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).
Latest commit 5483e3c Nov 5, 2015 @jedisct1 Indent


                                 .:. UCARP .:.
                        Documentation for version 1.5.2

           ------------------------ BLURB ------------------------

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.

Home page is http://www.ucarp.org/

        ------------------------ COMPILATION ------------------------

libpcap (http://www.tcpdump.org/) must be installed on your system, with
development files (headers).

Then, follow the boring traditional procedure:

make install-strip

For details, have a look at the INSTALL file.

The software has been successfully tested on Linux 2.4, Linux 2.6, MacOS X,
OpenBSD, MirBSD and NetBSD.

        ------------------------ REQUIREMENTS ------------------------

A couple of virtual hosts must be given:

- A shared virtual IP, which will be dynamically answered by one alive host.
Services that need high availability need to be assigned to that virtual IP.

- A real IP address for each host.

- A shared identifier for the virtual IP address, which is a number between 1
and 255.

- For each host : an advertisement interval, comprised of a base and skew value, 
which is the frequency the host will tell the other one that it's still alive. 
By default, base is 1 and skew is 0, which basically means one advertisement a 
second. The protocol is very light, a tiny packet every second won't have any 
noticeable impact on your network.

- A shared password (that will never go plaintext to the network).

- A script to bring the virtual address up when a host becomes the master.

- Another script to bring the virtual address down when a host is no more the

            ------------------------ USAGE ------------------------

The server will usually be installed as : /usr/local/sbin/ucarp
Everything is driven through command-line options.
In order to see the list of available options, try : /usr/local/sbin/ucarp -h

Better than a long technical discussion, here's a real-life setup example.

Your company has an internal mail relay whose IP address is Every
user has configured his mail client with that host or IP address and the
service must always be up and running without having to reconfigure every 
user's mail client in case of a failure.

Instead of assigning to a particular mail server, you decide
to use ucarp to allow two hosts to share this IP address.  Of course,
only one server can answer for this address at a time, while the other
sits idle.  However the other server will automatically become active in
case the first one fails.  Thus you're providing a simple but powerful
IP failover solution.

So you set up two mail servers hosts with an identical configuration.
Their real IP addresses are and

First, we will create a script that brings the virtual IP address up. Let's
save that file as /etc/vip-up.sh :

#! /bin/sh
/sbin/ip addr add dev eth0

Now another script to bring it down, /etc/vip-down.sh :

#! /bin/sh
/sbin/ip addr del dev eth0

Of course, anything can go in these scripts. For instance, you may want to add
routes, to add something to log files or to send mail. And last, but not
least, you can use a script that will connect to your switches and flush their
ARP cache. Some users reported that transitions were way faster when also
switching MAC addresses.

The called scripts are passed arguments, in this order:

<interface name> <virtual address> <optional extra parameter>

For instance, as the is passed as the first argument to the called scripts,
feel free to replace "eth0" with "$1" and by "$2" in the previous

Don't forget to make those files executable :

chmod +x /etc/vip-up.sh /etc/vip-down.sh

Right. What we need now is an identifier for the virtual IP. Let's take "42".
And we also need a password. Let's take "love".

Now, on the first host (whoose real IP is, run :

/usr/local/sbin/ucarp -v 42 -p love -a -s &

On the second host, whose real IP is, run :

/usr/local/sbin/ucarp -v 42 -p love -a -s &

You should see that one of those hosts quickly becomes the master, and the
other one the backup. Related scripts are spawned on change.

Now unplug the master. After a few seconds, the other host becomes the new

------------------------ MULTICAST IP SELECTION -------------------------

The '--vhid' virtual IP identifier field only is only eight bits, providing up
to 255 different virtual IPs on the same multicast group IP. For larger
deployments, and more flexibility in allocation, ucarp can optionally use a
different multicast IP. By default, ucarp will send/listen on, which
is the assigned IP for VRRP. If you want to use a different address, use the
'--mcast' option. Consult the available multicast addresses before deciding
which to use.


Addresses within should be most appropriate.

