Difference between revisions of "Dhcpcd"
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== Tips and tricks ==
== Tips and tricks ==
Revision as of 11:00, 5 April 2014
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.
- 1 Installation
- 2 Running
- 3 Configuration
- 4 Tips and tricks
- 5 Troubleshooting
- 6 Hooks
The official repositories. It is part of the group, so it is likely already installed on your system.package is available in the
You might be interested in wpa_supplicant). It features a configuration dialogue and the ability to enter a pass phrase for wireless networks.AUR, which is a GTK+ frontend for the dhcpcd daemon (and optionally
dhcpcd is usually controlled via the provided service file
dhcpcd@.service, which takes the interface name as an argument (see systemd#Using units for details):
# systemctl start firstname.lastname@example.org # systemctl enable email@example.com
To start dhcpcd manually, simply run the following command:
# dhcpcd interface
dhcpcd: version 5.1.1 starting dhcpcd: interface: broadcasting for a lease ... dhcpcd: interface: leased 192.168.1.70 for 86400 seconds
The main configuration is done in
dhcpcd.conf(5) 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, create a new dhcpcd hook-script in
/usr/lib/dhcpcd/dhcpcd-hooks. 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
ip route add 10.11.12.0/24 via 192.168.192.5
40 prefix means that it is the final hook-script to run when dhcpcd starts.
DHCP Client Identifier
The DHCP client may be uniquely identified in different ways by the server:
- hostname (or the hostname value sent by the client),
- MAC address of the network interface controller through which the connection is being made, linked to this is the third,
- Identity Association ID (IAID), which is an abstraction layer to differentiate different use-cases and/or interfaces on a single host,
- 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
hostnamesends the hostname set in
clientidsends the MAC address as identifier
IAID <interface>derives the IAID for the
interface <interface>to use for DHCP discovery. Both have to be used together, but more frequently the next option is used:
duidtriggers using a combination of DUID and IAID as identifier.
The DUID value is set in
Care must be taken on a network running Dynamic DNS to ensure that all three 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.
Speed up DHCP by disabling ARP probing
dhcpcd contains an implementation of a recommendation of the DHCP standard (RFC2131 section 2.2) to check via ARP if the assigned IP address is really not taken. This seems mostly useless in home networks, so you can save about 5 seconds on every connect by adding the following line to
This is equivalent to passing
dhcpcd, and disables the described ARP probing, speeding up connections to networks with DHCP.
Fallback static 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 such as Raspberry Pi, 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
static_eth0 profile with
192.168.1.1 as IP address,
192.168.1.23 as gateway and name server, and makes this profile fallback for interface
# define static profile profile static_eth0 static ip_address=192.168.1.1 static routers=192.168.1.23 static domain_name_servers=192.168.1.23 # fallback to static profile on eth0 interface eth0 fallback static_eth0
More complicated configurations are possible, for example combining with the
arping option. See
dhcpcd.conf(5) for details.
Tips and tricks
Remove old DHCP lease
interface is the name of the interface on which you have a lease, contains the actual DHCP lease reply sent by the DHCP server. It is used to determine the last lease from the server, and its
mtime 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 you are on a network with DHCPv4 that filters Client IDs based on MAC addresses, you may need to change the following line:
# Use the same DUID + IAID as set in DHCPv6 for DHCPv4 Client ID as per RFC4361. duid
# Use the hardware address of the interface for the Client ID (DHCPv4). clientid
Check DHCP problem by releasing IP first
Problem may occur when DHCP get wrong IP assignment. For example when two routers are tied together through VPN. The router that is connected to me by VPN may assigning IP address. To fix it. On a console, as root, release IP address:
# dhcpcd -k
Then request a new one:
Maybe you had to run those two commands many times.
Problems with incompliant routers
For some (incompliant) routers, you will not be able to connect properly unless you comment the line
/etc/dhcpcd.conf. This should not cause issues unless you have multiple DHCP servers on your network (not typical); see this page for more information.
dhcpcd, systemd & wpa_supplicant
archlinux dhcpcd package is delivered with a client configuration script to run wpa_supplicant (/lib/dhcpcd/dhcpcd-hooks/10-wpa_supplicant).
When you manage wpa_supplicant daemon with systemd this can result in unexpected behavior e.g.
- you stop wpa_supplicant via systemd and dhcpcd starts wpa_supplicant via hook
- dhcpcd starts wpa_supplicant daemon during boot (this can result in systemd-udevd error: "error changing net interface name wlan0 to wlp4s0: Device or resource busy" and can prevent "Predictable Network Interface Names" )
To disable the
wpa_supplicant hook, add
nohook wpa_supplicant into
dhcpcd executes all scripts found in
/usr/lib/dhcpcd/dhcpcd-hooks/ in a lexical order. See
dhcpcd-run-hooks(8) for details.