dhcpd(1M)
NAME | SYNOPSIS | DESCRIPTION | OPERATION | COMMAND LINE | CONFIGURATION | FILES | ATTRIBUTES | SEE ALSO | AUTHOR
NAME
- dhcpd- Dynamic Host Configuration
Protocol Server
SYNOPSIS
-
dhcpd [-p port] [-d ] [-q] [-cf config-file] [-lf lease-file] [-if0 [...ifN]]
DESCRIPTION
The Internet Software Consortium DHCP Server, dhcpd, implements the Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to request and be assigned IP addresses, and also to discover information about the network to which they are attached. BOOTP provides similar functionality, with certain restrictions.
OPERATION
The DHCP protocol allows a host which is unknown to the network administrator to be automatically assigned a new IP address out of a pool of IP addresses for its network. In order for this to work, the network administrator allocates address pools in each subnet and enters them into the dhcpd.conf file.
On startup, dhcpd reads the dhcpd.conf file and stores a list of available addresses on each subnet in memory. When a client requests an address using the DHCP protocol, dhcpd allocates an address for it. Each client is assigned a lease, which expires after an amount of time chosen by the administrator (by default, one day). Before leases expire, the clients to which leases are assigned are expected to renew them in order to continue to use the addresses. Once a lease has expired, the client to which that lease was assigned is no longer permitted to use the leased IP address.
In order to keep track of leases across system reboots and server restarts, dhcpd keeps a list of leases it has assigned in the dhcpd.leases file. Before dhcpd grants a lease to a host, it records the lease in this file and ensures that the contents of the file are flushed to disk. This ensures that even in the event of a system crash, dhcpd will not lose track of a lease that it has assigned. On startup, after reading the dhcpd.conf file, dhcpd reads the dhcpd.leases file to refresh its memory regarding leases that have been assigned.
New leases are appended to the end of the dhcpd.leases file. In order to prevent the file from becoming overly large, from time to time dhcpd creates a new dhcpd.leases file from its in-core lease database. Once this file has been written to disk, the old file is renamed dhcpd.leases~, and the new file is renamed dhcpd.leases. If the system crashes in the middle of this process, whichever dhcpd.leases file remains will contain all the lease information, so there is no need for a special crash recovery process.
BOOTP support is also provided by this server. Unlike DHCP, the BOOTP protocol does not provide a protocol for recovering dynamically-assigned addresses once they are no longer needed. It is still possible to assign addresses dynamically to BOOTP clients, but an administrative process for reclaiming addresses is required. By default, leases are granted to BOOTP clients in perpetuity, although the network administrator may set an earlier cutoff date or a shorter lease length for BOOTP leases if appropriate.
BOOTP clients may also be served in the old standard way, which is to simply provide a declaration in the dhcpd.conf file for each BOOTP client, permanently assigning an address to each client.
Whenever changes are made to the dhcpd.conf file, dhcpd must be killed and restarted. Kill dhcpd either with the akill C_INIT command, or by sending dhcpd a SIGTERM (signal 15). The dhcpd pid can be found in RUNDIR/dhcpd.pid. Because the DHCP server database is not as lightweight as a BOOTP database, dhcpd does not automatically restart itself when it detects a change to the dhcpd.conf file.
COMMAND LINE
The names of the network interfaces on which dhcpd should listen for broadcasts may be specified on the command line. This should be done on systems where dhcpd is unable to identify non-broadcast interfaces, but should not be required on other systems. If no interface names are specified on the command line dhcpd will identify all network interfaces which are up, elimininating non-broadcast interfaces if possible, and listen for DHCP broadcasts on each interface.
If dhcpd should listen on a port other than the standard (port 67), the -p flag may used. It should be followed by the udp port number on which dhcpd should listen. This is mainly useful for debugging purposes. If the -p flag is specified, the server will transmit responses to clients at a port number that is one greater than the one specified - that is, if you specify -p 67, then the server will listen on port 67 and transmit to port 68. Datagrams that must go through relay agents are sent to the port number specified with the -p flag. If you wish to use alternative port numbers, you must configure any relay agents you are using to use the same alternative port numbers.
