CARP(4) FreeBSD Kernel Interfaces Manual CARP(4)


carpCommon Address Redundancy Protocol


device carp


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):

Accept incoming carp packets. Enabled by default.
Allow virtual hosts to preempt each other. When enabled, a vhid in a backup state would preempt a master that is announcing itself with a lower advskew. Disabled by default.
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.
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, problematic 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.
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).
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).


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.


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 prefix on em0 and 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 
ifconfig em1 vhid 2 advskew 100 pass mekmitasdigoat

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

ifconfig em0 vhid 1 advskew 200 pass mekmitasdigoat 
ifconfig em1 vhid 2 advskew 200 pass mekmitasdigoat

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


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.
February 21, 2013 FreeBSD