1dhclient-script(8) System Manager's Manual dhclient-script(8)
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6 dhclient-script - DHCP client network configuration script
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9 The DHCP client network configuration script is invoked from time to
10 time by dhclient(8). This script is used by the dhcp client to set
11 each interface's initial configuration prior to requesting an address,
12 to test the address once it has been offered, and to set the inter‐
13 face's final configuration once a lease has been acquired. If no lease
14 is acquired, the script is used to test predefined leases, if any, and
15 also called once if no valid lease can be identified.
16 This script is not meant to be customized by the end user. If local
18 customizations are needed, they should be possible using the enter and
19 exit hooks provided (see HOOKS for details). These hooks will allow
20 the user to override the default behaviour of the client in creating a
21 /etc/resolv.conf file, and to handle DHCP options not handled by
22 default.
23 No standard client script exists for some operating systems, even
25 though the actual client may work, so a pioneering user may well need
26 to create a new script or modify an existing one. In general, cus‐
27 tomizations specific to a particular computer should be done in the
28 /etc/dhcp/dhclient.conf file. If you find that you can't make such a
29 customization without customizing /etc/dhcp/dhclient.conf or using the
30 enter and exit hooks, please submit a bug report.
31
33 When it starts, the client script first defines a shell function,
34 make_resolv_conf , which is later used to create the /etc/resolv.conf
35 file. To override the default behaviour, redefine this function in
36 the enter hook script.
37 On after defining the make_resolv_conf function, the client script
39 checks for the presence of an executable /etc/dhcp/dhclient-enter-hooks
40 script, and if present, it invokes the script inline, using the Bourne
41 shell ´.´ command. The entire environment documented under OPERATION
42 is available to this script, which may modify the environment if needed
43 to change the behaviour of the script. If an error occurs during the
44 execution of the script, it can set the exit_status variable to a
45 nonzero value, and /usr/sbin/dhclient-script will exit with that error
46 code immediately after the client script exits.
47 After all processing has completed, /usr/sbin/dhclient-script checks
49 for the presence of an executable /etc/dhcp/dhclient-exit-hooks script,
50 which if present is invoked using the ´.´ command. The exit status of
51 dhclient-script will be passed to dhclient-exit-hooks in the exit_sta‐
52 tus shell variable, and will always be zero if the script succeeded at
53 the task for which it was invoked. The rest of the environment as
54 described previously for dhclient-enter-hooks is also present. The
55 /etc/dhcp/dhclient-exit-hooks script can modify the valid of exit_sta‐
56 tus to change the exit status of dhclient-script.
57 Immediately after dhclient brings an interface UP with a new IP
59 address, subnet mask, and routes, in the REBOOT/BOUND states, it will
60 check for the existence of an executable /etc/dhcp/dhclient-up-hooks
61 script, and source it if found. This script can handle DHCP options in
62 the environment that are not handled by default. A per-interface.
63 /etc/dhcp/dhclient-${IF}-up-hooks script will override the generic
64 script and be sourced when interface $IF has been brought up.
65 Immediately before dhclient brings an interface DOWN, removing its IP
67 address, subnet mask, and routes, in the STOP/RELEASE states, it will
68 check for the existence of an executable /etc/dhcp/dhclient-down-hooks
69 script, and source it if found. This script can handle DHCP options in
70 the environment that are not handled by default. A per-interface
71 /etc/dhcp/dhclient-${IF}-down-hooks script will override the generic
72 script and be sourced when interface $IF is about to be brought down.
73
75 When dhclient needs to invoke the client configuration script, it
76 defines a set of variables in the environment, and then invokes
77 /usr/sbin/dhclient-script. In all cases, $reason is set to the name of
78 the reason why the script has been invoked. The following reasons are
79 currently defined: MEDIUM, PREINIT, BOUND, RENEW, REBIND, REBOOT,
80 EXPIRE, FAIL, STOP, RELEASE, NBI and TIMEOUT.
81
83 The DHCP client is requesting that an interface's media type be set.
84 The interface name is passed in $interface, and the media type is
85 passed in $medium.
86
88 The DHCP client is requesting that an interface be configured as
89 required in order to send packets prior to receiving an actual address.
90 For clients which use the BSD socket library, this means configuring
91 the interface with an IP address of 0.0.0.0 and a broadcast address of
92 255.255.255.255. For other clients, it may be possible to simply con‐
93 figure the interface up without actually giving it an IP address at
94 all. The interface name is passed in $interface, and the media type
95 in $medium.
96 If an IP alias has been declared in dhclient.conf, its address will be
98 passed in $alias_ip_address, and that ip alias should be deleted from
99 the interface, along with any routes to it.
100
102 The DHCP client has done an initial binding to a new address. The new
103 ip address is passed in $new_ip_address, and the interface name is
104 passed in $interface. The media type is passed in $medium. Any
105 options acquired from the server are passed using the option name
106 described in dhcp-options, except that dashes (´-´) are replaced by
107 underscores (´_´) in order to make valid shell variables, and the vari‐
108 able names start with new_. So for example, the new subnet mask would
109 be passed in $new_subnet_mask. The options that the client explicitly
110 requested via a PRL or ORO option are passed with the same option name
111 as above but prepended with requested_ and with a value of 1, or exam‐
112 ple requested_subnet_mask=1. No such variable is defined for options
113 not requested by the client or options that don't require a request
114 option, such as the ip address (*_ip_address) or expiration time
115 (*_expiry).
