DHCPCD(8) System Manager's Manual DHCPCD(8)
NAME
dhcpcd — a DHCP client
SYNOPSIS
dhcpcd [-146ABbDdEGgHJKLMNPpqTV] [-C, --nohook hook]
[-c, --script script] [-e, --env value] [-F, --fqdn FQDN]
[-f, --config file] [-h, --hostname hostname]
[-I, --clientid clientid] [-i, --vendorclassid vendorclassid]
[-j, --logfile logfile] [-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[/broadcast_address]]] [--inform6]
[-t, --timeout seconds] [-u, --userclass class]
[-v, --vendor code, value] [-W, --whitelist address[/cidr]] [-w]
[--waitip=[4 | 6]] [-y, --reboot seconds]
[-X, --blacklist address[/cidr]] [-Z, --denyinterfaces pattern]
[-z, --allowinterfaces pattern] [--inactive] [--configure]
[--noconfigure] [interface] [...]
dhcpcd -n, --rebind [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 local-
host, or force_hostname is YES or TRUE or 1 then dhcpcd sets the host-
name 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 re-
new 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 ob-
tain 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.
dhcpcd is also an implementation of the BOOTP client specified in RFC
951.
dhcpcd is also an implementation of the IPv6 Router Solicitor as speci-
fied in RFC 4861 and RFC 6106.
dhcpcd is also an implementation of the IPv6 Privacy Extensions to Auto-
Conf as specified in RFC 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 con-
figured, then a Non-temporary Address is requested.
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 Ether-
net interfaces that can be configured. When dhcpcd is 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 way.
If a single interface is given then dhcpcd only works for that interface
and runs as a separate instance to other dhcpcd processes. The -w,
--waitip option is enabled in this instance to maintain compatibility
with older versions. Using a single interface, optionally further lim-
ited to an address protocol, also affects the -k, -N, -n and -x options,
where the same interface and any address protocol will need to be speci-
fied, 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 -M,
--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
dhcpcd runs /usr/lib/dhcpcd/dhcpcd-run-hooks, or the script specified by
the -c, --script option. This script runs each script found in
/usr/lib/dhcpcd/dhcpcd-hooks in a lexical order. The default installa-
tion supplies the scripts 01-test, 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 cur-
rently ignores the exit code of the script.
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.
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
/usr/lib/dhcpcd/dhcpcd-run-hooks.
-D, --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 ad-
dress). 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 -I, --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.
-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.
--lastleaseextend
Same as the above, but the lease will be retained even if it ex-
pires. dhcpcd will 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.
-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 after-
wards. dhcpcd does not read a new configuration when this hap-
pens - you should rebind if you need that functionality.
-F, --fqdn fqdn
Requests that the DHCP server update 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 en-
codes the FQDN hostname as specified in RFC 1035.
-f, --config file
Specify a config to load instead of /etc/dhcpcd.conf. dhcpcd
always processes the config file before any command line op-
tions.
-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 (i.e., 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 DHCPv4 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.
-j, --logfile logfile
Writes to the specified logfile. dhcpcd still writes to
syslog(3). The logfile is reopened when dhcpcd receives the
SIGUSR2 signal.
-k, --release [interface]
This causes an existing dhcpcd process running on the interface
to release its lease and de-configure the interface regardless
of the -p, --persistent option. If no interface is specified
then this applies to all interfaces in Manager mode. If no in-
terfaces are left running, dhcpcd will exit.
-l, --leasetime seconds
Request a lease time of seconds. -1 represents an infinite
lease time. By default dhcpcd does not request any lease time
and leaves it in the hands of the DHCP server.
-M, --manager
Start dhcpcd in Manager mode even if only one interface speci-
fied on the command line. See the Multiple Interfaces section
above.
-m, --metric metric
Metrics are used to prefer an interface over another one, lowest
wins. dhcpcd will supply a default metric of 1000 +
if_nametoindex(3). This will be offset by 2000 for wireless in-
terfaces, with additional offsets of 1000000 for IPv4LL and
2000000 for roaming interfaces.
-n, --rebind [interface]
Notifies dhcpcd to reload its configuration and rebind the spec-
ified interface. If no interface is specified then this applies
to all interfaces in Manager mode. If dhcpcd is not running,
then it starts up as normal.
-N, --renew [interface]
Notifies dhcpcd to renew existing addresses on the specified
interface. If no interface is specified then this applies to
all interfaces in Manager mode. If dhcpcd is not running, then
it starts up as normal. Unlike the -n, --rebind option above,
the configuration for dhcpcd is not reloaded.
-o, --option option
Request the DHCP option variable for use in
/usr/lib/dhcpcd/dhcpcd-run-hooks.
-p, --persistent
dhcpcd de-configures the interface when it exits unless this op-
tion is enabled. Sometimes, this isn't desirable if, for exam-
ple, 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.
-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[/broadcast_address]]
Behaves like -r, --request as above, but sends a DHCP INFORM in-
stead 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 ad-
dress is not already configured on the interface. dhcpcd re-
mains 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.
--inform6
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 dhcpcd is not processing IPv6RA messages and
the need for DHCPv6 Information Request exists.
