UCARP is a portable implementation of the CARP protocol.

UCARP allows a couple of hosts to share common virtual IP
addresses in order to provide automatic failover. It is a
portable userland implementation of the secure and patent-free
Common Address Redundancy Protocol (CARP, OpenBSD's alternative
to the patents-bloated VRRP).

Strong points of the CARP protocol are: very low overhead,
cryptographically signed messages, interoperability between
different operating systems and no need for any dedicated extra
network link between redundant hosts.