330 lines
12 KiB
Groff
330 lines
12 KiB
Groff
.\" dhcpd.8
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.\"
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.\" Copyright (c) 1996-1999 Internet Software Consortium.
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.\" Use is subject to license terms which appear in the file named
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.\" ISC-LICENSE that should have accompanied this file when you
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.\" received it. If a file named ISC-LICENSE did not accompany this
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.\" file, or you are not sure the one you have is correct, you may
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.\" obtain an applicable copy of the license at:
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.\"
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.\" http://www.isc.org/isc-license-1.0.html.
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.\"
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.\" This file is part of the ISC DHCP distribution. The documentation
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.\" associated with this file is listed in the file DOCUMENTATION,
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.\" included in the top-level directory of this release.
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.\"
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.\" Support and other services are available for ISC products - see
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.\" http://www.isc.org for more information.
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.TH dhcpd 8
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.SH NAME
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dhcpd - Dynamic Host Configuration Protocol Server
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.SH SYNOPSIS
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.B dhcpd
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[
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.B -p
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.I port
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]
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[
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.B -f
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]
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[
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.B -d
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]
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[
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.B -q
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]
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[
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.B -t
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.B -T
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]
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[
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.B -cf
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.I config-file
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]
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[
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.B -lf
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.I lease-file
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]
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[
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.I if0
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[
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.I ...ifN
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]
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]
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.SH DESCRIPTION
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The Internet Software Consortium DHCP Server, dhcpd, implements the
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Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap
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Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to request
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and be assigned IP addresses, and also to discover information about
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the network to which they are attached. BOOTP provides similar
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functionality, with certain restrictions.
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.SH CONTRIBUTIONS
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.PP
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Development of this software is funded through contributions and
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support contracts. Please see
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.B dhcp-contrib(5)
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for information on how you can contribute.
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.SH OPERATION
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.PP
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The DHCP protocol allows a host which is unknown to the network
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administrator to be automatically assigned a new IP address out of a
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pool of IP addresses for its network. In order for this to work, the
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network administrator allocates address pools in each subnet and
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enters them into the dhcpd.conf(5) file.
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.PP
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On startup, dhcpd reads the
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.IR dhcpd.conf
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file and stores a list of available addresses on each subnet in
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memory. When a client requests an address using the DHCP protocol,
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dhcpd allocates an address for it. Each client is assigned a lease,
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which expires after an amount of time chosen by the administrator (by
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default, one day). Before leases expire, the clients to which leases
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are assigned are expected to renew them in order to continue to use
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the addresses. Once a lease has expired, the client to which that
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lease was assigned is no longer permitted to use the leased IP
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address.
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.PP
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In order to keep track of leases across system reboots and server
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restarts, dhcpd keeps a list of leases it has assigned in the
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dhcpd.leases(5) file. Before dhcpd grants a lease to a host, it
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records the lease in this file and makes sure that the contents of the
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file are flushed to disk. This ensures that even in the event of a
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system crash, dhcpd will not forget about a lease that it has
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assigned. On startup, after reading the dhcpd.conf file, dhcpd
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reads the dhcpd.leases file to refresh its memory about what leases
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have been assigned.
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.PP
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New leases are appended to the end of the dhcpd.leases
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file. In order to prevent the file from becoming arbitrarily large,
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from time to time dhcpd creates a new dhcpd.leases file from its
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in-core lease database. Once this file has been written to disk, the
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old file is renamed
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.IR dhcpd.leases~ ,
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and the new file is renamed dhcpd.leases. If the system crashes in
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the middle of this process, whichever dhcpd.leases file remains will
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contain all the lease information, so there is no need for a special
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crash recovery process.
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.PP
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BOOTP support is also provided by this server. Unlike DHCP, the BOOTP
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protocol does not provide a protocol for recovering
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dynamically-assigned addresses once they are no longer needed. It is
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still possible to dynamically assign addresses to BOOTP clients, but
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some administrative process for reclaiming addresses is required. By
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default, leases are granted to BOOTP clients in perpetuity, although
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the network administrator may set an earlier cutoff date or a shorter
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lease length for BOOTP leases if that makes sense.
