dhcrelay - Dynamic Host Configuration Protocol Relay Agent
dhcrelay [ -4 ] [ -dqaD ] [ -p
port | -rp relay-port ] [ -c count ] [
-A length ] [ -pf pid-file ] [ --no-pid ]
[ -m append | replace | forward | discard
] [ -i interface0 [ ... -i interfaceN ] ]
[ -iu interface0 [ ... -iu interfaceN ] ]
[ -id interface0 [ ... -id interfaceN ] ]
[ -U interface ] [ -g ipaddr ] server0 [
...serverN ]
dhcrelay -6 [ -dqI ] [ -p port |
-rp relay-port ] [ -c count ] [ -pf
pid-file ] [ --no-pid ] [ -s subscriber-id ]
-l lower0 [ ... -l lowerN ] -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:
- -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.
Incompatible with -rp.
- -rp relay-port
- Alternative source port for upstream (i.e toward the server) messages with
DHCPv4 RAI relay-port sub-option or DHCPv6 relay-source-port option. Relay
port support is only available if the code was compiled with (./configure
--enable-relay-port) and requires LPF or BPF link layer access.
- -q
- Quiet mode. Prevents dhcrelay6 from printing its network configuration on
startup.
- -pf pid-file
- Path to alternate pid file.
- --no-pid
- Option to disable writing pid files. By default the program will write a
pid file.
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.
- -g ipaddr
- When a package gets sent back to the client, replace the gateway's IP
address (giaddr) with the given ipaddr. This can be used as a
workaround for bogus clients like Solaris 11 grub, which use the giaddr
instead of the announced router (3) to setup its default route.
- -i ifname
- Listen for DHCPv4/BOOTP traffic 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.
- -iu ifname
- Specifies an upstream network interface: an interface from which replies
from servers and other relay agents will be accepted. Multiple interfaces
may be specified by using more than one -iu option. This argument
is
intended to be used in conjunction with one or more -i or -id
arguments.
- -id ifname
- Specifies a downstream network interface: an interface from which requests
from clients and other relay agents will be accepted. Multiple interfaces
may be specified by using more than one -id option. This argument
is intended to be used in conjunction with one or more -i or -iu
arguments.
- -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.
- -U ifname
- Enables the addition of a RFC 3527 compliant link selection suboption for
clients directly connected to the relay. This RFC allows a relay to
specify two different IP addresses: one for the server to use when
communicating with the relay (giaddr) the other for choosing the subnet
for the client (the suboption). This can be useful if the server is unable
to send packets to the relay via the address used for the subnet.
When enabled, dhcrelay will add an agent option (as per
-a above) that includes the link selection suboption to the
forwarded packet. This will only be done to packets received from
clients that are directly connected to the relay (i.e. giaddr is zero).
The address used in the suboption will be that of the link upon which
the inbound packet was received (which would otherwise be used for
giaddr). The value of giaddr will be set to that of interface
ifname.
Only one interface should be marked in this fashion. Currently
enabling this option on an interface causes the relay to process all
DHCP traffic similar to the -i option, in the future we may split
the two more completely.
This option is off by default. Note that enabling this option
automatically enables the -a option.
Keep in mind that using options such as -m replace or
-m discard on relays upstream from one using -U can pose
problems. The upstream relay will wipe out the initial agent option
containing the link selection while leaving the re-purposed giaddr value
in place, causing packets to go astray.
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.
- -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
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.
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.
dhcrelay(8) To learn more about Internet Systems
Consortium, see https://www.isc.org