Modifying /etc/hosts in Docker images for fun and profit

Update: The new Docker version 1.3.1 addresses some issues I describe in this post. Read about what problems Docker has solved.

Did you know that Docker doesn't let you (easily) modify /etc/hosts? You can't programmatically add aliases for localhost or new hostname-to-ip mappings. In short, Docker treats /etc/hosts like a static resource.

No problem. This will just take a quick Unix script to solve.

Attempt 1, the slate is wiped clean

No big deal, right? We simply write ourselves a little sed script to alias localhost and add a couple additional hostname mappings. We can run the sed script from our Dockerfile.

Well if you tried this at home, you'd quickly find out that it doesn't work. Each RUN command in a Dockerfile does another commit to the image. Ordinarily, that's just a space problem but Docker treats /etc/hosts special. It will overwrite it whenever it wants without any regard for your modifications. In a nutshell I wasn't able to persist any changes made during the Dockerfile.

Attempt 2, mount up

Okay, so we simply run our sed script when we run our container. Docker run will execute our startup script, running our sed script, updating /etc/hosts and finally running our server.

sed: cannot rename /etc/sedl8ySxL: Device or resource busy 

Unfortunately, that doesn't work either. The way sed -i works is to read from the original file, write to a temp file and then move the temp file over the original file. This doesn't work in this case because /etc/hosts is a mount point.

root@07734c969aa6:/usr/local/src# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs           19G  3.9G   14G  23% /
none             19G  3.9G   14G  23% /
tmpfs          1005M     0 1005M   0% /dev
shm              64M     0   64M   0% /dev/shm
/dev/sda1        19G  3.9G   14G  23% /etc/hosts      <----
none            1.1T  159G  881G  16% /code
tmpfs          1005M     0 1005M   0% /proc/kcore

Alternatives

How are other people solving this problem today? Some people seem to be using dnsmasq as a way of bypassing the problem of /etc/hosts. Running another dns server process as a workaround seems excessive, but I'm not sure I blame them. Doing a search of issues in the Docker Github repo found 125 issues which mentioned /etc/hosts.

On Stackoverflow there is the realization that /etc/hosts is now writable, but doesn't persist changes in running container.

Even so, I hadn't seen anyone provide a programmatic way to address the issue.

Attempt 3, Mungehosts is born

I took a step back and decided to write my own utility, Mungehosts. It solves my major pain points:

  • Programmatic interface
  • Can add aliases for localhost (ipv4 and ipv6)
  • Can add hostname-to-ip mappings
  • Works with Docker
  • Compiled executable (Linux binary in releases)
  • No dependencies except C runtime
  • Small size (167 KB)

Check it out and provide feedback. I know that the smart Docker folks will eventually get all of the features into the main application. When that happens Mungehosts will retire to pasture, but it will be a good workaround until then.

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