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 [ -p port ] [ -d ] [ -q ] [ -i if0 [ ... -i ifN ] ] [ -a ] [
10 -c count ] [ -A length ] [ -D ] [ -m append | replace | forward | dis‐
11 card ] server0 [ ...serverN ]
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14 The Internet Systems Consortium DHCP Relay Agent, dhcrelay, provides a
15 means for relaying DHCP and BOOTP requests from a subnet to which no
16 DHCP server is directly connected to one or more DHCP servers on other
17 subnets.
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20 The DHCP Relay Agent listens for DHCP and BOOTP queries and responses.
21 When a query is received from a client, dhcrelay forwards it to the
22 list of DHCP servers specified on the command line. When a reply is
23 received from a server, it is broadcast or unicast (according to the
24 relay agent's ability or the client's request) on the network from
25 which the original request came.
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28 The names of the network interfaces that dhcrelay should attempt to
29 configure may be specified on the command line using the -i option. If
30 no interface names are specified on the command line dhcrelay will
31 identify all network interfaces, elimininating non-broadcast interfaces
32 if possible, and attempt to configure each interface.
33 The -i flag can be used to specify the network interfaces on which the
35 relay agent should listen. In general, it must listen not only on
36 those network interfaces to which clients are attached, but also on
37 those network interfaces to which the server (or the router that
38 reaches the server) is attached. However, in some cases it may be
39 necessary to exclude some networks; in this case, you must list all
40 those network interfaces that should not be excluded using the -i flag.
41 In some cases it is helpful for the relay agent to forward requests
43 from networks on which a DHCP server is running to other DHCP servers.
44 This would be the case if two DHCP servers on different networks were
45 being used to provide backup service for each other's networks.
46 If dhcrelay should listen and transmit on a port other than the stan‐
48 dard (port 67), the -p flag may used. It should be followed by the udp
49 port number that dhcrelay should use. This is mostly useful for debug‐
50 ging purposes.
51 Dhcrelay will normally run in the foreground until it has configured an
53 interface, and then will revert to running in the background. To force
54 dhcrelay to always run as a foreground process, the -d flag should be
55 specified. This is useful when running dhcrelay under a debugger, or
56 when running it out of inittab on System V systems.
57 Dhcrelay will normally print its network configuration on startup.
59 This can be unhelpful in a system startup script - to disable this be‐
60 haviour, specify the -q flag.
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63 If the -a flag is set the relay agent will append an agent option field
64 to each request before forwarding it to the server. Agent option
65 fields in responses sent from servers to clients will be stripped
66 before forwarding such responses back to the client.
67 The agent option field will contain two agent options: the Circuit ID
69 suboption and the Remote ID suboption. Currently, the Circuit ID will
70 be the printable name of the interface on which the client request was
71 received. The client supports inclusion of a Remote ID suboption as
72 well, but this is not used by default.
73 When forwarding packets, dhcrelay discards packets which have reached a
75 hop count of 10. If a lower or higher threshold (up to 255) is
76 desired, depending on your environment, you can specify the max hop
77 count threshold as a number following the -c option.
78 Relay Agent options are added to a DHCP packet without the knowledge of
80 the DHCP client. The client may have filled the DHCP packet option
81 buffer completely, in which case there theoretically isn't any space to
82 add Agent options. However, the DHCP server may be able to handle a
83 much larger packet than most DHCP clients would send. The current
84 Agent Options draft requires that the relay agent use a maximum packet
85 size of 576 bytes.
86 It is recommended that with the Internet Systems Consortium DHCP
88 server, the maximum packet size be set to about 1400, allowing plenty
89 of extra space in which the relay agent can put the agent option field,
90 while still fitting into the Ethernet MTU size. This can be done by
91 specifying the -A flag, followed by the desired maximum packet size
92 (e.g., 1400).
93 Note that this is reasonably safe to do even if the MTU between the
95 server and the client is less than 1500, as long as the hosts on which
96 the server and client are running support IP fragmentation (and they
97 should). With some knowledge as to how large the agent options might
98 get in a particular configuration, this parameter can be tuned as
99 finely as necessary.
100 It is possible for a relay agent to receive a packet which already con‐
102 tains an agent option field. If this packet does not have a giaddr
103 set, the standard requires that the packet be discarded.
104 If giaddr is set, the server may handle the situation in one of four
106 ways: it may append its own set of relay options to the packet, leaving
107 the supplied option field intact. It may replace the existing agent
108 option field. It may forward the packet unchanged. Or, it may dis‐
109 card it.
110 Which of these behaviours is followed by the Internet Systems Consor‐
112 tium DHCP Relay Agent may be configured with the -m flag, followed by
113 one of the four keywords specified in italics above.
114 When the relay agent receives a reply from a server that it's supposed
116 to forward to a client, and Relay Agent Information option processing
117 is enabled, the relay agent scans the packet for Relay Agent Informa‐
118 tion options and removes them. As it's scanning, if it finds a Relay
119 Agent Information option field containing an Agent ID suboption that
120 matches one of its IP addresses, that option is recognized as its own.
121 If no such option is found, the relay agent can either drop the packet,
122 or relay it anyway. If the -D option is specified, all packets that
123 don't contain a match will be dropped.
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126 The name or IP address of at least one DHCP server to which DHCP and
127 BOOTP requests should be relayed must be specified on the command line.
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130 dhclient(8), dhcpd(8), RFC2132, RFC2131, draft-ietf-dhc-agent-
131 options-03.txt.
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134 It should be possible for the user to define the Circuit ID and Remote
135 ID values on a per-interface basis.
136 The relay agent should not relay packets received on a physical network
138 to DHCP servers on the same physical network - if they do, the server
139 will receive duplicate packets. In order to fix this, however, the
140 relay agent needs to be able to learn about the network topology, which
141 requires that it have a configuration file.
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144 dhcrelay(8) has been written for Internet Systems Consortium by Ted
145 Lemon in cooperation with Vixie Enterprises. To learn more about
146 Internet Systems Consortium, see http://www.isc.org/isc. To learn more
147 about Vixie Enterprises, see http://www.vix.com.
148 dhcrelay(8)