|DHCPCD(8)||System Manager's Manual||DHCPCD(8)|
dhcpcd — a DHCP
dhcpcd is an implementation of the DHCP
client specified in
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
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 when the lease begins to expire or the
DHCP server sends a message to renew early.
If any interface reports a working carrier then
dhcpcd will try to obtain a lease before forking to
the background, otherwise it will fork right away. This behaviour can be
modified with the
dhcpcd is also an implementation of the
BOOTP client specified in
dhcpcd is also an implementation of the
IPv6 Router Solicitor as specified in
RFC 4861 and
dhcpcd is also an implementation of the
IPv6 Privacy Extensions to AutoConf as specified in
4941. This feature needs to be enabled in the kernel and
dhcpcd will start using it.
dhcpcd is also an implementation of the
DHCPv6 client as specified in
RFC 3315. By default,
dhcpcd only starts DHCPv6 when instructed to do so
by an IPV6 Router Advertisement. If no Identity Association is configured,
then a Non-temporary Address is requested.
Local Link configuration¶
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
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
If a list of interfaces are given on the command line, then
dhcpcd only works with those interfaces, otherwise
dhcpcd discovers available Ethernet interfaces that
can be configured. When
dhcpcd not limited to one
interface on the command line, it is running in Manager mode. The
dhcpcd-ui project expects dhcpcd to be running this
If a single interface is given then
only works for that interface and runs as a separate instance to other
--waitip option is enabled in this instance to
maintain compatibility with older versions. Using a single interface also
-x options, where the
same interface will need to be specified, as a lack of an interface will
imply Manager mode which this is not. To force starting in Manager mode with
only one interface, the
--manager option can be used.
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.
Non-ethernet interfaces and some virtual ethernet interfaces such
as TAP and bridge are ignored by default, as is the FireWire interface. To
work with these devices they either need to be specified on the command
line, be listed in
--allowinterfaces or have an
interface directive in /etc/dhcpcd.conf.
Hooking into events¶
/usr/lib/dhcpcd/dhcpcd-run-hooks, or the script
specified by the
--script option. This script runs each script found
in /usr/lib/dhcpcd/dhcpcd-hooks in a lexical order.
The default installation supplies the scripts
30-hostname. You can disable each script by using
--nohook option. See
dhcpcd-run-hooks(8) for details on how these scripts work.
dhcpcd currently ignores the exit code of the
More scripts are supplied in
/usr/share/dhcpcd/hooks and need to be copied to
/usr/lib/dhcpcd/dhcpcd-hooks if you intend to use
them. For example, you could install
29-lookup-hostname so that
dhcpcd can lookup the hostname of the IP address in
DNS if no hostname is given by the lease and one is not already set.
You can fine-tune the behaviour of
with the following options:
- Background immediately. This is useful for startup scripts which don't disable link messages for carrier status.
- Use this script instead of the default /usr/lib/dhcpcd/dhcpcd-run-hooks.
--duid[ll | lt | uuid | value]
- Use a DHCP Unique Identifier. If a system UUID is available, that will be
used to create a DUID-UUID, otherwise if persistent storage is available
then a DUID-LLT (link local address + time) is generated, otherwise
DUID-LL is generated (link local address). The DUID type can be hinted as
an optional parameter if the file
/var/lib/dhcpcd/duid does not exist. If not
ll, lt or
uuid then value will be
converted from 00:11:22:33 format. This, plus the IAID will be used as the
--clientid. The DUID generated will be held in /var/lib/dhcpcd/duid and should not be copied to other hosts. This file also takes precedence over the above rules except for setting a value.
- Echo debug messages to the stderr and syslog.
dhcpcdcannot obtain a lease, then try to use the last lease acquired for the interface.
- Same as the above, but the lease will be retained even if it expires.
dhcpcdwill give it up if any other host tries to claim it for their own via ARP. This violates RFC 2131, section 3.7, which states the lease should be dropped once it has expired.
- 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
dhcpcdwill 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
dhcpcdto put things back afterwards.
dhcpcddoes not read a new configuration when this happens - you should rebind if you need that functionality.
- Requests that the DHCP server updates DNS using FQDN instead of just a
hostname. Valid values for fqdn are disable, none,
ptr and both.
dhcpcditself never does any DNS updates.
dhcpcdencodes the FQDN hostname as specified in
- Specify a config to load instead of
dhcpcdalways processes the config file before any command line options.
- 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 (i.e., contains a .) then it will be encoded as such.
- 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
dhcpcdsends a default clientid of the hardware family and the hardware address.
- Override the DHCPv4 vendorclassid field sent. The
dhcpcd-5.5.6:NetBSD-6.99.5:i386:i386If 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.
