraging
       users  to  register their DHCP clients.  To do this, you would set up a
       pair of pool declarations:

       subnet netmask {
         option routers;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers bogus.example.com;
           max-lease-time 300;
           allow unknown-clients;

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           deny unknown-clients;

       It is also possible to set up entirely different subnets for known  and
       unknown  clients - address pools exist at the level of shared networks,
       so address ranges within pool declarations can be on different subnets.

       As you can see in the preceding example, pools can  have  permit  lists
       that  control  which  clients  are allowed access to the pool and which
       aren't.  Each entry in a pool's permit  list  is  introduced  with  the
       allow  or  deny  keyword.  If a pool has a permit list, then only those
       clients that match specific entries on the permit list will be eligible
       to  be  assigned  addresses  from the pool.  If a pool has a deny list,
       then only those clients that do not match any entries on the deny  list
       will  be  eligible.    If  both permit and deny lists exist for a pool,
       then only clients that match the permit list and do not match the  deny
       list will be allowed access.

       The pool6 declaration is similar to the pool declaration.  Currently it
       is only allowed within a subnet6 declaration, and may not  be  included
       directly  in  a  shared network declaration.  In addition to the range6
       statement it allows the prefix6 statement  to  be  included.   You  may
       include range6 statements for both NA and TA and prefixy6 statements in
       a single pool6 statement.

       Address allocation is actually only done when a client is in  the  INIT
       state and has sent a DHCPDISCOVER message.  If the client thinks it has
       a valid lease and sends a DHCPREQUEST to initiate or renew that  lease,
       the server has only three choices - it can ignore the DHCPREQUEST, send
       a DHCPNAK to tell the client it should stop using the address, or  send
       a  DHCPACK,  telling  the  client to go ahead and use the address for a

       If the server finds the address the  client  is  requesting,  and  that
       address is available to the client, the server will send a DHCPACK.  If
       the address is no longer available, or the client  isn't  permitted  to
       have  it,  the server will send a DHCPNAK.  If the server knows nothing
       about the address, it will remain silent, unless the address is  incor-
       rect  for the network segment to which the client has been attached and
       the server is authoritative for that network segment, in which case the
       server  will  send  a  DHCPNAK  even  though  it doesn't know about the

       There may be a host declaration matching the  client's  identification.
       If  that  host  declaration  contains  a fixed-address declaration that
       lists an IP address that is valid for the network segment to which  the
       client  is  connected,  the  DHCP  server will never do dynamic address
       allocation.  In this case, the client is required to take  the  address
       specified  in  the host declaration.  If the client sends a DHCPREQUEST
       for some other address, the server will respond with a DHCPNAK.

       When the DHCP server allocates a new address for  a  client  (remember,
       this  only  happens  if  the  client has sent a DHCPDISCOVER), it first
       looks to see if the client already has a valid lease on an IP  address,
       or  if there is an old IP address the client had before that hasn't yet
       been reassigned.  In that case, the server will take that  address  and
       check  it  to  see  if the client is still permitted to use it.  If the
       client is no longer permitted to use it, the  lease  is  freed  if  the
       server  thought it was still in use - the fact that the client has sent
       a DHCPDISCOVER proves to the server that the client is no longer  using
       the lease.

       If no existing lease is found, or if the client is forbidden to receive
       the existing lease, then the server will look in the  list  of  address
       pools  for  the  network  segment to which the client is attached for a
       lease that is not in use and that the client is permitted to have.   It
       looks through each pool declaration in sequence (all range declarations
       that appear outside of pool declarations are grouped into a single pool
       with  no  permit  list).   If  the  permit list for the pool allows the
       client to be allocated an address from that pool, the pool is  examined
       to  see  if  there  is an address available.  If so, then the client is
       tentatively assigned that address.  Otherwise, the next pool is tested.
       If  no  addresses  are  found  that  can  be assigned to the client, no
       response is sent to the client.

       If an address is found that the client is permitted to have,  and  that
       has  never  been  assigned to any client before, the address is immedi-
       ately allocated to the client.  If the address is available for alloca-
       tion but has been previously assigned to a different client, the server
       will keep looking in hopes of finding an address that has never  before
       been assigned to a client.

       The  DHCP  server  generates  the list of available IP addresses from a
       hash table.  This means that the addresses are not sorted in  any  par-
       ticular  order, and so it is not possible to predict the order in which
       the DHCP server will allocate IP addresses.  Users of previous versions
       of  the  ISC  DHCP server may have become accustomed to the DHCP server
       allocating IP addresses in ascending order, but this is no longer  pos-
       sible, and there is no way to configure this behavior with version 3 of
       the ISC DHCP server.

       The DHCP server checks IP addresses to see if they are  in  use  before
       allocating  them  to  clients.   It  does  this by sending an ICMP Echo
       request message to the IP address being allocated.   If  no  ICMP  Echo
       reply  is  received within a second, the address is assumed to be free.
       This is only done for leases that have been specified in  range  state-
       ments, and only when the lease is thought by the DHCP server to be free
       - i.e., the DHCP server or its failover peer has not listed  the  lease
       as in use.

       If  a  response  is  received  to an ICMP Echo request, the DHCP server
       assumes that there is a configuration error - the IP address is in  use
       by  some  host  on the network that is not a DHCP client.  It marks the
       address as abandoned, and will not assign it to clients. The lease will
       remain abandoned for a minimum of abandon-lease-time seconds.

       If  a  DHCP  client tries to get an IP address, but none are available,
       but there are abandoned IP addresses, then the DHCP server will attempt
       to  reclaim  an abandoned IP address.  It marks one IP address as free,
       and then does the same ICMP Echo request  check  described  previously.
       If there is no answer to the ICMP Echo request, the address is assigned
       to the client.

       The DHCP server does not cycle through abandoned IP  addresses  if  the
       first  IP  address  it tries to reclaim is free.  Rather, when the next
       DHCPDISCOVER comes in from the client, it will attempt a new allocation
       using  the  same method described here, and will typically try a new IP

       This version of the ISC DHCP server supports the DHCP failover protocol
       as  documented  in draft-ietf-dhc-failover-12.txt.  This is not a final
       protocol document, and we have not done interoperability  testing  with
       other vendors' implementations of this protocol, so you must not assume
       that this implementation conforms to the standard.  If you wish to  use
       the  failover  protocol, make sure that both failover peers are running
       the same version of the ISC DHCP server.

       The failover protocol allows two DHCP servers (and no more than two) to
       share  a  common address pool.  Each server will have about half of the
       available IP addresses in the pool at any given  time  for  allocation.
       If one server fails, the other server will continue to renew leases out
       of the pool, and will allocate new addresses out of the roughly half of
       available  addresses  that  it  had  when communications with the other
       server were lost.

       It is possible during a prolonged failure to tell the remaining  server
       that  the other server is down, in which case the remaining server will
       (over time) reclaim all the addresses the other  server  had  available
       for  allocation,  and  begin to reuse them.  This is called putting the
       server into the PARTNER-DOWN state.

       You can put the server into the PARTNER-DOWN state either by using  the
       omshell  (1)  command  or  by  stopping  the  server,  editing the last
       failover state declaration  in  the  lease  file,  and  restarting  the
       server.  If you use this last method, change the "my state" line to:

       failover peer name state {
       my state partner-down;.
       peer state state at date;

       It is only required to change "my state" as shown above.

       When the other server comes back online, it should automatically detect
       that it has been offline and request a complete update from the  server
       that  was running in the PARTNER-DOWN state, and then both servers will
       resume processing together.

       It is possible to get into a dangerous situation: if you put one server
       into  the PARTNER-DOWN state, and then *that* server goes down, and the
       other server comes back up, the other server will  not  know  that  the
       first  server  was  in  the PARTNER-DOWN state, and may issue addresses
       previously issued by the other server to different  clients,  resulting
       in  IP  address  conflicts.   Before putting a server into PARTNER-DOWN
       state, therefore, make sure that the  other  server  will  not  restart

       The  failover  protocol  defines  a primary server role and a secondary
       server role.  There are some differences in how  primaries  and  secon-
       daries  act, but most of the differences simply have to do with provid-
       ing a way for each peer to behave in the opposite way from  the  other.
       So one server must be configured as primary, and the other must be con-
       figured as secondary, and it doesn't  matter  too  much  which  one  is

       When  a  server  starts  that  has not previously communicated with its
       failover peer, it must establish communications with its failover  peer
       and  synchronize  with it before it can serve clients.  This can happen
       either because you have just configured your DHCP  servers  to  perform
       failover  for  the  first time, or because one of your failover servers
       has failed catastrophically and lost its database.

       The initial recovery process  is  designed  to  ensure  that  when  one
       failover  peer  loses  its database and then resynchronizes, any leases
       that the failed server gave out before it failed will be honored.  When
       the  failed  server starts up, it notices that it has no saved failover
       state, and attempts to contact its peer.

       When it has established contact, it asks the peer for a  complete  copy
       its  peer's lease database.  The peer then sends its complete database,
       and sends a message indicating that it is done.  The failed server then
       waits until MCLT has passed, and once MCLT has passed both servers make
       the transition back into normal operation.  This waiting period ensures
       that  any leases the failed server may have given out while out of con-
       tact with its partner will have expired.

       While the failed server is recovering, its partner remains in the part-
       ner-down state, which means that it is serving all clients.  The failed
       server provides no service at all to DHCP clients until it has made the
       transition into normal operation.

       In  the case where both servers detect that they have never before com-
       municated with their partner, they both come up in this recovery  state
       and follow the procedure we have just described.  In this case, no ser-
       vice will be provided to DHCP clients until MCLT has expired.

       In order to configure failover, you need to write  a  peer  declaration
       that  configures the failover protocol, and you need to write peer ref-
       erences in each pool declaration for which you  want  to  do  failover.
       You  do  not  have to do failover for all pools on a given network seg-
       ment.   You must not tell one server it's doing failover on a  particu-
       lar  address  pool and tell the other it is not.  You must not have any
       common address pools on which you are not doing failover.  A pool  dec-
       laration that utilizes failover would look like this:

       pool {
            failover peer "foo";
            pool specific parameters

       The   server currently  does very  little  sanity checking,  so if  you
       configure it wrong, it will just  fail in odd ways.  I would  recommend
       therefore  that you either do  failover or don't do failover, but don't
       do any mixed pools.  Also,  use the same master configuration file  for
       both   servers,  and  have  a  separate file  that  contains  the  peer
       declaration and includes the master file.  This will help you to  avoid
       configuration   mismatches.  As our  implementation evolves,  this will
       become  less of  a  problem.  A  basic  sample dhcpd.conf  file for   a
       primary server might look like this:

       failover peer "foo" {
         address anthrax.rc.example.com;
         port 519;
         peer address trantor.rc.example.com;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

         [ primary | secondary ];

         This  determines  whether  the  server  is  primary  or secondary, as
         described earlier under DHCP FAILOVER.

       The address statement

         address address;

         The address statement declares the IP address or DNS  name  on  which
         the  server should listen for connections from its failover peer, and
         also the value to use for the DHCP Failover Protocol  server  identi-
         fier.   Because  this  value  is used as an identifier, it may not be

       The peer address statement

         peer address address;

         The peer address statement declares the IP address  or  DNS  name  to
         which  the  server  should  connect  to  reach  its failover peer for
         failover messages.

       The port statement

         port port-number;

         The port statement declares the TCP port on which the  server  should
         listen for connections from its failover peer.  This statement may be
         omitted, in which case the IANA assigned port number 647 will be used
         by default.

       The peer port statement

         peer port port-number;

         The  peer  port  statement  declares the TCP port to which the server
         should connect to reach its  failover  peer  for  failover  messages.
         This  statement  may be omitted, in which case the IANA assigned port
         number 647 will be used by default.

       The max-response-delay statement

         max-response-delay seconds;

         The max-response-delay statement tells the DHCP server how many  sec-
         onds  may  pass  without  receiving  a message from its failover peer
         before it assumes that connection has failed.  This number should  be
         small enough that a transient network failure that breaks the connec-
         tion will not result in the servers being out of communication for  a
         long  time,  but large enough that the server isn't constantly making
         and breaking connections.  This parameter must be specified.

       The max-unacked-updates statement

         max-unacked-updates count;

         The max-unacked-updates statement tells the remote  DHCP  server  how
         many BNDUPD messages it can send before it receives a BNDACK from the
         local system.  We don't have enough  operational  experience  to  say
         what  a good value for this is, but 10 seems to work.  This parameter
         must be specified.

       The mclt statement

         mclt seconds;

         The mclt statement defines the Maximum Client Lead Time.  It must  be
         specified  on the primary, and may not be specified on the secondary.
         This is the length of time for which a lease may be renewed by either
         failover peer without contacting the other.  The longer you set this,
         the longer it  will  take  for  the  running  server  to  recover  IP
         addresses  after moving into PARTNER-DOWN state.  The shorter you set
         it, the more load your servers will experience when they are not com-
         municating.   A  value of something like 3600 is probably reasonable,
         but again bear in mind that we have no  real  operational  experience
         with this.

       The split statement

         split bits;

         The  split statement specifies the split between the primary and sec-
         ondary for the purposes of load balancing.  Whenever a client makes a
         DHCP  request,  the DHCP server runs a hash on the client identifica-
         tion, resulting in value from 0 to 255.  This is  used  as  an  index
         into  a  256 bit field.  If the bit at that index is set, the primary
         is responsible.  If the bit at that index is not set,  the  secondary
         is  responsible.   The split value determines how many of the leading
         bits are set to one.  So, in practice, higher split values will cause
         the  primary  to  serve more clients than the secondary.  Lower split
         values, the converse.  Legal values are between 0 and 256  inclusive,
         of  which  the  most reasonable is 128.  Note that a value of 0 makes
         the secondary responsible for all clients and a value  of  256  makes
         the primary responsible for all clients.

