dhcpcd

dhcpcd is a DHCP and DHCPv6 client. It is currently the most feature-rich open source DHCP client; see the home page for the full list of features.

Note: Roy Marples' dhcpcd (DHCP client daemon) is not the same as Internet Systems Consortium's dhcpd (DHCP (server) daemon).

Related articles

Installation

Install the dhcpcd package.

dhcpcd-uiAUR is a GTK frontend for the dhcpcd daemon, and optionally wpa_supplicant. It features a configuration dialogue and the ability to enter a pass phrase for wireless networks.

dhcpcd-ui-patchedAUR is a patched version of the dhcpcd-uiAUR package. It uses AppIndicator instead of GtkStatusIcon, compiles with gtk3, and has a sharp tray icon when used with KDE.

Running

To start the daemon for all network interfaces, start/enable dhcpcd.service.

To start the daemon for a specific interface alone, start/enable the template unit , where interface can be found with Network configuration#Listing network interfaces.

Using the template unit is recommended; see #dhcpcd and systemd network interfaces for details. In either case, you will be assigned a dynamic IP address. To assign a static IP address, see #Static profile.

Configuration

The main configuration is done in . See for details. Some of the frequently used options are highlighted below.

DHCP static route(s)

If you need to add a static route client-side, add it to . The example shows a new hook-script which adds a static route to a VPN subnet on 10.11.12.0/24 via a gateway machine at 192.168.192.5:

You can add multiple routes to this file.

DHCP Client Identifier

The DHCP client may be uniquely identified in different ways by the server:

  1. hostname (or the hostname value sent by the client),
  2. MAC address of the network interface controller through which the connection is being made, linked to this is the third,
  3. Identity Association ID (IAID), which is an abstraction layer to differentiate different use-cases and/or interfaces on a single host,
  4. DHCP Unique Identifier (DUID).

For a further description, see RFC 3315.

It depends on the DHCP-server configuration which options are optional or required to request a DHCP IP lease.

If the dhcpcd default configuration fails to obtain an IP, the following options are available to use in :

  • sends the hostname set in
  • sends the MAC address as identifier
  • derives the IAID to use for DHCP discovery. It has to be used in an interface block (started by , see ), but more frequently the next option is used:
  • triggers using a combination of DUID and IAID as identifier.

The DUID value is set in /var/lib/dhcpcd/duid. For efficient DHCP lease operation it is important that it is unique for the system and applies to all network interfaces alike, while the IAID represents an identifier for each of the systems' interfaces (see RFC 4361).

Care must be taken on a network running Dynamic DNS to ensure that all three IDs are unique. If duplicate DUID values are presented to the DNS server, e.g. in the case where a virtual machine has been cloned and the hostname and MAC have been made unique but the DUID has not been changed, then the result will be that as each client with the duplicated DUID requests a lease the server will remove the predecessor from the DNS record.

Static profile

Required settings are explained in Network configuration. These typically include the network interface name, IP address, router address, and name server.

Configure a static profile for dhcpcd in , for example: 

/etc/dhcpcd.conf
interface eth0
static ip_address=192.168.0.10/24	
static routers=192.168.0.1
static domain_name_servers=192.168.0.1 8.8.8.8

More complicated configurations are possible, for example combining with the option. See for details.

Fallback profile

It is possible to configure a static profile within dhcpcd and fall back to it when DHCP lease fails. This is useful particularly for headless machines, where the static profile can be used as "recovery" profile to ensure that it is always possible to connect to the machine.

The following example configures a profile with as IP address, as gateway and name server, and makes this profile fallback for interface .

Hooks

dhcpcd executes all scripts found in in a lexical order. See and for details.

Note:
  • Each script can be disabled using the nohook option in dhcpcd.conf.
  • The env option can be used to set an environment variable for all hooks. For example, you can force the hostname hook to always set the hostname with env force_hostname=YES.

10-wpa_supplicant

Enable this hook by creating a symbolic link, which ensures the current version is used, even after package updates:

# ln -s /usr/share/dhcpcd/hooks/10-wpa_supplicant /usr/lib/dhcpcd/dhcpcd-hooks/

The 10-wpa_supplicant hook, if enabled, automatically launches wpa_supplicant on wireless interfaces. It is started only if:

  • no wpa_supplicant process is already listening on that interface.
  • a wpa_supplicant configuration file exists. dhcpcd checks
/etc/wpa_supplicant/wpa_supplicant-interface.conf
/etc/wpa_supplicant/wpa_supplicant.conf
/etc/wpa_supplicant-interface.conf
/etc/wpa_supplicant.conf

by default, in that order, but a custom path can be set by adding into .

If you manage wireless connections with wpa_supplicant itself, the hook may create unwanted connection events. For example, if you stop wpa_supplicant the hook may bring the interface up again. Also, if you use netctl-auto, wpa_supplicant is started automatically with for config, so starting it again from the hook is unnecessary and may result in boot-time parse errors of the file, which only contains dummy values in the default packaged version.

