dhcrelay - Dynamic Host Configuration Protocol Relay Agent
] [ -dqaD
] [ -p port
] [ -A length
] [ -pf
] [ --no-pid
] [ -m append
] [ -i
[ ... -i interfaceN
] ] server0
] [ -p port
] [ -c
] [ -pf pid-file
] [ --no-pid
] [ -s
] -l lower0
[ ... -l
] -u upper0
[ ... -u upperN
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.
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.
Protocol selection options:
- Run dhcrelay as a DHCPv6 relay agent. Incompatible with the -4
Specifying DHCPv4/BOOTP servers
- 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.
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
- Force dhcrelay to run as a foreground process. Useful when running
dhcrelay under a debugger, or running out of inittab on System V
- -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
- Quiet mode. Prevents dhcrelay6 from printing its network configuration on
- -pf pid-file
- Path to alternate pid file.
Options available in DHCPv4 mode only:
- Option to disable writing pid files. By default the program will write 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
- -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.
- 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.
Options available in DHCPv6 mode only:
- -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.
To use this option you must also enable the -a option.
- 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.
- -s subscriber-id
- Add an option with the specified subscriber-id into the packet. This
feature is for testing rather than production as it will put the same
subscriber-id into the packet for all clients.
- -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
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.
, RFC3315, RFC2132, RFC2131.
Using the same interface on both upper and lower sides may cause loops, so when
running this way, the maximum hop count should be set to a low value.
The loopback interface is not (yet) recognized as a valid interface.
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