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.
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:
hostname="hosta.example.org"
ifconfig_fxp0="inet 192.168.1.3 netmask 255.255.255.0"
cloned_interfaces="carp0"
ifconfig_carp0="vhid 1 pass testpass 192.168.1.50/24"
On hostb.example.org the following lines should be in rc.conf:
hostname="hostb.example.org"
ifconfig_fxp0="inet 192.168.1.4 netmask 255.255.255.0"
cloned_interfaces="carp0"
ifconfig_carp0="vhid 2 pass testpass 192.168.1.51/24"
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:
hostname="provider.example.org"
ifconfig_fxp0="inet 192.168.1.5 netmask 255.255.255.0"
cloned_interfaces="carp0 carp1"
ifconfig_carp0="vhid 1 advskew 100 pass testpass 192.168.1.50/24"
ifconfig_carp1="vhid 2 advskew 100 pass testpass 192.168.1.51/24"
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.