       The hba statement

         hba colon-separated-hex-list;

         The  hba  statement  specifies the split between the primary and sec-
         ondary as a bitmap rather than a cutoff, which  theoretically  allows
         for  finer-grained  control.   In practice, there is probably no need
         for such fine-grained control, however.  An example hba statement:

           hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:

         This is equivalent to a split 128;  statement,  and  identical.   The
         following two examples are also equivalent to a split of 128, but are
         not identical:

           hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:

           hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:

         They are equivalent, because half the bits are set to 0, half are set
         to  1  (0xa and 0x5 are 1010 and 0101 binary respectively) and conse-
         quently this would roughly divide the  clients  equally  between  the
         servers.  They are not identical, because the actual peers this would
         load balance to each server are different for each example.

         You must only have split or hba defined, never both.  For most cases,
         the  fine-grained  control that hba offers isn't necessary, and split
         should be used.

       The load balance max seconds statement

         load balance max seconds seconds;

         This statement allows you to configure a cutoff after which load bal-
         ancing  is  disabled.   The  cutoff is based on the number of seconds
         since the client sent its first DHCPDISCOVER or DHCPREQUEST  message,
         and only works with clients that correctly implement the secs field -
         fortunately most clients do.  We recommend setting this to  something
         like 3 or 5.  The effect of this is that if one of the failover peers
         gets into a state where it is responding to failover messages but not
         responding to some client requests, the other failover peer will take
         over its client load automatically as the clients retry.

         It is possible to disable load balancing  between  peers  by  setting
         this  value  to  0  on both peers.  Bear in mind that this means both
         peers will respond to all DHCPDISCOVERs or DHCPREQUESTs.

       The auto-partner-down statement

         auto-partner-down seconds;

         This statement instructs the server to initiate a  timed  delay  upon
         entering the communications-interrupted state (any situation of being
         out-of-contact with the remote failover peer).  At the conclusion  of
         the  timer,  the  server  will  automatically  enter the partner-down
         state.  This permits the server to allocate leases from the partner's
         free  lease  pool after an STOS+MCLT timer expires, which can be dan-
         gerous if the partner is in fact  operating  at  the  time  (the  two
         servers will give conflicting bindings).

         Think  very carefully before enabling this feature.  The partner-down
         and communications-interrupted states  are  intentionally  segregated
         because there do exist situations where a failover server can fail to
         communicate with its peer, but still has the ability to  receive  and
         reply to requests from DHCP clients.  In general, this feature should
         only be used in those deployments  where  the  failover  servers  are
         directly  connected  to one another, such as by a dedicated hardwired
         link ("a heartbeat cable").

         A  zero  value  disables  the  auto-partner-down  feature  (also  the
         default),  and  any  positive  value indicates the time in seconds to
         wait before automatically entering partner-down.

       The Failover pool balance statements.

          max-lease-misbalance percentage;
          max-lease-ownership percentage;
          min-balance seconds;
          max-balance seconds;

         This version of the DHCP Server evaluates pool balance on a schedule,
         rather  than  on demand as leases are allocated.  The latter approach
         proved to be slightly klunky when pool misbalanced reach total  satu-
         ration  --  when any server ran out of leases to assign, it also lost
         its ability to notice it had run dry.

         In order to understand pool balance, some elements of  its  operation
         first  need  to  be  defined.   First,  there are 'free' and 'backup'
         leases.  Both of these  are  referred  to  as  'free  state  leases'.
         'free'  and  'backup'  are  'the free states' for the purpose of this
         document.  The difference is that only the primary may allocate  from
         'free'  leases  unless under special circumstances, and only the sec-
         ondary may allocate 'backup' leases.

         When pool balance is performed, the only plausible expectation is  to
         provide  a  50/50  split  of  the  free  state leases between the two
         servers.  This is because no one can predict which server will  fail,
         regardless  of the relative load placed upon the two servers, so giv-
         ing each server half the leases gives both servers the same amount of
         'failure  endurance'.   Therefore,  there  is no way to configure any
         different behaviour, outside of  some  very  small  windows  we  will
         describe shortly.

         The  first  thing  calculated  on  any  pool  balance  run is a value
         referred to as 'lts', or "Leases To Send".  This, simply, is the dif-
         ference  in the count of free and backup leases, divided by two.  For
         the secondary, it is the difference in the backup  and  free  leases,
         divided  by  two.   The resulting value is signed: if it is positive,
         the local server is expected to hand out leases  to  retain  a  50/50
         balance.   If  it  is  negative, the remote server would need to send
         leases to balance the pool.  Once the lts  value  reaches  zero,  the
         pool  is perfectly balanced (give or take one lease in the case of an
         odd number of total free state leases).

         The current approach is still  something  of  a  hybrid  of  the  old
         approach,  marked  by the presence of the max-lease-misbalance state-
         ment.  This parameter configures what used to be a 10% fixed value in
         previous  versions:  if lts is less than free+backup * max-lease-mis-
         balance percent, then the server will skip balancing a given pool (it
         won't  bother  moving  any  leases,  even  if some leases "should" be
         moved).  The meaning of this value is also somewhat overloaded,  how-
         ever,  in  that  it also governs the estimation of when to attempt to
         balance the pool (which may then also be skipped over).   The  oldest
         leases  in  the  free  and backup states are examined.  The time they
         have resided in their respective queues is used  as  an  estimate  to
         indicate how much time it is probable it would take before the leases
         at the top of the list would be consumed (and thus, how long it would
         take  to  use all leases in that state).  This percentage is directly
         multiplied by this time, and fit into the schedule if it falls within
         the  min-balance  and  max-balance  configured values.  The scheduled
         pool check time is only moved in a downwards direction, it  is  never
         increased.  Lastly, if the lts is more than double this number in the
         negative direction, the local server  will  'panic'  and  transmit  a
         Failover  protocol POOLREQ message, in the hopes that the remote sys-
         tem will be woken up into action.

         Once the lts value exceeds  the  max-lease-misbalance  percentage  of
         total  free  state leases as described above, leases are moved to the
         remote server.  This is done in two passes.

         In the first pass, only leases whose most recent bound  client  would
         have been served by the remote server - according to the Load Balance
         Algorithm (see above split and hba configuration  statements)  -  are
         given  away  to  the  peer.  This first pass will happily continue to
         give away leases, decrementing the lts value by one for  each,  until
         the  lts value has reached the negative of the total number of leases
         multiplied by the max-lease-ownership percentage.  So it  is  through
         this  value that you can permit a small misbalance of the lease pools
         - for the purpose of giving the peer  more  than  a  50/50  share  of
         leases  in  the hopes that their clients might some day return and be
         allocated by the peer (operating normally).  This process is referred
         to  as  'MAC  Address  Affinity',  but  this is somewhat misnamed: it
         applies equally to DHCP Client Identifier options.   Note  also  that
         affinity  is  applied to leases when they enter the state 'free' from
         'expired' or 'released'.  In this case also, leases will not be moved
         from free to backup if the secondary already has more than its share.

         The  second  pass  is  only  entered  into if the first pass fails to
         reduce the lts underneath the total number of free state leases  mul-
         tiplied  by  the  max-lease-ownership  percentage.  In this pass, the
         oldest leases are given over to the peer without second thought about
         the  Load  Balance  Algorithm, and this continues until the lts falls
         under this value.  In this way, the local server  will  also  happily
         keep  a  small percentage of the leases that would normally load bal-
         ance to itself.

         So, the  max-lease-misbalance  value  acts  as  a  behavioural  gate.
         Smaller values will cause more leases to transition states to balance
         the pools over time, higher values will decrease the amount of change
         (but may lead to pool starvation if there's a run on leases).

         The  max-lease-ownership  value  permits a small (percentage) skew in
         the lease balance of a percentage of the total number of  free  state

         Finally,  the  min-balance and max-balance make certain that a sched-
         uled rebalance event happens within a reasonable timeframe (not to be
         thrown off by, for example, a 7 year old free lease).

         Plausible  values  for  the percentages lie between 0 and 100, inclu-
         sive, but values over 50 are indistinguishable from one another (once
         lts  exceeds  50% of the free state leases, one server must therefore
         have 100% of the leases in its respective free state).  It is  recom-
         mended  to  select a max-lease-ownership value that is lower than the
         value selected for the max-lease-misbalance value.   max-lease-owner-
         ship defaults to 10, and max-lease-misbalance defaults to 15.

         Plausible values for the min-balance and max-balance times also range
         from 0 to (2^32)-1 (or the limit of your  local  time_t  value),  but
         default  to  values 60 and 3600 respectively (to place balance events
         between 1 minute and 1 hour).

       Clients can be separated into classes, and treated differently  depend-
       ing on what class they are in.  This separation can be done either with
       a conditional statement, or with a match  statement  within  the  class
       declaration.   It is possible to specify a limit on the total number of
       clients within a particular class or subclass that may hold  leases  at
       one  time, and it is possible to specify automatic subclassing based on
       the contents of the client packet.

       Classing support for DHCPv6 clients was added in 4.3.0.  It follows the
       same  rules  as  for DHCPv4 except that support for billing classes has
       not been added yet.

       To add clients to classes based  on  conditional  evaluation,  you  can
       specify a matching expression in the class statement:

       class "ras-clients" {
         match if substring (option dhcp-client-identifier, 1, 3) = "RAS";

       Please  note  that  the  values used in match expressions may only come
       from data or options that are part of the client packet. It is not pos-
       sible  to  use values constructed through one or more executable state-
       ments.  This stems from the  fact  that  client  classification  occurs
       before  any  statements  are  executed.  Attempting to do so will yield
       indeterminate results.

       Note that whether you use matching expressions or  add  statements  (or
       both)  to  classify  clients, you must always write a class declaration
       for any class that you use.  If there will be no match statement and no
       in-scope statements for a class, the declaration should look like this:

       class "ras-clients" {

       In  addition  to classes, it is possible to declare subclasses.  A sub-
       class is a class with the same name as a regular class, but with a spe-
       cific  submatch expression which is hashed for quick matching.  This is
       essentially a speed hack - the main  difference  between  five  classes
       with  match  expressions  and one class with five subclasses is that it
       will be quicker to find the subclasses.  Subclasses work as follows:

       class "allocation-class-1" {
         match pick-first-value (option dhcp-client-identifier, hardware);

       class "allocation-class-2" {
         match pick-first-value (option dhcp-client-identifier, hardware);

       subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass "allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet netmask {
         pool {
           allow members of "allocation-class-1";
         pool {
           allow members of "allocation-class-2";

       The data following the class name in the subclass declaration is a con-
       stant  value  to  use  in  matching the match expression for the class.
       When class matching is done, the server will evaluate the match expres-
       sion  and  then  look  the  result up in the hash table.  If it finds a
       match, the client is considered a member of both the class and the sub-

       Subclasses  can  be declared with or without scope.  In the above exam-
       ple, the sole purpose of the subclass is to allow some  clients  access
       to  one address pool, while other clients are given access to the other
       pool, so these subclasses are declared without scopes.  If part of  the
       purpose  of  the subclass were to define different parameter values for
       some clients, you might want to declare some subclasses with scopes.

       In the above example, if you had a single client that needed some  con-
       figuration parameters, while most didn't, you might write the following
       subclass declaration for that client:

       subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
         option root-path "samsara:/var/diskless/alphapc";
         filename "/tftpboot/netbsd.alphapc-diskless";

       In this example, we've used subclassing as a  way  to  control  address
       allocation  on  a per-client basis.  However, it's also possible to use
       subclassing in ways that are not specific to clients - for example,  to
       use  the  value of the vendor-class-identifier option to determine what
       values to send in the vendor-encapsulated-options option.   An  example
       of  this  is  shown  under  the VENDOR ENCAPSULATED OPTIONS head in the
       dhcp-options(5) manual page.

       You may specify a limit to the number of clients in a class that can be
       assigned leases.  The effect of this will be to make it difficult for a
       new client in a class to get an address.  Once  a  class  with  such  a
       limit  has  reached  its limit, the only way a new client in that class
       can get a lease is for an existing  client  to  relinquish  its  lease,
       either  by  letting  it  expire,  or  by  sending a DHCPRELEASE packet.
       Classes with lease limits are specified as follows:

       class "limited-1" {
         lease limit 4;

       This will produce a class in which a maximum of four members may hold a
       lease at one time.

       It  is  possible  to  declare  a spawning class.  A spawning class is a
       class that automatically produces subclasses based on what  the  client
       sends.   The  reason  that spawning classes were created was to make it
       possible to create lease-limited classes on the  fly.   The  envisioned
       application  is  a cable-modem environment where the ISP wishes to pro-
       vide clients at a particular site with more than one  IP  address,  but
       does  not  wish to provide such clients with their own subnet, nor give
       them an unlimited number of IP addresses from the  network  segment  to
       which they are connected.

       Many  cable  modem  head-end  systems  can be configured to add a Relay
       Agent Information option to DHCP packets when relaying them to the DHCP
       server.   These  systems typically add a circuit ID or remote ID option
       that uniquely identifies the customer site.  To take advantage of this,
       you can write a class declaration as follows:

       class "customer" {
         spawn with option agent.circuit-id;
         lease limit 4;

       Now  whenever  a  request comes in from a customer site, the circuit ID
       option will be checked against the class's hash table.  If  a  subclass
       is  found that matches the circuit ID, the client will be classified in
       that subclass and treated accordingly.  If no subclass is found  match-
       ing  the  circuit  ID,  a  new  one  will  be created and logged in the
       dhcpd.leases file, and the client will be classified in this new class.
       Once  the  client  has been classified, it will be treated according to
       the rules of the class, including, in this case, being subject  to  the
       per-site limit of four leases.

