DHCPCD(8) BSD System Manager’s Manual DHCPCD(8)

NAME

dhcpcd — an RFC 2131 compliant DHCP client

SYNOPSIS

dhcpcd [−ABbDdEGgHJKkLnpqTVw] [−C−−nohook hook] [−c−−script script] [−e−−env value] [−F−−fqdn FQDN] [−f−−config file] [−h−−hostname hostname] [−I−−clientid clientid] [−i−−vendorclassid vendorclassid] [−l−−leasetime seconds] [−m−−metric metric] [−O−−nooption option] [−o−−option option] [−Q−−require option] [−r−−request address] [−S−−static value] [−s−−inform address[/cidr]] [−t−−timeout seconds] [−u−−userclass class] [−v−−vendor codevalue] [−W−−whitelist address[/cidr]] [−y−−reboot seconds] [−X−−blacklist address[/cidr]] [−Z−−denyinterfaces pattern] [−z−−allowinterfaces pattern] [interface] [...]
dhcpcd −k
, −−release [interface]
dhcpcd −U, −−dumplease
interface
dhcpcd −−version
dhcpcd −x
, −−exit [interface]

DESCRIPTION

dhcpcd is an implementation of the DHCP client specified in RFC 2131. dhcpcd gets the host information (

IP address, routes, etc ) from a DHCP server and configures the network interface of the machine on which it is running. dhcpcd then runs the configuration script which writes DNS information to resolvconf(8), if available, otherwise directly to /etc/resolv.conf. If the hostname is currently blank, (null) or localhost, or force_hostname is YES or TRUE or 1 then dhcpcd sets the hostname to the one supplied by the DHCP server. dhcpcd then daemonises and waits for the lease renewal time to lapse. It will then attempt to renew its lease and reconfigure if the new lease changes.

dhcpcd is also an implementation of the BOOTP client specified in RFC 951.

dhcpcd is also an implementation of an IPv6 Router Solicitor as specified in RFC 6106 with regard to the RDNSS and DNSSL options.

Local Link configuration
If dhcpcd failed to obtain a lease, it probes for a valid IPv4LL address (

aka ZeroConf, aka APIPA ). Once obtained it restarts the process of looking for a DHCP server to get a proper address.

When using IPv4LL, dhcpcd nearly always succeeds and returns an exit code of 0. In the rare case it fails, it normally means that there is a reverse ARP proxy installed which always defeats IPv4LL probing. To disable this behaviour, you can use the −L, −−noipv4ll option.

Multiple interfaces
If a list of interfaces are given on the command line, then dhcpcd only works with those interfaces, otherwise dhcpcd discovers available Ethernet interfaces. If any interface reports a working carrier then dhcpcd will try and obtain a lease before forking to the background, otherwise it will fork right away. This behaviour can be modified with the −b, −−background and −w, −−waitip options.

If a single interface is given then dhcpcd only works for that interface and runs as a separate instance. The −w, −−waitip option is enabled in this instance to maintain compatibility with older versions.

Interfaces are preferred by carrier, DHCP lease/IPv4LL and then lowest metric. For systems that support route metrics, each route will be tagged with the metric, otherwise dhcpcd changes the routes to use the interface with the same route and the lowest metric. See options below for controlling which interfaces we allow and deny through the use of patterns.

Hooking into DHCP events
dhcpcd
runs /lib/dhcpcd/dhcpcd-run-hooks, or the script specified by the −c, −−script option. This script runs each script found in /lib/dhcpcd/dhcpcd-hooks in a lexical order. The default installation supplies the scripts 01-test, 10-mtu, 20-resolv.conf and 30-hostname. You can disable each script by using the −C, −−nohook option. See dhcpcd-run-hooks(8) for details on how these scripts work. dhcpcd currently ignores the exit code of the script.

Fine tuning
You can fine-tune the behaviour of dhcpcd with the following options:

−b, −−background

Background immediately. This is useful for startup scripts which don’t disable link messages for carrier status.

−c, −−script script

Use this script instead of the default /lib/dhcpcd/dhcpcd-run-hooks.

−D, −−duid

Generate an RFC 4361 compliant clientid. This requires persistent storage and not all DHCP servers work with it so it is not enabled by default. dhcpcd generates the DUID and stores it in /etc/dhcpcd.duid. This file should not be copied to other hosts.

−d, −−debug

Echo debug messages to the stderr and syslog.

−E, −−lastlease

If dhcpcd cannot obtain a lease, then try to use the last lease acquired for the interface. If the −p, −−persistent option is not given then the lease is used if it hasn’t expired.

−e, −−env value

Push value to the environment for use in dhcpcd-run-hooks(8). For example, you can force the hostname hook to always set the hostname with −e force_hostname=YES.

