dhcpd

dhcpd(8)                     System Manager's Manual                    dhcpd(8)



NAME
       dhcpd - Dynamic Host Configuration Protocol Server

SYNOPSIS
       dhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -4 | -6 ] [ -4o6
       port ] [ -s server ] [ -cf config-file ] [ -lf lease-file ] [ -pf pid-
       file ] [ --no-pid ] [ -user user ] [ -group group ] [ -chroot dir ] [ -tf
       trace-output-file ] [ -play trace-playback-file ] [ if0 [ ...ifN ] ]

       dhcpd --version

DESCRIPTION
       The Internet Systems 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(5) file.

       There are two versions of the DHCP protocol DHCPv4 and DHCPv6.  At
       startup the server  may be started for one or the other via the -4 or -6
       arguments.

       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(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.

       New leases are appended to the end of the dhcpd.leases file.  In order to
       prevent the file from becoming arbitrarily 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
       dynamically 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 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
       restarted.  To restart dhcpd, send a SIGTERM (signal 15) to the process
       ID contained in RUNDIR/dhcpd.pid, and then re-invoke dhcpd.  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.

       Note: We get a lot of complaints about this.  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 limited, 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.

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, eliminating non-broadcast interfaces if possible, and
       listen for DHCP broadcasts on each interface.

COMMAND LINE OPTIONS
       -4     Run as a DHCP server. This is the default and cannot be combined
              with -6.

       -6     Run as a DHCPv6 server. This cannot be combined with -4.

       -4o6 port
              Participate in the DHCPv4 over DHCPv6 protocol specified by RFC
              7341.  This associates a DHCPv4 and a DHCPv6 server to allow the
              v4 server to receive v4 requests that were encapsulated in a v6
              packet.  Communication between the two servers is done on a pair
              of UDP sockets bound to ::1 port and port + 1. Both servers must
              be launched using the same port argument.

       -p port
              The UDP port number on which dhcpd should listen.  If unspecified
              dhcpd uses the default port of 67.  This is mostly useful for
              debugging purposes.

       -s address
              Specify an address or host name to which dhcpd should send replies
              rather than the broadcast address (255.255.255.255).  This option
              is only supported in IPv4.

       -f     Force dhcpd to run as a foreground process instead of 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.

       -d     Send log messages to the standard error descriptor.  This can be
              useful for debugging, and also at sites where a complete log of
              all dhcp activity must be kept but syslogd 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.  Note that -d implies -f (the daemon will not fork
              itself into the background).

       -q     Be quiet at startup.  This suppresses the printing of the entire
              copyright message during startup.  This might be desirable when
              starting dhcpd from a system startup script (e.g., /etc/rc).

       -t     Test the configuration file.  The server tests the configuration
              file for correct syntax, but will not attempt to perform any
              network operations.  This can be used to test a new configuration
              file automatically before installing it.

       -T     Test the lease file.  The server tests the lease file for correct
              syntax, but will not attempt to perform any network operations.
              In addition to reading the lease file it will also write the
              leases to a temporary lease file.  The current lease file will not
              be modified and the temporary lease file will be removed upon
              completion of the test. This can be used to test a new lease file
              automatically before installing it.

       -user user
              Setuid to user after completing privileged operations, such as
              creating sockets that listen on privileged ports.  This also
              causes the lease file to be owned by user.  This option is only
              available if the code was compiled with the PARANOIA patch
              (./configure --enable-paranoia).

       -group group
              Setgid to group after completing privileged operations, such as
              creating sockets that listen on privileged ports.  This also
              causes the lease file to use group.  This option is only available
              if the code was compiled with the PARANOIA patch (./configure
              --enable-paranoia).

       -chroot dir
              Chroot to directory.  This may occur before or after reading the
              configuration files depending on whether the code was compiled
              with the EARLY_CHROOT option enabled (./configure --enable-early-
              chroot).  This option is only available if the code was compiled
              with the PARANOIA patch (./configure --enable-paranoia).

