@MadMike comprehension is half the solution. If you don't understand how it's working, this requires one solution per configuration. Also recent kernels (5.something) have a per-namespace setting instead of a global all-namespaces-wide setting.

So, let me see if I understand this: In Debian 9, I installed Docker with apt, and ran my machine on the network, and everything was fine. In Debian 12, I'm going to need to go back to school for network engineering PhD to do the same thing? Am I understanding that right? Because if "comprehension is half the solution" then I am several years away from even halfway knowing how to get my network working agin. I'm sure this is all fascinating, but it doesn't begin to tell me how to get my network working and still run Docker like I used to easily do. Is that no longer possible on Debian?

@JohnSmith nothing changes here between Debian 9 and Debian 12. Same tools are available in both (iptables is not deprecated (though iptables-legacy might be) and won't disappear). What makes you think something changed? Can you give more details? libvirt will get problems the same using Debian 9 and Debian 12 (though libvirt might have added counter measures meanwhile: it does handle the NAT issue correctly for example).

Well, I set up a Debian 9 machine a few years ago, and ran Docker, and never had to think about any of this. It worked. I followed whatever instructions I found via google and the whole thing was fairly easy. Now, I installed docker on a fresh install of Debian12, both from official sources, and not only does my networking not work at all anymore, but in what had to be 8 or 10 hours of troubleshooting and googling and scratching my head over SE posts and the Debian Wiki, I am no closer to an answer. So, it seems to me, something has definitely changed.

@JohnSmith If you install Docker and only Docker, and don't do anything with bridges (did you use bridges on Debian 9 before installing Docker?), then I don't see how installing Docker will break things. If you do use other networking technologies but didn't when using Debian 9, then you're comparing apples and oranges.

Ok, thanks for your reply. I'm obviously way over my head in this. I definitely don't have any bridging going on, so maybe I misunderstood the question entirely. The only networking technology I'm using is the same ethernet cable that used to be plugged into my teeny, tiny little Dell Mini 9, which is now plugged into a MacBook pro that boots straight into Debian12, which was able to see my network over that cable just like the Mini 9, until I installed Docker. I'm just utterly exhausted and confused, I've been working on this for 17 hours today alone, and the 8 consecutive days previous.

I think the answer for me is to stick with an operating system that a simpleton like me, who's only been programming since 1979 and doing it professionally since the late 80s, can figure out how to set up with my meager knowledge and skill set.

Sorry. Frayed nerves and exhaustion. This has been the single most negative experience I've had since I started working with computers 44 years ago.

@JohnSmith You might need to ask your own question about it with details (intended to allow to reproduce your problem: it can be rare to suddenly think "oh I know that must be this" without experimenting with the problem first); because the problem you describe doesn't appear to be related to the problem described in this Q/A (which is related to bridging). I wouldn't know if SF is a good choice (rather than SU or UL SE) for this.

@JohnSmith personally I don't run Docker on my personal (including work-related "personal") host system: I run it in a VM to be sure it won't disrupt other tools (LXC, libvirt, ...) running beside it.

Moved to chat to be polite to others. Yeah, the whole reason I just spent 17 hours getting Debian running on a MacBook is because running a VM was an equally big nightmare on it's own. (Ever try to get cut & paste between host & guest enabled on a VMware or VirtualBox VM? I lost, I'm not kidding, four whole days and nights of my life down that black hole before I said, "Hmmmm, maybe relying on VMs is a REALLY BAD IDEA.")

As much as I can I ssh to my VM, without GUI. So I can do cut/pastes from my terminal as usual heh.

As to asking my own ?, I gotta be honest, every time I've ever posted online looking for help with a technical issue, I've sat on my hands without a solution for months, at this point I don't even waste my time with it.

The world is a big place... if there's a solution, somebody will have already posted it, and if nobody has posted it, there probably isn't one. At this point I've discovered I can restart the networking with a manual sudo dhclient enp1s0f0 every time I reboot, which is not a very good solution at all, but it'll get me by for a day or two until I came come up with an escape plan.

But really I see you are ranting, searching everywhere etc. But this solution has to be done methodically, to check you still have network interfaces (drivers), then check their configuration, etc. Requires using tcpdump on all involved sides. This is a debug thing, and Stackexchange is not well geared for doing debugging.

Dude. GUI. It's not 1980 anymore, you don't need to memorize every command and type it in by hand in a CLI. Or, you shouldn't have to. There's menus and panels and things.

How do you use Docker if not with commands?

Yeah, like I said, if this problem requires going back to school, I'm out. I have obviously done plenty of methodical differential testing to the best of my ability, but that's impossible when literally all the information online how to test those things on linux (online is my only way of getting linux information, unless I want to go back to school for it) is bad. Most of the time, I look up how to test something, and the attempt just spits out some random unintelligible error message,

so then I have to methodically test that, but everything I find on line about how to troubleshoot it doesn't work and just gives more intintelligible errors, and next thing you know I've lost 9 days of my life on trying to migrate 1 software package to a new machine, without success.

re docker, My docker container is already made and working, setting it up takes all of 5 minutes in the CLI... but the migration it takes hours of copying files, etc, and I much prefer to copy and paste a file path from an open window than to have to remember every file path and type them in letter by letter. I have no problem dipping into a CLI for quick things, I do it all the time, but you gotta have a GUI or a lot of tasks just take 10x as long and are 10x more liable to transcription errs.

