DHCPCD(8)

📦

version 10.5.2 · 17 versions

, in opensuse-leap 16.0

🦋 DHCPCD(8) — a DHCP client 🦋

from the System Manager's Manual, bestie

✨ NAME ✨

dhcpcd — a DHCP client. That's the name, and it's iconic.

✨ SYNOPSIS ✨

okay so here's the whole shape of it, no cap:

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 🌸

okay so listen: dhcpcd is an implementation of the DHCP client specified in RFC 2131. It 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. Then it runs the configuration script which writes DNS information to resolvconf(8), if available, otherwise directly to /etc/resolv.conf. slay.

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. Then dhcpcd 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. main character energy.

Bestie, 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 -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. iykyk.

🦋 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 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 limited 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 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 -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/libexec/dhcpcd-run-hooks, or the script specified by the -c, --script option. This script runs each script found in /usr/libexec/dhcpcd-hooks in a lexical order. The default installation 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 currently ignores the exit code of the script.

More scripts are supplied in /usr/share/dhcpcd/hooks and need to be copied to /usr/libexec/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/libexec/dhcpcd-run-hooks.
-D, --duid [ll | lt | uuid | value] — Use a DHCP Unique Identifier. 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 -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 expires. 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 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 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 encodes 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 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 (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 interfaces 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 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 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.
-n, --rebind [interface] — tells dhcpcd to reload its config and rebind the interface you named. No interface given? Then it's all interfaces, but only in Manager mode. And if dhcpcd isn't even running, it just starts up like normal. no drama 💅
-N, --renew [interface] — same vibe, different errand: this one tells dhcpcd to renew the addresses it already has on that interface. No interface = all interfaces in Manager mode. Not running at all? Starts up as usual. Big difference from -n, --rebind up there: the config for dhcpcd is not reloaded. keep that in mind bestie 🎀
-o, --option option — asks for the DHCP option variable so it can be used in /usr/libexec/dhcpcd-run-hooks.
-p, --persistent — by default dhcpcd de-configures the interface when it exits, unless this is enabled. And honestly sometimes that default is just not it — like if you've got root mounted over NFS, or SSH clients are connected to this host and they genuinely need to be notified that the host is shutting down. Use this option and that de-configuring stops happening. 🫶
-r, --request address — asks for that address in the DHCP DISCOVER message. Warning though: there is no guarantee this is the address the DHCP server will actually hand over 😭. Leave the address out and the first address currently assigned to the interface gets used.
-s, --inform address[/cidr[/broadcast_address]] — behaves like -r, --request above, except it sends a DHCP INFORM instead of DISCOVER/REQUEST. Heads up: this doesn't get a lease as such, it just notifies the DHCP server of the address in use. You should also include the optional cidr network number in case the address isn't already configured on the interface. dhcpcd stays running and pretends it has an infinite lease, and it will not de-configure the interface when it exits. And if dhcpcd fails to contact a DHCP server? It returns a failure instead of falling back on IPv4LL. no fallback era 💀
--inform6 — does a DHCPv6 Information Request. No address is requested or specified, but all the other DHCPv6 options are allowed. Normally this happens automatically when the IPv6 Router Advertises that the client should do this. You only need this option when dhcpcd isn't processing IPv6RA messages and the need for a DHCPv6 Information Request genuinely exists.
-S, --static value — configures a static DHCP value. Set ip_address and dhcpcd won't even try to obtain a lease — it just uses that value for the address with an infinite lease time. slay ✨

Here's an example configuring 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 exits if the -1, --oneshot option was given — otherwise it forks into the background and keeps on trying. Setting it to 0 seconds makes dhcpcd wait forever to get a lease. forever-forever. 🕰️
-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. king behavior 👑
-v, --vendor code,value — adds 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 below 👇

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 — displays both program version and copyright information. Then dhcpcd exits before doing any configuration at all. just a lil peek, no commitment 🙈
-w — waits for an address to be assigned before forking to the background. Does not take an argument, unlike the option right below it.
--waitip=[4 | 6] — waits 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. No argument given? dhcpcd waits for any address protocol to be assigned. And yes, it's possible to wait for more than one address protocol — dhcpcd will only fork to the background when all the waiting conditions are satisfied. patient queen 🌸
-x, --exit [interface] — signals an existing dhcpcd process running on the interface to exit. No interface specified? Then the above applies to all interfaces in Manager mode. Go peep 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 — allows reboot seconds before moving to the discover phase if we have an old lease to use, and allows reboot seconds before starting fallback states from the discover phase. IPv4LL is started when the first reboot timeout is reached. 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. Which is iconic of it honestly 🫶 but sometimes there are situations where you do not want things configured exactly how the DHCP server wants. So 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. And if the protocol is restricted as well, then the protocol needs to be included with the exit instruction. iykyk 💅

-1, --oneshot — exit after configuring an interface. Use the -w, --waitip option to specify which protocol(s) to configure before exiting. one and done 🎀
-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. 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. it's got you 💜
-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). so many nicknames for one thing 😭
-O, --nooption option — removes the option from the DHCP message before processing.
-P, --printpidfile — prints the pidfile dhcpcd will use, based on command-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 get displayed. Use this option another time and all console output is disabled. These messages are still logged via syslog(3) though, so nothing's really lost.
-T, --test — on receipt of DHCP messages, just call /usr/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. 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. very look-don't-touch 👀
-U, --dumplease [interface] — dumps the current lease for the interface to stdout. No interface 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 — and in this case, specifying an address family is mandatory.
-V, --variables — displays 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. whitelist supremacy ✨
-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). But if the same interface is matched in -Z, --denyinterfaces, then it is still denied. denied is denied bestie 💀

Last four option besties, coming right up — do not skip these, they’re lowkey important:

--inactive — don’t start any interfaces other than the ones specified on the command line. This lets dhcpcd start in Manager mode and then wait for later dhcpcd commands to start each interface as required. 💅
--configure — lets dhcpcd configure the system; this is the default behaviour and sets if_configured=true. ✨
--noconfigure — dhcpcd will not configure the system at all. This is only useful if the --script that dhcpcd calls at each network event configures the system instead. It’s different from -T, --test mode because it’s not one shot, and the only change to the environment is the addition of if_configured=false. 🫶
--nodev — don’t load any /dev management modules. 🎀

✨ 3RDPARTY LINK MANAGEMENT ✨

Some interfaces need a 3rd party to handle their config — think PPP or VPN. When an interface configuration in dhcpcd is marked as STATIC or INFORM without an address, 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 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. No cap, that’s just the vibe. ⭐

If you’re restricting dhcpcd to a single interface and optionally an 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 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/libexec/dhcpcd-run-hooks — Bourne shell script that is run to configure or de-configure an interface.
/usr/lib64/dhcpcd/dev — Linux /dev management modules.
/usr/libexec/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/interface.sock — control socket to per interface daemon. 🧚