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

     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 con-
     trol how frequently the host sends advertisements when it is the master
     for a virtual host, and pass which is used to authenticate carp adver-
     tisements.  The advbase parameter stands for ``advertisement base''.  It
     is measured in seconds and specifies the base of the advertisement inter-
     val.  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 con-
     figurations can be done using ifconfig(8), or through the SIOCSVH

     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

     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 inter-
                               faces.  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.

                               A read only value showing the status of preemp-
                               tion 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 connec-
     tions 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 config-
     ured 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 con-
     figure one virtual host for each physical host which would respond to ARP
     requests and thus handle the traffic.  In the following example, two vir-
     tual 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 vir-
     tual 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.

FreeBSD 6.2                      June 6, 2006                      FreeBSD 6.2


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