       The  use  of the subclass spawning mechanism is not restricted to relay
       agent options - this particular example is given only because it  is  a
       fairly straightforward one.

       In  some  cases,  it  may  be  useful to use one expression to assign a
       client to a particular class, and a second expression to put it into  a
       subclass of that class.  This can be done by combining the match if and
       spawn with statements, or the match if and match statements.  For exam-

       class "jr-cable-modems" {
         match if option dhcp-vendor-identifier = "jrcm";
         spawn with option agent.circuit-id;
         lease limit 4;

       class "dv-dsl-modems" {
         match if option dhcp-vendor-identifier = "dvdsl";
         spawn with option agent.circuit-id;
         lease limit 16;

       This  allows you to have two classes that both have the same spawn with
       expression without getting the clients in the two classes confused with
       each other.

       The  DHCP  server has the ability to dynamically update the Domain Name
       System.  Within the configuration files, you can define  how  you  want
       the  Domain Name System to be updated.  These updates are RFC 2136 com-
       pliant so any DNS server supporting RFC 2136 should be able  to  accept
       updates from the DHCP server.

       There  are two DNS schemes implemented.  The interim option is based on
       draft revisions of the DDNS documents  while  the  standard  option  is
       based on the RFCs for DHCP-DNS interaction and DHCIDs.  A third option,
       ad-hoc, was deprecated and has now been removed  from  the  code  base.
       The DHCP server must be configured to use one of the two currently-sup-
       ported methods, or not to do DNS updates.

       New installations should use the standard option.  Older  installations
       may want to continue using the interim option for backwards compatibil-
       ity with the DNS database until the database can be updated.  This  can
       be done with the ddns-update-style configuration parameter.

       the interim and standard DNS update schemes operate mostly according to
       work from the IETF.  The interim version was based  on  the  drafts  in
       progress at the time while the standard is based on the completed RFCs.
       The standard RFCs are:

                            RFC 4701 (updated by RF5494)
                                      RFC 4702
                                      RFC 4703

       And the corresponding drafts were:


       The basic framework for the two schemes is similar with the main  mate-
       rial  difference  being that a DHCID RR is used in the standard version
       while the interim versions uses a TXT RR.  The format of the TXT record
       bears  a  resemblance  to the DHCID RR but it is not equivalent (MD5 vs
       SHA2, field length differences etc).

       In these two schemes the DHCP server does not necessarily always update
       both the A and the PTR records.  The FQDN option includes a flag which,
       when sent by the client, indicates that the client wishes to update its
       own  A  record.   In  that case, the server can be configured either to
       honor the client's intentions or ignore them.  This is  done  with  the
       statement   allow  client-updates;  or  the  statement  ignore  client-
       updates;.  By default, client updates are allowed.

       If the server is configured to allow client updates, then if the client
       sends a fully-qualified domain name in the FQDN option, the server will
       use that name the client sent in the FQDN  option  to  update  the  PTR
       record.   For example, let us say that the client is a visitor from the
       "radish.org" domain, whose hostname is "jschmoe".  The  server  is  for
       the "example.org" domain.  The DHCP client indicates in the FQDN option
       that its FQDN is "jschmoe.radish.org.".   It  also  indicates  that  it
       wants  to  update its own A record.  The DHCP server therefore does not
       attempt to set up an A record for the client, but does  set  up  a  PTR
       record  for  the  IP  address  that  it assigns the client, pointing at
       jschmoe.radish.org.  Once the DHCP client has an  IP  address,  it  can
       update its own A record, assuming that the "radish.org" DNS server will
       allow it to do so.

       If the server is configured not to allow  client  updates,  or  if  the
       client doesn't want to do its own update, the server will simply choose
       a name for the client. By default, the server will choose from the fol-
       lowing three values:

            1. fqdn option (if present)
            2. hostname option (if present)
            3. Configured hostname option (if defined).

       If  these  defaults  for choosing the host name are not appropriate you
       can write your own statement to set the ddns-hostname variable  as  you
       wish.  If none of the above are found the server will use the host dec-
       laration name (if one) and use-host-decl-names is on.

       It will use its own domain name for the client.  It  will  then  update
       both the A and PTR record, using the name that it chose for the client.
       If the client sends a fully-qualified domain name in the  fqdn  option,
       the  server  uses  only  the  leftmost part of the domain name - in the
       example above, "jschmoe" instead of "jschmoe.radish.org".

       Further, if the ignore client-updates;  directive  is  used,  then  the
       server  will  in addition send a response in the DHCP packet, using the
       FQDN Option, that implies to the client that it should perform its  own
       updates  if it chooses to do so.  With deny client-updates;, a response
       is sent which indicates the client may not perform updates.

       Both the standard and interim options also include a  method  to  allow
       more  than  one DHCP server to update the DNS database without acciden-
       tally deleting A records that shouldn't be deleted nor failing to add A
       records that should be added.  For the standard option the method works
       as follows:

       When the DHCP server issues a client a new lease,  it  creates  a  text
       string  that  is an SHA hash over the DHCP client's identification (see
       RFCs 4701 & 4702 for details).  The update attempts to add an A  record
       with the name the server chose and a DHCID record containing the hashed
       identifier string (hashid).  If this update  succeeds,  the  server  is

       If  the update fails because the A record already exists, then the DHCP
       server attempts to add the A record with the  prerequisite  that  there
       must  be  a DHCID record in the same name as the new A record, and that
       DHCID record's contents must be equal to hashid.  If this  update  suc-
       ceeds,  then  the client has its A record and PTR record.  If it fails,
       then the name the client has been assigned (or requested)  is  in  use,
       and  can't  be used by the client.  At this point the DHCP server gives
       up trying to do a DNS update for the client until the client chooses  a
       new name.

       The  server  also  does not update very aggressively.  Because each DNS
       update involves a round trip to the DNS server, there is a cost associ-
       ated  with  doing  updates  even if they do not actually modify the DNS
       database.  So the DHCP server tracks whether or not it has updated  the
       record  in  the past (this information is stored on the lease) and does
       not attempt to update records that it thinks it has already updated.

       This can lead to cases where the DHCP server adds a  record,  and  then
       the  record  is  deleted  through  some other mechanism, but the server
       never again updates the DNS because  it  thinks  the  data  is  already
       there.   In  this  case  the data can be removed from the lease through
       operator intervention, and once this has been done,  the  DNS  will  be
       updated the next time the client renews.

       The  interim  DNS update scheme was written before the RFCs were final-
       ized and does not quite follow them.  The RFCs call  for  a  new  DHCID
       RRtype while the interim DNS update scheme uses a TXT record.  In addi-
       tion the ddns-resolution draft called for the  DHCP  server  to  put  a
       DHCID  RR  on the PTR record, but the interim update method does not do
       this.  In the final RFC this requirement was relaxed such that a server
       may add a DHCID RR to the PTR record.

       As described in RFC 4703, section 5.2, in order to perform DDNS in dual
       stack environments, both IPv4 and IPv6 servers would need to be config-
       ured  to  use  the standard update style and participating IPv4 clients
       MUST convey DUIDs as described in RFC  4361,  section  6.1.,  in  their

       In  a nutshell, this mechanism is intended to use globally unique DUIDs
       to idenfity both IPv4 and IPv6 clients, and where  a  device  has  both
       IPv4  and IPv6 leases it is identified by the same DUID.  This allows a
       dual stack client to use the same FQDN for both mappings,  while  being
       protected  from  updates  for  other  clients  by the rules of conflict

       However, not all IPv4 clients implement this behavior which makes  sup-
       porting  them dual stack environments problematic.  In order to address
       this issue ISC DHCP (as of 4.4.0) supports a new mode of DDNS  conflict
       resolution referred to as Dual Stack Mixed Mode (DSMM).

       The  concept behind DSMM is relatively simple.  All dhcp servers of one
       protocol (IPv4 or v6) use one ddns-update-style (interim  or  standard)
       while  all  servers  of the "other" protocol will use the "other" ddns-
       udpate-style.  In this way, all servers of a given protocol  are  using
       the  same  record type (TXT or DHCID) for their DHCID RR entries.  This
       allows conflict detection to be enforced within each  protocol  without
       interferring with the other's entries.

       DSMM modifications now ensure that IPv4 DSMM servers only ever modify A
       records, their associated PTR records and  DHCID  records,  while  DSMM
       IPv6 severs only modify AAAA records, their associated PTR records, and
       DHCID records.

       Note that DSMM is not a perfect solution, it is a compromise  that  can
       work  well  provided all participating DNS updaters play by DSMM rules.
       As with anything else in life, it only works as well as those who  par-
       ticpate behave.

       While conflict detection is enabled by default, DSMM is not.  To enable
       DSMM,  both  update-conflict-detection  and  ddns-dual-stack-mixed-mode
       must be true.

       Built  into  conflict  resolution  is  the  protection of manually made
       entries for static clients.  Per the rules of conflict  resolution,   a
       DNS  updater  may not alter forward DNS entries unless there is a DHCID
       RR which matches for whom the update is  being  made.   Therefore,  any
       forward  DNS entries without a corresponding DHCID RR cannot be altered
       by such an updater.

       In some environments, it may be desirable to use only  this  aspect  of
       conflict  resolution  and  allow  DNS updaters to overwrite entries for
       dynamic clients regardless of what client owns them.  In  other  words,
       the presence or lack of a DHCID RR is used to determine whether entries
       may or may not be overwritten.  Whether or not the client  matches  the
       data  value  of  the  DHCID  RR  is irrelevant.   This behavior, off by
       default, can be configured through the  parameter,  ddns-guard-id-must-
       match.   As with DSMM, this behavior is can only be enabled if conflict
       resolution is enabled.   This behavior should be  considered  carefully
       before electing to use it.

       There is an additional parameter that can be used with DSMM ddns-other-
       guard-is-dynamic.  When enabled along with DSMM, a server  will  regard
       the  presence  of a DHCID RR of the other style type as indicating that
       the forward DNS entries for that FQDN should  be  dynamic  and  may  be
       overwritten.   For  example,  such  a  server using interim style could
       overwrite the DNS entries for an FQDN if there is  only  a  DHDID  type
       DHDID  RR  for the FQDN.  Essentially, if there are dynamic entries for
       one protocol, that is enough  to  overcome  the  static  protection  of
       entries for the other protocol.  This behavior warrants careful consid-
       eration before electing to use it.

       When you set your DNS server up to allow updates from the DHCP  server,
       you  may  be  exposing  it to unauthorized updates.  To avoid this, you
       should use TSIG signatures -  a  method  of  cryptographically  signing
       updates  using a shared secret key.  As long as you protect the secrecy
       of this key, your updates should also be secure.  Note,  however,  that
       the  DHCP  protocol  itself  provides no security, and that clients can
       therefore provide information to the DHCP server which the DHCP  server
       will  then  use  in  its updates, with the constraints described previ-

       The DNS server must be configured to allow updates for  any  zone  that
       the DHCP server will be updating.  For example, let us say that clients
       in  the  sneedville.edu  domain  will  be  assigned  addresses  on  the  subnet.   In  that case, you will need a key declaration
       for the TSIG key you will be using, and also two  zone  declarations  -
       one  for the zone containing A records that will be updates and one for
       the zone containing PTR records - for ISC BIND, something like this:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone "example.org" {
            type master;
            file "example.org.db";
            allow-update { key DHCP_UPDATER; };

       zone "17.10.10.in-addr.arpa" {
            type master;
            file "10.10.17.db";
            allow-update { key DHCP_UPDATER; };

       You will also have to configure your DHCP server to do updates to these
       zones.   To  do  so,  you  need  to  add  something  like  this to your
       dhcpd.conf file:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone EXAMPLE.ORG. {
         key DHCP_UPDATER;

       zone 17.127.10.in-addr.arpa. {
         key DHCP_UPDATER;

       The primary statement specifies the IP address of the name server whose
       zone  information  is to be updated.  In addition to the primary state-
       ment there are also the primary6 , secondary and secondary6 statements.
       The  primary6  statement specifies an IPv6 address for the name server.
       The secondaries provide for additional addresses for name servers to be
       used  if  the primary does not respond.  The number of name servers the
       DDNS code will attempt to use before giving up is limited and  is  cur-
       rently set to three.

       Note that the zone declarations have to correspond to authority records
       in your name server - in the above example, there must be an SOA record
       for  "example.org."  and for "17.10.10.in-addr.arpa.".  For example, if
       there were a subdomain "foo.example.org"  with  no  separate  SOA,  you
       could not write a zone declaration for "foo.example.org."  Also keep in
       mind that zone names in your DHCP configuration should end  in  a  ".";
       this  is  the  preferred syntax.  If you do not end your zone name in a
       ".", the DHCP server will figure it out.  Also note that  in  the  DHCP
       configuration,  zone  names  are not encapsulated in quotes where there
       are in the DNS configuration.

       You should choose your own secret key, of course.  The ISC BIND 9  dis-
       tribution  comes  with  a  program  for  generating  secret keys called
       dnssec-keygen.  If you are using BIND 9's dnssec-keygen, the above  key
       would be created as follows:

            dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

       The  key  name, algorithm, and secret must match that being used by the
       DNS server. The DHCP server  currently  supports  the  following  algo-


       You  may  wish to enable logging of DNS updates on your DNS server.  To
       do so, you might write a logging statement like the following:

       logging {
            channel update_debug {
                 file "/var/log/update-debug.log";
                 severity  debug 3;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            channel security_info    {
                 file "/var/log/named-auth.info";
                 severity  info;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;

            category update { update_debug; };
            category security { security_info; };

       You  must  create  the  /var/log/named-auth.info  and  /var/log/update-
       debug.log  files before starting the name server.  For more information
       on configuring ISC BIND, consult the documentation that accompanies it.

