Re: Connecting back to the host through a dummy veth interface

On Sat, 20 Dec 2025 15:28:43 +0100

Felix Rubio wrote:

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Hey Stefano,

Thank you for your answer! I know I can run rootful containers, and that

Ok, things are starting to get clear. The problem was, I think, between the desk and the keyboard. * I have everything on a VM that I configure with Ansible. I have just taken everything down and started from scratch * I still have my containers without any ad-hoc network. They are binding only to network interface 10.255.255.1, which is a dummy ethernet. * My error was that I am running an LDAP server in one of these containers, and I was checking if it was working with a ldapwhoami. The client was replying that could not reach the server, which triggered all subsequent investigation, but the real cause was that the certificate offered by the server was not trusted by the client, and the latter broke the connection (without giving a more proper message - facepalm). Once fixed the problem with the certificates, everything seems to work. This means that: * I have a dns server in 10.255.255.1 that resolves ldap.host.internal to 10.255.255.1 * ldap server rootless container is listening to 10.255.255.1:1636 * ldap client is in another rootless container, and can reach directly ldap.host.internal:1636. ... Is this last point expected? the ldap server is started through podman as a regular user, without any network options... nothing fancy. The reason for me asking is that all I have read points in the direction that from a rootless container I should not be able to loopback to the host... but maybe this dummy interface is not identified as "the host" and therefore I can connect to services bound to it? On the LDAP side, the logs show that these connections are coming from the same 10.255.255.1. That would be actually convenient, because then I can put firewall rules in place that prevent connecting from that dummy ethernet back to the host at all. Thank you very much, and sorry for the initial confusing messages. Felix On Sunday, 21 December 2025 11:47:22 Central European Standard Time Stefano Brivio wrote: then

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I can access the host's network ns. However, this exposes a number of potential issues: * In case the an attacker manages to break out of the container, gets root * That enables connecting back to the host loopback, but then from that container any service listening to the loopback can be reached as well.

Sure. That's the whole point behind pasta(1) and rootless containers with Podman / rootlesskit. I certainly won't be the one suggesting that you'd run anything as root. :)

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The reason for looking for a way of binding those services to 10.255.255.1 (so that only exposed services will be in that interface) and running fully rootless, if works, provides a more secure system... in general.

Indeed.

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About the mapped ports, I am a bit lost: for what I have tested, running rootless disables the possibility to connect back to the host, right?

Hah, I see now. No, that's not the case. You can run rootless containers and connect to the host from them, in two ways:

1. disabled by default in Podman's pasta integration, not what you want: via the loopback interface, see -U / -T in 'man pasta' and --host-lo-to-ns-lo for the other way around.

In that case, packets appear to be local (source address is loopback) in the other namespace ("host" or initial namespace for packets from a container, and container for packets from host).

This gives you better throughput but making connections appear as if they were local is risky (cf. CVE-2021-20199), so it's disabled by default, and not what I'm suggesting (at least in general)

2. what you get as default in Podman: using pasta's --map-guest-addr.

The current description of this option in pasta(1) isn't great, hence https://bugs.passt.top/show_bug.cgi?id=132, but the idea is that you will reach the host from the container with a non-loopback address, as if the connection was coming from another host (which should represent the expected container usage).

So here's an example:

$ podman run --rm -ti -p 8089:80 traefik/whoami 2025/12/21 10:42:16 Starting up on port 80

[in another terminal] $ podman run --rm -ti fedora curl host.containers.internal:8089 Hostname: ab94f49b5042 IP: 127.0.0.1 IP: ::1 IP: **.***.*.*** IP: ****:***:***:***::* IP: ****::****:****:****:**** RemoteAddr: 169.254.1.2:46592 GET / HTTP/1.1 Host: host.containers.internal:8089 User-Agent: curl/8.15.0 Accept: */*

...doesn't that work for you? Note that you'll need somewhat recent versions of pasta (>= 2024_08_21.1d6142f) and Podman (>= 5.3).

-- Felix Rubio