CARP(4) BSD Kernel Interfaces Manual CARP(4)NAMEcarp — Common Address Redundancy Protocol
SYNOPSIS
pseudo-device carp [count]
DESCRIPTION
The carp interface is a pseudo-device which 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.
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 vir‐
tual host, and pass which is used to authenticate carp advertisements.
Finally carpdev is used to specify which interface the carp device
attaches to. If unspecified, the kernel attempts to set carpdev by look‐
ing for another interface with the same subnet. These configurations can
be done using ifconfig(8), or through the SIOCSVH ioctl.
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 Log bad carp packets. Disabled by default.
net.inet.carp.arpbalance Balance local traffic using ARP. Disabled by
default.
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 -w net.inet.carp.preempt=1
Assume that host A is the preferred master and 192.168.1.x/24 is config‐
ured 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 \
netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24 \
netmask 255.255.255.0
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 netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \
192.168.2.1 netmask 255.255.255.0
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 con‐
figure one virtual host for each physical host which would respond to ARP
requests and thus handle the traffic. In the following example, two vir‐
tual 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 \
netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \
192.168.1.10 netmask 255.255.255.0
The configuration for host B is identical, except the skew 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 netmask 255.255.255.0
# ifconfig carp1 create
# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10 \
netmask 255.255.255.0
Finally, the ARP balancing feature must be enabled on both hosts:
# sysctl -w 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 ALSOnetstat(1), sysctl(3), arp(4), arp(8), ifconfig(8), sysctl(8)HISTORY
The carp device first appeared in OpenBSD 3.5.
BSD October 16, 2003 BSD