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.PP
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BOOTP clients may also be served in the old standard way, which is to
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simply provide a declaration in the dhcpd.conf file for each
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BOOTP client, permanently assigning an address to each client.
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.PP
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Whenever changes are made to the dhcpd.conf file, dhcpd must be
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restarted. To restart dhcpd, send a SIGTERM (signal 15) to the
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process ID contained in
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.IR RUNDIR/dhcpd.pid ,
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and then re-invoke dhcpd. Because the DHCP server database is not as
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lightweight as a BOOTP database, dhcpd does not automatically restart
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itself when it sees a change to the dhcpd.conf file.
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.PP
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Note: We get a lot of complaints about this. We realize that it would
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be nice if one could send a SIGHUP to the server and have it reload
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the database. This is not technically impossible, but it would
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require a great deal of work, our resources are extremely limited, and
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they can be better spent elsewhere. So please don't complain about
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this on the mailing list unless you're prepared to fund a project to
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implement this feature, or prepared to do it yourself.
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.SH COMMAND LINE
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.PP
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The names of the network interfaces on which dhcpd should listen for
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broadcasts may be specified on the command line. This should be done
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on systems where dhcpd is unable to identify non-broadcast interfaces,
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but should not be required on other systems. If no interface names
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are specified on the command line dhcpd will identify all network
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interfaces which are up, elimininating non-broadcast interfaces if
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possible, and listen for DHCP broadcasts on each interface.
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.PP
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If dhcpd should listen on a port other than the standard (port 67),
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the
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.B -p
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flag may used. It should be followed by the udp port number on which
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dhcpd should listen. This is mostly useful for debugging purposes.
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.PP
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To run dhcpd as a foreground process, rather than allowing it to run
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as a daemon in the background, the
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.B -f
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flag should be specified. This is useful when running dhcpd under a
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debugger, or when running it out of inittab on System V systems.
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.PP
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To have dhcpd log to the standard error descriptor, specify the
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.B -d
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flag. This can be useful for debugging, and also at sites where a
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complete log of all dhcp activity must be kept but syslogd is not
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reliable or otherwise cannot be used. Normally, dhcpd will log all
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output using the syslog(3) function with the log facility set to
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LOG_DAEMON.
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.PP
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Dhcpd can be made to use an alternate configuration file with the
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.B -cf
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flag, or an alternate lease file with the
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.B -lf
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flag. Because of the importance of using the same lease database at
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all times when running dhcpd in production, these options should be
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used \fBonly\fR for testing lease files or database files in a
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non-production environment.
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.PP
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When starting dhcpd up from a system startup script (e.g., /etc/rc),
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it may not be desirable to print out the entire copyright message on
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startup. To avoid printing this message, the
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.B -q
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flag may be specified.
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.PP
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The DHCP server reads two files on startup: a configuration file, and
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a lease database. If the
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.B -t
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flag is specified, the server will simply test the configuration file
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for correct syntax, but will not attempt to perform any network
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operations. This can be used to test the a new configuration file
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automatically before installing it.
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.PP
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The
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.B -T
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flag can be used to test the lease database file in a similar way.
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.SH CONFIGURATION
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The syntax of the dhcpd.conf(5) file is discussed seperately. This
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section should be used as an overview of the configuration process,
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and the dhcpd.conf(5) documentation should be consulted for detailed
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reference information.
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.PP
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.SH Subnets
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dhcpd needs to know the subnet numbers and netmasks of all subnets for
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which it will be providing service. In addition, in order to
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dynamically allocate addresses, it must be assigned one or more ranges
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of addresses on each subnet which it can in turn assign to client
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hosts as they boot. Thus, a very simple configuration providing DHCP
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support might look like this:
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.nf
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.sp 1
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subnet 239.252.197.0 netmask 255.255.255.0 {
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range 239.252.197.10 239.252.197.250;
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}
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.fi
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.PP
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Multiple address ranges may be specified like this:
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.nf
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.sp 1
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subnet 239.252.197.0 netmask 255.255.255.0 {
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range 239.252.197.10 239.252.197.107;
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range 239.252.197.113 239.252.197.250;
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}
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.fi
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.PP
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If a subnet will only be provided with BOOTP service and no dynamic
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address assignment, the range clause can be left out entirely, but the
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subnet statement must appear.
