haproxy highly available by using ucarp - all conveniently wrapped in a Docker container
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haproxy + ucarp

This image runs a haproxy with ucarp to make it highly available.

This image uses host networking, which requires Docker >= 0.11.


To configure haproxy, you can either bind-mount a haproxy.cfg from your host system to /haproxy/haproxy.cfg in the container or create a custom image by using a Dockerfile like this:

FROM fish/haproxy-docker
ADD  haproxy.cfg /haproxy/haproxy.cfg

If you need graceful reloads and can't affort rebuilding and restarting haproxy completely, you can bind-mount a config from your host and use docker kill -s HUP ... to reload the config.


Start a container like this on two hosts:

$ docker run --cap-add=NET_ADMIN --net host fish/haproxy-docker foobar23 wlan0

Now you should have two haproxy running and ucarp running on the given interface, making sure only one listens on

You can set the IPS env var to bind additional IPs on the active system.


If you're using IP based vhosts, you need to set net.ipv4.ip_nonlocal_bind=1 to allow the slave haproxy to bind to the right addresses even if they are not configured on your interfaces.

Failure Scenarios

If any service haproxy goes down, the container is supposed to kill UCARP so the IP gets removed and the backup can take over.

If for any reasons this isn't working, kill the container manually. This should cause the backup to take over.

If this is not the case, there is a problem with the backup container. You might want to restart it.