       There are three kinds of events that can happen regarding a lease,  and
       it  is  possible  to  declare  statements  that occur when any of these
       events happen.  These events are the commit event, when the server  has
       made  a  commitment  of a certain lease to a client, the release event,
       when the client has released the server from its  commitment,  and  the
       expiry event, when the commitment expires.

       To  declare  a  set of statements to execute when an event happens, you
       must use the on statement, followed by the name of the event,  followed
       by  a  series of statements to execute when the event happens, enclosed
       in braces.

       The include statement

        include "filename";

       The include statement is used to read in a named file, and process  the
       contents of that file as though it were entered in place of the include

       The shared-network statement

        shared-network name {
          [ parameters ]
          [ declarations ]

       The shared-network statement is used to inform  the  DHCP  server  that
       some  IP subnets actually share the same physical network.  Any subnets
       in a shared network should be declared within a  shared-network  state-
       ment.   Parameters  specified  in  the shared-network statement will be
       used when booting clients on those subnets unless  parameters  provided
       at  the  subnet or host level override them.  If any subnet in a shared
       network has addresses available for dynamic allocation, those addresses
       are  collected  into a common pool for that shared network and assigned
       to clients as needed.  There is no way to distinguish on  which  subnet
       of a shared network a client should boot.

       Name  should be the name of the shared network.  This name is used when
       printing debugging messages, so it should be descriptive for the shared
       network.  The name may have the syntax of a valid domain name (although
       it will never be used as such),  or  it  may  be  any  arbitrary  name,
       enclosed in quotes.

       The subnet statement

        subnet subnet-number netmask netmask {
          [ parameters ]
          [ declarations ]

       The  subnet  statement is used to provide dhcpd with enough information
       to tell whether or not an IP address is on that subnet.  It may also be
       used   to  provide  subnet-specific  parameters  and  to  specify  what
       addresses may be dynamically allocated to clients booting on that  sub-
       net.  Such addresses are specified using the range declaration.

       The subnet-number should be an IP address or domain name which resolves
       to the subnet number of the subnet being described.  The netmask should
       be  an  IP  address or domain name which resolves to the subnet mask of
       the subnet being described.  The subnet number, together with the  net-
       mask,  are  sufficient  to determine whether any given IP address is on
       the specified subnet.

       Although a netmask must be given with every subnet declaration,  it  is
       recommended  that if there is any variance in subnet masks at a site, a
       subnet-mask option statement be used in each subnet declaration to  set
       the  desired  subnet  mask, since any subnet-mask option statement will
       override the subnet mask declared in the subnet statement.

       The subnet6 statement

        subnet6 subnet6-number {
          [ parameters ]
          [ declarations ]

       The subnet6 statement is used to provide dhcpd with enough  information
       to tell whether or not an IPv6 address is on that subnet6.  It may also
       be used to provide  subnet-specific  parameters  and  to  specify  what
       addresses  may be dynamically allocated to clients booting on that sub-

       The subnet6-number should be an IPv6 network identifier,  specified  as

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For  any  subnet on which addresses will be assigned dynamically, there
       must be at least one range statement.  The range  statement  gives  the
       lowest  and  highest  IP addresses in a range.  All IP addresses in the
       range should be in the subnet in which the range statement is declared.
       The  dynamic-bootp  flag may be specified if addresses in the specified
       range may be dynamically assigned to BOOTP  clients  as  well  as  DHCP
       clients.   When  specifying a single address, high-address can be omit-

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For any IPv6 subnet6 on which addresses will be  assigned  dynamically,
       there  must  be at least one range6 statement. The range6 statement can
       either be the lowest and highest IPv6 addresses in  a  range6,  or  use
       CIDR  notation,  specified as ip6-address/bits. All IP addresses in the
       range6 should be in the  subnet6  in  which  the  range6  statement  is

       The  temporary  variant makes the prefix (by default on 64 bits) avail-
       able for temporary (RFC 4941) addresses. A new address  per  prefix  in
       the  shared  network  is computed at each request with an IA_TA option.
       Release and Confirm ignores temporary addresses.

       Any IPv6 addresses given to hosts with fixed-address6 are excluded from
       the range6, as are IPv6 addresses on the server itself.

       The prefix6 statement

       prefix6 low-address high-address / bits;

       The  prefix6 is the range6 equivalent for Prefix Delegation (RFC 3633).
       Prefixes of bits length are  assigned  between  low-address  and  high-

       Any  IPv6  prefixes  given to static entries (hosts) with fixed-prefix6
       are excluded from the prefix6.

       This statement is currently global but it should have a  shared-network

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]

       The  host  declaration provides a way for the DHCP server to identify a
       DHCP or BOOTP client.  This allows the server to provide  configuration
       information including fixed addresses or, in DHCPv6, fixed prefixes for
       a specific client.

       If it is desirable to be able to boot a DHCP or BOOTP  client  on  more
       than  one  subnet with fixed v4 addresses, more than one address may be
       specified in the fixed-address  declaration,  or  more  than  one  host
       statement may be specified matching the same client.

       The  fixed-address6 declaration is used for v6 addresses.  At this time
       it only works with a single address.  For  multiple  addresses  specify
       multiple host statements.

       If  client-specific boot parameters must change based on the network to
       which the client is attached, then multiple host declarations should be
       used.   The  host declarations will only match a client if one of their
       fixed-address statements is viable on the subnet  (or  shared  network)
       where  the  client  is attached.  Conversely, for a host declaration to
       match a client being allocated a dynamic address, it must not have  any
       fixed-address  statements.   You  may  therefore need a mixture of host
       declarations for any given client...some  having  fixed-address  state-
       ments, others without.

       hostname  should  be a name identifying the host.  If a hostname option
       is not specified for the host, hostname is used.

       Host declarations are matched to actual DHCP or BOOTP clients by match-
       ing the dhcp-client-identifier option specified in the host declaration
       to the one supplied by the client, or, if the host declaration  or  the
       client  does  not  provide a dhcp-client-identifier option, by matching
       the hardware parameter in the host declaration to the network  hardware
       address  supplied by the client.  BOOTP clients do not normally provide
       a dhcp-client-identifier, so the hardware address must be used for  all
       clients that may boot using the BOOTP protocol.

       DHCPv6 servers can use the host-identifier option parameter in the host
       declaration, and specify any option with  a  fixed  value  to  identify

       Please  be  aware  that  only the dhcp-client-identifier option and the
       hardware address can be used to match a host declaration, or the  host-
       identifier option parameter for DHCPv6 servers.  For example, it is not
       possible to match a host declaration to a host-name  option.   This  is
       because  the host-name option cannot be guaranteed to be unique for any
       given client, whereas both the hardware address and dhcp-client-identi-
       fier  option  are  at  least theoretically guaranteed to be unique to a
       given client.

       The group statement

        group {
          [ parameters ]
          [ declarations ]

       The group statement is used simply to apply one or more parameters to a
       group of declarations.  It can be used to group hosts, shared networks,
       subnets, or even other groups.

       The allow and deny statements can be used to control  the  response  of
       the  DHCP server to various sorts of requests.  The allow and deny key-
       words actually have different meanings depending on the context.  In  a
       pool  context,  these  keywords  can be used to set up access lists for
       address allocation pools.  In other contexts, the keywords simply  con-
       trol  general  server  behavior with respect to clients based on scope.
       In a non-pool context, the ignore keyword can be used in place  of  the
       deny keyword to prevent logging of denied requests.

       The following usages of allow and deny will work in any scope, although
       it is not recommended that they be used in pool declarations.

       The unknown-clients keyword

        allow unknown-clients;
        deny unknown-clients;
        ignore unknown-clients;

       The unknown-clients flag is used to tell dhcpd whether or not to dynam-
       ically assign addresses to unknown clients.  Dynamic address assignment
       to unknown clients is allowed by default.  An unknown client is  simply
       a client that has no host declaration.

       The  use  of  this  option  is  now  deprecated.   If you are trying to
       restrict access on your network to known clients, you should  use  deny
       unknown-clients;  inside  of  your address pool, as described under the

       The bootp keyword

        allow bootp;
        deny bootp;
        ignore bootp;

       The bootp flag is used to tell dhcpd whether or not to respond to bootp
       queries.  Bootp queries are allowed by default.

       The booting keyword

        allow booting;
        deny booting;
        ignore booting;

       The  booting  flag  is  used to tell dhcpd whether or not to respond to
       queries from a particular client.  This keyword only has  meaning  when
       it  appears in a host declaration.  By default, booting is allowed, but
       if it is disabled for a particular client, then that client will not be
       able to get an address from the DHCP server.

       The duplicates keyword

        allow duplicates;
        deny duplicates;

       Host  declarations  can  match client messages based on the DHCP Client
       Identifier option or based on the client's network  hardware  type  and
       MAC  address.   If  the  MAC address is used, the host declaration will
       match any client with that MAC address - even  clients  with  different
       client identifiers.  This doesn't normally happen, but is possible when
       one computer has more than one operating system installed on it  -  for
       example, Microsoft Windows and NetBSD or Linux.

       The duplicates flag tells the DHCP server that if a request is received
       from a client that matches the MAC address of a host  declaration,  any
       other  leases  matching  that  MAC  address  should be discarded by the
       server, even if the UID is not the same.  This is a  violation  of  the
       DHCP  protocol, but can prevent clients whose client identifiers change
       regularly from holding many leases  at  the  same  time.   By  default,
       duplicates are allowed.

       The declines keyword

        allow declines;
        deny declines;
        ignore declines;

       The  DHCPDECLINE  message  is used by DHCP clients to indicate that the
       lease the server has offered is not valid.  When the server receives  a
       DHCPDECLINE  for  a  particular  address,  it  normally  abandons  that
       address, assuming that some unauthorized system is using it.   Unfortu-
       nately,  a  malicious  or buggy client can, using DHCPDECLINE messages,
       completely exhaust the DHCP server's allocation pool.  The server  will
       eventually  reclaim  these  leases, but not while the client is running
       through the pool. This may cause serious thrashing in the DNS,  and  it
       will also cause the DHCP server to forget old DHCP client address allo-

       The declines flag tells the DHCP server whether or not to honor DHCPDE-
       CLINE  messages.  If it is set to deny or ignore in a particular scope,
       the DHCP server will not respond to DHCPDECLINE messages.

       The declines flag is only supported by DHCPv4 servers.  Given the large
       IPv6  address  space  and  the  internal limits imposed by the server's
       address generation mechanism we don't think it is necessary for  DHCPv6
       servers at this time.

       Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
           a) Client renews a specific address:
           If a client using a given DUID submits a DHCP REQUEST containing
           the last address abandoned by that DUID, the address will be
           reassigned to that client.

           b) Upon the second restart following an address abandonment.  When
           an address is abandoned it is both recorded as such in the lease
           file and retained as abandoned in server memory until the server
           is restarted. Upon restart, the server will process the lease file
           and all addresses whose last known state is abandoned will be
           retained as such in memory but not rewritten to the lease file.
           This means that a subsequent restart of the server will not see the
           abandoned addresses in the lease file and therefore have no record
           of them as abandoned in memory and as such perceive them as free
           for assignment.

       The total number addresses in a pool, available for a given DUID value,
       is internally limited by the server's address generation mechanism.  If
       through  mistaken  configuration,  multiple  clients are using the same
       DUID they will competing for the same addresses causing the  server  to
       reach  this internal limit rather quickly.  The internal limit isolates
       this type of activity such that address  range  is  not  exhausted  for
       other  DUID  values.  The appearance of the following error log, can be
       an indication of this condition:

           "Best match for DUID <XX> is an abandoned address, This may be a
            result of multiple clients attempting to use this DUID"

           where <XX> is an actual DUID value depicted as colon separated
           string of bytes in hexadecimal values.

       The client-updates keyword

        allow client-updates;
        deny client-updates;

       The client-updates flag tells the DHCP server whether or not  to  honor
       the  client's  intention to do its own update of its A record.  See the
       documentation under the heading THE DNS UPDATE SCHEME for details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The leasequery flag tells the DHCP server whether or not to answer DHC-
       PLEASEQUERY  packets.  The  answer  to a DHCPLEASEQUERY packet includes
       information about a specific lease, such as when it was issued and when
       it  will expire. By default, the server will not respond to these pack-

       The uses of the allow and deny keywords shown in the  previous  section
       work  pretty much the same way whether the client is sending a DHCPDIS-
       COVER or a DHCPREQUEST message - an address will be  allocated  to  the
       client  (either  the old address it's requesting, or a new address) and
       then that address will be tested to see if it's okay to let the  client
       have  it.   If  the  client requested it, and it's not okay, the server
       will send a DHCPNAK message.  Otherwise, the  server  will  simply  not
       respond  to  the  client.   If  it  is  okay to give the address to the
       client, the server will send a DHCPACK message.

       The primary motivation behind pool  declarations  is  to  have  address
       allocation pools whose allocation policies are different.  A client may
       be denied access to one pool, but allowed access to another pool on the
       same network segment.  In order for this to work, access control has to
       be done during address allocation,  not  after  address  allocation  is

       When a DHCPREQUEST message is processed, address allocation simply con-
       sists of looking up the address the client is requesting and seeing  if
       it's  still  available  for the client.  If it is, then the DHCP server
       checks both the address pool permit lists  and  the  relevant  in-scope
       allow  and deny statements to see if it's okay to give the lease to the
       client.  In the case of a DHCPDISCOVER message, the allocation  process
       is done as described previously in the ADDRESS ALLOCATION section.

