Overview

Small python daemon that writes an entry in /etc/hosts for every running docker container. Goal is to provide a lightweight alternative for service discovery (like skydns) that also works on the host without having change DNS configuration.

A script modifying /etc/hosts is only feasible for smaller setups, but it has the advantage that when the script exits / crashes, resolving DNS names on the host keeps working.

The script adds [containerlabel].docker.local hostnames, but for containers launched using docker-compose, extra hostnames are generated: [number].[service].[group].docker.local (see below for an example).

How to

• make sure you have python 3 (/usr/bin/python3 in most linux distributions). The daemon does not work (yet) on python2, fixing this would be trivial.
• first install the script using python3 setup.py install
• then edit the /etc/hosts file, and add the following 2 lines (the script will only touch the section between these lines, and if the lines don't exist, the script will not touch /etc/hosts

#DOCKER_UPDATE_HOSTS_START
#DOCKER_UPDATE_HOSTS_END

Command line arguments

• '-n','--no-daemon' : do not run as a daemon but keep running in the foreground
• '-o','--once' : run once (updating /etc/hosts if needed) and exit (eg for running as a cron job)
• '-d','--debug' : run in foreground and do not log to syslog but stdout (and be more verbose)
• '-p','--pidfile' : write a pid file (default: '/tmp/docker_update_hosts.pid')

Example

running docker_hostsfile_updater with a docker_compose redmine/mariadb like this:

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
b630dab77b8d        redmine_redmine     "/docker-entrypoint.s"   10 days ago         Up 4 days           0.0.0.0:8080->3000/tcp   redmine_redmine_1
be3079da2e4f        mariadb             "docker-entrypoint.sh"   10 days ago         Up 4 days           3306/tcp                 redmine_db_1

will result in the following /etc/hosts:

127.0.0.1	localhost

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

#DOCKER_UPDATE_HOSTS_START
172.19.0.2	1.redmine.redmine.docker.local redmine_redmine_1.docker.local
172.19.0.3	1.db.redmine.docker.local redmine_db_1.docker.local
#DOCKER_UPDATE_HOSTS_END