       -tf tracefile
              Specify a file into which the entire startup state of the server
              and all the transactions it processes are logged.  This can be
              useful in submitting bug reports - if you are getting a core dump
              every so often, you can start the server with the -tf option and
              then, when the server dumps core, the trace file will contain all
              the transactions that led up to it dumping core, so that the
              problem can be easily debugged with -play.

       -play playfile
              Specify a file from which the entire startup state of the server
              and all the transactions it processed are read.  The -play option
              must be specified with an alternate lease file, using the -lf
              switch, so that the DHCP server doesn't wipe out your existing
              lease file with its test data.  The DHCP server will refuse to
              operate in playback mode unless you specify an alternate lease
              file.

       --version
              Print version number and exit.

       Modifying default file locations: The following options can be used to
       modify the locations dhcpd uses for its files.  Because of the importance
       of using the same lease database at all times when running dhcpd in
       production, these options should be used only for testing lease files or
       database files in a non-production environment.

       -cf config-file
              Path to alternate configuration file.

       -lf lease-file
              Path to alternate lease file.

       -pf pid-file
              Path to alternate pid file.

       --no-pid
              Option to disable writing pid files.  By default the program will
              write a pid file.  If the program is invoked with this option it
              will not check for an existing server process.

PORTS
       During operations the server may use multiple UDP and TCP ports to
       provide different functions.  Which ports are opened depends on both the
       way you compiled your code and the configuration you supply.  The
       following should provide you an idea of what ports may be in use.

       Normally a DHCPv4 server will open a raw UDP socket to receive and send
       most DHCPv4 packets.  It also opens a fallback UDP socket for use in
       sending unicast packets.  Normally these will both use the well known
       port number for BOOTPS.

       For each DHCPv4 failover peer you list in the configuration file there
       will be a TCP socket listening for connections on the ports specified in
       the configuration file.  When the peer connects there will be another
       socket for the established connection.  For the established connection
       the side (primary or secondary) opening the connection will use a random
       port.

       For DHCPv6 the server opens a UDP socket on the well known dhcpv6-server
       port.

       The server opens an icmp socket for doing ping requests to check if
       addresses are in use.

       If you have included an omapi-port statement in your configuration file
       then the server will open a TCP socket on that port to listen for OMPAI
       connections.  When something connects another port will be used for the
       established connection.

       When DDNS is enabled at compile time (see includes/site.h) the server
       will open both a v4 and a v6 UDP socket on random ports, unless DDNS
       updates are globally disabled by setting ddns-update-style to none in the
       configuration file.

CONFIGURATION
       The syntax of the dhcpd.conf(5) file is discussed separately.  This
       section 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 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 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 infrequently,
       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 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 leaseā€”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 (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 "/tftpboot/haagen.boot";
              option domain-name-servers 192.5.5.1;
              option domain-name "example.com";
            }

       A more complete description of the dhcpd.conf file syntax is provided in
       dhcpd.conf(5).

OMAPI
       The DHCP server provides the capability to modify some of its
       configuration while it is running, without stopping it, modifying its
       database files, and restarting it.  This capability is currently provided
       using OMAPI - an API for manipulating remote objects.  OMAPI clients
       connect to the server using TCP/IP, authenticate, and can then examine
       the server's current status and make changes to it.

       Rather than implementing the underlying OMAPI protocol directly, user
       programs should use the dhcpctl API or OMAPI itself.  Dhcpctl is a
       wrapper that handles some of the housekeeping chores that OMAPI does not
       do automatically.  Dhcpctl and OMAPI are documented in dhcpctl(3) and
       omapi(3).

       OMAPI exports objects, which can then be examined and modified.  The DHCP
       server exports the following objects: lease, host, failover-state and
       group.  Each object has a number of methods that are provided: lookup,
       create, and destroy.  In addition, it is possible to look at attributes
       that are stored on objects, and in some cases to modify those attributes.

THE LEASE OBJECT
       Leases can't currently be created or destroyed, but they can be looked up
       to examine and modify their state.

