CARP(4) MidnightBSD 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 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 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. Disabled by default.

net.inet.carp.suppress_preempt

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.

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 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 192.168.1.1/24
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 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 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 virtual 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), pfsync(4), rc.conf(5), ifconfig(8), sysctl(8)

HISTORY

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

MidnightBSD 0.3 May 15, 2005 MidnightBSD 0.3