CARP(4)                   OpenBSD Programmer's Manual                  CARP(4)

NAME
     carp - Common Address Redundancy Protocol

SYNOPSIS
     pseudo-device carp [count]

DESCRIPTION
     The carp interface is a pseudo-device which 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 setting up a hostname.if(5) configuration file for
     netstart(8).

     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.
     These configurations can be done using ifconfig(8), or through the
     SIOCSVH ioctl.

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

EXAMPLES
     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, 2 hosts
     are configured 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
           # ifconfig carp1 create
           # ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10

     The configuration for host B is identical, except the skew is on virtual
     host 1 rather than virtual host 2.

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

     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, 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.

     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 vir-
     tual host with ID 1, while Host B will tend to be the master for the vir-
     tual 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.

     Note: ARP balancing only works on the local network segment.  Connections
     which cross a router to arrive on the local network segment will all ap-
     pear to come from that router and be balanced to the same host.

SEE ALSO
     sysctl(3), inet(4), ifconfig(8), sysctl(8)

HISTORY
     The carp device first appeared in OpenBSD 3.5.

OpenBSD 3.5                    October 16, 2003                              2
