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NAME

     carp - Common Address Redundancy Protocol

SYNOPSIS

     device carp

DESCRIPTION

     The carp interface is a pseudo-device that 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 configuring it via cloned_interfaces in the
     /etc/rc.conf file.

     To use carp, the administrator needs to configure at minimum a common
     virtual host ID (VHID) 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 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
     configurations can be done using ifconfig(8), or through the SIOCSVH
     ioctl(2).

     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.  It
                               is also used to failover carp interfaces as a
                               group.  When the option is enabled and one of
                               the carp enabled physical interfaces goes down,
                               advskew is changed to 240 on all carp
                               interfaces.  See also the first example.
                               Disabled by default.

     net.inet.carp.log         Value of 0 disables any logging.  Value of 1
                               enables logging of bad carp packets.  Values
                               above 1 enable logging state changes of carp
                               interfaces.  Default value is 1.

     net.inet.carp.arpbalance  Balance local traffic using ARP (see below).
                               Disabled by default.

     net.inet.carp.suppress_preempt
                               A read only value showing the status of
                               preemption suppression.  Preemption can be
                               suppressed if link on an interface is down or
                               when pfsync(4) interface is not synchronized.
                               Value of 0 means that preemption is not
                               suppressed, since no problems are detected.
                               Every problem increments suppression counter.

ARP level load balancing

     The carp has limited abilities for load balancing the incoming
     connections between hosts in Ethernet network.  For load balancing
     operation, one needs several CARP interfaces that are configured to the
     same IP address, but to a different VHIDs.  Once an ARP request is
     received, the CARP protocol will use a hashing function against the
     source IP address in the ARP request to determine which VHID should this
     request belong to.  If the corresponding CARP interface is in master
     state, the ARP request will be replied, otherwise it will be ignored.
     See the EXAMPLES section for a practical example of load balancing.

     The ARP load balancing has some limitations.  First, ARP balancing only
     works on the local network segment.  It cannot balance traffic that
     crosses a router, because the router itself will always be balanced to
     the same virtual host.  Second, ARP load balancing can lead to asymmetric
     routing of incoming and outgoing traffic, and thus combining it with
     pfsync(4) is dangerous, because this creates a race condition between
     balanced routers and a host they are serving.  Imagine an incoming packet
     creating state on the first router, being forwarded to its destination,
     and destination replying faster than the state information is packed and
     synced with the second router.  If the reply would be load balanced to
     second router, it will be dropped due to no state.

EXAMPLES

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

           sysctl net.inet.carp.preempt=1

     Assume that host A is the preferred master and 192.168.1.x/24 is
     configured on one physical interface and 192.168.2.y/24 on another.  This
     is the setup for host A:

           ifconfig carp0 create
           ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24

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

           ifconfig carp0 create
           ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24

     Because of the preempt option, when one of the physical interfaces of
     host A fails, advskew is adjusted to 240 on all its carp interfaces.
     This will cause host B to preempt on both interfaces instead of just the
     failed one.

     In order to set up an ARP balanced virtual host, it is necessary to
     configure one virtual host for each physical host which would respond to
     ARP requests and thus handle the traffic.  In the following example, two
     virtual hosts are configured on two hosts 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 192.168.1.10/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24

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

           ifconfig carp0 create
           ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
           ifconfig carp1 create
           ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24

     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, the source IP
     address of the request is used to compute which virtual host should
     answer the request.  The host which is master of the selected virtual
     host will reply to the request, the other(s) will ignore it.

     This way, locally connected systems will receive different ARP replies
     and subsequent IP traffic will be balanced among the hosts.  If one of
     the hosts fails, the other will take over the virtual MAC address, and
     begin answering ARP requests on its behalf.

SEE ALSO

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

HISTORY

     The carp device first appeared in OpenBSD 3.5.  The carp device was
     imported into FreeBSD 5.4.