To have dhcpd log to the standard error descriptor, specify the -d flag. The -d flag forces dhcrelay to always run as a foreground process. This can be useful for debugging, and also at sites where a complete log of all dhcp activity must be kept, but where syslogd is not reliable or otherwise cannot be used. Messages are sent to stdout and to the microkernel log, which can be viewed using the debugger.
dhcpd can be made to use an alternative configuration file with the -cf flag, or an alternative lease file with the -lf flag. Because of the importance of using the same lease database at all times when running dhcpd in production, these options should only be used for testing lease files or database files in a non-production environment.
When starting dhcpd up from a system startup script (for example, /etc/rc), it may not be desirable to print out the entire copyright message on startup. Use the -q flag to avoid printing this message.
CONFIGURATION
The syntax of the dhcpd.conf file is discussed seperately. This section should be used as an overview of the configuration process, and the dhcpd.conf documentation should be consulted for detailed reference information.
Subnets
dhcpd needs to know the subnet numbers and netmasks of all subnets for which it will be providing service. In addition, in order to allocate addresses dynamically, it must be assigned one or more ranges of addresses on each subnet, which it can in turn assign to client hosts as they boot. Thus, a very simple configuration providing DHCP support might look like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.250;
}
Multiple address ranges may be specified like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.107;
range 239.252.197.113 239.252.197.250;
}
If a subnet will only be provided with BOOTP service and no dynamic address assignment, the range clause can be left out entirely, but the subnet statement must appear.
Lease Lengths
DHCP leases can be assigned any length from zero seconds to infinity. What lease length seems appropriate for any given subnet, or for any given installation, will vary depending on the kinds of hosts being served.
For example, in an office environment where systems are added from time to time and removed from time to time, but move relatively infrequently, it might seem appropriate to allow lease times of a month or more. In a final test environment on a manufacturing floor, it may make more sense to assign a maximum lease length of 30 minutes - enough time to go through a simple test procedure on a network appliance before packaging it up for delivery.
It is possible to specify two lease lengths: the default length that will be assigned if a client doesn't ask for any particular lease length, and a maximum lease length. These are specified as clauses to the subnet command:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.107;
default-lease-time 600;
max-lease-time 7200;
}
This particular subnet declaration specifies a default lease time of 600 seconds (ten minutes), and a maximum lease time of 7200 seconds (two hours). Other common values would be 86400 (one day), 604800 (one week) and 2592000 (30 days).
Each subnet need not have the same lease - in the case of an office environment and a manufacturing environment served by the same DHCP server, it might make sense to have widely disparate values for default and maximum lease times on each subnet.
BOOTP Support
Each BOOTP client must be explicitly declared in the dhcpd.conf file. A very basic client declaration will specify the client network interface's hardware address and the IP address to assign to that client. If the client needs to be able to load a boot file from the server, that file's name must be specified. A simple bootp client declaration might look like this:
host haagen {
hardware ethernet 08:00:2b:4c:59:23;
fixed-address 239.252.197.9;
filename "/tftpboot/haagen.boot";
}
Options
DHCP (and also BOOTP with Vendor Extensions) provide a mechanism whereby the server can provide the client with information about how to configure its network interface (for example, subnet mask), and also how the client can access various network services (for example, DNS or IP routers).
These options can be specified on a per-subnet basis, and, for BOOTP clients, also on a per-client basis. In the event that a BOOTP client declaration specifies options that are also specified in its subnet declaration, the options specified in the client declaration take precedence. A reasonably complete DHCP configuration might look something like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.250;
default-lease-time 600 max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 239.252.197.255;
option routers 239.252.197.1;
option domain-name-servers 239.252.197.2, 239.252.197.3;
option domain-name "isc.org";
}
A bootp host on that subnet that needs to be in a different domain and use a different name server might be declared as follows:
host haagen {
hardware ethernet 08:00:2b:4c:59:23;
fixed-address 239.252.197.9;
filename "/tftpboot/haagen.boot";
option domain-name-servers 192.5.5.1;
option domain-name "vix.com";
}
A more complete description of the dhcpd.conf file syntax is provided in dhcpd.conf.
FILES
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|---|---|
| Interface Stability | Evolving |
SEE ALSO
AUTHOR
dhcpd was originally written by Ted Lemon <mellon@vix.com> under a contract with Vixie Labs. Funding for this project was provided by the Internet Software Consortium. Information about the Internet Software Consortium can be found at http://www.isc.org/isc.
NAME | SYNOPSIS | DESCRIPTION | OPERATION | COMMAND LINE | CONFIGURATION | FILES | ATTRIBUTES | SEE ALSO | AUTHOR
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