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. It supports both DHCPv4/BOOTP and DHCPv6 protocols.
OPERATION
The DHCP Relay Agent listens for DHCPv4 or DHCPv6 queries from clients or
other relay agents on one or more interfaces, passing them along to
``upstream'' servers or relay agents as specified on the command line.
When a reply is received from upstream, it is multicast or unicast back
downstream to the source of the original request.
COMMAND LINE
Protocol selection options:
-6
Run dhcrelay as a DHCPv6 relay agent. Incompatible with the -4
option.
-4
Run dhcrelay as a DHCPv4/BOOTP relay agent. This is the default mode of
operation, so the argument is not necessary, but may be specified for
clarity. Incompatible with -6.
Specifying DHCPv4/BOOTP servers
In DHCPv4 mode, a list of one or more server addresses must be specified on
the command line, to which DHCP/BOOTP queries should be relayed.
Options available for both DHCPv4 and DHCPv6:
-c COUNT
Maximum hop count. When forwarding packets, dhcrelay discards packets
which have reached a hop count of COUNT. Default is 10. Maximum is 255.
-d
Force dhcrelay to run as a foreground process. Useful when running
dhcrelay under a debugger, or running out of inittab on System V systems.
-p PORT
Listen and transmit on port PORT. This is mostly useful for debugging
purposes. Default is port 67 for DHCPv4/BOOTP, or port 547 for DHCPv6.
-q
Quiet mode. Prevents dhcrelay6 from printing its network configuration
on startup.
Options available in DHCPv4 mode only:
-a
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.
-A LENGTH
Specify the maximum packet size to send to a DHCPv4/BOOTP server. This
might be done to allow sufficient space for addition of relay agent
options while still fitting into the Ethernet MTU size.
-D
Drop packets from upstream servers if they contain Relay Agent
Information options that indicate they were generated in response to
a query that came via a different relay agent. If this option is not
specified, such packets will be relayed anyway.
-i ifname
Listen for DHCPv4/BOOTP queries on interface ifname. Multiple
interfaces may be specified by using more than one -i option. If
no interfaces are specified on the command line, dhcrelay will identify
all network interfaces, eliminating non-broadcast interfaces if possible,
and attempt to listen on all of them.
-m append|replace|forward|discard
Control the handling of incoming DHCPv4 packets which already contain
relay agent options. If such a packet does not have giaddr set in
its header, the DHCP standard requires that the packet be discarded.
However, if giaddr is set, the relay agent may handle the situation
in 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.
Options available in DHCPv6 mode only:
-I
Force use of the DHCPv6 Interface-ID option. This option is
automatically sent when there are two or more downstream interfaces
in use, to disambiguate between them. The -I option causes
dhcrelay to send the option even if there is only one downstream
interface.
-l [address%]ifname[#index]
Specifies the ``lower'' network interface for DHCPv6 relay mode: the
interface on which queries will be received from clients or from other
relay agents. At least one -l option must be included in the command
line when running in DHCPv6 mode. The interface name ifname is a
mandatory parameter. The link address can be specified by address%;
if it isn't, dhcrelay will use the first non-link-local address configured
on the interface. The optional #index parameter specifies the
interface index.
-u [address%]ifname
Specifies the ``upper'' network interface for DHCPv6 relay mode: the
interface to which queries from clients and other relay agents should be
forwarded. At least one -u option must be included in the command
line when running in DHCPv6 mode. The interface name ifname is a
mandatory parameter. The destination unicast or multicast address can be
specified by address%; if not specified, the relay agent will forward
to the DHCPv6 All_DHCP_Relay_Agents_and_Servers multicast address.
It is possible to specify the same interface with different addresses
more than once, and even, when the system supports it, to use the same
interface as both upper and lower interfaces.