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 ] [ -pf
10 pid-file ] [ --no-pid ] [ -m append | replace | forward | discard ] [
11 -i interface0 [ ... -i interfaceN ] ] server0 [ ...serverN ]
12 dhcrelay -6 [ -dqI ] [ -p port ] [ -c count ] [ -pf pid-file ] [ --no-
14 pid ] -l lower0 [ ... -l lowerN ] -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 -pf pid-file
65 Path to alternate pid file.
66 --no-pid
68 Option to disable writing pid files. By default the program
69 will write a pid file.
70 Options available in DHCPv4 mode only:
72 -a Append an agent option field to each request before forwarding
74 it to the server. Agent option fields in responses sent from
75 servers to clients will be stripped before forwarding such
76 responses back to the client. The agent option field will con‐
77 tain two agent options: the Circuit ID suboption and the Remote
78 ID suboption. Currently, the Circuit ID will be the printable
79 name of the interface on which the client request was received.
80 The client supports inclusion of a Remote ID suboption as well,
81 but this is not used by default.
82 -A LENGTH
84 Specify the maximum packet size to send to a DHCPv4/BOOTP
85 server. This might be done to allow sufficient space for addi‐
86 tion of relay agent options while still fitting into the Ether‐
87 net MTU size.
88 -D Drop packets from upstream servers if they contain Relay Agent
90 Information options that indicate they were generated in
91 response to a query that came via a different relay agent. If
92 this option is not specified, such packets will be relayed any‐
93 way.
94 -i ifname
96 Listen for DHCPv4/BOOTP queries on interface ifname. Multiple
97 interfaces may be specified by using more than one -i option.
98 If no interfaces are specified on the command line, dhcrelay
99 will identify all network interfaces, eliminating non-broadcast
100 interfaces if possible, and attempt to listen on all of them.
101 -m append|replace|forward|discard
103 Control the handling of incoming DHCPv4 packets which already
104 contain relay agent options. If such a packet does not have
105 giaddr set in its header, the DHCP standard requires that the
106 packet be discarded. However, if giaddr is set, the relay agent
107 may handle the situation in four ways: It may append its own
108 set of relay options to the packet, leaving the supplied option
109 field intact; it may replace the existing agent option field; it
110 may forward the packet unchanged; or, it may discard it.
111 To use this option you must also enable the -a option.
113 Options available in DHCPv6 mode only:
115 -I Force use of the DHCPv6 Interface-ID option. This option is
117 automatically sent when there are two or more downstream inter‐
118 faces in use, to disambiguate between them. The -I option
119 causes dhcrelay to send the option even if there is only one
120 downstream interface.
121 -l [address%]ifname[#index]
123 Specifies the ``lower'' network interface for DHCPv6 relay mode:
124 the interface on which queries will be received from clients or
125 from other relay agents. At least one -l option must be
126 included in the command line when running in DHCPv6 mode. The
127 interface name ifname is a mandatory parameter. The link
128 address can be specified by address%; if it isn't, dhcrelay will
129 use the first non-link-local address configured on the inter‐
130 face. The optional #index parameter specifies the interface
131 index.
132 -u [address%]ifname
134 Specifies the ``upper'' network interface for DHCPv6 relay mode:
135 the interface to which queries from clients and other relay
136 agents should be forwarded. At least one -u option must be
137 included in the command line when running in DHCPv6 mode. The
138 interface name ifname is a mandatory parameter. The destination
139 unicast or multicast address can be specified by address%; if
140 not specified, the relay agent will forward to the DHCPv6
141 All_DHCP_Relay_Agents_and_Servers multicast address.
142 It is possible to specify the same interface with different addresses
144 more than once, and even, when the system supports it, to use the same
145 interface as both upper and lower interfaces.
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148 dhclient(8), dhcpd(8), RFC3315, RFC2132, RFC2131.
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151 Using the same interface on both upper and lower sides may cause loops,
152 so when running this way, the maximum hop count should be set to a low
153 value.
154 The loopback interface is not (yet) recognized as a valid interface.
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158 dhcrelay(8) To learn more about Internet Systems Consortium, see
159 https://www.isc.org
160 dhcrelay(8)