.Dd January 15, 2023
.Dt DHCLIENT 8
.Os
.Sh NAME
.Nm dhclient
.Nd dynamic host configuration protocol client
.Sh SYNOPSIS
.Nm
.Op Fl f Ar config
.Op Fl t
.Ar interface
.Sh DESCRIPTION
.Nm
dynamically configures the specified network
.Ar interface
.Xr ( if 4 )
and the Domain Name System (DNS) server list
via the Dynamic Host Configuration Protocol (DHCP) with settings obtained from
the local network's DHCP server.
.Pp
.Nm
is configured in
.Xr dhclient.conf 5 .
Each network setting can be configured to be automatically obtained from the
network (default), set manually, or not set at all.
If the interface is fully manually configured,
.Nm
sets the configuration and exits immediately.
.Pp
.Nm
broadcasts a request and selects a DHCP server to obtain the network
configuration from.
.Nm
leases an internet address as needed and remains running to occasionally renew
the release.
.Pp
The options are as follows:
.Bl -tag -width "12345678"
.It Fl f Ar config
Load the configuration from the
.Ar config
file instead as a shared configuration file instead of searching for it.
.It Fl t
Test the configuration is valid without configuring the network interface.
The
.Ar interface
can optionally be specified to search for its configuration file.
Exit 0 if the configuration is valid and non-zero otherwise.
.El
.Pp
Port 68 on the network interface is bound to send and receive DHCP messages.
.Nm
exits 0 immediately if the port is already bound to avoid interfering with other
.Nm
processes.
.Pp
.Nm
supports the Ethernet link layer
.Xr ether 4
with the Internet Protocol network layer
.Xr ip 4 .
The loopback interface
.Xr lo 4
does not need to be configured and
.Nm
immediately exits successfully.
.Pp
Network interfaces can be manually configured using
.Xr ifconfig 8
and the DNS servers using
.Xr dnsconfig 8 .
.Nm
will occasionally reset the configuration when the internet address lease is
renewed unless disabled.
.Pp
.Nm
is typically automatically started as the
.Sy dhclient
daemon, which is depended on by the
.Sy network
daemon per the
.Xr init 5
configuration.
.Sh IMPLEMENTATION NOTES
.Nm
signals readiness when it has attempted to configure the network interface and
continues retrying afterwards.
The first attempt fails if the network interface's link doesn't come up within
10 seconds.
Packets are transmitted 6 times with an exponential backoff before giving up
and the configuration restarts.
.Sh FILES
.Bl -tag -width "/etc/dhclient.${mac}.conf" -compact
.It Pa /etc/dhclient.${mac}.conf
The preferred
.Xr dhclient.conf 5
file for the interface with the hardware address
.Ar mac .
.It Pa /etc/dhclient.${if}.conf
The second priority
.Xr dhclient.conf 5
file for the interface with the device name
.Ar if .
.It Pa /etc/dhclient.conf
The lowest priority
.Xr dhclient.conf 5
file shared across all network interfaces.
.El
.Sh ASYNCHRONOUS EVENTS
.Bl -tag -width "SIGUSR1"
.It Dv SIGTERM
Request daemon termination.
.El
.Sh EXIT STATUS
.Nm
runs as a
.Xr daemon 7
until stopped by
.Dv SIGTERM .
.Nm
signals readiness on the
.Ev READYFD
file descriptor when the interface is configured, when waiting for the link to
come up times out, or when the DHCP server doesn't respond in time or
configuration fails.
.Nm
will exit 0 if the network interface doesn't need to be configured or is fully
manually configured and will exit non-zero on any fatal startup error.
.Sh EXAMPLES
See
.Xr dhclient.conf 5
for example configurations.
.Pp
See
.Xr init 5
for an example disabling the
.Sy dhclient
daemon by removing the
.Sy network
daemon's dependency on it.
.Sh SEE ALSO
.Xr ether 4 ,
.Xr if 4 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr dnsconfig 8 ,
.Xr ifconfig 8 ,
.Xr init 8
.Sh STANDARDS
.Rs
.%A R. Droms
.%D March 1997
.%R RFC 2131
.%T Dynamic Host Configuration Protocol
.%Q Network Working Group
.Re
.Pp
.Rs
.%A S. Alexander
.%A R. Droms
.%D March 1997
.%R RFC 2132
.%T DHCP Options and BOOTP Vendor Extensions
.%Q Network Working Group
.Re