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CARP(4)			 BSD 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 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 de-

     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 state changes of	carp inter-
			       faces.  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 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 ad-
     dress, 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 ad-
     dress 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 im-
     ported into FreeBSD 5.4.

BSD				January	5, 2010				   BSD


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