CARP

[Ryan McBride <mcbride@openbsd.org>]

As those of you who follow source-changes know, I have just imported
support for CARP, OpenBSD's Common Address Redundancy Protocol.

This protocol allows multiple hosts on the same local network to share a
set of IP addresses among them. Some of the functionality it provides is
similar to VRRP, although CARP differs in some significant aspects: CARP
has been designed to provide greater security and be protocol
independent (so we can support both IPv4 and IPv6). Finally, CARP allows
for some level of load balancing in addition to it's high-availability
functionality.

The basic configuration is as simple as running a command like the
following on all hosts which you want to have providing fail-over for
192.168.1.10:

# ifconfig carp0 vhid 1 192.168.1.10

One of the hosts will become "Master" of this IP address, while the
other hosts will go into "Backup" mode. If the master fails to advertise
after a certain period (3 times the advertisement interval of the Backup
host), the Backup will assume it is down and take over as Master,
responding to ARP requests for this IP address and accepting traffic
sent to it.

CARP specifies that the system which can advertise the most frequently
is most fit to be Master, one can encourage one system to be more likely
to become master by making it advertises more frequently relative to the
others. For example, the following command slows down the advertisement
frequency on this host;

# ifconfig carp0 advskew 100

Other hosts will advertise more frequently, and will be more likely to
become master.


See the man pages for more details: carp(4), ifconfig(8), sysctl(3).

A number of further developments are planned, including replay
detection, IPv6 support, and other fine-tuning.

-Ryan


P.S. If anyone has concerns about the Cisco's patent #5,473,599 and how
their claim that it applies to VRRP has forced us to design our own
incompatible protocol, don't talk to us. Instead, call Cisco's lawyer at
408-525-9706, or email him: rbarr@cisco.com.