Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
CARP(4)		       FreeBSD Kernel Interfaces Manual		       CARP(4)

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 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 vir-
     tual 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 adverisement	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 advertisments.  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 inter-
			       faces.  See also	the first example.  Disabled
			       by default.

     net.inet.carp.log	       Log bad carp packets.  Enabled by default.

     net.inet.carp.arpbalance  Balance local traffic using ARP.	 Disabled by
			       default.

     net.inet.carp.suppress_preempt
			       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.

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 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 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 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	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 vir-
     tual 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.

     Note: 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.

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

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

FreeBSD	10.1			 May 15, 2005			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | HISTORY

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=carp&sektion=4&manpath=FreeBSD+5.5-RELEASE>

home | help