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.\" Copyright (c) 2004-2007 by Internet Systems Consortium, Inc. ("ISC")
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.\" Copyright (c) 1996-2003 by Internet Software Consortium
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
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.\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\" Internet Systems Consortium, Inc.
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.\" 950 Charter Street
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.\" Redwood City, CA 94063
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.\" http://www.isc.org/
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.\" This software has been written for Internet Systems Consortium
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.\" by Ted Lemon in cooperation with Vixie Enterprises and Nominum, Inc.
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.\" To learn more about Internet Systems Consortium, see
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.\" ``http://www.isc.org/''. To learn more about Vixie Enterprises,
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.\" see ``http://www.vix.com''. To learn more about Nominum, Inc., see
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.\" ``http://www.nominum.com''.
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.\" $Id: dhcpd.8,v 1.29 2008/07/17 06:17:57 each Exp $
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dhcpd - Dynamic Host Configuration Protocol Server
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.I trace-playback-file
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The Internet Systems 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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This software is free software. At various times its development has
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been underwritten by various organizations, including the ISC and
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Vixie Enterprises. The development of 3.0 has been funded almost
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entirely by Nominum, Inc.
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At this point development is being shepherded by Ted Lemon, and hosted
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by the ISC, but the future of this project depends on you. If you
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have features you want, please consider implementing them.
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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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On startup, dhcpd reads the
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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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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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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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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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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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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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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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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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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, eliminating non-broadcast interfaces if
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possible, and listen for DHCP broadcasts on each interface.
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The server either operates as a DHCPv6 server or a DHCP server, but
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not both at the same time. To run as a DHCPv6 server, use the
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flag. To run as a DHCP server, use the
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flag. If neither is used, the default is to run as a DHCPv6 server.
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If dhcpd should listen on a port other than the standard (port 67),
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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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If dhcpd should send replies to an address other than the broadcast
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address (255.255.255.255), the
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flag may be used. It is followed by either the IP address or the host
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name to send replies to. This option is only supported in IPv4.
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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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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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To have dhcpd log to the standard error descriptor, specify the
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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. Note that -d implies -f (the daemon will not fork
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itself into the background).
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Dhcpd can be made to use an alternate configuration file with the
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flag, an alternate lease file with the
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flag, or an alternate pid file with the
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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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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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flag may be specified.
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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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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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flag can be used to test the lease database file in a similar way.
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The \fB-tf\fR and \fB-play\fR options allow you to specify a file into
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which the entire startup state of the server and all the transactions
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it processes are either logged or played back from. This can be
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useful in submitting bug reports - if you are getting a core dump
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every so often, you can start the server with the \fB-tf\fR option and
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then, when the server dumps core, the trace file will contain all the
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transactions that led up to it dumping core, so that the problem can
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be easily debugged with \fB-play\fR.
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The \fB-play\fR option must be specified with an alternate lease file,
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using the \fB-lf\fR switch, so that the DHCP server doesn't wipe out
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your existing lease file with its test data. The DHCP server will
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refuse to operate in playback mode unless you specify an alternate
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To find the version of dhcpd that will run, use the
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argument. Instead of running, the version will be printed.
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The syntax of the dhcpd.conf(5) file is discussed separately. 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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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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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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Multiple address ranges may be specified like this:
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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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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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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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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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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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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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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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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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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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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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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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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. A reasonably complete DHCP configuration might
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look something like this:
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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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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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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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A more complete description of the dhcpd.conf file syntax is provided
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The DHCP server provides the capability to modify some of its
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configuration while it is running, without stopping it, modifying its
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database files, and restarting it. This capability is currently
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provided using OMAPI - an API for manipulating remote objects. OMAPI
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clients connect to the server using TCP/IP, authenticate, and can then
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examine the server's current status and make changes to it.
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Rather than implementing the underlying OMAPI protocol directly, user
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programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
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wrapper that handles some of the housekeeping chores that OMAPI does
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not do automatically. Dhcpctl and OMAPI are documented in \fBdhcpctl(3)\fR
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OMAPI exports objects, which can then be examined and modified. The
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DHCP server exports the following objects: lease, host,
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failover-state and group. Each object has a number of methods that
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are provided: lookup, create, and destroy. In addition, it is
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possible to look at attributes that are stored on objects, and in some
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cases to modify those attributes.