       When declaring permit lists for address allocation pools, the following
       syntaxes are recognized following the allow or deny keywords:


       If specified, this statement either allows or prevents allocation  from
       this  pool  to any client that has a host declaration (i.e., is known).
       A client is known if it has a host declaration in any scope,  not  just
       the current scope.


       If  specified, this statement either allows or prevents allocation from
       this pool to any client that has no  host  declaration  (i.e.,  is  not

        members of "class";

       If  specified, this statement either allows or prevents allocation from
       this pool to any client that is a member of the named class.

        dynamic bootp clients;

       If specified, this statement either allows or prevents allocation  from
       this pool to any bootp client.

        authenticated clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any client that has  been  authenticated  using  the  DHCP
       authentication protocol.  This is not yet supported.

        unauthenticated clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any client that has not been authenticated using the  DHCP
       authentication protocol.  This is not yet supported.

        all clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to all clients.  This can be used when you want  to  write  a
       pool  declaration  for some reason, but hold it in reserve, or when you
       want to renumber your network quickly, and  thus  want  the  server  to
       force  all clients that have been allocated addresses from this pool to
       obtain new addresses immediately when they next renew.

        after time;

       If specified, this statement either allows or prevents allocation  from
       this  pool  after  a given date. This can be used when you want to move
       clients from one pool to another. The server adjusts the regular  lease
       time  so  that  the  latest expiry time is at the given time+min-lease-
       time.  A short min-lease-time enforces a step change, whereas a  longer
       min-lease-time  allows  for  a  gradual  change.  time is either second
       since epoch, or a UTC time string e.g.   4  2007/08/24  09:14:32  or  a
       string  with  time  zone  offset  in seconds e.g. 4 2007/08/24 11:14:32

       The abandon-lease-time statement

         abandon-lease-time time;

         Time should be the maximum amount of time (in seconds) that an  aban-
         doned  IPv4  lease  remains  unavailable  for assignment to a client.
         Abandoned leases will only be offered to clients if there are no free
         leases.  If not defined, the default abandon lease time is 86400 sec-
         onds (24 hours).  Note the abandoned lease time for a given lease  is
         preserved  across  server restarts.  The parameter may only be set at
         the global scope and is evaluated only once during server startup.

         Values less than sixty seconds are not recommended as this  is  below
         the  ping  check  threshold  and  can cause leases once abandoned but
         since returned to the free  state  to  not  be  pinged  before  being
         offered.   If the requested time is larger than 0x7FFFFFFF - 1 or the
         sum of the current  time  plus  the  abandoned  time  isgreater  than
         0x7FFFFFFF it is treated as infinite.

       The adaptive-lease-time-threshold statement

         adaptive-lease-time-threshold percentage;

         When  the  number  of  allocated leases within a pool rises above the
         percentage given in this statement, the  DHCP  server  decreases  the
         lease  length for new clients within this pool to min-lease-time sec-
         onds. Clients renewing an already valid (long) leases  get  at  least
         the  remaining  time  from the current lease. Since the leases expire
         faster, the server may either recover  more  quickly  or  avoid  pool
         exhaustion  entirely.  Once the number of allocated leases drop below
         the threshold, the server reverts back to normal lease times.   Valid
         percentages are between 1 and 99.

       The always-broadcast statement

         always-broadcast flag;

         The  DHCP  and BOOTP protocols both require DHCP and BOOTP clients to
         set the broadcast bit in the flags field of the BOOTP message header.
         Unfortunately, some DHCP and BOOTP clients do not do this, and there-
         fore may not receive responses from the DHCP server.  The DHCP server
         can  be  made to always broadcast its responses to clients by setting
         this flag to 'on' for the relevant scope; relevant  scopes  would  be
         inside  a  conditional statement, as a parameter for a class, or as a
         parameter for a host declaration.  To avoid creating excess broadcast
         traffic  on  your  network, we recommend that you restrict the use of
         this option to as few clients as possible.  For example,  the  Micro-
         soft  DHCP client is known not to have this problem, as are the Open-
         Transport and ISC DHCP clients.

       The always-reply-rfc1048 statement

         always-reply-rfc1048 flag;

         Some BOOTP clients expect RFC1048-style responses, but do not  follow
         RFC1048  when  sending their requests.  You can tell that a client is
         having this problem if it is not getting the options you have config-
         ured  for  it  and  if  you  see in the server log the message "(non-
         rfc1048)" printed with each BOOTREQUEST that is logged.

         If you want to send rfc1048 options to such a client, you can set the
         always-reply-rfc1048  option  in  that client's host declaration, and
         the DHCP server will respond with an  RFC-1048-style  vendor  options
         field.   This  flag  can  be  set  in  any scope, and will affect all
         clients covered by that scope.

       The authoritative statement


         not authoritative;

         The DHCP server will normally assume that the configuration  informa-
         tion  about a given network segment is not known to be correct and is
         not authoritative.  This is so that if a naive user installs  a  DHCP
         server  not fully understanding how to configure it, it does not send
         spurious DHCPNAK messages to clients  that  have  obtained  addresses
         from a legitimate DHCP server on the network.

         Network  administrators  setting  up  authoritative  DHCP servers for
         their networks should always write authoritative; at the top of their
         configuration file to indicate that the DHCP server should send DHCP-
         NAK messages to misconfigured clients.  If this is not done,  clients
         will  be  unable  to  get a correct IP address after changing subnets
         until their old lease has expired, which  could  take  quite  a  long

         Usually,  writing  authoritative; at the top level of the file should
         be sufficient.  However, if a DHCP server is to be set up so that  it
         is aware of some networks for which it is authoritative and some net-
         works for which it is not, it may  be  more  appropriate  to  declare
         authority on a per-network-segment basis.

         Note  that the most specific scope for which the concept of authority
         makes any sense is the physical network segment -  either  a  shared-
         network  statement or a subnet statement that is not contained within
         a shared-network statement.  It is not meaningful to specify that the
         server is authoritative for some subnets within a shared network, but
         not authoritative for others, nor is it meaningful  to  specify  that
         the  server  is authoritative for some host declarations and not oth-

         In order for DHCPINFORMs to be responded to by the server, they  must
         match  to subnets over which the server has authority; otherwise they
         will be ignored and logged.  To minimize the impact on  logging  vol-
         ume,  only  the  first  and  every  subsequent 100th occurrence of an
         ignored DHCPINFORM is logged.

       The boot-unknown-clients statement

         boot-unknown-clients flag;

         If the boot-unknown-clients statement is present and has a  value  of
         false  or  off,  then  clients for which there is no host declaration
         will not be allowed to obtain IP addresses.  If this statement is not
         present  or has a value of true or on, then clients without host dec-
         larations will be allowed to obtain IP addresses, as  long  as  those
         addresses  are  not  restricted  by  allow and deny statements within
         their pool declarations.

       The check-secs-byte-order statement

         check-secs-byte-order flag;

         When check-secs-byte-order is enabled,  the  server  will  check  for
         DHCPv4  clients  that  do  the byte ordering on the secs field incor-
         rectly. This field should be in network byte order but  some  clients
         get  it wrong. When this parameter is enabled the server will examine
         the secs field and if it looks wrong (high byte non zero and low byte
         zero)  swap  the  bytes.   The default is disabled. This parameter is
         only useful when doing load  balancing  within  failover.  (Formerly,
         this  behavior had to be enabled during compilation configuration via

         The db-time-format statement

            db-time-format [ default | local ] ;

            The DHCP server software outputs several timestamps  when  writing
            leases   to  persistent  storage.   This  configuration  parameter
            selects one of two output formats.  The default format prints  the
            day, date, and time in UTC, while the local format prints the sys-
            tem seconds-since-epoch, and helpfully provides the day  and  time
            in  the  system  timezone  in  a  comment.   The  time formats are
            described in detail in the dhcpd.leases(5) manpage.

         The ddns-hostname statement

            ddns-hostname name;

            The name parameter should be the hostname that  will  be  used  in
            setting up the client's A and PTR records.  If no ddns-hostname is
            specified in scope, then the server will derive the hostname auto-
            matically,  using an algorithm that varies for each of the differ-
            ent update methods.

         The ddns-domainname statement

            ddns-domainname name;

            The name parameter should be the domain name that will be appended
            to  the  client's  hostname  to form a fully-qualified domain-name

         The ddns-dual-stack-mixed-mode statement

            ddns-dual-stack-mixed-mode flag;

            The ddns-dual-stack-mixed-mode parameter controls whether  or  not
            the  server  applies  Dual Stack Mixed Mode rules during DDNS con-
            flict resolution.  This parameter is off by default, has no effect
            unless update-conflict-detection is enabled, and may only be spec-
            ified at the global scope.

         The ddns-guard-id-must-match statement

            ddns-guard-id-must-match flag;

            The ddns-guard-id-must-match parameter controls whether or  not  a
            the  client  id  within  a  DHCID  RR  must  match that of the DNS
            update's client to permit DNS entries associated with  that  DHCID
            RR  to  be  ovewritten.   Proper  conflict  resolution requires ID
            matching and should only be disabled after careful  consideration.
            When disabled, it is allows any DNS updater to replace DNS entries
            that have an associated DHCID RR, regardless of  client  identity.
            This  parameter is on by default, has no effect unless update-con-
            flict-detection is enabled, and  may  only  be  specified  at  the
            global scope.

         The dns-local-address4 and dns-local-address6 statements

            ddns-local-address4 address;

            ddns-local-address6 address;

            The address parameter should be the local IPv4 or IPv6 address the
            server should use as the from address  when  sending  DDNS  update

         The ddns-other-guard-is-dynamic statement

            ddns-other-guard-is-dynamic flag;

            The  ddns-other-guard-is-dynamic parameter controls whether or not
            a a server running DSMM will consider the presence  of  the  other
            update  style  DHCID  RR as an indcation that a DNS entries may be
            overwritten. It should only be enabled after careful study  as  it
            allows DNS entries that would otherwise be protected as static, to
            be overwritten in certain cases. This paramater is off by default,
            has  no  effect  unless ddns-dual-stack-mixed-mode is enabled, and
            may only be specified at the global scope.

         The ddns-rev-domainname statement

            ddns-rev-domainname name;

            The name parameter should be the domain name that will be appended
            to  the  client's reversed IP address to produce a name for use in
            the client's PTR record.  By default, this is "in-addr.arpa.", but
            the default can be overridden here.

            The  reversed  IP address to which this domain name is appended is
            always the IP address of the  client,  in  dotted  quad  notation,
            reversed  -  for example, if the IP address assigned to the client
            is, then the reversed IP address is  So a
            client  with  that  IP  address  would, by default, be given a PTR
            record of

         The ddns-update-style parameter

            ddns-update-style style;

            The style parameter must be one of standard, interim or none.  The
            ddns-update-style  statement is only meaningful in the outer scope
            - it is evaluated once after reading the dhcpd.conf  file,  rather
            than  each time a client is assigned an IP address, so there is no
            way to use different DNS update styles for different clients.  The
            default is none.

         The ddns-updates statement

             ddns-updates flag;

            The ddns-updates parameter controls whether or not the server will
            attempt to do a DNS update when a lease is confirmed.  Set this to
            off  if  the server should not attempt to do updates within a cer-
            tain scope.  The ddns-updates parameter is on by default.  To dis-
            able  DNS updates in all scopes, it is preferable to use the ddns-
            update-style statement, setting the style to none.

         The default-lease-time statement

            default-lease-time time;

            Time should be the length in seconds that will be  assigned  to  a
            lease  if  the client requesting the lease does not ask for a spe-
            cific expiration time.  This is used for both  DHCPv4  and  DHCPv6
            leases  (it is also known as the "valid lifetime" in DHCPv6).  The
            default is 43200 seconds.

         The delayed-ack and max-ack-delay statements

            delayed-ack count;

            max-ack-delay microseconds;

            Count should be an integer value from zero to 2^16-1 and  defaults
            to 0, which means that the feature is disabled.  Otherwise, 28 may
            be a sensible starting point for  many  configurations  (SO_SNDBUF
            size  /  576 bytes.)  The count represents how many DHCPv4 replies
            maximum will be queued pending transmission until after a database
            commit  event.  If this number is reached, a database commit event
            (commonly resulting in  fsync()  and  representing  a  performance
            penalty)  will  be made, and the reply packets will be transmitted
            in a batch afterwards.  This preserves the RFC2131 direction  that
            "stable  storage" be updated prior to replying to clients.  Should
            the DHCPv4 sockets "go dry" (select() returns immediately with  no
            read  sockets),  the  commit  is  made  and any queued packets are

            Similarly, microseconds indicates how many microseconds  are  per-
            mitted  to  pass  inbetween queuing a packet pending an fsync, and
            performing the fsync.  Valid values range from 0  to  2^32-1,  and
            defaults to 250,000 (1/4 of a second).

            The delayed-ack feature is compiled in by default, but can be dis-
            abled at compile time  with  './configure  --disable-delayed-ack'.
            Please note that the delayed-ack feature is not currently compati-
            ble with support for DHPCv4-over-DHCPv6 so when a 4to6 port ommand
            line  argument enables this in the server the delayed-ack value is
            reset to 0.

         The dhcp-cache-threshold statement

            dhcp-cache-threshold percentage;

            The dhcp-cache-threshold statement  takes  one  integer  parameter
            with  allowed  values  between  0 and 100. The default value is 25
            (25% of the lease time). This parameter expresses  the  percentage
            of the total lease time, measured from the beginning, during which
            a client's attempt to renew its lease will result in  getting  the
            already  assigned lease, rather than an extended lease.  This fea-
            ture is supported for both IPv4 and IPv6  and  down  to  the  pool
            level and for IPv6 all three pool types: NA, TA and PD.