       Leases have the following attributes:

       state integer lookup, examine
            1 = free
            2 = active
            3 = expired
            4 = released
            5 = abandoned
            6 = reset
            7 = backup
            8 = reserved
            9 = bootp

       ip-address data lookup, examine
            The IP address of the lease.

       dhcp-client-identifier data lookup, examine, update
            The client identifier that the client used when it acquired the
            lease.  Not all clients send client identifiers, so this may be
            empty.

       client-hostname data examine, update
            The value the client sent in the host-name option.

       host handle examine
            the host declaration associated with this lease, if any.

       subnet handle examine
            the subnet object associated with this lease (the subnet object is
            not currently supported).

       pool handle examine
            the pool object associated with this lease (the pool object is not
            currently supported).

       billing-class handle examine
            the handle to the class to which this lease is currently billed, if
            any (the class object is not currently supported).

       hardware-address data examine, update
            the hardware address (chaddr) field sent by the client when it
            acquired its lease.

       hardware-type integer examine, update
            the type of the network interface that the client reported when it
            acquired its lease.

       ends time examine
            the time when the lease's current state ends, as understood by the
            client.

       tstp time examine
            the time when the lease's current state ends, as understood by the
            server.
       tsfp time examine
            the adjusted time when the lease's current state ends, as understood
            by the failover peer (if there is no failover peer, this value is
            undefined).  Generally this value is only adjusted for expired,
            released, or reset leases while the server is operating in partner-
            down state, and otherwise is simply the value supplied by the peer.
       atsfp time examine
            the actual tsfp value sent from the peer.  This value is forgotten
            when a lease binding state change is made, to facilitate
            retransmission logic.

       cltt time examine
            The time of the last transaction with the client on this lease.

THE HOST OBJECT
       Hosts can be created, destroyed, looked up, examined and modified.  If a
       host declaration is created or deleted using OMAPI, that information will
       be recorded in the dhcpd.leases file.  It is permissible to delete host
       declarations that are declared in the dhcpd.conf file.

       Hosts have the following attributes:

       name data lookup, examine, modify
            the name of the host declaration.  This name must be unique among
            all host declarations.

       group handle examine, modify
            the named group associated with the host declaration, if there is
            one.

       hardware-address data lookup, examine, modify
            the link-layer address that will be used to match the client, if
            any.  Only valid if hardware-type is also present.

       hardware-type integer lookup, examine, modify
            the type of the network interface that will be used to match the
            client, if any.  Only valid if hardware-address is also present.

       dhcp-client-identifier data lookup, examine, modify
            the dhcp-client-identifier option that will be used to match the
            client, if any.

       ip-address data examine, modify
            a fixed IP address which is reserved for a DHCP client that matches
            this host declaration.  The IP address will only be assigned to the
            client if it is valid for the network segment to which the client is
            connected.

       statements data modify
            a list of statements in the format of the dhcpd.conf file that will
            be executed whenever a message from the client is being processed.

       known integer examine, modify
            if nonzero, indicates that a client matching this host declaration
            will be treated as known in pool permit lists.  If zero, the client
            will not be treated as known.

THE GROUP OBJECT
       Named groups can be created, destroyed, looked up, examined and modified.
       If a group declaration is created or deleted using OMAPI, that
       information will be recorded in the dhcpd.leases file.  It is permissible
       to delete group declarations that are declared in the dhcpd.conf file.

       Named groups currently can only be associated with hosts - this allows
       one set of statements to be efficiently attached to more than one host
       declaration.

       Groups have the following attributes:

       name data
            the name of the group.  All groups that are created using OMAPI must
            have names, and the names must be unique among all groups.

       statements data
            a list of statements in the format of the dhcpd.conf file that will
            be executed whenever a message from a client whose host declaration
            references this group is processed.

THE CONTROL OBJECT
       The control object allows you to shut the server down.  If the server is
       doing failover with another peer, it will make a clean transition into
       the shutdown state and notify its peer, so that the peer can go into
       partner down, and then record the "recover" state in the lease file so
       that when the server is restarted, it will automatically resynchronize
       with its peer.

