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

NAME

     carp -- Common Address Redundancy Protocol

SYNOPSIS

     device carp

DESCRIPTION

     The CARP allows multiple hosts on the same local network to share a set of IPv4 and/or IPv6 addresses.  Its primary purpose is to ensure that
     these addresses are always available.

     To use carp, the administrator needs to configure at a minimum a common virtual host ID (vhid), and attach at least one IP address to this
     vhid on each machine which is to take part in the virtual group.  Additional parameters can also be set on a per-vhid basis: advbase and
     advskew, which are used to control how frequently the host sends advertisements when it is the master for a virtual 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 advertisement 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 advertisements.  These values can be configured using ifconfig(8), or through the SIOCSVH ioctl(2).

     CARP virtual hosts can be configured on multicast-capable interfaces: Ethernet, layer 2 VLAN, FDDI and Token Ring.  An arbitrary number of
     virtual host IDs can be configured on an interface.  An arbitrary number of IPv4 or IPv6 addresses can be attached to a particular vhid.  It
     is important that all hosts participating in a vhid have the same list of prefixes configured on the vhid, since all prefixes are included in
     the cryptographic checksum supplied in each advertisement.  Multiple vhids running on one interface participate in master/backup elections
     independently.

     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.  When enabled, a vhid in a backup state would preempt a mas-
					   ter that is announcing itself with a lower advskew.	Disabled by default.

     net.inet.carp.log			   Determines what events relating to carp vhids are logged.  A value of 0 disables any logging.  A value
					   of 1 enables logging state changes of carp vhids.  Values above 1 enable logging of bad carp packets.
					   The default value is 1.

     net.inet.carp.demotion		   This value shows current level of CARP demotion.  The value is added to the actual advskew sent in
					   announcements for all vhids.  At normal system operation the demotion factor is zero.  However, prob-
					   lematic conditions raise its level: when carp experiences problem with sending announcements, when an
					   interface running a vhid goes down, or while the pfsync(4) interface is not synchronized.  The demotion
					   factor can be adjusted writing to the sysctl oid.  The signed value supplied to the sysctl(8) command
					   is added to current demotion factor.  This allows to control carp behaviour depending on some external
					   conditions, for example on the status of some daemon utility.

     net.inet.carp.ifdown_demotion_factor  This value is added to net.inet.carp.demotion when an interface running a vhid goes down.  The default
					   value is 240 (the maximum advskew value).

     net.inet.carp.senderr_demotion_factor
					   This value is added to net.inet.carp.demotion when carp experiences errors sending its announcements.
					   The default value is 240 (the maximum advskew value).

STATE CHANGE NOTIFICATIONS

     Sometimes it is useful to get notified about carp status change events.  This can be accomplished by using devd(8) hooks.	Master/slave
     events are signalled under system CARP.  The subsystem specifies the vhid and name of the interface where the master/slave event occurred.
     The type of the message displays the new state of the vhid.  Please see devd.conf(5) and the EXAMPLES section for more information.

EXAMPLES

     For firewalls and routers with multiple interfaces, it is desirable to failover all of the addresses running carp together, when one of the
     physical interfaces goes down.  This is achieved by the use of the preempt option.  Enable it on both hosts A and B:

	   sysctl net.inet.carp.preempt=1

     Assume that host A is the preferred master and we are running the 192.168.1.0/24 prefix on em0 and 192.168.2.0/24 on em1.	This is the setup
     for host A (advskew is above 0 so it could be overwritten in the emergency situation from the other host):

	   ifconfig em0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
	   ifconfig em1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24

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

	   ifconfig em0 vhid 1 advskew 200 pass mekmitasdigoat 192.168.1.1/24
	   ifconfig em1 vhid 2 advskew 200 pass mekmitasdigoat 192.168.2.1/24

     When one of the physical interfaces of host A fails, advskew is demoted to a configured value on all its carp vhids.  Due to the preempt
     option, host B would start announcing itself, and thus preempt host A on both interfaces instead of just the failed one.

     Processing of carp status change events can be set up by using the following devd.conf rule:

	   notify 0 {
		   match "system"	   "CARP";
		   match "subsystem"	   "[0-9]+@[0-9a-z]+";
		   match "type" 	   "(MASTER|BACKUP)";
		   action "/root/carpcontrol.sh $subsystem $type";
	   };

     To see carp packets decoded in tcpdump(8) output, one needs to specify -T carp option, otherwise tcpdump(8) tries to interpret them as VRRP
     packets:

	   tcpdump -npi vlan0 -T carp

SEE ALSO

     inet(4), pfsync(4), devd.conf(5), rc.conf(5), ifconfig(8), sysctl(8), tcpdump(8)

HISTORY

     The carp device first appeared in OpenBSD 3.5.  The carp device was imported into FreeBSD 5.4.  In FreeBSD 10.0, carp was significantly
     rewritten, and is no longer a pseudo-interface.

BSD
								 February 21, 2013							       BSD