Trick a docker containers DNS to resolve a different url

Question

I am not sure I know the terminology to describe this correctly, my apologies.

But for a contrived example. Say I have a docker container that runs code that accesses "google.com". But I want all requests that the container sets to "google.com" to actually go to "facebook.com". In other words I want the DNS lookup of one domain name to use another domain when reloving. Is there a way to do this with docker or docker-compose?

My ask is a bit more than this, because I know that often multiple websites are hosted on the same server (same IP), so I need a way for the conversion to happen at the domain name level, not at the IP level.

Thank you

Answer 1

If you want a DNS lookup of one domain to resolve to another one, you have 2 options.

1. Modify the Container /etc/hosts file. By default any DNS lookup is done there. You schould do that in the Dockerfile.

Modify /etc/hosts in a Dockerfile

2. Maintain your own DNS servers with all mapping records. On container runtime just pass the DNS server.

Docker docs

--dns The IP address of a DNS server.

Answer 2

Is this what you are trying to do?

$ docker run --add-host foo:127.0.0.42 alpine ping foo -c 3
PING foo (127.0.0.42): 56 data bytes
64 bytes from 127.0.0.42: seq=0 ttl=64 time=0.045 ms
64 bytes from 127.0.0.42: seq=1 ttl=64 time=0.061 ms
64 bytes from 127.0.0.42: seq=2 ttl=64 time=0.070 ms

--- foo ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.045/0.058/0.070 ms

Look for --add-host parameter for docker run or extra_hosts for docker-compose.

Answer 3

The extra_hosts setting in docker-compose might help you. Docs