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Leases can't currently be created or destroyed, but they can be looked
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up to examine and modify their state.
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Leases have the following attributes:
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.B state \fIinteger\fR lookup, examine
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.B ip-address \fIdata\fR lookup, examine
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The IP address of the lease.
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.B dhcp-client-identifier \fIdata\fR lookup, examine, update
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client identifier that the client used when it acquired the lease.
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Not all clients send client identifiers, so this may be empty.
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.B client-hostname \fIdata\fR examine, update
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The value the client sent in the host-name option.
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.B host \fIhandle\fR examine
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the host declaration associated with this lease, if any.
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.B subnet \fIhandle\fR examine
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the subnet object associated with this lease (the subnet object is not
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currently supported).
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.B pool \fIhandle\fR examine
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the pool object associated with this lease (the pool object is not
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currently supported).
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.B billing-class \fIhandle\fR examine
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the handle to the class to which this lease is currently billed, if
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any (the class object is not currently supported).
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.B hardware-address \fIdata\fR examine, update
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the hardware address (chaddr) field sent by the client when it
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.B hardware-type \fIinteger\fR examine, update
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the type of the network interface that the client reported when it
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.B ends \fItime\fR examine
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the time when the lease's current state ends, as understood by the
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.B tstp \fItime\fR examine
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the time when the lease's current state ends, as understood by the
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.B tsfp \fItime\fR examine
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the adjusted time when the lease's current state ends, as understood by
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the failover peer (if there is no failover peer, this value is
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undefined). Generally this value is only adjusted for expired, released,
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or reset leases while the server is operating in partner-down state, and
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otherwise is simply the value supplied by the peer.
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.B atsfp \fItime\fR examine
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the actual tsfp value sent from the peer. This value is forgotten when a
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lease binding state change is made, to facilitate retransmission logic.
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.B cltt \fItime\fR examine
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The time of the last transaction with the client on this lease.
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Hosts can be created, destroyed, looked up, examined and modified.
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If a host declaration is created or deleted using OMAPI, that
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information will be recorded in the dhcpd.leases file. It is
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permissible to delete host declarations that are declared in the
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Hosts have the following attributes:
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.B name \fIdata\fR lookup, examine, modify
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the name of the host declaration. This name must be unique among all
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.B group \fIhandle\fR examine, modify
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the named group associated with the host declaration, if there is one.
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.B hardware-address \fIdata\fR lookup, examine, modify
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the link-layer address that will be used to match the client, if any.
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Only valid if hardware-type is also present.
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.B hardware-type \fIinteger\fR lookup, examine, modify
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the type of the network interface that will be used to match the
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client, if any. Only valid if hardware-address is also present.
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.B dhcp-client-identifier \fIdata\fR lookup, examine, modify
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the dhcp-client-identifier option that will be used to match the
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.B ip-address \fIdata\fR examine, modify
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a fixed IP address which is reserved for a DHCP client that matches
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this host declaration. The IP address will only be assigned to the
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client if it is valid for the network segment to which the client is
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.B statements \fIdata\fR modify
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a list of statements in the format of the dhcpd.conf file that will be
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executed whenever a message from the client is being processed.
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.B known \fIinteger\fR examine, modify
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if nonzero, indicates that a client matching this host declaration
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will be treated as \fIknown\fR in pool permit lists. If zero, the
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client will not be treated as known.
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Named groups can be created, destroyed, looked up, examined and
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modified. If a group declaration is created or deleted using OMAPI,
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that information will be recorded in the dhcpd.leases file. It is
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permissible to delete group declarations that are declared in the
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Named groups currently can only be associated with
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hosts - this allows one set of statements to be efficiently attached
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to more than one host declaration.
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Groups have the following attributes:
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the name of the group. All groups that are created using OMAPI must
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have names, and the names must be unique among all groups.
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.B statements \fIdata\fR
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a list of statements in the format of the dhcpd.conf file that will be
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executed whenever a message from a client whose host declaration
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references this group is processed.
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.SH THE CONTROL OBJECT
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The control object allows you to shut the server down. If the server
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is doing failover with another peer, it will make a clean transition
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into the shutdown state and notify its peer, so that the peer can go
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into partner down, and then record the "recover" state in the lease
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file so that when the server is restarted, it will automatically
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resynchronize with its peer.
