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PSA: Docker nukes your firewall rules and replaces them with its own.
(sh.itjust.works)
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this post was submitted on 11 Mar 2024
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This is standard, but often unwanted, behavior of docker.
Docker creates a bunch of chain rules, but IIRC, doesn't modify actual incoming rules (at least it doesn't for me) it just will make a chain rule for every internal docker network item to make sure all of the services can contact each other.
Yes it is a security risk, but if you don't have all ports forwarded, someone would still have to breach your internal network IIRC, so you would have many many more problems than docker.
I think from the dev's point of view (not that it is right or wrong), this is intended behavior simply because if docker didn't do this, they would get 1,000 issues opened per day of people saying containers don't work when they forgot to add a firewall rules for a new container.
Option to disable this behavior would be 100x better then current, but what do I know lol
That assumes you're on some VPS with a hardware firewall in front.
Often enough you're on a dedicated server that's directly exposed to the internet, with those iptables rules being the only thing standing between your services and the internet.
What difference does it make if you open the ports yourself for the services you expose, or Docker does it for you? That's all that Docker is meant to do, act as convenience so you don't have to add/remove rules as the containers go up/down, or remember Docker interfaces.
If by any chance you are making services listen on 0.0.0.0 and covering them up with a firewall that's very bad practice.
There's no alternative for 0.0.0.0 and a firewall if you're e.g. using kubernetes.
I'm fairly sure you can find an alternative to whatever problem you're having.
You need to be able to have multiple nodes in one LAN access ports on each others' containers without exposing those to the world and without using additional firewalls in front of the nodes.
That's why kubernetes ended up removing docker support and instead recommends podman or using containerd natively.