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 ] [ -g ipaddr ] 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.
31
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 re‐
46 layed.
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 de‐
61 bugging purposes. Default is port 67 for DHCPv4/BOOTP, or port
62 547 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 re‐
69 quires 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 re‐
86 sponses back to the client. The agent option field will contain
87 two agent options: the Circuit ID suboption and the Remote ID
88 suboption. Currently, the Circuit ID will be the printable name
89 of the interface on which the client request was received. The
90 client supports inclusion of a Remote ID suboption as well, but
91 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 re‐
101 sponse 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 -g ipaddr
106 When a package gets sent back to the client, replace the gate‐
107 way's IP address (giaddr) with the given ipaddr. This can be
108 used as a workaround for bogus clients like Solaris 11 grub,
109 which use the giaddr instead of the announced router (3) to
110 setup its default route.
111 -i ifname
113 Listen for DHCPv4/BOOTP traffic on interface ifname. Multiple
114 interfaces may be specified by using more than one -i option.
115 If no interfaces are specified on the command line, dhcrelay
116 will identify all network interfaces, eliminating non-broadcast
117 interfaces if possible, and attempt to listen on all of them.
118 -iu ifname
120 Specifies an upstream network interface: an interface from which
121 replies from servers and other relay agents will be accepted.
122 Multiple interfaces may be specified by using more than one -iu
123 option. This argument is
124 intended to be used in conjunction with one or more -i or -id
125 arguments.
126 -id ifname
128 Specifies a downstream network interface: an interface from
129 which requests from clients and other relay agents will be ac‐
130 cepted. Multiple interfaces may be specified by using more than
131 one -id option. This argument is intended to be used in con‐
132 junction with one or more -i or -iu arguments.
133 -m append|replace|forward|discard
135 Control the handling of incoming DHCPv4 packets which already
136 contain relay agent options. If such a packet does not have gi‐
137 addr set in its header, the DHCP standard requires that the
138 packet be discarded. However, if giaddr is set, the relay agent
139 may handle the situation in four ways: It may append its own
140 set of relay options to the packet, leaving the supplied option
141 field intact; it may replace the existing agent option field; it
142 may forward the packet unchanged; or, it may discard it.
143 -U ifname
145 Enables the addition of a RFC 3527 compliant link selection sub‐
146 option for clients directly connected to the relay. This RFC
147 allows a relay to specify two different IP addresses: one for
148 the server to use when communicating with the relay (giaddr) the
149 other for choosing the subnet for the client (the suboption).
150 This can be useful if the server is unable to send packets to
151 the relay via the address used for the subnet.
152 When enabled, dhcrelay will add an agent option (as per -a
154 above) that includes the link selection suboption to the for‐
155 warded packet. This will only be done to packets received from
156 clients that are directly connected to the relay (i.e. giaddr is
157 zero). The address used in the suboption will be that of the
158 link upon which the inbound packet was received (which would
159 otherwise be used for giaddr). The value of giaddr will be set
160 to that of interface ifname.
161 Only one interface should be marked in this fashion. Currently
163 enabling this option on an interface causes the relay to process
164 all DHCP traffic similar to the -i option, in the future we may
165 split the two more completely.
166 This option is off by default. Note that enabling this option
168 automatically enables the -a option.
169 Keep in mind that using options such as -m replace or -m discard
171 on relays upstream from one using -U can pose problems. The up‐
172 stream relay will wipe out the initial agent option containing
173 the link selection while leaving the re-purposed giaddr value in
174 place, causing packets to go astray.
175 Options available in DHCPv6 mode only:
178 -I Force use of the DHCPv6 Interface-ID option. This option is au‐
180 tomatically sent when there are two or more downstream inter‐
181 faces in use, to disambiguate between them. The -I option
182 causes dhcrelay to send the option even if there is only one
183 downstream interface.
184 -s subscriber-id
186 Add an option with the specified subscriber-id into the packet.
187 This feature is for testing rather than production as it will
188 put the same subscriber-id into the packet for all clients.
189 -l [address%]ifname[#index]
191 Specifies the ``lower'' network interface for DHCPv6 relay mode:
192 the interface on which queries will be received from clients or
193 from other relay agents. At least one -l option must be in‐
194 cluded in the command line when running in DHCPv6 mode. The in‐
195 terface name ifname is a mandatory parameter. The link address
196 can be specified by address%; if it isn't, dhcrelay will use the
197 first non-link-local address configured on the interface. The
198 optional #index parameter specifies the interface index.
199 -u [address%]ifname
201 Specifies the ``upper'' network interface for DHCPv6 relay mode:
202 the interface to which queries from clients and other relay
203 agents should be forwarded. At least one -u option must be in‐
204 cluded in the command line when running in DHCPv6 mode. The in‐
205 terface name ifname is a mandatory parameter. The destination
206 unicast or multicast address can be specified by address%; if
207 not specified, the relay agent will forward to the DHCPv6
208 All_DHCP_Relay_Agents_and_Servers multicast address.
209 It is possible to specify the same interface with different addresses
211 more than once, and even, when the system supports it, to use the same
212 interface as both upper and lower interfaces.
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215 dhclient(8), dhcpd(8), RFC3315, RFC2132, RFC2131.
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218 Using the same interface on both upper and lower sides may cause loops,
219 so when running this way, the maximum hop count should be set to a low
220 value.
221 The loopback interface is not (yet) recognized as a valid interface.
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225 dhcrelay(8) To learn more about Internet Systems Consortium, see
226 https://www.isc.org
227 dhcrelay(8)