Setting Up Docker Dnsmasq

Question

I'm trying to set up a docker dnsmasq container so that I can have all my docker containers look up the domain names rather than having hard-coded IPs (if they are on the same host). This fixes an issue with the fact that one cannot alter the /etc/hosts file in docker containers, and this allows me to easily update all my containers in one go, by altering a single file that the dnsmasq container references.

It looks like someone has already done the hard work for me and created a dnsmasq container. Unfortunately, it is not "working" for me. I wrote a bash script to start the container as shown below:

name="dnsmasq_"
timenow=$(date +%s)
name="$name$timenow"

sudo docker run \
-v="$(pwd)/dnsmasq.hosts:/dnsmasq.hosts" \
--name=$name \
-p='127.0.0.1:53:5353/udp' \
-d sroegner/dnsmasq

Before running that, I created the dnsmasq.hosts directory and inserted a single file within it called hosts.txt with the following contents:

192.168.1.3 database.mydomain.com

Unfortunately whenever I try to ping that domain from within:

• the host
• The dnsmasq container
• another container on the same host

I always receive the ping: unknown host error message.

I tried starting the dnsmasq container without daemon mode so I could debug its output, which is below:

dnsmasq: started, version 2.59 cachesize 150
dnsmasq: compile time options: IPv6 GNU-getopt DBus i18n DHCP TFTP conntrack IDN
dnsmasq: reading /etc/resolv.dnsmasq.conf
dnsmasq: using nameserver 8.8.8.8#53
dnsmasq: read /etc/hosts - 7 addresses
dnsmasq: read /dnsmasq.hosts//hosts.txt - 1 addresses

I am guessing that I have not specified the -p parameter correctly when starting the container. Can somebody tell me what it should be for other docker containers to lookup the DNS, or whether what I am trying to do is actually impossible?

Answer 1

The build script for the docker dnsmasq service needs to be changed in order to bind to your server's public IP, which in this case is 192.168.1.12 on my eth0 interface

#!/bin/bash

NIC="eth0"

name="dnsmasq_"
timenow=$(date +%s)
name="$name$timenow"

MY_IP=$(ifconfig $NIC | grep 'inet addr:'| grep -v '127.0.0.1' | cut -d: -f2 | awk '{ print $1}')


sudo docker run \
-v="$(pwd)/dnsmasq.hosts:/dnsmasq.hosts" \
--name=$name \
-p=$MY_IP:53:5353/udp \
-d sroegner/dnsmasq

On the host (in this case ubuntu 12), you need to update the resolv.conf or /etc/network/interfaces file so that you have registered your public IP (eth0 or eth1 device) as the nameserver.

You may want to set a secondary nameserver to be google for whenever the container is not running, by changing the line to be dns-nameservers xxx.xxx.xxx.xxx 8.8.8.8 E.g. there is no comma or another line.

You then need to restart your networking service sudo /etc/init.d/networking restart if you updated the /etc/network/interfaces file so that this auto updates the /etc/resolve.conf file that docker will copy to the container during the build.

Now restart all of your containers

• sudo docker stop $CONTAINER_ID
• sudo docker start $CONTAINER_ID

This causes their /etc/resolv.conf files update so they point to the new nameserver settings.

DNS lookups in all your docker containers (that you built since making the changes) should now work using your dnsmasq container!

As a side note, this means that docker containers on other hosts can also take advantage of your dnsmasq service on this host as long as their host's nameserver settings is set to using this server's public IP.