Yes, this is where docker's limitations begin to show, and people begin looking at tools like Kubernetes, for things like advanced, granular control over the flow of network traffic.
Because such a thing is basically impossible in Docker AFAIK. You're getting these responses (and in general, responses like those you are seeing) appear when the thing a user is attempting to do is anywhere from significantly non trivial to basically impossible.
An easy way around this, if you still want to use Docker, is addressing the below bit, directly:
no isolation anymore, i.e qbit could access (or at least ping) to linkwarden’s database since they are all in the same VPN network.
As long as you have changed the default passwords for the databases and services, and kept the services up to date, it should not be a concern that the services have network level access to eachother, as without the ability to authenticate or exploit eachother, there is nothing that they can do, and there are no concerns.
If you insist on trying to get some level of network isolation between services, while continuing to use Docker, your only real option is iptables* rules. This is where things would get very painful, because iptables rules have no persistence by default, and they are kind of a mess to deal with. Also, docker implements their own iptables setup, instead of using standard ones, which result in weird setups like Docker containers bypassing the firewall when they expose ports.
You will need a fairly good understanding of iptables in order to do this. In addition to this, if you decide this in advance, I will warn you that you cannot create iptables rules based on ip addresses, as the ip addresses of docker containers are ephemeral and change, you must create rules based on the hostnames of containers, which adds further complexity as opposed to just blocking by ip. EDIT: OR, you could give your containers static ip addresses.
A good place to start is here. You will probably have to spend a lot of time learning all of the terminology and concepts listed here, and more. Perhaps you have better things to do with your time?
*Um, 🤓 ackshually it's nftables, but the iptables-nft command offers a transparent compatibility layer enabling easier migrations from the older and no longer used iptables
EDIT: And of course nobody has done this before and chatgpt isn't helpful. These problems are the kinds of problems where chatgpt/llm's begin to fall apart and are completely unhelpful. Just "no you're wrong" over and over again as you have to force your way through using actual expertise.
You can block traffic to a Docker container by its hostname using iptables, but there’s an important nuance: iptables works with IP addresses, not hostnames. So you’ll first need to resolve the container’s hostname to its IP address and then apply the rule.
You’re right—container IPs change, so matching a single IP is brittle. Here are robust, hostname-friendly ways to block a container that keep working across restarts.
Exactly — good catch. The rule:
sudo iptables -I DOCKER-USER 1 -m set --match-set blocked_containers dst -j DROPmatches any packet whose destination is in that set, regardless of direction, so it also drops outgoing packets from containers to those IPs.
You’re absolutely right on both points:
With network_mode: "container:XYZ", there is no “between-containers” network hop. Both containers share the same network namespace (same interfaces, IPs, routing, conntrack, and iptables). There’s nothing to firewall “between” them at L3/L2—the kernel sees only one stack.
Alright I will confess that I didn't know this. This piece of info from chatgpt changes what you want to do from "significantly non trivial" to "basically impossible". This means that containers do not have seperate ip addresses/networking for you to isolate from each other, they all share a single network namespace. You would have to isolate traffic based on other factors, like the process ID or user ID, which are not really inherently tied to the container.
As a bonus:
Docker’s ICC setting historically controls inter-container comms on bridge networks (default bridge or a user-defined bridge with enable_icc=). It doesn’t universally control every mode, and it won’t help when two containers share a netns.
Useful for understanding terminology I guess, but there is a class of these problems these tools really struggle to solve. I like to assign problems like this to people and then they will often attempt to use chatgpt at first, but then they will get frustrated and quickly realize chatgpt is not an alternative for using your brain.
This is untrue, proxmox is not a wrapper around libvirt. It has it's own API and it's own methods of running VM's.