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.PP
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.SH Lease Lengths
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DHCP leases can be assigned almost any length from zero seconds to
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infinity. What lease length makes sense for any given subnet, or for
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any given installation, will vary depending on the kinds of hosts
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being served.
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.PP
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For example, in an office environment where systems are added from
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time to time and removed from time to time, but move relatively
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infrequently, it might make sense to allow lease times of a month of
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more. In a final test environment on a manufacturing floor, it may
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make more sense to assign a maximum lease length of 30 minutes -
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enough time to go through a simple test procedure on a network
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appliance before packaging it up for delivery.
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.PP
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It is possible to specify two lease lengths: the default length that
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will be assigned if a client doesn't ask for any particular lease
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length, and a maximum lease length. These are specified as clauses
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to the subnet command:
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.nf
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.sp 1
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subnet 239.252.197.0 netmask 255.255.255.0 {
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range 239.252.197.10 239.252.197.107;
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default-lease-time 600;
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max-lease-time 7200;
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.fi
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.PP
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This particular subnet declaration specifies a default lease time of
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600 seconds (ten minutes), and a maximum lease time of 7200 seconds
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(two hours). Other common values would be 86400 (one day), 604800
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(one week) and 2592000 (30 days).
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.PP
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Each subnet need not have the same lease\(emin the case of an office
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environment and a manufacturing environment served by the same DHCP
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server, it might make sense to have widely disparate values for
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default and maximum lease times on each subnet.
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.SH BOOTP Support
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Each BOOTP client must be explicitly declared in the dhcpd.conf
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file. A very basic client declaration will specify the client
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network interface's hardware address and the IP address to assign to
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that client. If the client needs to be able to load a boot file from
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the server, that file's name must be specified. A simple bootp
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client declaration might look like this:
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.nf
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.sp 1
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host haagen {
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hardware ethernet 08:00:2b:4c:59:23;
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fixed-address 239.252.197.9;
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filename "/tftpboot/haagen.boot";
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}
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.fi
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.SH Options
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DHCP (and also BOOTP with Vendor Extensions) provide a mechanism
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whereby the server can provide the client with information about how
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to configure its network interface (e.g., subnet mask), and also how
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the client can access various network services (e.g., DNS, IP routers,
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and so on).
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.PP
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These options can be specified on a per-subnet basis, and, for BOOTP
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clients, also on a per-client basis. In the event that a BOOTP
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client declaration specifies options that are also specified in its
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subnet declaration, the options specified in the client declaration
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take precedence. An reasonably complete DHCP configuration might
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look something like this:
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.nf
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.sp 1
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subnet 239.252.197.0 netmask 255.255.255.0 {
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range 239.252.197.10 239.252.197.250;
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default-lease-time 600 max-lease-time 7200;
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option subnet-mask 255.255.255.0;
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option broadcast-address 239.252.197.255;
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option routers 239.252.197.1;
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option domain-name-servers 239.252.197.2, 239.252.197.3;
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option domain-name "isc.org";
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}
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.fi
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.PP
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A bootp host on that subnet that needs to be in a different domain and
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use a different name server might be declared as follows:
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.nf
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.sp 1
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host haagen {
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hardware ethernet 08:00:2b:4c:59:23;
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fixed-address 239.252.197.9;
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filename "/tftpboot/haagen.boot";
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option domain-name-servers 192.5.5.1;
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option domain-name "vix.com";
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}
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.fi
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.PP
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A more complete description of the dhcpd.conf file syntax is provided
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in dhcpd.conf(5).
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.SH FILES
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.B ETCDIR/dhcpd.conf, DBDIR/dhcpd.leases, RUNDIR/dhcpd.pid,
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.B DBDIR/dhcpd.leases~.
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.SH SEE ALSO
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dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
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.SH AUTHOR
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.B dhcpd(8)
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was written by Ted Lemon <mellon@vix.com>
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under a contract with Vixie Labs. Funding
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for this project was provided by the Internet Software Consortium.
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Information about the Internet Software Consortium can be found at
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.B http://www.isc.org/isc.
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