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On shutdown the server will also attempt to cleanly shut down all
610
OMAPI connections. If these connections do not go down cleanly after
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five seconds, they are shut down preemptively. It can take as much
612
as 25 seconds from the beginning of the shutdown process to the time
613
that the server actually exits.
615
To shut the server down, open its control object and set the state
617
.SH THE FAILOVER-STATE OBJECT
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The failover-state object is the object that tracks the state of the
619
failover protocol as it is being managed for a given failover peer.
620
The failover object has the following attributes (please see
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for explanations about what these attributes mean):
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.B name \fIdata\fR examine
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Indicates the name of the failover peer relationship, as described in
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the server's \fBdhcpd.conf\fR file.
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.B partner-address \fIdata\fR examine
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Indicates the failover partner's IP address.
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.B local-address \fIdata\fR examine
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Indicates the IP address that is being used by the DHCP server for
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.B partner-port \fIdata\fR examine
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Indicates the TCP port on which the failover partner is listening for
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failover protocol connections.
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.B local-port \fIdata\fR examine
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Indicates the TCP port on which the DHCP server is listening for
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failover protocol connections for this failover pair.
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.B max-outstanding-updates \fIinteger\fR examine
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Indicates the number of updates that can be outstanding and
656
unacknowledged at any given time, in this failover relationship.
659
.B mclt \fIinteger\fR examine
661
Indicates the maximum client lead time in this failover relationship.
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.B load-balance-max-secs \fIinteger\fR examine
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Indicates the maximum value for the secs field in a client request
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before load balancing is bypassed.
670
.B load-balance-hba \fIdata\fR examine
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Indicates the load balancing hash bucket array for this failover
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.B local-state \fIinteger\fR examine, modify
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Indicates the present state of the DHCP server in this failover
679
relationship. Possible values for state are:
686
3 - communications interrupted
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5 - potential conflict
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10 - resolution interrupted
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(Note that some of the above values have changed since DHCP 3.0.x.)
704
In general it is not a good idea to make changes to this state.
705
However, in the case that the failover partner is known to be down, it
706
can be useful to set the DHCP server's failover state to partner
707
down. At this point the DHCP server will take over service of the
708
failover partner's leases as soon as possible, and will give out
709
normal leases, not leases that are restricted by MCLT. If you do put
710
the DHCP server into the partner-down when the other DHCP server is
711
not in the partner-down state, but is not reachable, IP address
712
assignment conflicts are possible, even likely. Once a server has
713
been put into partner-down mode, its failover partner must not be
714
brought back online until communication is possible between the two
718
.B partner-state \fIinteger\fR examine
720
Indicates the present state of the failover partner.
723
.B local-stos \fIinteger\fR examine
725
Indicates the time at which the DHCP server entered its present state
726
in this failover relationship.
729
.B partner-stos \fIinteger\fR examine
731
Indicates the time at which the failover partner entered its present state.
734
.B hierarchy \fIinteger\fR examine
736
Indicates whether the DHCP server is primary (0) or secondary (1) in
737
this failover relationship.
740
.B last-packet-sent \fIinteger\fR examine
742
Indicates the time at which the most recent failover packet was sent
743
by this DHCP server to its failover partner.
746
.B last-timestamp-received \fIinteger\fR examine
748
Indicates the timestamp that was on the failover message most recently
749
received from the failover partner.
752
.B skew \fIinteger\fR examine
754
Indicates the skew between the failover partner's clock and this DHCP
758
.B max-response-delay \fIinteger\fR examine
760
Indicates the time in seconds after which, if no message is received
761
from the failover partner, the partner is assumed to be out of
765
.B cur-unacked-updates \fIinteger\fR examine
767
Indicates the number of update messages that have been received from
768
the failover partner but not yet processed.
771
.B ETCDIR/dhcpd.conf, DBDIR/dhcpd.leases, RUNDIR/dhcpd.pid,
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.B DBDIR/dhcpd.leases~.
774
dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
777
was originally written by Ted Lemon under a contract with Vixie Labs.
778
Funding for this project was provided by Internet Systems
779
Consortium. Version 3 of the DHCP server was funded by Nominum, Inc.
780
Information about Internet Systems Consortium is available at
781
.B http://www.isc.org/\fR.
782
Information about Nominum can be found at \fBhttp://www.nominum.com/\fR.
added
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server/dhcpd.8
