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 | -rp relay-port ] [ -c count ] [
10 -A length ] [ -pf pid-file ] [ --no-pid ] [ -m append | replace | for‐
11 ward | discard ] [ -i interface0 [ ... -i interfaceN ] ] [ -iu inter‐
12 face0 [ ... -iu interfaceN ] ] [ -id interface0 [ ... -id interfaceN
13 ] ] [ -U interface ] server0 [ ...serverN ]
14 dhcrelay -6 [ -dqI ] [ -p port | -rp relay-port ] [ -c count ] [ -pf
16 pid-file ] [ --no-pid ] [ -s subscriber-id ] -l lower0 [ ... -l lowerN
17 ] -u upper0 [ ... -u upperN ]
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20 The Internet Systems Consortium DHCP Relay Agent, dhcrelay, provides a
21 means for relaying DHCP and BOOTP requests from a subnet to which no
22 DHCP server is directly connected to one or more DHCP servers on other
23 subnets. It supports both DHCPv4/BOOTP and DHCPv6 protocols.
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26 The DHCP Relay Agent listens for DHCPv4 or DHCPv6 queries from clients
27 or other relay agents on one or more interfaces, passing them along to
28 ``upstream'' servers or relay agents as specified on the command line.
29 When a reply is received from upstream, it is multicast or unicast back
30 downstream to the source of the original request.
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33 Protocol selection options:
34 -6 Run dhcrelay as a DHCPv6 relay agent. Incompatible with the -4
36 option.
37 -4 Run dhcrelay as a DHCPv4/BOOTP relay agent. This is the default
39 mode of operation, so the argument is not necessary, but may be
40 specified for clarity. Incompatible with -6.
41 Specifying DHCPv4/BOOTP servers
43 In DHCPv4 mode, a list of one or more server addresses must be speci‐
45 fied on the command line, to which DHCP/BOOTP queries should be
46 relayed.
47 Options available for both DHCPv4 and DHCPv6:
49 -c count
51 Maximum hop count. When forwarding packets, dhcrelay discards
52 packets which have reached a hop count of COUNT. Default is 10.
53 Maximum is 255.
54 -d Force dhcrelay to run as a foreground process. Useful when run‐
56 ning dhcrelay under a debugger, or running out of inittab on
57 System V systems.
58 -p port
60 Listen and transmit on port PORT. This is mostly useful for
61 debugging purposes. Default is port 67 for DHCPv4/BOOTP, or
62 port 547 for DHCPv6. Incompatible with -rp.
63 -rp relay-port
65 Alternative source port for upstream (i.e toward the server)
66 messages with DHCPv4 RAI relay-port sub-option or DHCPv6 relay-
67 source-port option. Relay port support is only available if the
68 code was compiled with (./configure --enable-relay-port) and
69 requires LPF or BPF link layer access.
70 -q Quiet mode. Prevents dhcrelay6 from printing its network con‐
72 figuration on startup.
73 -pf pid-file
75 Path to alternate pid file.
76 --no-pid
78 Option to disable writing pid files. By default the program
79 will write a pid file.
80 Options available in DHCPv4 mode only:
82 -a Append an agent option field to each request before forwarding
84 it to the server. Agent option fields in responses sent from
85 servers to clients will be stripped before forwarding such
86 responses back to the client. The agent option field will con‐
87 tain two agent options: the Circuit ID suboption and the Remote
88 ID suboption. Currently, the Circuit ID will be the printable
89 name of the interface on which the client request was received.
90 The client supports inclusion of a Remote ID suboption as well,
91 but this is not used by default.
92 -A length
94 Specify the maximum packet size to send to a DHCPv4/BOOTP
95 server. This might be done to allow sufficient space for addi‐
96 tion of relay agent options while still fitting into the Ether‐
97 net MTU size.
98 -D Drop packets from upstream servers if they contain Relay Agent
100 Information options that indicate they were generated in
101 response to a query that came via a different relay agent. If
102 this option is not specified, such packets will be relayed any‐
103 way.
104 -i ifname
106 Listen for DHCPv4/BOOTP traffic on interface ifname. Multiple
107 interfaces may be specified by using more than one -i option.
108 If no interfaces are specified on the command line, dhcrelay
109 will identify all network interfaces, eliminating non-broadcast
110 interfaces if possible, and attempt to listen on all of them.
