Using DNSMASQ to Containers DNS Resolution
All these experiments have been done on Centos:7, this will
elaborate the Containers running on the Docker Host will have DNS resolution
with DNSMASQ running on Docker Host
It’s a lightweight and
straightforward solution for service discovery which doesn’t
require running additional containers
First, we need to install
dnsmasq on Docker Host
yum install
dnsmasq.x86_64
Create a new Docker-related config file in /etc/dnsmasq.d:
vi /etc/dnsmasq.d/docker-dns
Paste the following lines:
addn-hosts=/docker-container-hosts
interface=docker0
#bind-interfaces
- The first line specifies an
additional hosts file to use for container domain
name lookup.
- The second adds the docker0 interface to listen on.
- Uncomment the bind-interfaces line
to keep the port 53 from exposing (beware, it may conflict with starting containers on boot and/or using
Docker’s restart policies).
Restart the Dnsmasq service
service dnsmasq restart
Now let’s determine the host IP address for
the docker0 interface:
ip addr show dev docker0 | awk -F'[ /]*' '/inet /{print $3}'
Most likely it will print 172.17.0.1.
For demonstration purposes, create two synthetic Docker
containers specifying the determined IP address as DNS, and also host name and
a container name:
docker run -itd --dns=172.17.0.1 --hostname=businesswork1
--name=bw1 tibco:3.0.0
docker run -itd --dns=172.17.0.1 --hostname=businesswork2
--name=bw2 tibco:3.0.0
Docker PS
# docker ps
CONTAINER ID IMAGE
COMMAND CREATED
STATUS
PORTS
NAMES
e95e45ca99e1 tibco:3.0.0
"/opt/tmo/docker-entr"
3 seconds ago Up 2 seconds
bw2
6b4a3026c0df tibco:3.0.0
"/opt/tmo/docker-entr"
10 seconds ago Up 9 seconds
bw1
Now we are going to create a simple shell script for updating
additional host entries based on running containers:
vi ~/update-docker-dns.sh
#!/bin/bash
# Domain name for containers
CONTAINER_DOMAIN=containers.demo.com
# Path to the addn-hosts file
CONTAINER_HOSTS=/docker-container-hosts
echo "# Auto-generated by $0" > $CONTAINER_HOSTS
for CID in `docker ps -q`; do
IP=`docker inspect --format '{{
.NetworkSettings.IPAddress }}' $CID`
NAME=`docker inspect --format '{{ .Config.Hostname
}}' $CID`
echo "$IP $NAME.$CONTAINER_DOMAIN"
>> $CONTAINER_HOSTS
done
# Ask dnsmasq to reload addn-hosts
pkill -x -HUP dnsmasq
Make the script executable:
chmod +x ~/update-docker-dns.sh
And run it
~/update-docker-dns.sh
Let’s check the contents of the generated hosts file:
cat /docker-container-hosts
[root@sdnrhel00561 ~]# cat /docker-container-hosts
# Auto-generated by ./update-docker-dns.sh
172.17.0.4 businesswork2.containers.demo.com
172.17.0.3 businesswork1.containers.demo.com
Check that BW2 container is accessible from BW1:
# docker exec -it bw1 /bin/bash
# ping businesswork1.containers.demo.com
PING businesswork1.containers.demo.com (172.17.0.3) 56(84) bytes
of data.
64 bytes from businesswork1 (172.17.0.3): icmp_seq=1 ttl=64
time=0.034 ms
64 bytes from businesswork1 (172.17.0.3): icmp_seq=2 ttl=64
time=0.043 ms
^C
--- businesswork1.containers.demo.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.034/0.038/0.043/0.007 ms
Just do not forget to provide correct --dns, --name and
--hostname flags in the docker run command line! And Run the Script.
The script will update the static DNS Container Hostnames when
you run it.