            Clients  that  attempt  renewal frequently can cause the server to
            update and write the database frequently resulting  in  a  perfor-
            mance  impact  on  the server.  The dhcp-cache-threshold statement
            instructs the DHCP server to avoid updating leases too  frequently
            thus  avoiding this behavior.  Instead the server replies with the
            same lease (i.e. reuses it) with no modifications except for  CLTT
            (Client Last Transmission Time) and for IPv4:

                the lease time sent to the client is shortened by the age of
                the lease

            while for IPv6:

                the preferred and valid lifetimes sent to the client are
                shortened by the age of the lease.

            None of these changes require writing the lease to disk.

            When an existing lease is matched to a renewing client, it will be
            reused if all of the following conditions are true:
                1. The dhcp-cache-threshold is larger than zero
                2. The current lease is active
                3. The percentage of the lease time that has elapsed is less than
                4. The client information provided in the renewal does not alter
                any of the following:
                   a. DNS information and DNS updates are enabled
                   b. Billing class to which the lease is associated (IPv4 only)
                   c. The host declaration associated with the lease (IPv4 only)
                   d. The client id - this may happen if a client boots without
                      a client id and then starts using one in subsequent
                      requests. (IPv4 only)

            While lease data is not written to disk when a  lease  is  reused,
            the server will still execute any on-commit statements.

            Note  that  the  lease  can be reused if the options the client or
            relay agent sends are changed.  These changes will not be recorded
            in  the  in-memory  or  on-disk  databases until the client renews
            after the threshold time is reached.

         The do-forward-updates statement

            do-forward-updates flag;

            The do-forward-updates statement instructs the DHCP server  as  to
            whether  it should attempt to update a DHCP client's A record when
            the client acquires or renews a  lease.   This  statement  has  no
            effect  unless  DNS  updates  are  enabled.   Forward  updates are
            enabled by default.  If this statement is used to disable  forward
            updates, the DHCP server will never attempt to update the client's
            A record, and will only ever attempt to update  the  client's  PTR
            record if the client supplies an FQDN that should be placed in the
            PTR record using the fqdn option.  If forward updates are enabled,
            the DHCP server will still honor the setting of the client-updates

         The dont-use-fsync statement

            dont-use-fsync flag;

            The dont-use-fsync statement  instructs  the  DHCP  server  if  it
            should  call  fsync()  when  writing leases to the lease file.  By
            default and if the flag is set  to  false  the  server  will  call
            fsync().  Suppressing the call to fsync() may increase the perfor-
            mance of the server but it also adds a risk that a lease will  not
            be  properly  written  to  the  disk after it has been issued to a
            client and before the server stops.  This can  lead  to  duplicate
            leases  being  issued  to different clients.  Using this option is
            not recommended.

         The dynamic-bootp-lease-cutoff statement

            dynamic-bootp-lease-cutoff date;

            The dynamic-bootp-lease-cutoff statement sets the ending time  for
            all  leases  assigned dynamically to BOOTP clients.  Because BOOTP
            clients do not have any way of renewing  leases,  and  don't  know
            that  their leases could expire, by default dhcpd assigns infinite
            leases to all BOOTP clients.  However, it may make sense  in  some
            situations  to  set a cutoff date for all BOOTP leases - for exam-
            ple, the end of a school term, or the time at night when a  facil-
            ity is closed and all machines are required to be powered off.

            Date  should  be  the date on which all assigned BOOTP leases will
            end.  The date is specified in the form:

                                  W YYYY/MM/DD HH:MM:SS

            W is the day of the week expressed as a number from zero  (Sunday)
            to  six  (Saturday).  YYYY is the year, including the century.  MM
            is the month expressed as a number from 1 to 12.  DD is the day of
            the  month, counting from 1.  HH is the hour, from zero to 23.  MM
            is the minute and SS is the second.  The time is always in Coordi-
            nated Universal Time (UTC), not local time.

         The dynamic-bootp-lease-length statement

            dynamic-bootp-lease-length length;

            The dynamic-bootp-lease-length statement is used to set the length
            of leases dynamically assigned to BOOTP clients.  At  some  sites,
            it  may  be possible to assume that a lease is no longer in use if
            its holder has not used BOOTP or DHCP to get its address within  a
            certain  time period.  The period is specified in length as a num-
            ber of seconds.  If a client reboots using BOOTP during the  time-
            out  period,  the  lease  duration  is reset to length, so a BOOTP
            client that boots frequently enough will  never  lose  its  lease.
            Needless  to  say,  this parameter should be adjusted with extreme

         The echo-client-id statement

            echo-client-id flag;

            The echo-client-id statement is used to enable or disable RFC 6842
            compliant  behavior.   If  the echo-client-id statement is present
            and has a value of true or on, and a DHCP DISCOVER or  REQUEST  is
            received  which contains the client identifier option (Option code
            61), the server will copy the option into its response  (DHCP  ACK
            or  NAK)  per  RFC  6842.   In other words if the client sends the
            option it will receive it back. By default, this flag is  off  and
            client identifiers will not echoed back to the client.

         The filename statement

            filename "filename";

            The filename statement can be used to specify the name of the ini-
            tial boot file which is to be loaded by a  client.   The  filename
            should be a filename recognizable to whatever file transfer proto-
            col the client can be expected to use to load the file.

         The fixed-address declaration

            fixed-address address [, address ... ];

            The fixed-address declaration is used to assign one or more  fixed
            IP  addresses to a client.  It should only appear in a host decla-
            ration.  If more than one  address  is  supplied,  then  when  the
            client  boots, it will be assigned the address that corresponds to
            the network on which it is booting.  If none of the  addresses  in
            the fixed-address statement are valid for the network to which the
            client is connected, that client will not match the host  declara-
            tion  containing  that fixed-address declaration.  Each address in
            the fixed-address declaration should be either an IP address or  a
            domain name that resolves to one or more IP addresses.

         The fixed-address6 declaration

            fixed-address6 ip6-address ;

            The  fixed-address6  declaration  is  used  to assign a fixed IPv6
            addresses to a client.  It should only appear in a  host  declara-

         The fixed-prefix6 declaration

            fixed-prefix6 low-address / bits;

            The  fixed-prefix6 declaration is used to assign a fixed IPv6 pre-
            fix to a client.  It should only appear in a host declaration, but
            multiple fixed-prefix6 statements may appear in a single host dec-

            The low-address specifies the start of the  prefix  and  the  bits
            specifies the size of the prefix in bits.

            If  there  are multiple prefixes for a given host entry the server
            will choose one that matches the requested prefix size or, if none
            match, the first one.

            If  there  are  multiple  host declarations the server will try to
            choose a declaration where the fixed-address6 matches the client's
            subnet.   If  none  match  it  will choose one that doesn't have a
            fixed-address6 statement.

            Note Well: Unlike the fixed address the fixed prefix does not need
            to  match a subnet in order to be served.  This allows you to pro-
            vide a prefix to a client that is outside of the subnet  on  which
            the client makes the request to the the server.

         The get-lease-hostnames statement

            get-lease-hostnames flag;

            The get-lease-hostnames statement is used to tell dhcpd whether or
            not to look up the domain name corresponding to the IP address  of
            each  address  in the lease pool and use that address for the DHCP
            hostname option.  If flag is true, then this lookup  is  done  for
            all  addresses  in  the  current scope.  By default, or if flag is
            false, no lookups are done.

         The hardware statement

            hardware hardware-type hardware-address;

            In order for a BOOTP client to be recognized, its network hardware
            address  must  be  declared  using  a  hardware clause in the host
            statement.  hardware-type must be the name of a physical  hardware
            interface type.  Currently, only the ethernet and token-ring types
            are recognized, although support for a  fddi  hardware  type  (and
            others) would also be desirable.  The hardware-address should be a
            set of hexadecimal octets (numbers from 0 through ff) separated by
            colons.  The hardware statement may also be used for DHCP clients.

         The host-identifier option statement

            host-identifier option option-name option-data;


            host-identifier v6relopt number option-name option-data;

            This  identifies a DHCPv6 client in a host statement.  option-name
            is any option, and option-data is the value for  the  option  that
            the  client  will  send. The option-data must be a constant value.
            In the v6relopts case the additional number is the relay to  exam-
            ine  for  the specified option name and value.  The values are the
            same as for the v6relay option.  0 is a  no-op,  1  is  the  relay
            closest  to  the client, 2 the next one in and so on.  Values that
            are larger than the maximum number of relays (currently 32)  indi-
            cate the relay closest to the server independent of number.

         The ignore-client-uids statement

            ignore-client-uids flag;

            If  the ignore-client-uids statement is present and has a value of
            true or on, the UID for clients will not  be  recorded.   If  this
            statement  is  not  present  or  has a value of false or off, then
            client UIDs will be recorded.

         The infinite-is-reserved statement

            infinite-is-reserved flag;

            ISC DHCP now supports  'reserved'  leases.   See  the  section  on
            RESERVED  LEASES below.  If this flag is on, the server will auto-
            matically reserve leases allocated to clients which  requested  an
            infinite (0xffffffff) lease-time.

            The default is off.

         The lease-file-name statement

            lease-file-name name;

            Name  Where  name  is the name of the DHCP server's lease file. By
            default, this is DBDIR/dhcpd.leases.  This statement  must  appear
            in  the  outer  scope of the configuration file - if it appears in
            some other scope, it will have no effect.  The value must  be  the
            absolute  path  of  the  file to use.  The order of precedence the
            server uses for the lease file name is:

                1. lease-file-name configuration file statement.
                2. -lf command line flag.
                3. PATH_DHCPD_DB environment variable.

         The dhcpv6-lease-file-name statement

            dhcpv6-lease-file-name name;

            Where name is the name of the DHCP server's lease  file  when  the
            server is running DHCPv6. By default, this is DBDIR/dhcpd6.leases.
            This statement must appear in the outer scope of the configuration
            file  - if it appears in some other scope, it will have no effect.
            The value must be the absolute path of the file to use.  The order
            of precedence the server uses for the lease file name is:

                1. dhcpv6-lease-file-name configuration file statement.
                2. -lf command line flag.
                3. PATH_DHCPD6_DB environment variable.

         The lease-id-format parameter

            lease-id-format format;

            The  format parameter must be either octal or hex.  This parameter
            governs the format used to write certain values  to  lease  files.
            With  the  default  format,  octal,  values  are written as quoted
            strings in which non-printable characters are represented as octal
            escapes  -  a  backslash character followed by three octal digits.
            When the hex  format  is  specified,  values  are  written  as  an
            unquoted  series  of  pairs  of  hexadecimal  digits, separated by

            Currently, the values written out based on lease-id-format are the
            server-duid,  the  uid  (DHCPv4 leases), and the IAID_DUID (DHCPv6
            leases).  Note the server automatically reads the values in either

         The limit-addrs-per-ia statement

            limit-addrs-per-ia number;

            By default, the DHCPv6 server will limit clients to one IAADDR per
            IA option, meaning one address.  If you wish to permit clients  to
            hang  onto multiple addresses at a time, configure a larger number

            Note that there is no present method to configure  the  server  to
            forcibly  configure the client with one IP address per each subnet
            on a shared network.  This is left to future work.

         The local-port statement

            local-port port;

            This statement causes the DHCP server to listen for DHCP  requests
            on the UDP port specified in port, rather than on port 67.

         The local-address statement

            local-address address;

            This  statement causes the DHCP server to listen for DHCP requests
            sent to the specified address, rather than requests  sent  to  all
            addresses.   Since  serving directly attached DHCP clients implies
            that the server must respond to requests sent to the  all-ones  IP
            address,  this  option  cannot  be used if clients are on directly
            attached networks; it is only realistically useful  for  a  server
            whose  only  clients  are  reached  via unicasts, such as via DHCP
            relay agents.

            Note:  This statement is only effective if the server was compiled
            using  the  USE_SOCKETS  #define  statement, which is default on a
            small number of operating systems, and must be  explicitly  chosen
            at compile-time for all others.  You can be sure if your server is
            compiled with USE_SOCKETS if you  see  lines  of  this  format  at

             Listening on Socket/eth0

            Note  also  that since this bind()s all DHCP sockets to the speci-
            fied address, that only one address may be supported in  a  daemon
            at a given time.

         The local-address6 and bind-local-address6 statements

            local-address6 address;

            bind-local-address6 flag;

            The  local-address6  statement causes the DHCP server to send IPv6
            packets as originating from the  specified  IPv6  address,  rather
            than leaving the kernel to fill in the source address field.

            When bind-local-address6 is present and has a value of true or on,
            service sockets are bound to address too.

            By default address is the undefined address  and  the  bind-local-
            address6 is disabled, both may only be set at the global scope.

         The log-facility statement

            log-facility facility;

            This  statement causes the DHCP server to do all of its logging on
            the specified log facility once the dhcpd.conf file has been read.
            By  default the DHCP server logs to the daemon facility.  Possible
            log facilities include auth, authpriv, cron,  daemon,  ftp,  kern,
            lpr,  mail,  mark,  news,  ntp,  security, syslog, user, uucp, and
            local0 through local7.  Not all of these facilities are  available
            on  all  systems,  and  there may be other facilities available on
            other systems.

