1dhcrelay(8) System Manager's Manual dhcrelay(8)
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6 dhcrelay - Dynamic Host Configuration Protocol Relay Agent
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9 dhcrelay [ -4 ] [ -dqaD ] [ -p port ] [ -c count ] [ -A length ] [ -m
10 append | replace | forward | discard ] [ -i interface0 [ ... -i inter‐
11 faceN ] ] server0 [ ...serverN ]
12 dhcrelay -6 [ -dqI ] [ -p port ] [ -c count ] -l lower0 [ ... -l low‐
14 erN ] -u upper0 [ ... -u upperN ]
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17 The Internet Systems Consortium DHCP Relay Agent, dhcrelay, provides a
18 means for relaying DHCP and BOOTP requests from a subnet to which no
19 DHCP server is directly connected to one or more DHCP servers on other
20 subnets. It supports both DHCPv4/BOOTP and DHCPv6 protocols.
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23 The DHCP Relay Agent listens for DHCPv4 or DHCPv6 queries from clients
24 or other relay agents on one or more interfaces, passing them along to
25 ``upstream'' servers or relay agents as specified on the command line.
26 When a reply is received from upstream, it is multicast or unicast back
27 downstream to the source of the original request.
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30 Protocol selection options:
31 -6 Run dhcrelay as a DHCPv6 relay agent. Incompatible with the -4
33 option.
34 -4 Run dhcrelay as a DHCPv4/BOOTP relay agent. This is the default
36 mode of operation, so the argument is not necessary, but may be
37 specified for clarity. Incompatible with -6.
38 Specifying DHCPv4/BOOTP servers
40 In DHCPv4 mode, a list of one or more server addresses must be speci‐
42 fied on the command line, to which DHCP/BOOTP queries should be
43 relayed.
44 Options available for both DHCPv4 and DHCPv6:
46 -c COUNT
48 Maximum hop count. When forwarding packets, dhcrelay discards
49 packets which have reached a hop count of COUNT. Default is 10.
50 Maximum is 255.
51 -d Force dhcrelay to run as a foreground process. Useful when run‐
53 ning dhcrelay under a debugger, or running out of inittab on
54 System V systems.
55 -p PORT
57 Listen and transmit on port PORT. This is mostly useful for
58 debugging purposes. Default is port 67 for DHCPv4/BOOTP, or
59 port 547 for DHCPv6.
60 -q Quiet mode. Prevents dhcrelay6 from printing its network con‐
62 figuration on startup.
63 Options available in DHCPv4 mode only:
65 -a Append an agent option field to each request before forwarding
67 it to the server. Agent option fields in responses sent from
68 servers to clients will be stripped before forwarding such
69 responses back to the client. The agent option field will con‐
70 tain two agent options: the Circuit ID suboption and the Remote
71 ID suboption. Currently, the Circuit ID will be the printable
72 name of the interface on which the client request was received.
73 The client supports inclusion of a Remote ID suboption as well,
74 but this is not used by default.
75 -A LENGTH
77 Specify the maximum packet size to send to a DHCPv4/BOOTP
78 server. This might be done to allow sufficient space for addi‐
79 tion of relay agent options while still fitting into the Ether‐
80 net MTU size.
81 -D Drop packets from upstream servers if they contain Relay Agent
83 Information options that indicate they were generated in
84 response to a query that came via a different relay agent. If
85 this option is not specified, such packets will be relayed any‐
86 way.
87 -i ifname
89 Listen for DHCPv4/BOOTP queries on interface ifname. Multiple
90 interfaces may be specified by using more than one -i option.
91 If no interfaces are specified on the command line, dhcrelay
92 will identify all network interfaces, eliminating non-broadcast
93 interfaces if possible, and attempt to listen on all of them.
94 -m append|replace|forward|discard
96 Control the handling of incoming DHCPv4 packets which already
97 contain relay agent options. If such a packet does not have
98 giaddr set in its header, the DHCP standard requires that the
99 packet be discarded. However, if giaddr is set, the relay agent
100 may handle the situation in four ways: It may append its own
101 set of relay options to the packet, leaving the supplied option
102 field intact; it may replace the existing agent option field; it
103 may forward the packet unchanged; or, it may discard it.
104 Options available in DHCPv6 mode only:
106 -I Force use of the DHCPv6 Interface-ID option. This option is
108 automatically sent when there are two or more downstream inter‐
109 faces in use, to disambiguate between them. The -I option
110 causes dhcrelay to send the option even if there is only one
111 downstream interface.
112 -l [address%]ifname[#index]
114 Specifies the ``lower'' network interface for DHCPv6 relay mode:
115 the interface on which queries will be received from clients or
116 from other relay agents. At least one -l option must be
117 included in the command line when running in DHCPv6 mode. The
118 interface name ifname is a mandatory parameter. The link
119 address can be specified by address%; if it isn't, dhcrelay will
120 use the first non-link-local address configured on the inter‐
121 face. The optional #index parameter specifies the interface
122 index.
123 -u [address%]ifname
125 Specifies the ``upper'' network interface for DHCPv6 relay mode:
126 the interface to which queries from clients and other relay
127 agents should be forwarded. At least one -u option must be
128 included in the command line when running in DHCPv6 mode. The
129 interface name ifname is a mandatory parameter. The destination
130 unicast or multicast address can be specified by address%; if
131 not specified, the relay agent will forward to the DHCPv6
132 All_DHCP_Relay_Agents_and_Servers multicast address.
133 It is possible to specify the same interface with different addresses
135 more than once, and even, when the system supports it, to use the same
136 interface as both upper and lower interfaces.
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139 dhclient(8), dhcpd(8), RFC3315, RFC2132, RFC2131.
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142 Using the same interface on both upper and lower sides may cause loops,
143 so when running this way, the maximum hop count should be set to a low
144 value.
145 The loopback interface is not (yet) recognized as a valid interface.
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149 dhcrelay(8) To learn more about Internet Systems Consortium, see
150 https://www.isc.org
151 dhcrelay(8)