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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 | dis-
card ] 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
You must have the Berkeley Packet Filter (bpf) configured in your
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 stan-
dard (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 debug-
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 be-
haviour, 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
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 con-
tains 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 dis-
Which of these behaviours is followed by the Internet Systems Consor-
tium 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 Informa-
tion 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.
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.
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.