Provided by: isc-dhcp-relay_4.4.1-2.1ubuntu5.20.04.5_amd64 
NAME
dhcrelay - Dynamic Host Configuration Protocol Relay Agent
SYNOPSIS
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 ] 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 ]
DESCRIPTION
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. 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.
-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.
SEE ALSO
dhclient(8), dhcpd(8), RFC3315, RFC2132, RFC2131.
BUGS
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.
AUTHOR
dhcrelay(8) To learn more about Internet Systems Consortium, see https://www.isc.org
dhcrelay(8)