If ucarp isn't working on a different IP, check that your networking gear is
set up to handle it. tcpdump on each host can be handy for diagnosis:

tcpdump -n 'net'

------------------------ MASTER ELECTION PROCESS ------------------------

When ucarp first runs, it starts as a backup and listens to the network
to determine if it should become the master.  If at any time more than
three times the node's advertising interval (defined as the advertising
base (seconds) plus a fudge factor, the advertising skew) passes without
hearing a peer's CARP advertisement, the node will transition itself to
being a master.

Transitioning from backup to master means:
1. running the specified up script to assign the vip to the local system
2. sending a gratuitous arp to the network to claim the vip
3. continuously sending CARP advertisements to the network every interval.

Transitioning from master to backup means:
1. running the specified down script to remove the vip from the local system

To understand how ucarp works, it's important to note that the
advertisement interval is not only used as the time in between which
each CARP advertisement is sent by the master, but also as a priority
mechanism where shorter (i.e. more frequent) is better.  The interval
base and skew values are stored in the CARP advertisement and are used
by other nodes to make certain decisions.

By default, once a node becomes the master, it will continue on
indefinitely as the master.  If you like/want/need this behavior, or don't
have a preferred master, then choose the same interval on all hosts.
If for whatever reason you were to choose different intervals on the
hosts, then over time the one with the shortest interval would tend to
become the master as machines are rebooted, after failures, etc.

Also of note is a conflict resolution algorithm that in case a master
hears another, equal (in terms of its advertised interval) master, the
one with the lower IP address will remain master and the other will
immediately demote itself.  This is simply to eliminate flapping and
quickly determine who should remain master.  This situation should not
happen very often but it can.

If you want a "preferred" master to always be the master (even if another
host is already the master), add the preempt switch (--preempt or -P) and
assign a shorter interval via the advertisement base (--advbase or -b) and
skew (--advskew or -k).  This will cause the preferred node to ignore a
master who is advertising a longer interval and promote itself to master.
The old master will quickly hear the preferred node advertising a shorter
interval and immediately demote itself.

In summary, a backup will become master if:
- no one else advertises for 3 times its own advertisement interval
- you specified --preempt and it hears a master with a longer interval

and a master will become backup if:
- another master advertises a shorter interval
- another master advertises the same interval, and has a lower IP address

      ------------------------ OTHER NOTES ------------------------

Specify the --neutral (-n) switch for ucarp to not run the downscript
at startup.

--shutdown (-z) will run the downscript at exit, unless ucarp is already in
the backup state. 

The "dead ratio" (--deadratio=...) knob basically changes how long a backup
server will wait for an unresponsive master before considering it as dead, and
becoming the new master. In the original protocol, the ratio is 3. This is
also the default when this command-line switch is missing.

Notices are sent both to stderr/stdout and to the syslog daemon (with the
"daemon" facility) by default. stderr/stdout are bypassed if the daemon is
started in background (--daemonize). Facilities can be changed with the
--syslog switch. Use --syslog=none to disable syslog logging, for instance if
prefer using something like multilog.

You can send the ucarp process a SIGUSR1 to have it log a status line to syslog, 
Jan  7 17:38:22 localhost ucarp[6103]: [INFO] BACKUP on eth0 id 198

You can send the ucarp process a SIGUSR2 to have it 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.

--ignoreifstate (-S) option tells ucarp to ignore unplugged network cable. It 
is useful when you connect ucarp nodes with a crossover patch cord (not via a 
hub or a switch). Without this option the node in MASTER state will switch to
BACKUP state when the other node is powered down, because network interface 
shows that cable is unplugged (NO-CARRIER). Some network interface drivers 
don't support NO-CARRIER feature, and this option is not needed for these 
network cards. The card that definitely supports this feature is Realtek 8139.

        ------------------------ TRANSLATIONS ------------------------

UCARP can speak your native language through gettext / libintl.
If you want to translate the software, have a look at the po/ directory.
Copy the ucarp.pot file to <your locale name>.po and use software like Kbabel
or Emacs to update the file.
Better use use your local charset than UTF-8.

      ------------------------ DOWNLOADING UCARP ------------------------

UCARP home page is: http://www.ucarp.org/ .

UCARP mailing-list: http://www.ucarp.org/ml/ .  Be sure to scroll all the way 
down to see the UCARP mailing list info!

Thank you, 

                        -Frank DENIS "Jedi/Sector One" <j at ucarp dot org> .