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CARP(4) Device Drivers Manual CARP(4)


carpCommon Address Redundancy Protocol


device carp


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 (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 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):
Accept incoming carp packets. Enabled by default.
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.
Value of 0 disables any logging. Value of 1 enables logging state changes of carp interfaces. Values above 1 enable logging of bad carp packets. Default value is 1.
Balance local traffic using ARP (see below). Disabled by default.
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.

ARP level load balancing

The carp has limited abilities for load balancing the incoming connections between hosts in Ethernet network. For load balancing operation, one needs several CARP interfaces that are configured to the same IP address, but to a different VHIDs. Once an ARP request is received, the CARP protocol will use a hashing function against the source IP address in the ARP request to determine which VHID should this request belong to. If the corresponding CARP interface is in master state, the ARP request will be replied, otherwise it will be ignored. See the EXAMPLES section for a practical example of load balancing.
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.


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 
ifconfig carp1 create 
ifconfig carp1 vhid 2 pass mekmitasdigoat
The setup for host B is identical, but it has a higher advskew:
ifconfig carp0 create 
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 
ifconfig carp1 create 
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat
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
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 
ifconfig carp1 create 
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat
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 
ifconfig carp1 create 
ifconfig carp1 vhid 2 pass mekmitasdigoat
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, 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.


inet(4), pfsync(4), rc.conf(5), ifconfig(8), sysctl(8)


The carp device first appeared in OpenBSD 3.5. The carp device was imported into FreeBSD 5.4.
January 5, 2010 Debian