From Linux Man Pages
dhcrelay - Dynamic Host Configuration Protocol Relay Agent
SYNOPSIS
dhcrelay [ -p port ] [ -d ] [ -q ] [ -i if0 [ ... -i ifN ] ] [ -a ] [ -c count ] [ -A length ] [ -D ] [ -m
append | replace | forward | discard ] server0 [ ...serverN ]
DESCRIPTION
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 subnets.
OPERATION
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 net-
work from which the original request came.
COMMAND LINE
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 run-
ning 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 pur-
poses.
Dhcrelay will normally run in the foreground until it has configured an interface, and then will revert to run-
ning in the background. To force dhcrelay to always run as a foreground process, the -d flag should be speci-
fied. This is useful when running dhcrelay under a debugger, or when running it out of inittab on System V sys-
tems.
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.
RELAY AGENT INFORMATION OPTIONS
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 necessary.
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 it.
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 pack-
ets that don't contain a match will be dropped.
SPECIFYING DHCP SERVERS
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.
RELATED
dhclient(8), dhcpd(8), RFC2132, RFC2131, draft-ietf-dhc-agent-options-03.txt.
BUGS
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 net-
work - 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.
CATEGORY