Ubuntu Manpage: dhcpcd — a DHCP client

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]]  [--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]
              [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
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 when the lease beings to expire or the DHCP
server sends message to renew early.

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.

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

dhcpcd is also an implementation of the IPv6 Router Solicitor as specified in RFC 4861 and RFC 6106.

dhcpcd is also an implementation of the IPv6 Privacy Extensions to AutoConf 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
configured, 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 Ethernet interfaces that can be configured. When dhcpcd is
operating on more than one interface, it is called Master mode. and this behaviour can be forced with the
-M, --master option so that an individual interface can start dhcpcd but only one instance is running.
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.
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 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, 02-dump, 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.

More scripts are supplied in /usr/share/dhcpcd/hooks and need to be copied to /lib/dhcpcd/dhcpcd-hooks if
you intend to use them. For example, you could install 10-wpa_supplicant so that dhcpcd can ensure that
wpa_supplicant(8) is always running on a hot-plugged wireless interface.

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 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 rather than syslog(3). The logfile is truncated when opened and
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. If no interfaces are left running, dhcpcd will exit.

-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, --master
Start dhcpcd in master mode even if only one interface specified 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 metic of 200 + if_nametoindex(3). An extra 100 will be added for wireless interfaces.

-n, --rebind [interface]
Notifies dhcpcd to reload its configuration and rebind the specified interface. If no interface
is specified then this applies to all interfaces. If dhcpcd is not running, then it starts up as
normal. This may also cause wpa_supplicant(8) to reload its configuration for each interface as
well if the relevant hook script has been installed.

-N, --renew [interface]
Notifies dhcpcd to renew existing addresses on the specified interface. If no interface is
specified then this applies to all interfaces. 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 /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 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]]
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.

--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. A setting of 0 seconds causes dhcpcd to wait
forever to get a lease. If dhcpcd is working on a single interface then dhcpcd will exit when a
timeout occurs, otherwise dhcpcd will fork into the background.

-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 background. Does not take an argument,
unlike the below option. -fl -waitip 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, dhcpcd will wait for any address protocol to be assigned. It is possible to wait for more
than one address protocol 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. 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 the DHCP server wants. Here are some options that
deal with turning these bits off.

-1, --oneshot
Exit after configuring an interface. Use the -w, --waitip option 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, -FL -printpidfile
Print the pidfile dhcpcd will use based on commmand-line arguments to stdout.

-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.

-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. 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 before starting dhcpcd.

-U, --dumplease interface
Dumps the last lease for the interface to stdout. If omitted, standard input is used to read a
DHCP wire formatted message. Use the -4 or -6 flags to specify an address family.

-V, --variables
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.

-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.

--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 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 for all IPv4 configuration.

       If  restricting  dhcpcd to a single interface and optionally address family via the command-line then all
       futher calls to dhcpcd to rebind, reconfigure or exit need to include the same restrictive flags so  that
       dhcpcd knows which process to signal.

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.

/etc/dhcpcd.secret
Text file that holds a secret key known only to the host.

/lib/dhcpcd/dhcpcd-run-hooks
Bourne shell script that is run to configure or de-configure an interface.

/lib/x86_64-linux-gnu/dhcpcd/dev
/dev management modules.

/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/dhcpcd5/dhcpcd-interface-ssid.lease
The actual DHCP message sent by the server. We use this when reading the last lease and use the files
mtime as when it was issued.

/var/lib/dhcpcd5/dhcpcd-interface-ssid.lease6
The actual DHCPv6 message sent by the server. We use this when reading the last lease and use the files
mtime as when it was issued.

/var/lib/dhcpcd5/dhcpcd-rdm.monotonic
Stores the monotonic counter used in the replay field in Authentication Options.

/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.

/var/run/dhcpcd.sock
Control socket to the master daemon.

/var/run/dhcpcd.unpriv.sock
Unpriviledged socket to the master daemon, only allows state retrieval.

/var/run/dhcpcd-interface.sock
Control socket to per interface daemon.

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 6603, RFC 6704, RFC 7217, RFC 7550.

AUTHORS

       Roy Marples <roy@marples.name>

BUGS

       Please report them to http://roy.marples.name/projects/dhcpcd

Debian                                          January 14, 2016                                       DHCPCD(8)