Using --add-host or extra_hosts with docker-compose

Question

I am using docker-compose to run a test environment, that consists of about 5 different containers. The inter-container links and the shared volumes (volumes-from) works wonderfully. I also expose some ports up to the host machine, which works nicely.

What I am missing is a way to link some of my real servers into this environment, without hardcoding ip address. With docker run, you could use --add-host to add another line in your /etc/hosts file. Is there any way to do something similar with docker-compose?

Answer 1

I have great news: this will be in Compose 1.3!

I'm using it in the current RC (RC1) like this:

rng:
  build: rng
  extra_hosts:
    seed: 1.2.3.4
    tree: 4.3.2.1

Answer 2

https://docs.docker.com/compose/compose-file/#extra_hosts

extra_hosts - Add hostname mappings. Uses the same values as the docker client --add-host parameter.

extra_hosts:
 - "somehost:162.242.195.82"
 - "otherhost:50.31.209.229"

An entry with the ip address and hostname will be created in /etc/hosts > inside containers for this service, e.g:

162.242.195.82  somehost
50.31.209.229   otherhost

Answer 3

This is in the feature backlog for Compose but it doesn't look like work has been started yet. Github issue.

Answer 4

It seems like it should be made possible to say:

extra_hosts:
 - "loghost:localhost"

So if the part after the colon (normally an IP address) doesn't start with a digit, then name resolution will be performed to look up an IP for localhost, and add something like to the container's /etc/hosts:

127.0.0.1 loghost

...assuming that localhost resolves to 127.0.0.1 on the host system.

It looks like it'd be really easy to add in docker-compose's source code: compose/config/types.py's parse_extra_hosts function would likely do it.

For docker itself, this would probably be addable in opts/hosts.go's ValidateExtraHost function, though then it's not strictly validating anymore, so the function would be a little misnamed.

It might actually be a little better to add this to docker, not docker-compose - docker-compose might just get it automatically if docker gets it.

Sadly, this would probably require a container bounce to change an IP address.