            In addition to setting this value, you may  need  to  modify  your
            syslog.conf  file  to  configure  logging of the DHCP server.  For
            example, you might add a line like this:

                 local7.debug /var/log/dhcpd.log

            The syntax of the syslog.conf file may be different on some  oper-
            ating  systems  -  consult the syslog.conf manual page to be sure.
            To get syslog to start logging to the new  file,  you  must  first
            create  the  file with correct ownership and permissions (usually,
            the same  owner  and  permissions  of  your  /var/log/messages  or
            /usr/adm/messages  file  should be fine) and send a SIGHUP to sys-
            logd.  Some systems support log rollover using a shell  script  or
            program called newsyslog or logrotate, and you may be able to con-
            figure this as well so that your log file doesn't grow  uncontrol-

            Because  the  log-facility setting is controlled by the dhcpd.conf
            file, log messages printed while parsing the  dhcpd.conf  file  or
            before parsing it are logged to the default log facility.  To pre-
            vent this, see the README file included  with  this  distribution,
            which  describes  BUG:  where is that mentioned in README?  how to
            change the default log facility.  When this parameter is used, the
            DHCP server prints its startup message a second time after parsing
            the configuration file, so that the log will  be  as  complete  as

         The log-threshold-high and log-threshold-low statements

            log-threshold-high percentage;

            log-threshold-low percentage;

            The  log-threshold-low  and log-threshold-high statements are used
            to control when a message is output about pool usage.   The  value
            for  both  of  them  is the percentage of the pool in use.  If the
            high threshold is 0 or has not been specified, no messages will be
            produced.   If a high threshold is given, a message is output once
            the pool usage passes that level.  After that,  no  more  messages
            will be output until the pool usage falls below the low threshold.
            If the low threshold is not given, it default to a value of zero.

            A special case occurs when the low threshold is set  to  be  higer
            than  the  high threshold.  In this case, a message will be gener-
            ated each time a lease is acknowledged  when  the  pool  usage  is
            above the high threshold.

            Note  that  threshold  logging  will be automatically disabled for
            shared subnets whose total number  of  addresses  is  larger  than
            (2^64)-1.   The  server  will emit a log statement at startup when
            threshold logging is disabled as shown below:

                "Threshold logging  disabled  for  shared  subnet  of  ranges:

            This  is  likely  to  have no practical runtime effect as CPUs are
            unlikely to support a server actually reaching such a large number
            of leases.

         The max-lease-time statement

            max-lease-time time;

            Time should be the maximum length in seconds that will be assigned
            to a lease.  If not defined, the default  maximum  lease  time  is
            86400.   The  only  exception  to this is that Dynamic BOOTP lease
            lengths, which are not specified by the client, are not limited by
            this maximum.

         The min-lease-time statement

            min-lease-time time;

            Time should be the minimum length in seconds that will be assigned
            to a lease.  The default is the minimum of  300  seconds  or  max-

         The min-secs statement

            min-secs seconds;

            Seconds  should  be  the  minimum number of seconds since a client
            began trying to acquire a new lease before the  DHCP  server  will
            respond  to  its  request.  The number of seconds is based on what
            the client reports, and the maximum  value  that  the  client  can
            report is 255 seconds.  Generally, setting this to one will result
            in the DHCP server not responding to the client's  first  request,
            but always responding to its second request.

            This  can  be  used  to set up a secondary DHCP server which never
            offers an address to a client until the primary  server  has  been
            given  a  chance  to  do  so.   If the primary server is down, the
            client will bind to the secondary server,  but  otherwise  clients
            should  always  bind  to the primary.  Note that this does not, by
            itself, permit a primary server and a secondary server to share  a
            pool of dynamically-allocatable addresses.

         The next-server statement

            next-server server-name;

            The  next-server  statement is used to specify the host address of
            the server from which the initial  boot  file  (specified  in  the
            filename  statement)  is  to  be  loaded.  Server-name should be a
            numeric IP address or a domain name.

         The omapi-port statement

            omapi-port port;

            The omapi-port statement causes the  DHCP  server  to  listen  for
            OMAPI  connections  on  the  specified  port.   This  statement is
            required to enable the OMAPI protocol, which is  used  to  examine
            and modify the state of the DHCP server as it is running.

         The one-lease-per-client statement

            one-lease-per-client flag;

            If this flag is enabled, whenever a client sends a DHCPREQUEST for
            a particular lease, the server will automatically free  any  other
            leases the client holds.  This presumes that when the client sends
            a DHCPREQUEST, it has forgotten any lease  not  mentioned  in  the
            DHCPREQUEST - i.e., the client has only a single network interface
            and it does not remember leases it's holding on networks to  which
            it  is  not  currently attached.  Neither of these assumptions are
            guaranteed or provable, so we urge caution  in  the  use  of  this

         The persist-eui-64-leases statement

            persist-eui-64-leases flag;

            When  this  flag  is  enabled,  the server will write EUI-64 based
            leases to the leases file. Since such leases  can  only,  ever  be
            valid  for  a single DUID value it can be argued that writing them
            to the leases file isn't essential and not doing so may have  per-
            fomance  advantages.  See use-eui-64 statement for more details on
            EUI-64 based address allocation.  The flag is enabled  by  default
            and may only be set at the global scope.

         The pid-file-name statement

            pid-file-name name;

            Name  should  be  the  name  of the DHCP server's process ID file.
            This is the file in which the DHCP server's process ID  is  stored
            when  the  server  starts.   By default, this is RUNDIR/dhcpd.pid.
            Like the lease-file-name statement, this statement must appear  in
            the outer scope of the configuration file. The order of precedence
            used by the server is:

                1. pid-file-name configuration file statement.
                2. -lf command line flag.
                3. PATH_DHCPD_PID environment variable.

            The dhcpv6-pid-file-name statement

              dhcpv6-pid-file-name name;

              Name is the name of the pid file to  use  if  and  only  if  the
              server   is  running  in  DHCPv6  mode.   By  default,  this  is
              DBDIR/dhcpd6.pid.   This  statement,  like  pid-file-name,  must
              appear  in the outer scope of the configuration file.  The order
              of precedence used by the server is:

                  1. dhcpv6-pid-file-name configuration file statement.
                  2. -lf command line flag.
                  3. PATH_DHCPD6_PID environment variable.

            The ping-check statement

              ping-check flag;

              When the DHCP server is considering dynamically allocating an IP
              address  to  a  client,  it  first sends an ICMP Echo request (a
              ping) to the address being assigned.  It waits for a second, and
              if no ICMP Echo response has been heard, it assigns the address.
              If a response is heard, the lease is abandoned, and  the  server
              does not respond to the client.  The lease will remain abandoned
              for a minimum of abandon-lease-time seconds.

              If a there are no free addressses but  there  are  abandoned  IP
              addresses,  the DHCP server will attempt to reclaim an abandoned
              IP address regardless of the value of abandon-lease-time.

              This  ping  check  introduces  a  default  one-second  delay  in
              responding  to DHCPDISCOVER messages, which can be a problem for
              some clients.  The default delay of one second may be configured
              using  the ping-timeout parameter.  The ping-check configuration
              parameter can be used to control checking  -  if  its  value  is
              false, no ping check is done.

            The ping-cltt-secs statement

              ping-cltt-secs seconds;

              The server will conduct a ping check if all the following condi-
              tions are true:

              1. Ping checking is enabled.

              2. The server is responding to a DISCOVER.

              3. The lease to be offered is neither static  nor  active  (i.e.
              still a valid lease).

              4. And any of the following are true:
                  a. This will be the first offer of this lease (CLTT is 0).
                  b.  The  lease  is  being offered to a client other than its
              previous owner
                  c. The lease is being offered to its previous owner and more
                  ping-cltt-secs  have  elapsed  since  CLTT  of  the original
                  d. The lease was abandoned and the server is  attempting  to
              reclaim it.

              The  ping-cltt-secs  statement  allows  the  user to specify the
              amount of time that must elaspe after CLTT before a  ping  check
              will be conducted.  The default value is sixty seconds.

            The ping-timeout statement

              ping-timeout seconds;

              If  the  DHCP  server  determined  it  should  send an ICMP echo
              request (a ping) because the ping-check statement is true, ping-
              timeout allows you to configure how many seconds the DHCP server
              should wait for an ICMP Echo response to be heard,  if  no  ICMP
              Echo  response  has been received before the timeout expires, it
              assigns the address.  If a response is heard, the lease is aban-
              doned,  and  the  server  does not respond to the client.  If no
              value is set, ping-timeout defaults to 1 second. (See also ping-
              timeout-ms below)

            The ping-timeout-ms statement

              ping-timeout-ms milliseconds;

              Allows  you  to  specify the ping timeout in milliseconds rather
              than seconds.  If this value is greater than  zero,  the  server
              will  use  it  in  place  of ping-timeout.  The default value is

            The preferred-lifetime statement

              preferred-lifetime seconds;

              IPv6 addresses have  'valid'  and  'preferred'  lifetimes.   The
              valid  lifetime  determines at what point at lease might be said
              to have expired, and is no longer useable.  A preferred lifetime
              is  an  advisory  condition to help applications move off of the
              address and onto currently valid addresses (should  there  still
              be any open TCP sockets or similar).

              The preferred lifetime defaults to 5/8 the default lease time.

            The prefix-length-mode statement

              prefix-length-mode mode;

              According  to  RFC  3633, DHCPv6 clients may specify preferences
              when soliciting prefixes by including  an  IA_PD  Prefix  option
              within  the IA_PD option. Among the preferences that may be con-
              veyed is the  "prefix-length".  When  non-zero  it  indicates  a
              client's  desired  length  for offered prefixes.  The RFC states
              that servers "MAY choose to use the information...to select pre-
              fix(es)" but does not specify any particular rules for doing so.
              The prefix-length-mode statement can be used to set  the  prefix
              selection rules employed by the server, when clients send a non-
              zero prefix-length value. The mode  parameter  must  be  one  of
              ignore, prefer, exact, minimum, or maximum where:

              1.  ignore  -  The  requested length is ignored. The server will
              offer the first available prefix.

              2. prefer - The server will offer  the  first  available  prefix
              with the same length as the requested length.  If none are found
              then it will offer the first available  prefix  of  any  length.
              This is the default behavior.

              3. exact - The server will offer the first available prefix with
              the same length as the requested length.  If none are found,  it
              will return a status indicating no prefixes available.

              4.  minimum  -  The server will offer the first available prefix
              with the same length as  the  requested  length.   If  none  are
              found, it will return the first available prefix whose length is
              greater than (e.g. longer than), the requested value.   If  none
              of  those  are found, it will return a status indicating no pre-
              fixes available.  For example, if client requests  a  length  of
              /60,  and  the  server has available prefixes of lengths /56 and
              /64, it will offer prefix of length /64.

              5. maximum - The server will offer the  first  available  prefix
              with  the  same  length  as  the  requested length.  If none are
              found, it will return the first available prefix whose length is
              less  than (e.g. shorter than), the requested value.  If none of
              those are found, it will return a status indicating no  prefixes
              available.  For example, if client requests a length of /60, and
              the server has available prefixes of lengths  /56  and  /64,  it
              will offer a prefix of length /56.

              In general "first available" is determined by the order in which
              pools are defined in the server's configuration.   For  example,
              if a subnet is defined with three prefix pools A,B, and C:

              subnet 3000::/64 {
                   # pool A
                   pool6 {
                   # pool B
                   pool6 {
                   # pool C
                   pool6 {

              then  the  pools will be checked in the order A, B, C. For modes
              prefer, minimum, and maximum this may mean checking the pools in
              that  order  twice.  A first pass through is made looking for an
              available prefix of exactly the preferred length.  If  none  are
              found,  then a second pass is performed starting with pool A but
              with appropriately adjusted length criteria.

            The release-on-roam statement

              release-on-roam flag;

              When enabled and the dhcpd server detects that a  DHCPv6  client
              (IAID+DUID)  has  roamed  to  a new network, it will release the
              pre-existing leases on the old network and emit a log  statement
              similiar to the following:

                    "Client:  <id>  roamed  to  new  network, releasing lease:

              The server will carry out all of the same steps that would  nor-
              mally  occur  when  a  client explicitly releases a lease.  When
              release-on-roam is disabled (the default) the server makes  such
              leases unavailable until they expire or the server is restarted.
              Clients that need leases in  multiple  networks  must  supply  a
              unique IAID in each IA.  This parameter may only be specified at
              the global level.

            The remote-port statement

              remote-port port;

              This statement causes the DHCP server to transmit DHCP responses
              to DHCP clients upon the UDP port specified in port, rather than
              on port 68.  In the event that the UDP response  is  transmitted
              to  a  DHCP Relay, the server generally uses the local-port con-
              figuration value.  Should the DHCP Relay happen to be  addressed
              as, however, the DHCP Server transmits its response to
              the remote-port configuration value.   This  is  generally  only
              useful for testing purposes, and this configuration value should
              generally not be used.

            The server-identifier statement

              server-identifier hostname;

              The server-identifier statement can be used to define the  value
              that  is  sent  in the DHCP Server Identifier option for a given
              scope.  The value specified must be an IP address for  the  DHCP
              server, and must be reachable by all clients served by a partic-
              ular scope.

              The use of the server-identifier statement is not recommended  -
              the  only  reason  to  use it is to force a value other than the
              default value to be sent on occasions where  the  default  value
              would  be  incorrect.  The default value is the first IP address
              associated with the physical  network  interface  on  which  the
              request arrived.

              The usual case where the server-identifier statement needs to be
              sent is when a physical interface has more than one IP  address,
              and  the one being sent by default isn't appropriate for some or
              all clients served by that interface.  Another  common  case  is
              when  an alias is defined for the purpose of having a consistent
              IP address for the DHCP server,  and  it  is  desired  that  the
              clients use this IP address when contacting the server.

              Supplying  a  value  for  the  dhcp-server-identifier  option is
              equivalent to using the server-identifier statement.

