Debian 12.1 upstream networking breaks in SSH sessions after running any Docker app

N00b to Docker here - sorry to ask but I’m not actually sure what to search for or what to start with…

Debian 12.1 host - SSH is stock (OpenSSH_9.2p1). Installed Docker from standard instructions on the main site (so current release as of 29-08-23)…

I SSH into the system and do this – all is fine. Then after I run any Docker application (just running “ubuntu bash” for the test) – suddenly all subsequent SSH sessions lose upstream network access but I can still SSH into the host and existing sessions continue, just their own access to network resources breaks. This only happens for SSH sessions – if I’m at the machine’s console no issue, no loss of network access. If I run a Docker image from the machine’s console – this also didn’t seem to break network access for existing or future SSH sessions.

My intended use case is to run the server headless on a Debian 12.1 host on baremetal (I’m doing my testing in ESXi 6.5 guests) – I ran Ubuntu Server, Mint and Debian 11.7 to test this problem, only Debian 11.7 had the same behaviour and I’ve seen mentions that I think may be related but I’m not actually quite sure what to search for or where to actually begin.

I suspect the problem is with Debian which TBH really surprised me; given all this can anyone point me in the right direction? I don’t quite know what to look for.

Can you explain this with more details?

After running the docker application - I can no longer connect to any network resources (zero traffic out) from subsequent SSH sessions on the same host. Happens for all users, that said if I start an terminal session on the same host (no SSH) it works. It seems to tie in to the SSH daemon - it’s like the sessions conducted via that daemon lose access upstream. They can’t ping, lynx can’t browse, can’t traceroute, the works. They still work locally but anything off localhost is dead.

Seems similar to a problem I have here … initially networking works in container, but after maybe 10 seconds or so of network traffic a couple extra routes show up to dev veth* which breaks routing for both container and host.

Debian 12.4, docker-ce v25.0.3 build 4debf41