UCARP(8) System Manager's Manual UCARP(8)

ucarp — Automatic IP failover

ucarp [-i, --interface=INTERFACE] [-s, --srcip=IPADDRESS]
[-v, --vhid=VHID] [-p, --pass=PASSWORD] [-o, --passfile=PASSFILE]
[-P, --preempt] [-n, --neutral] [-a, --addr=IPADDR] [-h, --help]
[-b, --advbase=SECS] [-k, --advskew=SKEW] [-u, --upscript=SCRIPT]
[-d, --downscript=SCRIPT] [-r, --deadratio=RATIO] [-z, --shutdown]
[-B, --daemonize] [-f, --facility=FACILITY] [-x, --xparam PARAM]
[-S, --ignoreifstate] [-M, --nomcast]

ucarp allows a pair of hosts to share common IP addresses in order to provide automatic failover of an address from one machine to another. It is a portable userland implementation of the secure and patent-free Common Address Redundancy Protocol, (CARP), OpenBSD's alternative to VRRP.

ucarp supports the following command line options:

The network interface to bind to.
The persistent source address, (real IP), associated with this interface.
The id of the virtual server [1-255].
The shared password, (this gets encrypted and is not sent in the clear).
File to read the shared password from. The file specified should contain the password on the first line of the file.
Turn on preemptive failover. This causes an instance of ucarp to assume master status right immediately.
Do not run the downscript on startup when the initial state is backup.
The IP address of the virtual server.
Display a brief summary of the command line options.
Interval in seconds that advertisements will occur, (defaults to 1 second).
Advertisement skew [1-255], (defaults to 0).
Specifies the command to run after ucarp has successfully become master, the interface name gets passed as an argument. Typically a script used to bring up the virtual address, log the result, add routes, clear arp cache entries, etc.
Specifies the command that is run after ucarp has transitioned to the backup state, the interface name is passed as an argument. This is typically a script used to bring down the virtual interface, log the action, remove routes, etc.
Ratio used by the backup to determine how long to wait for an unresponsive master before considering it dead.
Use of this command causes the command specified by the -d argument to be invoked when ucarp shuts down.
Causes ucarp to detach from the terminal and run in the background as a daemon.
Set the syslog facility, defaults to daemon.
Specify an extra parameter to be supplied to the up/down scripts.
Ignore unplugged network cables. This option is useful when ucarp nodes are connected with a crossover cable. Without this option the master will transition to backup when the other node is powered down, as it no longer has a link (NO-CARRIER).
Use broadcast instead of multicast advertisements.

A host with a real IP of configured to be the master in a preemptive configuration with a virtual IP of

ucarp -i eth0 -s -v 10 -p secret -a \ 
--upscript=/usr/local/sbin/vip-up.sh --downscript=/usr/local/sbin/vip-down.sh -P 

The backup might be configured something like this.

ucarp -i eth0 -s -v 10 -p secret -a \ 
--upscript=/usr/local/sbin/vip-up.sh --downscript=/usr/local/sbin/vip-down.sh 

A machine with a real IP of is the preferred master for a virtual IP of, broadcasts are sent every 5 seconds.

ucarp -b 5 -s -v 27 -p badpass -a \ 
-u /usr/local/sbin/vip-up.sh -d /usr/local/sbin/vip-down.sh -z 

The hot standby with an IP of uses the following command, (note the advskew of 50 putting it at a disadvantage and making the first machine preferred).

ucarp -b 5 -k 50 -s -v 27 -p badpass -a \ 
-u /usr/local/sbin/vip-up.sh -d /usr/local/sbin/vip-down.sh -z 

Sending the ucarp process a SIGUSR1 will have it log a status line to syslog, eg "Sep 13 12:59:56 localhost ucarp[2654]: [INFO] MASTER on eth0 id 1" or "Sep 13 13:00:25 localhost ucarp[2644]: [INFO] BACKUP on eth0 id 1"

Sending the ucarp process a SIGUSR2 will cause it to demote itself from master to backup, pause 3 seconds, then proceed as usual to listen for other masters, and promote itself if necessary. This could be useful if you wish another node to take over master.

ucarp was written by Frank Denis, <j@ucarp.org>.

This manual page was written by Eric Evans <eevans@debian.org> for Debian and adapted to Archlinux by Sébastien Luttringer <seblu@archlinux.org>. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU General Public License, Version 2 or any later version published by the Free Software Foundation.

On Archlinux systems, the complete text of the GNU General Public License can be found in /usr/share/licenses/common/GPL.