            The server-id-check statement

              server-id-check flag;

              The server-id-check statement is used to control whether or  not
              a  server, participating in failover, verifies that the value of
              the dhcp-server-identifier  option  in  received  DHCP  REQUESTs
              match  the  server's id before processing the request. Server id
              checking is disabled by default.  Setting this flag  enables  id
              checking  and  thereafter  the server will only process requests
              that match.  Note the flag setting should be consistent  between
              failover partners.

              Unless overridden by use of the server-identifier statement, the
              value the server uses as its id will be  the  first  IP  address
              associated  with  the  physical  network  interface on which the
              request arrived.

              In order to reduce runtime overhead the server only checks for a
              server  id  option in the global and subnet scopes.  Complicated
              configurations may result in different server ids for this check
              and  when  the server id for a reply packet is determined, which
              would prohibit the server from responding.

              The primary use for this option is when a  client  broadcasts  a
              request  but  requires  that  the  response come from a specific
              failover peer.  An example  of  this  would  be  when  a  client
              reboots  while  its  lease  is  still active - in this case both
              servers will normally respond.  Most  of  the  time  the  client
              won't  check  the server id and can use either of the responses.
              However if the client does check the server id it may reject the
              response  if it came from the wrong peer.  If the timing is such
              that the "wrong" peer responds first most of the time the client
              may not get an address for some time.

              Care should be taken before enabling this option.

            The server-duid statement

              server-duid LLT [ hardware-type timestamp hardware-address ] ;

              server-duid EN enterprise-number enterprise-identifier ;

              server-duid LL [ hardware-type hardware-address ] ;

              The  server-duid  statement  configures the server DUID. You may
              pick either LLT (link local address plus time), EN (enterprise),
              or LL (link local).

              If  you  choose LLT or LL, you may specify the exact contents of
              the DUID.  Otherwise the server will  generate  a  DUID  of  the
              specified type.

              If you choose EN, you must include the enterprise number and the

              If there is a server-duid statement in the lease  file  it  will
              take  precedence  over the server-duid statement from the config
              file and a dhcp6.server-id option in the config file will  over-
              ride both.

              The default server-duid type is LLT.

            The server-name statement

              server-name name ;

              The  server-name  statement  can be used to inform the client of
              the name of the server from which it is booting.  Name should be
              the name that will be provided to the client.

            The dhcpv6-set-tee-times statement

              dhcpv6-set-tee-times flag;

              The  dhcpv6-set-tee-times statement enables setting T1 and T2 to
              the values recommended in RFC 3315 (Section 22.4).  When setting
              T1  and  T2,  the  server  will  use dhcp-renewal-time and dhcp-
              rebinding-time, respectively.  A value of zero tells the  client
              it may choose its own value.

              When  those  options  are not defined then values will be set to
              zero unless the global dhcpv6-set-tee-times  is  enabled.   When
              this  option  is enabled the times are calculated as recommended
              by RFC 3315, Section 22.4:

                    T1 will be set to 0.5 times the shortest  preferred  life-
                    in the reply.  If the "shortest" preferred lifetime is
                    0xFFFFFFFF,  T1 will set to 0xFFFFFFFF.

                    T2  will  be set to 0.8 times the shortest preferred life-
                    in the reply.  If the "shortest" preferred lifetime is
                    0xFFFFFFFF,  T2 will set to 0xFFFFFFFF.

              Keep in mind that given sufficiently small lease lifetimes,  the
              above  calculations  will  result in the two values being equal.
              For example, a 9 second lease lifetime would yield T1 = T2  =  4
              seconds,  which  would  cause clients to issue rebinds only.  In
              such a case it would likely be better to explicitly  define  the

              Note  that  dhcpv6-set-tee-times  is intended to be transitional
              and will likely be removed in a future release. Once removed the
              behavior  will  be  to use the configured values when present or
              calculate them per the RFC. If you want zeros,  define  them  as

            The site-option-space statement

              site-option-space name ;

              The  site-option-space  statement  can be used to determine from
              what option space site-local options will be taken.  This can be
              used  in much the same way as the vendor-option-space statement.
              Site-local options in DHCP are those options whose numeric codes
              are  greater than 224.  These options are intended for site-spe-
              cific uses, but are frequently used by vendors of embedded hard-
              ware  that contains DHCP clients.  Because site-specific options
              are allocated on an ad hoc basis, it is quite possible that  one
              vendor's DHCP client might use the same option code that another
              vendor's client uses, for different purposes.  The  site-option-
              space  option can be used to assign a different set of site-spe-
              cific options for each such vendor, using conditional evaluation
              (see dhcp-eval (5) for details).

            The stash-agent-options statement

              stash-agent-options flag;

              If the stash-agent-options parameter is true for a given client,
              the server will record the relay agent information options  sent
              during  the client's initial DHCPREQUEST message when the client
              was in the SELECTING state and behave as if  those  options  are
              included  in  all  subsequent  DHCPREQUEST  messages sent in the
              RENEWING state.  This works around a problem  with  relay  agent
              information  options,  which  is that they usually not appear in
              DHCPREQUEST messages sent by the client in the  RENEWING  state,
              because such messages are unicast directly to the server and not
              sent through a relay agent.

            The update-conflict-detection statement

              update-conflict-detection flag;

              If the update-conflict-detection parameter is true,  the  server
              will  perform  standard DHCID multiple-client, one-name conflict
              detection.  If the parameter has been set false, the server will
              skip  this check and instead simply tear down any previous bind-
              ings to install the new binding without question.   The  default
              is  true  and this parameter may only be specified at the global

            The update-optimization statement

              update-optimization flag;

              If the  update-optimization  parameter  is  false  for  a  given
              client,  the  server  will  attempt a DNS update for that client
              each time the client renews its lease, rather than only attempt-
              ing  an update when it appears to be necessary.  This will allow
              the DNS to heal from database inconsistencies more  easily,  but
              the  cost is that the DHCP server must do many more DNS updates.
              We recommend leaving this option enabled, which is the  default.
              If  this parameter is not specified, or is true, the DHCP server
              will only update when the client information changes, the client
              gets a different lease, or the client's lease expires.

            The update-static-leases statement

              update-static-leases flag;

              The  update-static-leases  flag,  if  enabled,  causes  the DHCP
              server to do DNS updates for clients even if those  clients  are
              being  assigned their IP address using a fixed-address or fixed-
              address6 statement - that is, the client is being given a static
              assignment.   It  is not recommended because the DHCP server has
              no way to tell that the update has been done, and therefore will
              not  delete  the record when it is not in use.  Also, the server
              must attempt the update each time the client renews  its  lease,
              which  could  have  a significant performance impact in environ-
              ments that place heavy demands on the DHCP server.  This feature
              is  supported for both DHCPv4 and DHCPv6, and update modes stan-
              dard or interim. It is disabled by default.

            The use-eui-64 statement

              use-eui-64 flag;

              (Support for this must be enabled at compile time, see EUI_64 in

              The use-eui-64 flag, if enabled, instructs the  server  to  con-
              struct  an  address  using  the client's EUI-64 DUID (Type 3, HW
              Type EUI-64), rather than creating an address using the  dynamic
              algorithm.   This  means  that a given DUID will always generate
              the same address for a given pool and further that  the  address
              is  guaranteed to be unique to that DUID.  The IPv6 address will
              be calculated from the EUI-64 link layer address, conforming  to
              RFC 2373, unless there is a host declaration for the client-id.

              The range6 statement for EUI-64 must define full /64 bit ranges.
              Invalid ranges will be flagged during configuration  parsing  as
              errors.  See the following example:

                  subnet6 fc00:e4::/64 {
                      use-eui-64 true;
                      range6 fc00:e4::/64;

              The  statement may be specified down to the pool level, allowing
              a mixture of dynamic and EUI-64 based pools.

              During lease file parsing, any leases which  map  to  an  EUI-64
              pool, that have a non-EUI-64 DUID or for which the lease address
              is not the EUI-64 address for that DUID in that  pool,  will  be

              If a host declaration exists for the DUID, the server grants the
              address (fixed-prefix6, fixed-address6) according  to  the  host
              declaration, regardless of the DUID type of the client (even for
              EUI-64 DUIDs).

              If a client request's an EUI-64 lease for a given  network,  and
              the  resultant  address  conflicts with a fixed address reserva-
              tion, the server will send the client a "no addresses available"

              Any client with a non-conforming DUID (not type 3 or not hw type
              EUI-64) that is not linked to a host declaration, which requests
              an  address  from an EUI-64 enabled pool will be ignored and the
              event will be logged.

              Pools that are configured for EUI-64 will be skipped for dynamic
              allocation.   If  there  are no pools in the shared network from
              which to allocate, the client  will  get  back  a  no  addresses
              available status.

              On  an  EUI-64  enabled  pool, any client with a DUID 3, HW Type
              EUI-64, requesting a solicit/renew and including IA_NA  that  do
              not match the EUI-64 policy, they will be treated as though they
              are "outside" the subnet for a given client message:

                  Solicit - Server will advertise with  EUI-64  ia  suboption,
              but with rapid
                  commit off
                  Request  - Server will send "an address not on link status",
              and no ia
                  suboption Renew/Rebind -  Server  will  send  the  requested
              address ia
                  suboption with lifetimes of 0, plus an EUI-64 ia

              Whether or not  EUI-64 based leases are written out to the lease
              database may be controlled by persist-eui-64-leases statement.

            The use-host-decl-names statement

              use-host-decl-names flag;

              If the use-host-decl-names parameter is true in a  given  scope,
              then for every host declaration within that scope, the name pro-
              vided for the host declaration will be supplied to the client as
              its hostname.  So, for example,

                  group {
                    use-host-decl-names on;

                    host joe {
                      hardware ethernet 08:00:2b:4c:29:32;
                      fixed-address joe.example.com;

              is equivalent to

                    host joe {
                      hardware ethernet 08:00:2b:4c:29:32;
                      fixed-address joe.example.com;
                      option host-name "joe";

              Additionally,  enabling use-host-decl-names instructs the server
              to use the host declaration name in the the forward DNS name, if
              no  other values are available.  This value selection process is
              discussed in more detail under DNS updates.

              An option host-name statement within  a  host  declaration  will
              override the use of the name in the host declaration.

              It should be noted here that most DHCP clients completely ignore
              the host-name option sent by the DHCP server, and  there  is  no
              way  to  configure them not to do this.  So you generally have a
              choice of either not having any hostname to  client  IP  address
              mapping  that  the  client will recognize, or doing DNS updates.
              It is beyond the scope of this document to describe how to  make
              this determination.

            The use-lease-addr-for-default-route statement

              use-lease-addr-for-default-route flag;

              If  the  use-lease-addr-for-default-route parameter is true in a
              given scope, then instead of sending the value specified in  the
              routers  option  (or sending no value at all), the IP address of
              the lease being assigned is sent to the client.  This supposedly
              causes  Win95 machines to ARP for all IP addresses, which can be
              helpful if your router is configured for proxy ARP.  The use  of
              this  feature is not recommended, because it won't work for many
              DHCP clients.

            The vendor-option-space statement

              vendor-option-space string;

              The vendor-option-space parameter determines  from  what  option
              space  vendor  options are taken.  The use of this configuration
              parameter is illustrated in the dhcp-options(5) manual page,  in
              the VENDOR ENCAPSULATED OPTIONS section.

       Sometimes  it's  helpful  to  be able to set the value of a DHCP server
       parameter based on some value that the client has sent.   To  do  this,
       you  can  use  expression  evaluation.   The  dhcp-eval(5)  manual page
       describes how to write expressions.  To assign the result of an evalua-
       tion to an option, define the option as follows:

         my-parameter = expression ;

       For example:

         ddns-hostname = binary-to-ascii (16, 8, "-",
                                          substring (hardware, 1, 6));

       It's  often  useful to allocate a single address to a single client, in
       approximate perpetuity.  Host  statements  with  fixed-address  clauses
       exist  to  a  certain  extent  to  serve this purpose, but because host
       statements are intended to  approximate  'static  configuration',  they
       suffer from not being referenced in a littany of other Server Services,
       such as dynamic DNS, failover, 'on events' and so forth.

       If a standard dynamic lease, as from any  range  statement,  is  marked
       'reserved', then the server will only allocate this lease to the client
       it is identified by (be that by client identifier or hardware address).

       In practice, this means that the lease follows the normal state engine,
       enters  ACTIVE  state  when  the  client is bound to it, expires, or is
       released, and any events or services that would  normally  be  supplied
       during  these  events are processed normally, as with any other dynamic
       lease.  The only difference is that  failover  servers  treat  reserved
       leases  as  special  when  they  enter the FREE or BACKUP states - each
       server applies the lease into the state it may allocate from - and  the
       leases  are  not  placed  on the queue for allocation to other clients.
       Instead they may only be 'found' by client  identity.   The  result  is
       that the lease is only offered to the returning client.

       Care  should  probably  be taken to ensure that the client only has one
       lease within a given subnet that it is identified by.

       Leases may be set 'reserved'  either  through  OMAPI,  or  through  the
       'infinite-is-reserved'  configuration  option (if this is applicable to
       your environment and mixture of clients).

       It should also be noted that leases marked 'reserved'  are  effectively
       treated the same as leases marked 'bootp'.

       DHCP  option  statements  are  documented in the dhcp-options(5) manual

       Expressions used in DHCP option statements and elsewhere are documented
       in the dhcp-eval(5) manual page.

       dhcpd(8),   dhcpd.leases(5),  dhcp-options(5),  dhcp-eval(5),  RFC2132,

       dhcpd.conf(5) is maintained by ISC.  Information about Internet Systems
       Consortium can be found at https://www.isc.org.