Docker does not allow you to freely modify /etc/hosts inside a container. Attempts to do so usually end with permission errors, which has caused a lot of discussion over time.
Until Docker offers a cleaner built-in solution, one practical workaround is to use dnsmasq for local name resolution instead.
The example below assumes a CentOS 6 container.
1. Install dnsmasq
1 | yum install dnsmasq |
If you want dnsmasq to start automatically whenever the container starts, you can append the startup command to /.bashrc:
1 | echo "service dnsmasq start &>/dev/null" >> /.bashrc |
2. Configure dnsmasq
Edit /etc/dnsmasq.conf and add:
1 | listen-address=127.0.0.1 |
This tells dnsmasq to listen on localhost and use /etc/dnsmasq.hosts as an additional hosts file.
3. Add custom host entries
Create or edit /etc/dnsmasq.hosts and add the hostnames you want to resolve. The format is the same as a normal hosts file.
4. Exit the container and save its changes
5. Start the container with custom DNS
When starting the container, add --dns so the container uses the local dnsmasq, plus a second DNS server for everything else:
1 | docker run -t -i --dns=127.0.0.1 --dns=114.114.114.114 knktc/centos6 /bin/bash |
At that point the container can resolve your custom local names just like it would with /etc/hosts.
Known issue
dnsmasq takes a little time to start. If you run a command immediately when the container starts, some lookups may fail early.
A simple workaround is to add a short delay:
1 | docker run -t -i --dns=127.0.0.1 --dns=114.114.114.114 knktc/centos6 /bin/bash -ic "sleep 5;python myapp.py" |
