Add containers to host's /etc/hosts file

add_containers_to_hosts.rb

require 'json'
require 'hosts'

loop do
  hosts = Hosts::File.read('/etc/hosts')
  host_elements = hosts.elements
  all_aliases = []
  `docker ps`.split(/\n/).each do |line|
    cols = line.split(/\s/)
    next if cols[0] =~ /[^a-f0-9]/
    inspect = `docker inspect #{cols[0]}`
    JSON.parse(inspect)[0]['NetworkSettings']['Networks'].each do |name, settings|
      ip = settings['IPAddress']
      next unless settings['Aliases'] # Container might be running with "network_mode: host"
      settings['Aliases'].each do |_alias|
        all_aliases << _alias
        comment = "Docker: #{_alias}"
        existing = host_elements.select { |e| e.respond_to?(:comment) && e.comment == comment }
        if existing.empty?
          new_element = Hosts::Entry.new(ip, _alias, comment: comment)
          hosts.elements << new_element
        else
          existing.each do |element|
            element.address = ip
          end
        end
      end
    end
  end
  # Remove old Docker containers
  hosts.elements.each do |element|
    next if !element.respond_to?(:comment) || element.comment !~ /Docker: /
    _alias = element.comment.match(/Docker: (.*)/)[1]
    next if all_aliases.include?(_alias)
    hosts.elements.delete(element)
  end
  hosts.write
  sleep 5
end

This is a quick-and-dirty script that should be run on a Docker host to add containers' IP address to the host's /etc/hosts file. It needs to be run as root, obviously. I know Docker has an API, but this works well enough for me.

It relies on the hosts gem: gem install hosts

Please let me know what you think!