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.
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 value.
The loopback interface is not (yet) recognized as a valid interface.
To learn more about Internet Systems Consortium, see
https://www.isc.org