- Writes to the specified logfile.
dhcpcdstill writes to syslog(3). The logfile is reopened when
- This causes an existing
dhcpcdprocess running on the interface to release its lease and de-configure the interface regardless of the
--persistentoption. If no interface is specified then this applies to all interfaces in Manager mode. If no interfaces are left running,
- Request a lease time of seconds.
-1 represents an infinite lease time. By default
dhcpcddoes not request any lease time and leaves it in the hands of the DHCP server.
dhcpcdin Manager mode even if only one interface specified on the command line. See the Multiple Interfaces section above.
- Metrics are used to prefer an interface over another one, lowest wins.
dhcpcdwill supply a default metric of 1000 + if_nametoindex(3). This will be offset by 2000 for wireless interfaces, with additional offsets of 1000000 for IPv4LL and 2000000 for roaming interfaces.
dhcpcdto reload its configuration and rebind the specified interface. If no interface is specified then this applies to all interfaces in Manager mode. If
dhcpcdis not running, then it starts up as normal.
dhcpcdto renew existing addresses on the specified interface. If no interface is specified then this applies to all interfaces in Manager mode. If
dhcpcdis not running, then it starts up as normal. Unlike the
--rebindoption above, the configuration for
dhcpcdis not reloaded.
- Request the DHCP option variable for use in /usr/lib/dhcpcd/dhcpcd-run-hooks.
dhcpcdnormally de-configures the interface and configuration when it exits. Sometimes, this isn't desirable if, for example, you have root mounted over NFS or SSH clients connect to this host and they need to be notified of the host shutting down. You can use this option to stop this from happening.
- 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.
- Behaves like
--requestas 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.
dhcpcdremains running and pretends it has an infinite lease.
dhcpcdwill not de-configure the interface when it exits. If
dhcpcdfails to contact a DHCP server then it returns a failure instead of falling back on IPv4LL.
- Performs a DHCPv6 Information Request. No address is requested or
specified, but all other DHCPv6 options are allowed. This is normally
performed automatically when the IPv6 Router Advertises that the client
should perform this operation. This option is only needed when
dhcpcdis not processing IPv6RA messages and the need for DHCPv6 Information Request exists.
- Configures a static DHCP value. If you set
dhcpcdwill 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
You cannot presently set static DHCPv6 values. Use the
- Timeout after seconds, instead of the default 30. A
setting of 0 seconds causes
dhcpcdto wait forever to get a lease. If
dhcpcdis working on a single interface then
dhcpcdwill exit when a timeout occurs, otherwise
dhcpcdwill fork into the background.
- Tags the DHCPv4 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.
- 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 eth0Set the vendor option 02 with a hex code.dhcpcd -v 02,01:02:03:04:05 eth0Set the vendor option 03 with an IP address as a string.dhcpcd -v 03,\"192.168.0.2\" eth0Set un-encapsulated vendor option to hello world.dhcpcd -v ,"hello world" eth0
- Display both program version and copyright information.
dhcpcdthen exits before doing any configuration.
- Wait for an address to be assigned before forking to the background. Does not take an argument, unlike the below option.
--waitip=[4 | 6]
- Wait for an address to be assigned before forking to the background. 4
means wait for an IPv4 address to be assigned. 6 means wait for an IPv6
address to be assigned. If no argument is given,
dhcpcdwill wait for any address protocol to be assigned. It is possible to wait for more than one address protocol and
dhcpcdwill only fork to the background when all waiting conditions are satisfied.
- This will signal an existing
dhcpcdprocess running on the interface to exit. If no interface is specified, then the above is applied to all interfaces in Manager mode. See the
--persistentoption to control configuration persistence on exit, which is enabled by default in dhcpcd.conf(5).
dhcpcdthen waits until this process has exited.
- Allow reboot seconds before moving to the discover
phase if we have an old lease to use. Allow reboot
seconds before starting fallback states from the discover phase. IPv4LL is
started when the first reboot timeout is reached.
The default is 5 seconds. A setting of 0 seconds causes
dhcpcdto skip the reboot phase and go straight into discover. This has no effect on DHCPv6 other than skipping the reboot phase.
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 DHCP server wants. Here are some
options that deal with turning these bits off.
Note that when
dhcpcd is restricted to a
single interface then the interface also needs to be specified when asking
dhcpcd to exit using the commandline. If the
protocol is restricted as well then the protocol needs to be included with
the exit instruction.
- Exit after configuring an interface. Use the
--waitipoption to specify which protocol(s) to configure before exiting.
- Configure IPv4 only.
- Configure IPv6 only.
- Don't request or claim the address by ARP. This also disables IPv4LL.