To disable the hook remove the symbolic link you added, or add to .

Tips and tricks

Speed up DHCP by disabling ARP probing

dhcpcd contains an implementation of a recommendation of the DHCP standard (RFC 2131) to verify via ARP if the assigned IP is not used by something else. This is usually not needed in home networks, so it is possible to save about 5 seconds on every connect by disabling it:

This is equivalent to passing to , and disables the described ARP probing, speeding up connections to networks with DHCP.

Remove old DHCP lease

The file /var/lib/dhcpcd/interface.lease, where is the name of the interface on which you have a lease, contains the actual DHCP lease reply sent by the DHCP server. For a wireless interface, the filename is , where ssid is the name of the wireless network. It is used to determine the last lease from the server, and its attribute is used to determine when it was issued. This last lease information is then used to request the same IP address previously held on a network, if it is available. If you do not want that, simply delete this file.

If the DHCP server still assigns the same IP address, this may happen because it is configured to keep the assignment stable and recognizes the requesting DHCP client id or DUID (see #DHCP Client Identifier). You can test it by stopping dhcpcd and removing or renaming /var/lib/dhcpcd/duid. dhcpcd will generate a new one on next run.

Keep in mind that the DUID is intended as persistent machine identifier across reboots and interfaces. If you are transferring the system to new computer, preserving this file should make it appear as old one.

Different IPs when multi-booting

If you are dualbooting Arch and macOS or Windows and want each to receive different IP addresses, you can exert control about the IPs leased by specifying a different DUID in each operating system installation.

In Windows the DUID should be stored in the 

\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters\Dhcpv6DUID

registry key.

On macOS it is directly accessible in .

If you are using a dnsmasq DHCP server, the different DUIDs can be used in appropriate rules in its configuration.

/etc/resolv.conf

If resolvconf is available DNS information will be sent to it, if not, then dhcpcd itself will write to .

overwriting can be stopped by disabling the hook . Do so by adding the following to the last section of : 
nohook resolv.conf

Alternatively, you can create a file called containing your DNS servers. dhcpcd will prepend this file to the beginning of .

Or you can configure dhcpcd to use the same DNS servers every time. To do this, add the following line at the end of your , where is a space separated list of DNS IP addresses.

static domain_name_servers=dns-server-ip-addresses

For example, to set it to Google's DNS servers:

static domain_name_servers=8.8.8.8 8.8.4.4
Tip: When using openresolv, DNS servers can instead be set in /etc/resolvconf.conf. This way they will not get overwritten by any resolvconf supporting software.

Troubleshooting

Client ID

If you are on a network with DHCPv4 that filters Client IDs based on MAC addresses, you may need to change the following line:

To:

Else, you may not obtain a lease since the DHCP server may not read your DHCPv6-style Client ID correctly. See RFC 4361 for more information.

Check DHCP problem by releasing IP first

A problem may occur when DHCP gets a wrong IP assignment, such as when two routers are tied together through a VPN. The router that is connected through the VPN may be assigning IP address. To fix it, as root, release the IP address:

# dhcpcd -k

Then request a new one:

# dhcpcd

You may have to run those two commands many times.

Problems with noncompliant routers

For some (noncompliant) routers, you will not be able to connect properly unless you comment the line

require dhcp_server_identifier

in . This should not cause issues unless you have multiple DHCP servers on your network (not typical); see this page for more information.

dhcpcd and systemd network interfaces

dhcpcd.service can be enabled without specifying an interface. This may, however, create a race condition at boot with systemd-udevd trying to apply a predictable network interface name: 

error changing net interface name wlan0 to wlp4s0: Device or resource busy"

To avoid this problem use or in to stop dhcpcd from binding to kernel names, for example:

denyinterfaces wlan* eth*

It is also possible to enable dhcpcd on a per interface basis as described in #Running. The downside of the template unit is, however, that it does not support hot-plugging of a wired connection and will fail if the network cable is not connected. To work-around the failure, see #Timeout delay.

Timeout delay

If dhcpcd operates on a single interface and fails to obtain a lease after 30 seconds (for example when the server is not ready or the cable not plugged), it will exit with an error.

To have dhcpcd wait indefinitely for one-time, edit the unit and set the timeout option to :

To have it wait indefinitely, let the unit restart after it exited:

Known issues

dhcpcd@.service causes slow startup

By default the waits to get an IP address before forking into the background via the flag for dhcpcd. If the unit is enabled, this may cause the boot to wait for an IP address before continuing. To fix this, create a drop-in file for the unit with the following:

/etc/systemd/system/dhcpcd@.service.d/no-wait.conf
[Service]
ExecStart=
ExecStart=/usr/bin/dhcpcd -b -q %I

See also .

gollark: It's not the programmers' fault if a language is just unsuited for some tasks.
gollark: Writing a web app or something in C is awful because you lose memory safety and type safety (→ security), as well as the convenient abstractions.
gollark: Also its poor memory safety generally is bad.
gollark: Yes.
gollark: Not really. The abstractions make it *not low level*.

See also
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