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 4861 and RFC 6106. dhcpcd can optionally handle address and route management itself, and will do so by default if Router Solicitation is disabled in the kernel. If dhcpcd is managing routes, dhcpcd sends Neighbor Solicitions to each advertising router periodically and will expire the ones that do not respond.
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
/libexec/dhcpcd-run-hooks, or the script specified by the
-c,
--script option. This script runs each script found in
/libexec/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 /libexec/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>:<os>:<machine>:<platform>. For example
dhcpcd-5.5.6:NetBSD-6.99.5:i386:i386
If not set then none is sent. Some badly configured DHCP servers reject unknown vendorclassids. To work around it, try and impersonate Windows by using the MSFT vendorclassid.
-
-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 /libexec/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 /libexec/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.