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carp - Common Address Redundancy Protocol
A interface can be created at runtime using the ifconfig carp N create command or by configuring it via cloned_interfaces in the /etc/rc.conf file.
To use , the administrator needs to configure at minimum a common virtual host ID (VHID) 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 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):
The ARP load balancing has some limitations. First, 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. Second, ARP load balancing can lead to asymmetric routing of incoming and outgoing traffic, and thus combining it with pfsync(4) is dangerous, because this creates a race condition between balanced routers and a host they are serving. Imagine an incoming packet creating state on the first router, being forwarded to its destination, and destination replying faster than the state information is packed and synced with the second router. If the reply would be load balanced to second router, it will be dropped due to no state.
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 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 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.
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