How does DNS resoluton work?

[ur5us]

I’d like to map a few domains/hosts to certain IP address, e.g. to load test a new server. I tried the following things:

• docker run --rm -it --add-host example.com:10.0.0.1 nakabonne/ali ali example.com
• docker run --rm -it -v $(pwd)/hosts:/etc/hosts nakabonne/ali ali example.com with a custom hosts file mapped into the container
• docker run --rm -it --network=host nakabonne/ali ali example.com with a modified hosts file on the docker host

but none of the above worked for me. Instead, ali would always hit the original server.

[nakabonne]

@ur5us Hi, sorry for the delay. I'm planning to support the --resolvers option which specifies custom DNS resolver addresses to use for name resolution instead of the ones configured by the operating system. It may be resolved with it.

[nakabonne]

@ur5us It may be an issue related to golang/go#22846. Could you give it a try to do this solution? Alternatively, try to use cgo-enabled Go binary which can be installed with go get github.com/nakabonne/ali.

[nakabonne]

@ur5us I'd say the root cause is the DNS cache. Clearing the cache and then running it should reference the domain in the hosts file.

For instance, on macOS:

$ sudo killall -HUP mDNSResponder
$ docker run --rm -it -v /etc/hosts:/etc/hosts nakabonne/ali ali http://example.com

[ur5us]

@nakabonne If you look at the 2nd example this is exactly what I did!

docker run --rm -it -v $(pwd)/hosts:/etc/hosts nakabonne/ali ali example.com

The only difference is that I used a custom hosts file ($(pwd)/hosts) instead of /etc/hosts. That custom hosts file simply contains one line with the custom entry I needed. Note, the whole issue only occurred with Docker. I haven’t tried running it on the host (without Docker). However, I did try modifying the host’s /etc/hosts file as per 3rd example. On the host itself using curl for instance or other programs would correctly resolve as per /etc/hosts file! Thus, I don’t think I’ve tried clearing the DNS cache as everything DNS related worked as expected otherwise. I don’t know if Docker ignores/bypasses the host’s /etc/hosts entirely but then again that’s why I tried mapping a custom $(pwd)/hosts file into the Docker container which didn’t work either. I’ll try a few more scenarios and explicitly clearing the host’s DNS cache though I’d still expect a custom hosts file mapped to /etc/hosts in the container to always work no matter what.

[nakabonne]

@ur5us Your 3rd example (with modifying the host’s /etc/hosts file) was worked for me! What error does it emit? Your example looks to lack the http:// scheme. It may be why.

docker run --rm -it --network=host nakabonne/ali ali http://example.com

[nakabonne]

And I was able to repro the issue about custom hosts file mapped into the container with v $(pwd)/hosts:/etc/hosts. While I don't get in detail, Docker looks to refer to the host's /etc/hosts file by default.