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PSA: Docker nukes your firewall rules and replaces them with its own. (sh.itjust.works)
submitted 9 months ago* (last edited 9 months ago) by [email protected] to c/[email protected]
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I use nftables to set my firewall rules. I typically manually configure the rules myself. Recently, I just happened to dump the ruleset, and, much to my surprise, my config was gone, and it was replaced with an enourmous amount of extremely cryptic firewall rules. After a quick examination of the rules, I found that it was Docker that had modified them. And after some brief research, I found a number of open issues, just like this one, of people complaining about this behaviour. I think it's an enourmous security risk to have Docker silently do this by default.

I have heard that Podman doesn't suffer from this issue, as it is daemonless. If that is true, I will certainly be switching from Docker to Podman.

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[–] [email protected] 2 points 9 months ago* (last edited 9 months ago) (3 children)

So uh, I just spun up a vps a couple days ago, few docker containers, usual security best practices... I used ufw to block all and open only ssh and a couple others, as that's what I've been told all I need to do. Should I be panicking about my containers fucking with the firewall?

[–] [email protected] 6 points 9 months ago

Probably not an issue, but you should check. If the port opened is something like 127.0.0.1:portnumber, then it's only bound to localhost, and only that local machine can access it. If no address is specified, then anyone with access to the server can access that service.

An easy way to see containers running is: docker ps, where you can look at forwarded ports.

Alternatively, you can use the nmap tool to scan your own server for exposed ports. nmap -A serverip does the slowest, but most indepth scan.

[–] [email protected] 3 points 9 months ago

Docker will have only exposed container ports if you told it to.

If you used -p 8080:80 (cli) or - 8080:80 (docker-compose) then docker will have dutifully NAT'd those ports through your firewall. You can either not do either of those if it's a port you don't want exposed or as @[email protected] says below you can ensure it's only mapped to localhost (or an otherwise non-public) IP.

[–] [email protected] 3 points 9 months ago

Actually, ufw has its own separate issue you may need to deal with. (Or bind ports to localhost/127.0.0.1 as others have stated.)