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Updated: Wednesday, February 10, 2021

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.   In  this  case,  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.

       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";

       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.

       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

         adandon-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-

       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 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  system  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 set-
         ting up the client's A and PTR records.  If no ddns-hostname is spec-
         ified  in  scope,  then the server will derive the hostname automati-
         cally, using an algorithm that  varies  for  each  of  the  different
         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 (FQDN).

       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-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

       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

       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 certain
         scope.  The ddns-updates parameter is on by default.  To disable  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 specific expi-
         ration time.  This is used for both DHCPv4 and DHCPv6 leases  (it  is
         also  known as the "valid lifetime" in DHCPv6).  The default is 43200

       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
         28.   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 transmitted.

         Similarly, microseconds indicates how many microseconds are permitted
         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 not compiled in by default,  but  must  be
         enabled  at  compile  time  with  './configure --enable-delayed-ack'.
         While we no longer consider it experimental and we don't know of  any
         issues  with it, in order to minimize problems with existing configu-
         ration files we have left it disabled by default.

       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.

         Clients  that  attempt  renewal  frequently  can  cause the server to
         update and write the database frequently resulting in  a  performance
         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  assigns  the same lease (i.e.
         reuses it) with no modifications except for CLTT (Client Last  Trans-
         mission  Time)  which  does not require disk operations. This feature
         applies to IPv4 only.

         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
                c. The host declaration associated with the lease
                d. The client id - this may happen if a client boots without
                a client id and then starts using one in subsequent requests.

         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 flag.

       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().  Suppress-
         ing the call to fsync() may increase the performance  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 example, the end of a school
         term, or the time at night when a facility 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 Coordinated  Uni-
         versal 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 number of seconds.
         If a client reboots using BOOTP during the timeout 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 caution.

       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 initial
         boot  file which is to be loaded by a client.  The filename should be
         a filename recognizable to whatever file transfer protocol 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  declaration.
         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 state-
         ment are valid for the network to which the client is connected, that
         client  will  not  match  the host declaration 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 declaration.

       The fixed-prefix6 declaration

         fixed-prefix6 low-address / bits;

         The  fixed-prefix6  declaration is used to assign a fixed IPv6 prefix
         to a client.  It should only appear in a host declaration, but multi-
         ple fixed-prefix6 statements may appear in a single host declaration.

         The low-address specifies the start of the prefix and the bits speci-
         fies 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

         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  provide  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  state-
         ment.   hardware-type  must be the name of a physical hardware inter-
         face 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 examine 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) indicate 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 state-
         ment 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  automatically
         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 should be 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.  Furthermore, it has no effect if overridden
         by the -lf flag or the PATH_DHCPD_DB environment variable.

       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 here.

         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 dhcpv6-lease-file-name statement

         dhcpv6-lease-file-name name;

         Name is the name of the lease file to use if and only if  the  server
         is  running in DHCPv6 mode.  By default, this is DBDIR/dhcpd6.leases.
         This statement, like lease-file-name, must appear in the outer  scope
         of the configuration file.  It has no effect if overridden by the -lf
         flag or the PATH_DHCPD6_DB environment  variable.   If  dhcpv6-lease-
         file-name  is not specified, but lease-file-name is, the latter value
         will be used.

       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 colons.

         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 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  net-
         works;  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  com-
         pile-time for all others.  You can be sure if your server is compiled
         with USE_SOCKETS if you see lines of this format at startup:

          Listening on Socket/eth0

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

       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  sys-

         In  addition  to setting this value, you may need to modify your sys-
         log.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 operating
         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 syslogd.  Some  systems  support
         log  rollover  using  a  shell  script or program called newsyslog or
         logrotate, and you may be able to configure this as well so that your
         log file doesn't grow uncontrollably.

         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 prevent
         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 configura-
         tion file, so that the log will be as complete as possible.

       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 pro-
         duced.  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  generated  each
         time  a  lease  is acknowledged when the pool usage is above the high

         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

       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-lease-

       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 sec-
         onds.   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-allocat-
         able 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 DHCPRE-
         QUEST, 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 statement.

       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.  It has no effect if  overridden  by
         the -pf flag or the 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.  It has no effect if overridden by  the
            -pf   flag   or  the  PATH_DHCPD6_PID  environment  variable.   If
            dhcpv6-pid-file-name is not specified, but pid-file-name  is,  the
            latter value will be used.

         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 min-
            imum 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 respond-
            ing 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  parame-
            ter  can  be  used to control checking - if its value is false, no
            ping check is done.

         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 abandoned, and
            the server does not respond to the client.  If no  value  is  set,
            ping-timeout defaults to 1 second.

         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  conveyed  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 prefix(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.

            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.  This is
            the default behavior.

            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  prefixes  avail-
            able.   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 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  configura-
            tion  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  gener-
            ally 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  particu-
            lar 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 equiva-
            lent 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 asso-
            ciated  with  the  physical network interface on which the request

            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

            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 override 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-rebind-
            ing-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-timesis  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 lifetime
                  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 lifetime
                  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 sec-
            onds, which would cause clients to issue rebinds only.  In such  a
            case it would likely be better to explicitly define the values.

            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 cal-
            culate them per the RFC. If you want zeros, define them as zeros.

         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-specific
            uses, but are frequently used by vendors of embedded hardware that
            contains DHCP clients.  Because site-specific  options  are  allo-
            cated  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-specific 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 RENEW-
            ING  state.  This works around a problem with relay agent informa-
            tion 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 bindings
            to install the new binding without question.  The default is true.

         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 attempting 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 leav-
            ing  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 statement  -  that
            is, the client is being given a static assignment.  It is not rec-
            ommended 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 signifi-
            cant performance impact in environments that place  heavy  demands
            on the DHCP server.

         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  dis-
            cussed in more detail under DNS updates.

            An option host-name statement within a host declaration will over-
            ride 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

         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.

       See attributes(7) for descriptions of the following attributes:

       |ATTRIBUTE TYPE |       ATTRIBUTE VALUE         |
       |Availability   | service/network/dhcp/isc-dhcp |
       |Stability      | Uncommitted                   |
       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.

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source                was                downloaded                from

       Further information about this software can be found on the open source
       community website at http://www.isc.org/software/dhcp/.