DHCPD(8)                OpenBSD System Manager's Manual               DHCPD(8)

NAME
     dhcpd - Dynamic Host Configuration Protocol Server

SYNOPSIS
     dhcpd [-dfn] [-A abandoned_ip_table] [-C changed_ip_table]
           [-c config-file] [-L leased_ip_table] [-l lease-file]
           [-Y synctarget] [-y synclisten] [if0 [... ifN]]

DESCRIPTION
     The Internet Software Consortium DHCP Server, dhcpd, implements the Dy-
     namic Host Configuration Protocol (DHCP) and the Internet Bootstrap Pro-
     tocol (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.

     The DHCP protocol allows a host which is unknown to the network adminis-
     trator 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 admin-
     istrator allocates address pools in each subnet and enters them into the
     dhcpd.conf(5) file.

     On startup, dhcpd reads the dhcpd.conf file and stores a list of avail-
     able addresses on each subnet in memory.  When a client requests an ad-
     dress 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).  When a leased IP address is
     assigned to a new hardware address, dhcpd may delete the leased IP from
     certain pf(4) tables.  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(5) file.  Before dhcpd grants a lease to a host, it records
     the lease in this file and makes sure that the contents of the file are
     flushed to disk.  This ensures that even in the event of a system crash,
     dhcpd will not forget about a lease that it has assigned.  On startup,
     after reading the dhcpd.conf file, dhcpd reads the dhcpd.leases file to
     refresh its memory about what leases have been assigned.

     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 dynam-
     ically assign addresses to BOOTP clients, but some 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 that
     makes sense.

     BOOTP clients may also be served in the old standard way, which is simply
     to 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 restart-
     ed.  Because the DHCP server database is not as lightweight as a BOOTP
     database, dhcpd does not automatically restart itself when it sees a
     change to the dhcpd.conf file.

     DHCP traffic always bypasses IPsec.  Otherwise there could be situations
     when a server has an IPsec SA for the client and sends replies over that,
     which a newly booted client would not be able to grasp.

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 spec-
     ified on the command line, dhcpd will identify all network interfaces
     which are up, eliminating non-broadcast interfaces if possible, and lis-
     ten for DHCP broadcasts on each interface.

     The options are as follows:

     -A abandoned_ip_table
             When an address is abandoned for some reason, add it to the pf(4)
             table named abandoned_ip_table.  This can be used to defend
             against machines "camping" on an address without obtaining a
             lease.  When an address is properly leased, dhcpd will remove the
             address from this table.

     -C changed_ip_table
             When an address is leased to a different hardware address, delete
             it from the pf(4) table named changed_ip_table.  This feature
             complements the overload table in a stateful pf(4) rule.  If a
             host appears to be misbehaving, it can be quarantined by using
             the overload feature.  When the address is leased to a different
             machine, dhcpd can remove the address from the overload table,
             thus allowing a well-behaved machine to reuse the address.

     -c config-file
             Use an alternate configuration file, config-file.  Because of the
             importance of using the same lease database at all times when
             running dhcpd in production, this option should be used only for
             testing database files in a non-production environment.

     -d      Force dhcpd to log to stderr.  This can be useful for debugging,
             and also at sites where a complete log of all dhcp activity must
             be kept, but syslogd(8) is not reliable or otherwise cannot be
             used.  Normally, dhcpd will log all output using the syslog(3)
             function with the log facility set to LOG_DAEMON.

     -f      Run dhcpd as a foreground process, rather than allowing it to run
             as a daemon in the background.  This is useful when running dhcpd
             under a debugger, or when running it out of inittab on System V
             systems.

     -L leased_ip_table
             When an address is leased dhcpd will insert it into the pf(4)
             table named leased_ip_table.  Addresses are removed from the
             table when the lease expires.  Combined with the table of aban-
             doned addresses, this can help enforce a requirement to use DHCP
             on a network, or can place DHCP users in a different class of
             service.  Users are cautioned against placing much trust in Eth-
             ernet or IP addresses; ifconfig(8) can be used to trivially
             change the interface's address, and on a busy DHCP network, IP
             addresses will likely be quickly recycled.