- 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.
- Don't run this hook script. Matches full name, or prefixed with 2 numbers
optionally ending with .sh.
So to stop
dhcpcdfrom touching your DNS settings you would do:-dhcpcd -C resolv.conf eth0
- Don't set any default routes.
- Use the last four bytes of the hardware address as the DHCP xid instead of a randomly generated number.
- 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,
dhcpcdwill set this automatically.
- Don't receive link messages for carrier status. You should only have to
use this with buggy device drivers or running
dhcpcdthrough a network manager.
- Don't use IPv4LL (aka APIPA, aka Bonjour, aka ZeroConf).
- Removes the option from the DHCP message before processing.
- Print the pidfile
dhcpcdwill use based on command-line arguments to stdout.
- Requires the option to be present in all DHCP
messages, otherwise the message is ignored. To enforce that
dhcpcdonly responds to DHCP servers and not BOOTP servers, you can
dhcpcdon the command line, only warnings and errors will be displayed. If this option is used another time then all console output is disabled. These messages are still logged via syslog(3).
- On receipt of DHCP messages just call
/usr/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. The rapid_commit option is not
sent in TEST mode so that the server does not lease an address. To test
INFORM the interface needs to be configured with the desired address
- Dumps the current lease for the interface to stdout.
If no interface is given then all interfaces are
dumped. Use the
-6flags to specify an address family. If a lease is piped in via standard input then that is dumped. In this case, specifying an address family is mandatory.
- Display a list of option codes, the associated variable and encoding 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.
- Only accept packets from address[/cidr].
--blacklistis ignored if
- Ignore all packets from address[/cidr].
- When discovering interfaces, the interface name must not match pattern which is a space or comma separated list of patterns passed to fnmatch(3).
- 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
--denyinterfacesthen it is still denied.
- Don't start any interfaces other than those specified on the command line.
dhcpcdto be started in Manager mode and then wait for subsequent
dhcpcdcommands to start each interface as required.
dhcpcdto configure the system. This is the default behaviour and sets
dhcpcdwill not configure the system at all. This is only of use if the
dhcpcdcalls at each network event configures the system instead. This is different from
--testmode in that it's not one shot and the only change to the environment is the addition of
- Don't load any /dev management modules.
3RDPARTY LINK MANAGEMENT¶
Some interfaces require configuration by 3rd parties, such as PPP
or VPN. When an interface configuration in
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.
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 for all IPv4 configuration.
dhcpcd to a single
interface and optionally address family via the command-line then all
further calls to
dhcpcd to rebind, reconfigure or
exit need to include the same restrictive flags so that
dhcpcd knows which process to signal.
Some DHCP servers implement ClientID filtering. If
dhcpcd is replacing an in-use DHCP client then you
might need to adjust the clientid option
sends to match. If using a DUID in place of the ClientID, edit
- Configuration file for dhcpcd. If you always use the same options, put them here.
- Bourne shell script that is run to configure or de-configure an interface.
- Linux /dev management modules.
- A directory containing bourne shell scripts that are run by the above
script. Each script can be disabled by using the
--nohookoption described above.
- Text file that holds the DUID used to identify the host.
- Text file that holds a secret key known only to the host.
- The actual DHCP message sent by the server. We use this when reading the last lease and use the file's mtime as when it was issued.
- The actual DHCPv6 message sent by the server. We use this when reading the last lease and use the file's mtime as when it was issued.
- Stores the monotonic counter used in the replay field in Authentication Options.
- Stores the PID of
dhcpcdrunning on all interfaces.
- Stores the PID of
dhcpcdrunning on the interface.
- Control socket to the manager daemon.
- Unprivileged socket to the manager daemon, only allows state retrieval.
- Control socket to per interface daemon.
- Unprivileged socket to per interface daemon, only allows state retrieval.
RFC 951, RFC 1534, RFC 2104, RFC 2131, RFC 2132, RFC 2563, RFC 2855, RFC 3004, RFC 3118, RFC 3203, RFC 3315, RFC 3361, RFC 3633, RFC 3396, RFC 3397, RFC 3442, RFC 3495, RFC 3925, RFC 3927, RFC 4039, RFC 4075, RFC 4242, RFC 4361, RFC 4390, RFC 4702, RFC 4074, RFC 4861, RFC 4833, RFC 4941, RFC 5227, RFC 5942, RFC 5969, RFC 6106, RFC 6334, RFC 6355, RFC 6603, RFC 6704, RFC 7217, RFC 7550, RFC 7844.
Roy Marples <email@example.com>
Please report them to http://roy.marples.name/projects/dhcpcd
|August 23, 2021||Debian|