111 -iu ifname
113 Specifies an upstream network interface: an interface from which
114 replies from servers and other relay agents will be accepted.
115 Multiple interfaces may be specified by using more than one -iu
116 option. This argument is
117 intended to be used in conjunction with one or more -i or -id
118 arguments.
119 -id ifname
121 Specifies a downstream network interface: an interface from
122 which requests from clients and other relay agents will be
123 accepted. Multiple interfaces may be specified by using more
124 than one -id option. This argument is intended to be used in
125 conjunction with one or more -i or -iu arguments.
126 -m append|replace|forward|discard
128 Control the handling of incoming DHCPv4 packets which already
129 contain relay agent options. If such a packet does not have
130 giaddr set in its header, the DHCP standard requires that the
131 packet be discarded. However, if giaddr is set, the relay agent
132 may handle the situation in four ways: It may append its own
133 set of relay options to the packet, leaving the supplied option
134 field intact; it may replace the existing agent option field; it
135 may forward the packet unchanged; or, it may discard it.
136 -U ifname
138 Enables the addition of a RFC 3527 compliant link selection sub‐
139 option for clients directly connected to the relay. This RFC
140 allows a relay to specify two different IP addresses: one for
141 the server to use when communicating with the relay (giaddr) the
142 other for choosing the subnet for the client (the suboption).
143 This can be useful if the server is unable to send packets to
144 the relay via the address used for the subnet.
145 When enabled, dhcrelay will add an agent option (as per -a
147 above) that includes the link selection suboption to the for‐
148 warded packet. This will only be done to packets received from
149 clients that are directly connected to the relay (i.e. giaddr is
150 zero). The address used in the suboption will be that of the
151 link upon which the inbound packet was received (which would
152 otherwise be used for giaddr). The value of giaddr will be set
153 to that of interface ifname.
154 Only one interface should be marked in this fashion. Currently
156 enabling this option on an interface causes the relay to process
157 all DHCP traffic similar to the -i option, in the future we may
158 split the two more completely.
159 This option is off by default. Note that enabling this option
161 automatically enables the -a option.
162 Keep in mind that using options such as -m replace or -m discard
164 on relays upstream from one using -U can pose problems. The
165 upstream relay will wipe out the initial agent option containing
166 the link selection while leaving the re-purposed giaddr value in
167 place, causing packets to go astray.
168 Options available in DHCPv6 mode only:
171 -I Force use of the DHCPv6 Interface-ID option. This option is
173 automatically sent when there are two or more downstream inter‐
174 faces in use, to disambiguate between them. The -I option
175 causes dhcrelay to send the option even if there is only one
176 downstream interface.
177 -s subscriber-id
179 Add an option with the specified subscriber-id into the packet.
180 This feature is for testing rather than production as it will
181 put the same subscriber-id into the packet for all clients.
182 -l [address%]ifname[#index]
184 Specifies the ``lower'' network interface for DHCPv6 relay mode:
185 the interface on which queries will be received from clients or
186 from other relay agents. At least one -l option must be
187 included in the command line when running in DHCPv6 mode. The
188 interface name ifname is a mandatory parameter. The link
189 address can be specified by address%; if it isn't, dhcrelay will
190 use the first non-link-local address configured on the inter‐
191 face. The optional #index parameter specifies the interface
192 index.
193 -u [address%]ifname
195 Specifies the ``upper'' network interface for DHCPv6 relay mode:
196 the interface to which queries from clients and other relay
197 agents should be forwarded. At least one -u option must be
198 included in the command line when running in DHCPv6 mode. The
199 interface name ifname is a mandatory parameter. The destination
200 unicast or multicast address can be specified by address%; if
201 not specified, the relay agent will forward to the DHCPv6
202 All_DHCP_Relay_Agents_and_Servers multicast address.
203 It is possible to specify the same interface with different addresses
205 more than once, and even, when the system supports it, to use the same
206 interface as both upper and lower interfaces.
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209 dhclient(8), dhcpd(8), RFC3315, RFC2132, RFC2131.
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212 Using the same interface on both upper and lower sides may cause loops,
213 so when running this way, the maximum hop count should be set to a low
214 value.
215 The loopback interface is not (yet) recognized as a valid interface.
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219 dhcrelay(8) To learn more about Internet Systems Consortium, see
220 https://www.isc.org
221 dhcrelay(8)