     -l lease-file
             Use an alternate lease file, lease-file.  Because of the impor-
             tance of using the same lease database at all times when running
             dhcpd in production, this option should be used only for testing
             lease files in a non-production environment.

     -n      Only test configuration, do not run dhcpd.

     -Y synctarget
             Add target synctarget to receive synchronisation messages.
             synctarget can be either an IPv4 address for unicast messages or
             a network interface name followed optionally by a colon and a nu-
             meric TTL value for multicast messages to the group 224.0.1.240.
             If the multicast TTL is not specified, a default value of 1 is
             used.  This option can be specified multiple times.  See also
             SYNCHRONISATION below.

     -y synclisten
             Listen on synclisten for incoming synchronisation messages.  The
             format for synclisten is the same as for synctarget, above.  This
             option can be specified only once.  See also SYNCHRONISATION be-
             low.

CONFIGURATION
     The syntax of the dhcpd.conf(5) file is discussed separately.  This sec-
     tion should be used as an overview of the configuration process, and the
     dhcpd.conf(5) 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
          dynamically allocate addresses, 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 pro-
          viding 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 almost any length from zero seconds to
          infinity.  What lease length makes sense 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 in-
          frequently, it might make sense 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 ap-
          pliance 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 de-
          fault and maximum lease times on each subnet.

     BOOTP Support
          Each BOOTP client must be explicitly declared in the dhcpd.conf(5)
          file.  A very basic client declaration will specify the client net-
          work 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 "haagen.boot";
                }

     Options
          DHCP (and also BOOTP with Vendor Extensions) provides a mechanism
          whereby the server can provide the client with information about how
          to configure its network interface (e.g., subnet mask), and also how
          the client can access various network services (e.g., DNS, IP
          routers, and so on).

          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 "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(5).

SYNCHRONISATION
     dhcpd supports realtime synchronisation of the lease allocations to a
     number of dhcpd daemons running on multiple machines, using the -Y and -y
     options.

     The following example will accept incoming multicast and unicast synchro-
     nisation messages, and send outgoing multicast messages through the net-
     work interface em0:

           # /usr/sbin/dhcpd -y em0 -Y em0

     The second example will increase the multicast TTL to a value of 2, add
     the unicast targets foo.somewhere.org and bar.somewhere.org, and accept
     incoming unicast messages sent to example.somewhere.org only.

           # /usr/sbin/dhcpd -y example.somewhere.org -Y em0:2 \
                   -Y foo.somewhere.org -Y bar.somewhere.org

     If the file /var/db/dhcpd.key exists, dhcpd will calculate the message-
     digest fingerprint (checksum) for the file and use it as a shared key to
     authenticate the synchronisation messages.  The file itself can contain
     any data.  For example, to create a secure random key:

           # dd if=/dev/arandom of=/var/db/dhcpd.key bs=2048 count=1

     The file needs to be copied to all hosts sending or receiving synchroni-
     sation messages.

     All hosts using synchronisation must use the same configuration in the
     /etc/dhcpd.conf file.

FILES
     /etc/dhcpd.conf          DHCPD configuration file.
     /var/db/dhcpd.leases     DHCPD lease file.

SEE ALSO
     pf(4), dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcp(8), dhcrelay(8),
     pxeboot(8)

AUTHORS
     dhcpd was written by Ted Lemon <mellon@vix.com> under a contract with
     Vixie Labs.

     The current implementation was reworked by Henning Brauer
     <henning@openbsd.org>.

BUGS
     We realize that it would be nice if one could send a SIGHUP to the server
     and have it reload the database.  This is not technically impossible, but
     it would require a great deal of work, our resources are extremely limit-
     ed, and they can be better spent elsewhere.  So please don't complain
     about this on the mailing list unless you're prepared to fund a project
     to implement this feature, or prepared to do it yourself.

OpenBSD 4.7                   September 14, 2009                             5
