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     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 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
                                           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

     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).

                                           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/ $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


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