32.13 Common Address Redundancy Protocol (CARP)

Contributed by Tom Rhodes.

The Common Address Redundancy Protocol, or CARP allows multiple hosts to share the same IP address. In some configurations, this may be used for availability or load balancing. Hosts may use separate IP addresses as well, as in the example provided here.

To enable support for CARP, the FreeBSD kernel must be rebuilt as described in Chapter 9 with the following option:

device	carp

Alternatively, the if_carp.ko module can be loaded at boot time. Add the following line to the /boot/loader.conf:


CARP functionality should now be available and may be tuned via several sysctl OIDs:

OID Description
net.inet.carp.allow Accept incoming CARP packets. Enabled by default.
net.inet.carp.preempt This option downs all of the CARP interfaces on the host when one of them goes down. Disabled by default
net.inet.carp.log A value of 0 disables any logging. A Value of 1 enables logging of bad CARP packets. Values greater than 1 enables logging of state changes for the CARP interfaces. The default value is 1.
net.inet.carp.arpbalance Balance local network traffic using ARP. Disabled by default.
net.inet.carp.suppress_preempt A read only OID showing the status of preemption suppression. Preemption can be suppressed if link on an interface is down. A value of 0, means that preemption is not suppressed. Every problem increments this OID.

The CARP devices themselves may be created via the ifconfig command:

# ifconfig carp0 create

In a real environment, these interfaces will need unique identification numbers known as a VHID. This VHID or Virtual Host Identification will be used to distinguish the host on the network.

32.13.1 Using CARP for Server Availability (CARP)

One use of CARP, as noted above, is for server availability. This example will provide failover support for three hosts, all with unique IP addresses and providing the same web content. These machines will act in conjunction with a Round Robin DNS configuration. The failover machine will have two additional CARP interfaces, one for each of the content server's IPs. When a failure occurs, the failover server should pick up the failed machine's IP address. This means the failure should go completely unnoticed to the user. The failover server requires identical content and services as the other content servers it is expected to pick up load for.

The two machines should be configured identically other than their issued hostnames and VHIDs. This example calls these machines hosta.example.org and hostb.example.org respectively. First, the required lines for a CARP configuration have to be added to rc.conf. For hosta.example.org, the rc.conf file should contain the following lines:

ifconfig_fxp0="inet netmask"
ifconfig_carp0="vhid 1 pass testpass"

On hostb.example.org the following lines should be in rc.conf:

ifconfig_fxp0="inet netmask"
ifconfig_carp0="vhid 2 pass testpass"

Note: It is very important that the passwords, specified by the pass option to ifconfig, are identical. The carp devices will only listen to and accept advertisements from machines with the correct password. The VHID must also be different for each machine.

The third machine, provider.example.org, should be prepared so that it may handle failover from either host. This machine will require two carp devices, one to handle each host. The appropriate rc.conf configuration lines will be similar to the following:

ifconfig_fxp0="inet netmask"
cloned_interfaces="carp0 carp1"
ifconfig_carp0="vhid 1 advskew 100 pass testpass"
ifconfig_carp1="vhid 2 advskew 100 pass testpass"

Having the two carp devices will allow provider.example.org to notice and pick up the IP address of either machine should it stop responding.

Note: The default FreeBSD kernel may have preemption enabled. If so, provider.example.org may not relinquish the IP address back to the original content server. In this case, an administrator may have to manually force the IP back to the master. The following command should be issued on provider.example.org:

# ifconfig carp0 down && ifconfig carp0 up

This should be done on the carp interface which corresponds to the correct host.

At this point, CARP should be completely enabled and available for testing. For testing, either networking has to be restarted or the machines need to be rebooted.

More information is always available in the carp(4) manual page.