dhcpcd - an RFC 2131 compliant DHCP client
dhcpcd [-dknpAEGHMLNRSTY] [-c, --script script] [-h, --hostname hostname]
[-i, --classid classid] [-l, --leasetime seconds]
[-m, --metric metric] [-r, --request address]
[-t, --timeout seconds] [-u, --userclass class] [-F, --fqdn FQDN]
[-I, --clientid clientid] interface
dhcpcd -k, --release interface
dhcpcd -x, --exit interface
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 will then write DNS information to resolvconf(8), if
available, otherwise directly to /etc/resolv.conf. dhcpcd will also
configure /etc/yp.conf and /etc/ntpd.conf with NIS and NTP information if
the DHCP server provided them. If those file contents changed, then
dhcpcd will also attempt to restart the respective services to notify
them of the change. If the hostname is currenly blank, (null) or
localhost then dhcpcd will set the hostname to the one supplied by the
DHCP server, or look it up in DNS if none supplied. dhcpcd then
daemonises and waits for the lease renewal time to lapse. Then it
attempts to renew its lease and reconfigure if the new lease changes.
Local Link configuration
If dhcpcd failed to obtain a lease, it will probe for a valid IPv4LL
address (aka Zeroconf, aka APIPA). Once obtained it will probe every 10
seconds for a DHCP server to get a proper address.
Even when dhcpcd obtains a proper lease, it will still add a Local Link
route (18.104.22.168/16) so that the host can communicate with clients
using these addresses.
When using IPv4LL, dhcpcd will always succeed and return a 0 exit code.
To disable this behaviour, you can use the -L, --noipv4ll option.
Hooking into DHCP events
dhcpcd will run /etc/dhcpcd.sh, or the script specified by the -c,
--script option. It will set $1 to a shell compatible file that holds
various configuration settings obtained from the DHCP server and $2 to
either up, down or new depending on the state of dhcpcd. dhcpcd ignores
the exist code of the script.
You can fine tune the behaviour of dhcpcd with the following options :-
Echo debug and informational messages to the console. Subsequent
debug options stop dhcpcd from daemonising.
-h, --hostname hostname
By default, dhcpcd will send the current hostname to the DHCP
server so it can register in DNS. You can use this option to
specify the hostname sent, or an empty string to stop any
hostname from being sent.
-i, --classid classid
Override the DHCP vendor classid field we send. The default is
This causes an existing dhcpcd process running on the interface
to release it's lease, deconfigure the interface and then exit.
-l, --leasetime seconds
Request a specific lease time in seconds. By default dhcpcd does
not request any lease time and leaves the it in the hands of the
-m, --metric metric
Added routes will use the metric on systems where this is
supported (presently only Linux). Route metrics allow the
addition of routes to the same destination across different
interfaces, the lower the metric the more it is preferred.
Notifies an existing dhcpcd process running on the interface to
renew it's lease. If dhcpcd is not running, then it starts up as
dhcpcd normally deconfigures the interface and configuration when
it exits. Sometimes, this isn't desirable if for example you
have root mounted over NFS. You can use this option to stop this
-r, --request [address]
dhcpcd normally sends a DHCP Broadcast to find servers to offer
an address. dhcpcd will then request the address used. You can
use this option to skip the broadcast step and just request an
address. The downside is if you request an address the DHCP
server does not know about or the DHCP server is not authorative,
it will remain silent. In this situation, we go back to the init
state and broadcast again. If no address is given then we use
the first address currently assigned to the interface.
-s, --inform [address [/ cidr]]
Behaves exactly like -r, --request as above, but sends a DHCP
inform instead of a request. This requires the interface to be
configured first. This does not get a lease as such, just
notifies the DHCP server of the address we are using.
-t, --timeout seconds
Timeout after seconds, instead of the default 20. A setting of 0
seconds causes dhcpcd to wait forever to get a lease.
-u, --userclass class
Tags the DHCP message with the userclass class. DHCP servers use
this give memebers of the class DHCP options other than the
default, without having to know things like hardware address or
hostname. infinityRequests that the DHCP server updates DNS
using FQDN instead of just a hostname. Valid values for fqdn are
none, ptr and both. dhcpcd dhcpcd itself never does any DNS
Forces dhcpcd to set the hostname as supplied by the DHCP server.
Because some OS's and users prefer to have just the hostname, or
the full FQDN more -H, ---sethostname options change the
behaviour. Below is the list of possible combinations:-
-H set the hostname to the full FQDN.
-HH strip the domain if it matches the dns domain.
-HHH strip the domain regardless.
-HHHH same as -H but force hostname lookup via DNS.
-HHHHH same as above, but strip the domain if it matches the dns
same as above, but strip the domain regardless.
-I, --clientid clientid
Send clientid as a client identifier string. If clientid matches
a hardware address format, such as 01:00:01:02:03:04:05 then we
encode it as that, otherwise as a string. You need to specify the
hardware type in the first byte. Ethernet is 01, and the hardware
address in the example is 00:01:02:03:04:05. If the clientid is a
blank string, then we disable DUID support and use a clientid as
Microsoft have their own code for Classless Static Routes (RFC
3442). You can use this option to request this as well as the
normal CSR. Another instace of this option only requests the
Microsoft CSR to prevent DHCP message over-running its maximum
size. DHCP server administrators should update their CSR code
from the Microsoft specific one to the RFC compliant one as the
content is fully compatible.
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 option that deal
with turning these bits off.
Don't request or claim the address by ARP.
Don't set any default routes.
Don't use IPv4LL at all.
Don't set the MTU of the interface.
Don't touch /etc/ntpd.conf or restart the ntp service.
Don't send DNS information to resolvconf or touch
On receipt of discover messages, simply print the contents of the
DHCP message to the console. dhcpcd will not configure the
interface, touch any files or restart any services.
Don't touch /etc/yp.conf or restart the ypbind service.
Because dhcpcd supports InfiniBand, we put a Node-specific Client
Identifier in the ClientID field. This is required by RFC 4390. It's also
required for DHCP IPv6 which dhcpcd should support one day. However, some
DHCP servers have no idea what this is and reject the message as they do
not understand type 255. This is not conformant with RFC 2132 and the
server should be fixed. Also, some DHCP server configurations require an
ethernet hardware address of 6 hexacdecimal numbers in the ClientID which
is the default behaviour of most other DHCP clients. If your DHCP server
is as desribed above, you should fix the server, or if that is not an
option you can compile DUID support out of dhcpcd or use the -I,
--clientid clientid option and set clientid to ''.
ISC dhcpd, dnsmasq, udhcpd and Microsoft DHCP server 2003 default
configurations work just fine with the default dhcpcd configuration.
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.
Bourne shell script that is run when we configure or deconfigure an
Text file that holds the DUID used to identify the host.
Bourne shell file that holds the DHCP values used in configuring the
interface. This path is passed as the first argument to /etc/dhcpcd.sh.
ntp(1), resolv.conf(5), resolvconf(8), yp.conf(5), ypbind(8)
RFC 2131, RFC 2132, RFC 2855, RFC 3004, RFC 3361, RFC 3397, RFC 3442, RFC
3927, RFC 4361, RFC 4390, RFC 4702.
Roy Marples <email@example.com>
Please report them to http://bugs.marples.name