alias a docker container's ip to /ect/hosts

add_docker_host.py

#!/usr/bin/env python

# Script to alias a docker container's primary IPv4 address
# with the desired host_name in /ect/hosts
#
# Usage: add_container_to_hosts <container_name> <host_name>
#
# CAVEAT: Currently won't work for IPv6 addresses and will only work
# on platforms, where the hosts file can be found under /etc/hosts 
# 
#
# Note: Could easily be replaced by https://github.com/gliderlabs/resolvable
# but that won't work, when one container requires being addressed using
# more than two domain names
#
# Author: Florian Lentsch <office@florian-lentsch.at>

import sys, re, os
from subprocess import Popen, PIPE, call
from os.path import isfile

HOSTS_FILE = '/etc/hosts'

def die(msg):
	sys.stderr.write("Error: " + msg + "\n")
	exit(1)

def cmd(cmd_arr):
	p = Popen(cmd_arr, stdout=PIPE)
	out, err = p.communicate()
	if err:
		return None
	return out

class HostsFileModifier:
    def __init__(self, container_name, host_name):
        self.host_name = host_name
        self.container_name = container_name
    
    def get_container_ip(self):
        ip = cmd(["docker", "inspect", "-f", "{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}", self.container_name])
        
        if isinstance(ip, basestring) and re.match(r"^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$", ip.strip()):
            return ip.strip()
        
        ip = cmd(["docker", "inspect", "--format", "{{ .NetworkSettings.IPAddress }}", self.container_name])
        if isinstance(ip, basestring) and re.match(r"^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$", ip.strip()):
            return ip.strip()
        
        die("Could not determine container IPv4 address of container id/name " + self.container_name)
    
    def read_hosts_file(self):
        if not isfile(HOSTS_FILE):
            die(HOSTS_FILE + " not found")
                
        try:
            self.hosts_file = open(HOSTS_FILE, 'r')
            return self.hosts_file.read()
        except IOError:
            die("Could not read from " + HOSTS_FILE)
    
    def replace_or_add_domain_ip(self, hosts_str, new_ip):
        correct_content = new_ip + "\t"  + self.host_name
        
        regex = re.compile("^((?:[0-9]{1,3}\.){3}[0-9]{1,3})\s+(" + re.escape(self.host_name) + ")$", flags=re.MULTILINE)
        md = regex.search(hosts_str)
        if md:
            if md.group(1) == new_ip:
                return hosts_str
            
            if os.getuid() == 0:
            	print  "Changing '%s' into '%s'" % (md.group(0), correct_content)  
            return regex.sub(correct_content, hosts_str)
        else:
        	# in case the host is present in a line with many other hosts 
        	# just remove it from there before adding it back in at the bottom:
        	hosts_str = re.sub("\s+" + re.escape(self.host_name), '', hosts_str)
        		
        	if os.getuid() == 0:
        		print "Appending '%s'" % correct_content 
        	return hosts_str + "\n" + correct_content + "\n"
                    
    def change_hosts_file(self, content):
        self.hosts_file.close
        
        if os.getuid() != 0:
        	print "re-running as root (required to change hosts file)...	"
        	call(['sudo', sys.argv[0], sys.argv[1], sys.argv[2] ])
        	exit(0)
        
        try:
            with open(HOSTS_FILE, 'w') as self.hosts_file:    
                self.hosts_file.write(content)
        except IOError:
            die("Change required, but could not write " + HOSTS_FILE + ". Did you forget to use sudo?")
    
    def set_domain_ip(self,ip):
        hosts_str = self.read_hosts_file()
        modified_hosts_str = self.replace_or_add_domain_ip(hosts_str, ip)
        
        if not modified_hosts_str:
            die("Error parsing hosts file")
        
        if hosts_str == modified_hosts_str:
            print "Hosts file does not need changing"
            exit(0)
        
            
        self.change_hosts_file(modified_hosts_str)
        exit(0)
        
if len(sys.argv) != 3:
	die('Usage: ' + sys.argv[0] + ' <container_name> <host_name>')
    
modifier = HostsFileModifier(sys.argv[1], sys.argv[2])
ip = modifier.get_container_ip()
modifier.set_domain_ip(ip)