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CARP(4)                  BSD Kernel Interfaces Manual                  CARP(4)

NAME
     carp -- Common Address Redundancy Protocol

SYNOPSIS
     device carp

DESCRIPTION
     The carp interface is a pseudo-device that implements and controls the
     CARP protocol.  CARP 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 carp can also
     provide load balancing functionality.

     A carp interface can be created at runtime using the ifconfig carpN
     create command or by configuring it via cloned_interfaces in the
     /etc/rc.conf file.

     To use carp, 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: advbase and advskew, which are used to control how
     frequently the host sends advertisements when it is the master for a vir-
     tual host, and pass which is used to authenticate carp advertisements.
     The advbase parameter stands for "advertisement base".  It is measured in
     seconds and specifies the base of the adverisement interval.  The advskew
     parameter stands for "advertisement skew".  It is measured in 1/256 of
     seconds.  It is added to the base advertisement interval to make one host
     advertise a bit slower that the other does.  Both advbase and advskew are
     put inside CARP advertisments.  These configurations can be done using
     ifconfig(8), or through the SIOCSVH ioctl(2).

     Additionally, there are a number of global parameters which can be set
     using sysctl(8):

     net.inet.carp.allow       Accept incoming carp packets.  Enabled by
                               default.

     net.inet.carp.preempt     Allow virtual hosts to preempt each other.  It
                               is also used to failover carp interfaces as a
                               group.  When the option is enabled and one of
                               the carp enabled physical interfaces goes down,
                               advskew is changed to 240 on all carp inter-
                               faces.  See also the first example.  Disabled
                               by default.

     net.inet.carp.log         Log bad carp packets.  Enabled by default.

     net.inet.carp.arpbalance  Balance local traffic using ARP.  Disabled by
                               default.

EXAMPLES
     For firewalls and routers with multiple interfaces, it is desirable to
     failover all of the carp 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:

           sysctl net.inet.carp.preempt=1

     Assume that host A is the preferred master and 192.168.1.x/24 is config-
     ured on one physical interface and 192.168.2.y/24 on another.  This is
     the setup for host A:

           ifconfig carp0 create
           ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24

     The setup for host B is identical, but it has a higher advskew:

           ifconfig carp0 create
           ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24

     Because of the preempt option, when one of the physical interfaces of
     host A fails, advskew is adjusted to 240 on all its carp interfaces.
     This will cause host B to preempt on both interfaces instead of just the
     failed one.

     In order to set up an ARP balanced virtual host, it is necessary to con-
     figure one virtual host for each physical host which would respond to ARP
     requests and thus handle the traffic.  In the following example, two vir-
     tual hosts are configured on two hosts to provide balancing and failover
     for the IP address 192.168.1.10.

     First the carp interfaces on host A are configured.  The advskew of 100
     on the second virtual host means that its advertisements will be sent out
     slightly less frequently.

           ifconfig carp0 create
           ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24

     The configuration for host B is identical, except the advskew is on vir-
     tual host 1 rather than virtual host 2.

           ifconfig carp0 create
           ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24

     Finally, the ARP balancing feature must be enabled on both hosts:

           sysctl net.inet.carp.arpbalance=1

     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.

     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.

     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.

SEE ALSO
     inet(4), rc.conf(5), ifconfig(8), sysctl(8)

HISTORY
     The carp device first appeared in OpenBSD 3.5.  The carp device was
     imported into FreeBSD 5.4.

BSD                              April 7, 2005                             BSD