I even will dip into the CLI to use Rsync rather than copy folders on the desktop, if they're big. But, the folder paths, I copy from the desktop into the terminal app, I don't type them out.

As I said, I can cut/paste without problem through an (host) terminal including paths. Through the GUI it requires addons which depends on the hypervisor (vmware, virtualbox or libvirt (I have practical GUI experience only about libvirt, because at work none of the VMs have a GUI, why would they?). So if not using a GUI tool from the VM, I don't see the need to have a GUI on the VM.

Anyway for your problem, you'll have to consider removing Docker after the lengthy backup. It appears (from one of your comments) you installed upstream's Docker rather than Debian-provided Docker. Debian's one, even of less recent, might have behaved differently because it's supposed to be better integrated by those who provided it. If you have to restart dhclient, then something has removed some of the network configuration on the host and that doesn't seem to be a big problem anyway.

Fair enough. Different needs. I also have very frequently this week tripped over Linux commands that just won't run over ssh, they want a screen present for some reason, I'm not really familiar enough with it to work that one out. But that's happened countless times in just the last few days. I assume a seasoned linux pro knows ways around that. But the last time I set up a virtually identical Debian box to what I'm trying to do now, only difference is..... wait, "Debian's docker"?

Aha. Yes. I didn't know debian had their own release. In fact, the troubleshooting instructions I found specifically said to make sure to install on Debian using Docker's repositories and instructions.

I will definitely give that a try. Thanks. I had no idea at all.

guides are more often wrong than correct. Of course Docker (upstream) would recommend to install their release, how could they support problems with one they didn't supply else?

Yeah, well, unfortunately without guides or Q&A sites, both of which seem to mostly be wrong, I have no resources at all.

Although apparently cornering an experienced & knowledgable (and patient) user in conversation sometimes might pan out.

you're in luck, sometimes I take comments like the one you did as an attack on my answer and won't behave so well, that's human nature. But as I couldn't see how my answer could have been the cause of anything here and got some spare time, I did behave.

Anyway... what I was going to say before.... when I set up almost this same exact setup a few years ago, it was not hard. In fact, back then I had to do all the initial; configuration on the docker app, now it runs on it's own, I just had to migrate it. So this should have been easy-peasy. That was a week and a half ago, I said that to myself.

LOL, yeah, I hate being so crabby, but, man, this really is the worst experience of my entire very long career in IT (well, I did have a battle with a scanner driver back in the MacOS 9 days that was noteworthy) and frankly, if I had seen someone driven to the brink of insanity by this sort of stuff posting all over Stack Exchange, I wouldn't have gone down that road... so I justify the snark. Anyway, I do appreciate you taking the time out.

Debian's docker is directly available using apt-get: apt-get install docker.io. But you'd have to remove the other one first (using purge), verify there are no configuration files left, reboot (which means you have to backup your own work first) and then install it.

Yeah, at this point I've fingured out how to pretty well clean out apt packages, in fact if I really need to I can just completely do a clean install (although I'd have to sit and try to remember all the other troubleshooting I went through before noticing the docker problem, there was a bit more wrong that I didn't think to write down because I assumed I'd only be doing it once).

And, nice thing about being an IT pro, somewhere along the line you generally have learn the hard way to make a million backups as you work. So I do have a ton of backups I can roll back to, once I can stomach the thought of losing all the troubleshooting I did since then. But the important data, the app & settings I was trying to migrate, I have a million recent working copies of.

It's even possible this was not Docker but an other action who knows. Looks like reinstalling could be faster than spending more days on it. Just do it a few steps at a time, and verify it's still working (reboot) at each step.

I'll delete my comments from the main site btw.

Well, I found a lot of people complaining about networking issues after installing docker, and I really can't think of anything else that might have done it. I hadn't installed any other packages at all.

Umm, up to you if you want to delete your comments or leave them, I thought it was all perfectly fine.

But, yeah, I'll probably do exactly what you said. Start clean, reboot more often. I'll tell you what, for the few days of hell I spent with trying the VMs, I very quickly got spoiled by being able to take snapshots easily.

Kinda missed it as soon as I got the actual debian install up on the the MacBook. First thing I did when I saw it running was say, "Ok, now, how do I take a snapshot here? Wait.... D'oh!"

Ah, I didn't check to what got moved to chat & what didn't.

Damn. daylight again. Listen, thanks for the tips & interesting convo, I'll let you go.

So, hey... cleaning out Docker, rebooting, and reinstalling from Debian's repo, like you suggested, took all of 3 minutes, and it actually worked. That tip probably saved me hours more work. It all looks good now, Thanks!

Your case might not be an isolated case. So I'll be sure when reading people complain about this, that they tell which Docker they did install. It's possible upstream-provided Docker does break Debian 12.