.\"	$OpenBSD: carp.4,v 1.11 2004/07/26 09:20:14 jmc Exp $
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.Dd October 16, 2003
.Dt CARP 4
.Os
.Sh NAME
.Nm carp
.Nd Common Address Redundancy Protocol
.Sh SYNOPSIS
.Cd "pseudo-device carp" Op Ar count
.Sh DESCRIPTION
The
.Nm
interface is a pseudo-device which implements and controls the
CARP protocol.
.Nm
allows multiple hosts on the same local network to share a set of IP addresses.
Its primary purpose is to ensure that these
addresses are always available, but in some configurations
.Nm
can also provide load balancing functionality.
.Pp
A
.Nm
interface can be created at runtime using the
.Ic ifconfig carpN create
command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
.Pp
To use
.Nm ,
the administrator needs to configure at minimum a common virtual host ID and
virtual host IP address on each machine which is to take part in the virtual
group.
Additional parameters can also be set on a per-interface basis:
.Ar advbase
and
.Ar advskew ,
which are used to control how frequently the host sends advertisements when it
is the master for a virtual host, and
.Ar pass
which is used to authenticate carp advertisements.
These configurations can be done using
.Xr ifconfig 8 ,
or through the
.Dv SIOCSVH
ioctl.
.Pp
Additionally, there are a number of global parameters which can be set using
.Xr sysctl 8 .
.Sh EXAMPLES
In order to set up an ARP balanced virtual host, it is necessary to configure
one virtual host for each physical host which would respond to ARP
requests and thus handle the traffic.
In the following example, 2 hosts are configured to provide balancing and
failover for the IP address 192.168.1.10.
.Pp
First the
.Nm
interfaces on Host A are configured.
The
.Ar advskew
of 100 on the second virtual host means that its advertisements will be sent
out slightly less frequently.
.Bd -literal -offset indent
# ifconfig carp0 create
# ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10 \e
	255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e
	192.168.1.10 255.255.255.0
.Ed
.Pp
The configuration for host B is identical, except the skew is on
virtual host 1 rather than virtual host 2.
.Bd -literal -offset indent
# ifconfig carp0 create
# ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat \e
	192.168.1.10 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10 \e
	255.255.255.0
.Ed
.Pp
Finally, the ARP balancing feature must be enabled on both hosts:
.Pp
.Dl # sysctl net.inet.carp.arpbalance=1
.Pp
When the hosts receive an ARP request for 192.168.1.10, they both select
one of the virtual hosts based on the source IP address in the request.
The host which is master of that virtual host will reply to the request, the
other will ignore it.
.Pp
Because the host which advertises most often will tend to be the master, the
result of this is that Host A will tend to be the master for the virtual host
with ID 1, while Host B will tend to be the master for the virtual host with ID
2, and therefore arp requests and subsequent IP traffic are balanced across the
two hosts.
If one of the hosts fails, the other will take over the virtual MAC address,
and begin answering ARP requests on its behalf.
.Pp
Note: ARP balancing only works on the local network segment.
Connections which cross a router to arrive on the local network segment
will all appear to come from that router and be balanced to the same host.
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr inet 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr netstart 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
device first appeared in
.Ox 3.5 .