−g, −−reconfigure

dhcpcd will re-apply IP address, routing and run dhcpcd-run-hooks(8) for each interface. This is useful so that a 3rd party such as PPP or VPN can change the routing table and / or DNS, etc and then instruct dhcpcd to put things back afterwards. dhcpcd does not read a new configuration when this happens - you should rebind if you need that functionality.

−F, −−fqdn fqdn

Requests that the DHCP server updates DNS using FQDN instead of just a hostname. Valid values for fqdn are disable, none, ptr and both. dhcpcd itself never does any DNS updates. dhcpcd encodes the FQDN hostname as specified in RFC1035.

−f, −−config file

Specify a config to load instead of /etc/dhcpcd.conf. dhcpcd always processes the config file before any command line options.

−h, −−hostname hostname

Sends hostname to the DHCP server so it can be registered in DNS. If hostname is an empty string then the current system hostname is sent. If hostname is a FQDN (ie, contains a .) then it will be encoded as such.

−I, −−clientid clientid

Send the clientid. If the string is of the format 01:02:03 then it is encoded as hex. For interfaces whose hardware address is longer than 8 bytes, or if the clientid is an empty string then dhcpcd sends a default clientid of the hardware family and the hardware address.

−i, −−vendorclassid vendorclassid

Override the vendorclassid field sent. The default is dhcpcd <version>. If not set then none is sent.

−k, −−release

This causes an existing dhcpcd process running on the interface to release its lease, de-configure the interface and then exit. dhcpcd then waits until this process has exited.

−l, −−leasetime seconds

Request a specific lease time in seconds. By default dhcpcd does not request any lease time and leaves it in the hands of the DHCP server.

−m, −−metric metric

Metrics are used to prefer an interface over another one, lowest wins. dhcpcd will supply a default metic of 200 + if_nametoindex(3). An extra 100 will be added for wireless interfaces.

−n, −−rebind

Notifies dhcpcd to reload its configuration and rebind its interfaces. If dhcpcd is not running, then it starts up as normal.

−o, −−option option

Request the DHCP option variable for use in /lib/dhcpcd/dhcpcd-run-hooks.

−p, −−persistent

dhcpcd normally de-configures the interface and configuration when it exits. Sometimes, this isn’t desirable if, for example, you have root mounted over NFS. You can use this option to stop this from happening.

−r, −−request [address]

Request the address in the DHCP DISCOVER message. There is no guarantee this is the address the DHCP server will actually give. If no address is given then the first address currently assigned to the interface is used.

−s, −−inform [address[/cidr]]

Behaves like −r, −−request as above, but sends a DHCP INFORM instead of DISCOVER/REQUEST. This does not get a lease as such, just notifies the DHCP server of the address in use. You should also include the optional cidr network number in case the address is not already configured on the interface. dhcpcd remains running and pretends it has an infinite lease. dhcpcd will not de-configure the interface when it exits. If dhcpcd fails to contact a DHCP server then it returns a failure instead of falling back on IPv4LL.

−t, −−timeout seconds

Timeout after seconds, instead of the default 30. A setting of 0 seconds causes dhcpcd to wait forever to get a lease.

−u, −−userclass class

Tags the DHCP message with the userclass class. DHCP servers use this to give members of the class DHCP options other than the default, without having to know things like hardware address or hostname.

−v, −−vendor code,value

Add an encapsulated vendor option. code should be between 1 and 254 inclusive. To add a raw vendor string, omit code but keep the comma. Examples.

Set the vendor option 01 with an IP address.

dhcpcd −v 01,192.168.0.2 eth0

Set the vendor option 02 with a hex code.

dhcpcd −v 02,01:02:03:04:05 eth0

Set the vendor option 03 with an IP address as a string.

dhcpcd −v 03,\"192.168.0.2\" eth0

Set un-encapsulated vendor option to hello world.

dhcpcd −v ,"hello world" eth0

−−version

Display both program version and copyright information. dhcpcd then exits before doing any configuration.

−w, −−waitip

Wait for an address to be assigned before forking to the background.

−x, −−exit

This will signal an existing dhcpcd process running on the interface to de-configure the interface and exit. dhcpcd then waits until this process has exited.

−y, −−reboot seconds

Allow reboot seconds before moving to the discover phase if we have an old lease to use. The default is 5 seconds. A setting of 0 seconds causes dhcpcd to skip the reboot phase and go straight into discover.

Restricting behaviour
dhcpcd
will try to do as much as it can by default. However, there are sometimes situations where you don’t want the things to be configured exactly how the the DHCP server wants. Here are some options that deal with turning these bits off.

−A, −−noarp

Don’t request or claim the address by ARP. This also disables IPv4LL.

−B, −−nobackground

Don’t run in the background when we acquire a lease. This is mainly useful for running under the control of another process, such as a debugger or a network manager.

−C, −−nohook script

Don’t run this hook script. Matches full name, or prefixed with 2 numbers optionally ending with .sh.

