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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 | for-
ward | discard ] [ -i interface0 [ ... -i interfaceN ] ] [ -iu inter-
face0 [ ... -iu interfaceN ] ] [ -id interface0 [ ... -id interfaceN
] ] [ -U interface ] 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
-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 speci-
fied on the command line, to which DHCP/BOOTP queries should be
Options available for both DHCPv4 and DHCPv6:
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 run-
ning dhcrelay under a debugger, or running out of inittab on
System V systems.
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.
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 con-
figuration on startup.
Path to alternate pid file.
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 con-
tain 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.
Specify the maximum packet size to send to a DHCPv4/BOOTP
server. This might be done to allow sufficient space for addi-
tion of relay agent options while still fitting into the Ether-
net 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 any-
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.
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
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.
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.
Enables the addition of a RFC 3527 compliant link selection sub-
option 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 for-
warded 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 inter-
faces in use, to disambiguate between them. The -I option
causes dhcrelay to send the option even if there is only one
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.
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 inter-
face. The optional #index parameter specifies the interface
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.
dhclient(8), dhcpd(8), 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
The loopback interface is not (yet) recognized as a valid interface.
dhcrelay(8) To learn more about Internet Systems Consortium, see