.\"	$NetBSD: carp.4,v 1.8 2020/10/12 15:18:48 roy Exp $
.\"	$OpenBSD: carp.4,v 1.19 2005/08/09 09:52:12 jmc Exp $
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.Dd October 12, 2020
.Dt CARP 4
.Os
.Sh NAME
.Nm carp
.Nd Common Address Redundancy Protocol
.Sh SYNOPSIS
.Cd pseudo-device carp
.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 carp Ns Ar N Ic create
command.
.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:
.Cm advbase
and
.Cm advskew ,
which are used to control how frequently the host sends advertisements when it
is the master for a virtual host, and
.Cm pass
which is used to authenticate carp advertisements.
Finally
.Cm carpdev
is used to specify which interface the
.Nm
device attaches to.
If unspecified, the kernel attempts to set carpdev by looking for
another interface with the same subnet.
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 :
.Bl -tag -width xxxxxxxxxxxxxxxxxxxxxxxxxx
.It net.inet.carp.allow
Accept incoming
.Nm
packets.
Enabled by default.
.It net.inet.carp.preempt
Allow virtual hosts to preempt each other.
It is also used to failover
.Nm
interfaces as a group.
When the option is enabled and one of the
.Nm
enabled physical interfaces
goes down, advskew is changed to 240 on all
.Nm
interfaces.
See also the first example.
Disabled by default.
.It net.inet.carp.log
Log bad
.Nm
packets.
Disabled by default.
.It net.inet.carp.arpbalance
Balance local traffic using ARP.
Disabled by default.
.El
.Sh EXAMPLES
For firewalls and routers with multiple interfaces, it is desirable to
failover all of the
.Nm
interfaces together, when one of the physical interfaces goes down.
This is achieved by the preempt option.
Enable it on both host A and B:
.Pp
.Dl # sysctl -w net.inet.carp.preempt=1
.Pp
Assume that host A is the preferred master and 192.168.1.x/24 is
configured on one physical interface and 192.168.2.y/24 on another.
This is the setup for host A:
.Bd -literal -offset indent
# ifconfig carp0 create
# ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1 \e
	netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1 \e
	netmask 255.255.255.0
.Ed
.Pp
The setup for host B is identical, but it has a higher advskew:
.Bd -literal -offset indent
# ifconfig carp0 create
# ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat \e
	192.168.1.1 netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e
	192.168.2.1 netmask 255.255.255.0
.Ed
.Pp
Because of the preempt option, when one of the physical interfaces of
host A fails, advskew is adjusted to 240 on all its
.Nm
interfaces.
This will cause host B to preempt on both interfaces instead of
just the failed one.
.Pp
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, two virtual hosts are configured on two hosts to
provide balancing and failover for the IP address 192.168.1.10.
.Pp
First the
.Nm
interfaces on Host A are configured.
The
.Cm 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
	netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e
	192.168.1.10 netmask 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 netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10 \e
	netmask 255.255.255.0
.Ed
.Pp
Finally, the ARP balancing feature must be enabled on both hosts:
.Pp
.Dl # sysctl -w net.inet.carp.arpbalance=1
.Pp
When the hosts receive an ARP request for 192.168.1.10, the source IP address
of the request is used to compute which virtual host should answer the request.
The host which is master of the selected virtual host will reply to the
request, the other(s) will ignore it.
.Pp
This way, locally connected systems will receive different ARP replies and
subsequent IP traffic will be balanced among the 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.
It cannot balance traffic that crosses a router, because the router
itself will always be balanced to the same virtual host.
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr sysctl 3 ,
.Xr arp 4 ,
.Xr arp 8 ,
.Xr ifconfig 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
device first appeared in
.Ox 3.5 .