-S, --static value
Configures a static DHCP 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
You cannot presently set static DHCPv6 values. Use the -e,
--env option instead.
-t, --timeout seconds
Timeout after seconds, instead of the default 30. On timeout,
dhcpcd will exit if the -1, --oneshot option has been given,
otherwise it will fork into the background and keep on trying.
A setting of 0 seconds causes dhcpcd to wait forever to get a
lease.
-u, --userclass class
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.
-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 Wait for an address to be assigned before forking to the back-
ground. Does not take an argument, unlike the below option.
--waitip=[4 | 6]
Wait for an address to be assigned before forking to the back-
ground. 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, dhcpcd will wait for any address protocol to be as-
signed. It is possible to wait for more than one address proto-
col and dhcpcd will only fork to the background when all waiting
conditions are satisfied.
-x, --exit [interface]
This will signal an existing dhcpcd process running on the
interface to exit. If no interface is specified, then the above
is applied to all interfaces in Manager mode. See the -p,
--persistent option to control configuration persistence on
exit, which is enabled by default in dhcpcd.conf(5). 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. 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 dhcpcd to skip the reboot phase
and go straight into discover. This has no effect on DHCPv6
other than skipping the reboot phase.
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 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 in-
terface 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.
-1, --oneshot
Exit after configuring an interface. Use the -w, --waitip op-
tion to specify which protocol(s) to configure before exiting.
-4, --ipv4only
Configure IPv4 only.
-6, --ipv6only
Configure IPv6 only.
-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 settings you would do:-
dhcpcd -C resolv.conf 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
Removes the option from the DHCP message before processing.
-P, --printpidfile
Print the pidfile dhcpcd will use based on command-line argu-
ments to stdout.
-Q, --require option
Requires the option to be present in all DHCP messages, other-
wise the message is ignored. To enforce that dhcpcd only re-
sponds 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. If this option is used another time then all con-
sole output is disabled. These messages are still logged via
syslog(3).
-T, --test
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 ad-
dress before starting dhcpcd.
-U, --dumplease [interface]
Dumps the current lease for the interface to stdout. If no
interface is given then all interfaces are dumped. Use the -4
or -6 flags to specify an address family. If a lease is piped
in via standard input then use the special interface named - to
dump it. In this case, specifying an address family is manda-
tory.
-V, --variables
Display a list of option codes, the associated variable and en-
coding 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 di-
rectly requested.
-W, --whitelist address[/cidr]
Only accept packets from address[/cidr]. -X, --blacklist is ig-
nored 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.
--inactive
Don't start any interfaces other than those specified on the
command line. This allows dhcpcd to be started in Manager mode
and then wait for subsequent dhcpcd commands to start each in-
terface as required.
--configure
Allows dhcpcd to configure the system. This is the default be-
haviour and sets if_configured=true.
--noconfigure
dhcpcd will not configure the system at all. This is only of
use if the --script that dhcpcd calls at each network event con-
figures the system instead. This is different from -T, --test
mode in that it's not one shot and the only change to the envi-
ronment is the addition of if_configured=false.
--nodev
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 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 inter-
face is configured for INFORM, then dhcpcd unicasts INFORM to the desti-
nation, otherwise it defaults to STATIC.
NOTES
dhcpcd requires a Berkeley 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.
If restricting dhcpcd to a single interface and optionally address fam-
ily via the command-line then all further calls to dhcpcd to rebind, re-
configure 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
dhcpcd sends to match. If using a DUID in place of the ClientID, edit
/var/lib/dhcpcd/duid accordingly.
FILES
/etc/dhcpcd.conf
Configuration file for dhcpcd. If you always use the same options, put
them here.
/usr/lib/dhcpcd/dhcpcd-run-hooks
Bourne shell script that is run to configure or de-configure an inter-
face.
/usr/lib/x86_64-linux-gnu/dhcpcd/dev
Linux /dev management modules.
/usr/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/duid
Text file that holds the DUID used to identify the host.
/var/lib/dhcpcd/secret
Text file that holds a secret key known only to the host.
/var/lib/dhcpcd/interface-ssid.lease
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.
/var/lib/dhcpcd/interface-ssid.lease6
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.
/var/lib/dhcpcd/rdm_monotonic
Stores the monotonic counter used in the replay field in Authentication
Options.
/run/dhcpcd/pid
Stores the PID of dhcpcd running on all interfaces.
/run/dhcpcd/interface.pid
Stores the PID of dhcpcd running on the interface.
/run/dhcpcd/sock
Control socket to the manager daemon.
/run/dhcpcd/unpriv.sock
Unprivileged socket to the manager daemon, only allows state retrieval.
/run/dhcpcd/interface.sock
Control socket to per interface daemon.
/run/dhcpcd/interface.unpriv.sock
Unprivileged socket to per interface daemon, only allows state re-
trieval.
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 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.
AUTHORS
Roy Marples <roy@marples.name>
BUGS
Please report them to https://roy.marples.name/projects/dhcpcd
Debian June 17, 2024 DHCPCD(8)
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