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     carp — Common Address Redundancy Protocol


     device carp


     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 state
                               changes of carp interfaces.  Values above 1 enable logging of bad
                               carp packets.  Default value is 1.

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

                               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.


     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
           ifconfig carp1 create
           ifconfig carp1 vhid 2 pass mekmitasdigoat

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

           ifconfig carp0 create
           ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat
           ifconfig carp1 create
           ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat

     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

     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
           ifconfig carp1 create
           ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat

     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
           ifconfig carp1 create
           ifconfig carp1 vhid 2 pass mekmitasdigoat

     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, 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.


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


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