       On shutdown the server will also attempt to cleanly shut down all OMAPI
       connections.  If these connections do not go down cleanly after five
       seconds, they are shut down preemptively.  It can take as much as 25
       seconds from the beginning of the shutdown process to the time that the
       server actually exits.

       To shut the server down, open its control object and set the state
       attribute to 2.

THE FAILOVER-STATE OBJECT
       The failover-state object is the object that tracks the state of the
       failover protocol as it is being managed for a given failover peer.  The
       failover object has the following attributes (please see dhcpd.conf (5)
       for explanations about what these attributes mean):

       name data examine
            Indicates the name of the failover peer relationship, as described
            in the server's dhcpd.conf file.

       partner-address data examine
            Indicates the failover partner's IP address.

       local-address data examine
            Indicates the IP address that is being used by the DHCP server for
            this failover pair.

       partner-port data examine
            Indicates the TCP port on which the failover partner is listening
            for failover protocol connections.

       local-port data examine
            Indicates the TCP port on which the DHCP server is listening for
            failover protocol connections for this failover pair.

       max-outstanding-updates integer examine
            Indicates the number of updates that can be outstanding and
            unacknowledged at any given time, in this failover relationship.

       mclt integer examine
            Indicates the maximum client lead time in this failover
            relationship.

       load-balance-max-secs integer examine
            Indicates the maximum value for the secs field in a client request
            before load balancing is bypassed.

       load-balance-hba data examine
            Indicates the load balancing hash bucket array for this failover
            relationship.

       local-state integer examine, modify
            Indicates the present state of the DHCP server in this failover
            relationship.  Possible values for state are:

                 1   - startup
                 2   - normal
                 3   - communications interrupted
                 4   - partner down
                 5   - potential conflict
                 6   - recover
                 7   - paused
                 8   - shutdown
                 9   - recover done
                 10  - resolution interrupted
                 11  - conflict done
                 254 - recover wait

            (Note that some of the above values have changed since DHCP 3.0.x.)

            In general it is not a good idea to make changes to this state.
            However, in the case that the failover partner is known to be down,
            it can be useful to set the DHCP server's failover state to partner
            down.  At this point the DHCP server will take over service of the
            failover partner's leases as soon as possible, and will give out
            normal leases, not leases that are restricted by MCLT.  If you do
            put the DHCP server into the partner-down when the other DHCP server
            is not in the partner-down state, but is not reachable, IP address
            assignment conflicts are possible, even likely.  Once a server has
            been put into partner-down mode, its failover partner must not be
            brought back online until communication is possible between the two
            servers.

       partner-state integer examine
            Indicates the present state of the failover partner.

       local-stos integer examine
            Indicates the time at which the DHCP server entered its present
            state in this failover relationship.

       partner-stos integer examine
            Indicates the time at which the failover partner entered its present
            state.

       hierarchy integer examine
            Indicates whether the DHCP server is primary (0) or secondary (1) in
            this failover relationship.

       last-packet-sent integer examine
            Indicates the time at which the most recent failover packet was sent
            by this DHCP server to its failover partner.

       last-timestamp-received integer examine
            Indicates the timestamp that was on the failover message most
            recently received from the failover partner.

       skew integer examine
            Indicates the skew between the failover partner's clock and this
            DHCP server's clock

       max-response-delay integer examine
            Indicates the time in seconds after which, if no message is received
            from the failover partner, the partner is assumed to be out of
            communication.

       cur-unacked-updates integer examine
            Indicates the number of update messages that have been received from
            the failover partner but not yet processed.

FILES
       ETCDIR/dhcpd.conf, DBDIR/dhcpd.leases, RUNDIR/dhcpd.pid,
       DBDIR/dhcpd.leases~.

SEE ALSO
       dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)

AUTHOR
       dhcpd(8) was originally written by Ted Lemon under a contract with Vixie
       Labs.  Funding for this project was provided by Internet Systems
       Consortium.  Version 3 of the DHCP server was funded by Nominum, Inc.
       Information about Internet Systems Consortium is available at
       https://www.isc.org/.



                                                                        dhcpd(8)