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

       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

       dhcrelay [ -p port ] [ -d ] [ -q ] [ -i if0 [ ...  -i ifN ] ] [ -a ] [ -c
       count ] [ -A length ] [ -D ] [ -m append | replace | forward | discard ]
       server0 [ ...serverN ]

       The Internet Systems Consortium DHCP Relay Agent, dhcrelay, provides a
       means for relaying DHCP and BOOTP requests from a subnet to which no DHCP
       server is directly connected to one or more DHCP servers on other

       The DHCP Relay Agent listens for DHCP and BOOTP queries and responses.
       When a query is received from a client, dhcrelay forwards it to the list
       of DHCP servers specified on the command line.  When a reply is received
       from a server, it is broadcast or unicast (according to the relay agent's
       ability or the client's request) on the network from which the original
       request came.

       The names of the network interfaces that dhcrelay should attempt to
       configure may be specified on the command line using the -i option.  If
       no interface names are specified on the command line dhcrelay will
       identify all network interfaces, elimininating non-broadcast interfaces
       if possible, and attempt to configure each interface.

       The -i flag can be used to specify the network interfaces on which the
       relay agent should listen.   In general, it must listen not only on those
       network interfaces to which clients are attached, but also on those
       network interfaces to which the server (or the router that reaches the
       server) is attached.   However, in some cases it may be necessary to
       exclude some networks; in this case, you must list all those network
       interfaces that should not be excluded using the -i flag.

       In some cases it is helpful for the relay agent to forward requests from
       networks on which a DHCP server is running to other DHCP servers.   This
       would be the case if two DHCP servers on different networks were being
       used to provide backup service for each other's networks.

       If dhcrelay should listen and transmit on a port other than the standard
       (port 67), the -p flag may used.  It should be followed by the udp port
       number that dhcrelay should use.  This is mostly useful for debugging

       Dhcrelay will normally run in the foreground until it has configured an
       interface, and then will revert to running in the background.  To force
       dhcrelay to always run as a foreground process, the -d flag should be
       specified.  This is useful when running dhcrelay under a debugger, or
       when running it out of inittab on System V systems.

       Dhcrelay will normally print its network configuration on startup.  This
       can be unhelpful in a system startup script - to disable this behaviour,
       specify the -q flag.

       If the -a flag is set the relay agent will append an agent option field
       to each request before forwarding it to the server.   Agent option fields
       in responses sent from servers to clients will be stripped before
       forwarding such responses back to the client.

       The agent option field will contain two agent options: the Circuit ID
       suboption and the Remote ID suboption.  Currently, the Circuit ID will be
       the printable name of the interface on which the client request was
       received.  The client supports inclusion of a Remote ID suboption as
       well, but this is not used by default.

       When forwarding packets, dhcrelay discards packets which have reached a
       hop count of 10.  If a lower or higher threshold (up to 255) is desired,
       depending on your environment, you can specify the max hop count
       threshold as a number following the -c option.

       Relay Agent options are added to a DHCP packet without the knowledge of
       the DHCP client.   The client may have filled the DHCP packet option
       buffer completely, in which case there theoretically isn't any space to
       add Agent options.   However, the DHCP server may be able to handle a
       much larger packet than most DHCP clients would send.   The current Agent
       Options draft requires that the relay agent use a maximum packet size of
       576 bytes.

       It is recommended that with the Internet Systems Consortium DHCP server,
       the maximum packet size be set to about 1400, allowing plenty of extra
       space in which the relay agent can put the agent option field, while
       still fitting into the Ethernet MTU size.  This can be done by specifying
       the -A flag, followed by the desired maximum packet size (e.g., 1400).

       Note that this is reasonably safe to do even if the MTU between the
       server and the client is less than 1500, as long as the hosts on which
       the server and client are running support IP fragmentation (and they
       should).  With some knowledge as to how large the agent options might get
       in a particular configuration, this parameter can be tuned as finely as

       It is possible for a relay agent to receive a packet which already
       contains an agent option field.  If this packet does not have a giaddr
       set, the standard requires that the packet be discarded.

       If giaddr is set, the server may handle the situation in one of four
       ways: it may append its own set of relay options to the packet, leaving
       the supplied option field intact.   It may replace the existing agent
       option field.  It may forward the packet unchanged.   Or, it may discard

       Which of these behaviours is followed by the Internet Systems Consortium
       DHCP Relay Agent may be configured with the -m flag, followed by one of
       the four keywords specified in italics above.

       When the relay agent receives a reply from a server that it's supposed to
       forward to a client, and Relay Agent Information option processing is
       enabled, the relay agent scans the packet for Relay Agent Information
       options and removes them.   As it's scanning, if it finds a Relay Agent
       Information option field containing an Agent ID suboption that matches
       one of its IP addresses, that option is recognized as its own.   If no
       such option is found, the relay agent can either drop the packet, or
       relay it anyway.   If the -D option is specified, all packets that don't
       contain a match will be dropped.

       The name or IP address of at least one DHCP server to which DHCP and
       BOOTP requests should be relayed must be specified on the command line.

       dhclient(8), dhcpd(8), RFC2132, RFC2131, draft-ietf-dhc-agent-

       It should be possible for the user to define the Circuit ID and Remote ID
       values on a per-interface basis.

       The relay agent should not relay packets received on a physical network
       to DHCP servers on the same physical network - if they do, the server
       will receive duplicate packets.   In order to fix this, however, the
       relay agent needs to be able to learn about the network topology, which
       requires that it have a configuration file.

       dhcrelay(8) has been written for Internet Systems Consortium by Ted Lemon
       in cooperation with Vixie Enterprises.  To learn more about Internet
       Systems Consortium, see http://www.isc.org/isc.  To learn more about
       Vixie Enterprises, see http://www.vix.com.