Clarification about ports in docker

[markwilkinson]

Hi!

Your instructions for Docker are a bit... opaque...

$ docker run --restart always -d -p 8000:8000 bulletmark/corsproxy 8000=http://192.168.1.98

It appears that your software, inside of the container, is listening on port 8000 - is that correct? So does the 8000 in the argument refer to the external 8000 port, or the internal 8000 port?

If i wanted to receive requests on port 5000 and forward them to 192.168.1.1:9000, what would the equivalent docker command look like? I'm having problems understanding when everything is 8000 :-)

Cheers!

[bulletmark]

I converted this to a discussion since you are asking a question, not raising a bug/issue.

Your first question is asking about the format of docker command line argument, specifically about how ports are published. Refer to the docker documentation which is pretty clear.

Your second question is whether the 8000= argument refers to the internal or external port. The corsproxy software is running within the container and can't know about anything external so it's given 8000= argument is the internal port that it can see, i.e. within the environment where that software is running.

Your third question is about how to receive on port 5000 and forward to port 9000. The corsproxy documentation and examples show the format of the argument is [host:]port=http://server[:targetport]. The following command should do what you want.

$ docker run --restart always -d -p 5000:8000 bulletmark/corsproxy 8000=http://192.168.1.1:9000

I.e. the 8000 in both places above can be anything really. It's just a hidden internal port we use to plug through into the container. I'd probably use 5000 for that too but you may not like that ;)