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NetBSD 6.1.5 - man page for carp (netbsd section 4)

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CARP(4) 			   BSD Kernel Interfaces Manual 			  CARP(4)

NAME
     carp -- Common Address Redundancy Protocol

SYNOPSIS
     pseudo-device carp [count]

DESCRIPTION
     The carp interface is a pseudo-device which 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.

     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.  Addi-
     tional 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.	Finally carpdev
     is used to specify which interface the carp device attaches to.  If unspecified, the kernel
     attempts to set carpdev by looking for another interface with the same subnet.  These con-
     figurations can be done using ifconfig(8), or through the SIOCSVH ioctl.

     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		 Log bad carp packets.	Disabled by default.

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

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 -w 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 \
		   netmask 255.255.255.0
	   # ifconfig carp1 create
	   # ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24 \
		   netmask 255.255.255.0

     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 netmask 255.255.255.0
	   # ifconfig carp1 create
	   # ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \
		   192.168.2.1 netmask 255.255.255.0

     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 vir-
     tual 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 \
		   netmask 255.255.255.0
	   # ifconfig carp1 create
	   # ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \
		   192.168.1.10 netmask 255.255.255.0

     The configuration for host B is identical, except the skew 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 netmask 255.255.255.0
	   # ifconfig carp1 create
	   # ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10 \
		   netmask 255.255.255.0

     Finally, the ARP balancing feature must be enabled on both hosts:

	   # sysctl -w 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
     netstat(1), sysctl(3), arp(4), arp(8), ifconfig(8), sysctl(8)

HISTORY
     The carp device first appeared in OpenBSD 3.5.

BSD					 October 16, 2003				      BSD
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