dt-dns-config

#!/usr/bin/python

import argparse
import socket
import docker

def parse_arguments():
        class DefaultList(list):
                def __copy__(self):
                        return []

	parser = argparse.ArgumentParser(description="Rewrite the DNS config file for docker containers.", formatter_class=argparse.ArgumentDefaultsHelpFormatter)
	parser.add_argument("--hosts-file", type=str, default="/etc/hosts.docker", help="output file for hosts")
	parser.add_argument("--domain", type=str, default=DefaultList([socket.getfqdn()]), action="append", dest="domains", help="domain name to append to each host. may be specified multiple times")
	parser.add_argument("--dry-run", action="store_true", help="only print to stdout instead of writing to file")

	return parser.parse_args()


def get_docker_hosts(args):
	client = docker.Client()
	containers = client.containers()

	hosts = []
	for container in containers:
		inspect = client.inspect_container(container["Id"])
		name = inspect["Name"].lstrip("/")

		host = {}
		host["ip"] = inspect["NetworkSettings"]["IPAddress"]
		host["names"] = set()
		host["names"].add(name)
		for domain in args.domains:
			host["names"].add(name + "." + domain)
	  	if inspect["Config"]["Hostname"]:
			host["names"].add(inspect["Config"]["Hostname"])

		hosts.append(host)

	return hosts


def write_hosts_file(args, hosts):
	name_widths = [len(name) for host in hosts for name in host["names"]]
	max_name_width = max(name_widths) + 4
	lines = ["%(ip)-20s%(names)s" % {"ip": host["ip"], "names": "".join(["%-*s" % (max_name_width, name) for name in host["names"]])} for host in hosts] 
	output = "\n".join(lines)

	if args.dry_run:
		print output
	else:
		with open(args.hosts_file, "w") as f:
			f.write(output)
		print "Replaced DNS config in %s" % args.hosts_file


args = parse_arguments()
hosts = get_docker_hosts(args)
write_hosts_file(args, hosts)