116 Before actually configuring the address, dhclient-script should somehow
118 ARP for it and exit with a nonzero status if it receives a reply. In
119 this case, the client will send a DHCPDECLINE message to the server and
120 acquire a different address. This may also be done in the RENEW,
121 REBIND, or REBOOT states, but is not required, and indeed may not be
122 desirable.
123 When a binding has been completed, a lot of network parameters are
125 likely to need to be set up. A new /etc/resolv.conf needs to be cre‐
126 ated, using the values of $new_domain_name and $new_domain_name_servers
127 (which may list more than one server, separated by spaces). A default
128 route should be set using $new_routers, and static routes may need to
129 be set up using $new_static_routes.
130 If an IP alias has been declared, it must be set up here. The alias
132 IP address will be written as $alias_ip_address, and other DHCP options
133 that are set for the alias (e.g., subnet mask) will be passed in vari‐
134 ables named as described previously except starting with $alias_
135 instead of $new_. Care should be taken that the alias IP address not
136 be used if it is identical to the bound IP address ($new_ip_address),
137 since the other alias parameters may be incorrect in this case.
138
140 When a binding has been renewed, the script is called as in BOUND,
141 except that in addition to all the variables starting with $new_, and
142 $requested_ there is another set of variables starting with $old_.
143 Persistent settings that may have changed need to be deleted - for
144 example, if a local route to the bound address is being configured, the
145 old local route should be deleted. If the default route has changed,
146 the old default route should be deleted. If the static routes have
147 changed, the old ones should be deleted. Otherwise, processing can be
148 done as with BOUND.
149
151 The DHCP client has rebound to a new DHCP server. This can be handled
152 as with RENEW, except that if the IP address has changed, the ARP table
153 should be cleared.
154
156 The DHCP client has successfully reacquired its old address after a
157 reboot. This can be processed as with BOUND.
158
160 The DHCP client has failed to renew its lease or acquire a new one, and
161 the lease has expired. The IP address must be relinquished, and all
162 related parameters should be deleted, as in RENEW and REBIND.
163
165 The DHCP client has been unable to contact any DHCP servers, and any
166 leases that have been tested have not proved to be valid. The parame‐
167 ters from the last lease tested should be deconfigured. This can be
168 handled in the same way as EXPIRE.
169
171 The dhclient has been informed to shut down gracefully, the dhclient-
172 script should unconfigure or shutdown the interface as appropriate.
173
175 The dhclient has been executed using the -r flag, indicating that the
176 administrator wishes it to release its lease(s). dhclient-script
177 should unconfigure or shutdown the interface.
178
180 No-Broadcast-Interfaces...dhclient was unable to find any interfaces
181 upon which it believed it should commence DHCP. What dhclient-script
182 should do in this situation is entirely up to the implementor.
183
185 The DHCP client has been unable to contact any DHCP servers. However,
186 an old lease has been identified, and its parameters have been passed
187 in as with BOUND. The client configuration script should test these
188 parameters and, if it has reason to believe they are valid, should exit
189 with a value of zero. If not, it should exit with a nonzero value.
190 The usual way to test a lease is to set up the network as with REBIND
192 (since this may be called to test more than one lease) and then ping
193 the first router defined in $routers. If a response is received, the
194 lease must be valid for the network to which the interface is currently
195 connected. It would be more complete to try to ping all of the
196 routers listed in $new_routers, as well as those listed in
197 $new_static_routes, but current scripts do not do this.
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200 Each operating system should generally have its own script file,
201 although the script files for similar operating systems may be similar
202 or even identical. The script files included in Internet Systems Con‐
203 sortium DHCP distribution appear in the distribution tree under
204 client/scripts, and bear the names of the operating systems on which
205 they are intended to work.
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208 If more than one interface is being used, there's no obvious way to
209 avoid clashes between server-supplied configuration parameters - for
210 example, the stock dhclient-script rewrites /etc/resolv.conf. If more
211 than one interface is being configured, /etc/resolv.conf will be
212 repeatedly initialized to the values provided by one server, and then
213 the other. Assuming the information provided by both servers is
214 valid, this shouldn't cause any real problems, but it could be confus‐
215 ing.
216 Normally, if dhclient was compiled with libcap-ng support, dhclient
218 drops most capabilities immediately upon startup. While more secure,
219 this greatly restricts the additional actions that hooks in dhclient-
220 script can take. For example, any daemons that dhclient-script starts
221 or restarts will inherit the restricted capabilities as well, which may
222 interfere with their correct operation. Thus, the -nc option can be
223 used to prevent dhclient from dropping capabilities.
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226 dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and
227 dhclient.leases(5).
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230 dhclient-script(8) has been written for Internet Systems Consortium by
231 Ted Lemon in cooperation with Vixie Enterprises. To learn more about
232 Internet Systems Consortium, see https://www.isc.org. To learn more
233 about Vixie Enterprises, see http://www.vix.com.
234 dhclient-script(8)