So to stop dhcpcd from touching your DNS or MTU settings you would do:-

dhcpcd -C resolv.conf -C mtu eth0

−G, −−nogateway

Don’t set any default routes.

−H, −−xidhwaddr

Use the last four bytes of the hardware address as the DHCP xid instead of a randomly generated number.

−J, −−broadcast

Instructs the DHCP server to broadcast replies back to the client. Normally this is only set for non Ethernet interfaces, such as FireWire and InfiniBand. In most instances, dhcpcd will set this automatically.

−K, −−nolink

Don’t receive link messages for carrier status. You should only have to use this with buggy device drivers or running dhcpcd through a network manager.

−L, −−noipv4ll

Don’t use IPv4LL (aka APIPA, aka Bonjour, aka ZeroConf).

−O, −−nooption option

Don’t request the specified option. If no option given, then don’t request any options other than those to configure the interface and routing.

−Q, −−require option

Requires the option to be present in all DHCP messages, otherwise the message is ignored. To enforce that dhcpcd only responds to DHCP servers and not BOOTP servers, you can −Q dhcp_message_type.

−q, −−quiet

Quiet dhcpcd on the command line, only warnings and errors will be displayed. The messages are still logged though.

−S, −−static value

Configures a static value. If you set ip_address then dhcpcd will not attempt to obtain a lease and just use the value for the address with an infinite lease time.

Here is an example which configures a static address, routes and dns.

dhcpcd -S ip_address=192.168.0.10/24 \
-S routers=192.168.0.1 \
-S domain_name_servers=192.168.0.1 \
eth0

−T, −−test

On receipt of DHCP messages just call /lib/dhcpcd/dhcpcd-run-hooks with the reason of TEST which echos the DHCP variables found in the message to the console. The interface configuration isn’t touched and neither are any configuration files. To test INFORM the interface needs to be configured with the desired address before starting dhcpcd.

−U, −−dumplease interface

Dumps the last lease for the interface to stdout. interface could also be a path to a DHCP wire formatted file.

−V, −−variables

Display a list of option codes and the associated variable for use in dhcpcd-run-hooks(8). Variables are prefixed with new_ and old_ unless the option number is -. Variables without an option are part of the DHCP message and cannot be directly requested.

−W, −−whitelist address[/cidr]

Only accept packets from address[/cidr]. −X, −−blacklist is ignored if −W, −−whitelist is set.

−X, −−blacklist address[/cidr]

Ignore all packets from address[/cidr].

−Z, −−denyinterfaces pattern

When discovering interfaces, the interface name must not match pattern which is a space or comma separated list of patterns passed to fnmatch(3).

−z, −−allowinterfaces pattern

When discovering interfaces, the interface name must match pattern which is a space or comma separated list of patterns passed to fnmatch(3). If the same interface is matched in −Z, −−denyinterfaces then it is still denied.

3RDPARTY LINK MANAGEMENT

Some interfaces require configuration by 3rd parties, such as PPP or VPN. When an interface configuration in dhcpcd is marked as STATIC or INFORM without an address then dhcpcd will monitor the interface until an address is added or removed from it and act accordingly. For point to point interfaces (like PPP), a default route to its destination is automatically added to the configuration. If the point to point interface is configured for INFORM, then dhcpcd unicasts INFORM to the destination, otherwise it defaults to STATIC.

NOTES

dhcpcd requires a Berkley Packet Filter, or BPF device on BSD based systems and a Linux Socket Filter, or LPF device on Linux based systems.

FILES
/etc/dhcpcd.conf

Configuration file for dhcpcd. If you always use the same options, put them here.

/etc/dhcpcd.duid

Text file that holds the DUID used to identify the host.

/lib/dhcpcd/dhcpcd-run-hooks

Bourne shell script that is run to configure or de-configure an interface.

/lib/dhcpcd/dhcpcd-hooks

A directory containing bourne shell scripts that are run by the above script. Each script can be disabled by using the −C, −−nohook option described above.

/var/lib/dhcpcd/dhcpcd−interface.lease

The actual DHCP message send by the server. We use this when reading the last lease and use the files mtime as when it was issued.

/var/run/dhcpcd.pid

Stores the PID of dhcpcd running on all interfaces.

/var/run/dhcpcd−interface.pid

Stores the PID of dhcpcd running on the interface.

SEE ALSO

fnmatch(3), if_nametoindex(3), dhcpcd.conf(5), resolv.conf(5), dhcpcd-run-hooks(8), resolvconf(8)

STANDARDS

RFC 951, RFC 1534, RFC 2131, RFC 2132, RFC 2855, RFC 3004, RFC 3361, RFC 3396, RFC 3397, RFC 3442, RFC 3927, RFC 4361, RFC 4390, RFC 4702, RFC 5969, RFC 6106.

AUTHORS

Roy Marples ⟨roy@marples.name⟩

BUGS

Please report them to

http://roy.marples.name/projects/dhcpcd

BSD March 19, 2012 BSD


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