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

     carp - Common 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

     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 master 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, 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.

     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).

     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][email protected][0-9a-z]+";
                   match "type"            "(MASTER|BACKUP)";
                   action "/root/ $subsystem $type";

     To see carp packets decoded in tcpdump(1) output, one needs to specify -T
     carp option, otherwise tcpdump(1) tries to interpret them as VRRP

           tcpdump -npi vlan0 -T carp

     tcpdump(1), inet(4), pfsync(4), devd.conf(5), rc.conf(5), ifconfig(8),

     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.

FreeBSD 11.1-RELEASE-p4        February 21, 2013       FreeBSD 11.1-RELEASE-p4
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