What NetworkManager does to VPN devices?

[Dan Fruehauf <malkodan gmail com>]

G'day,

I hope that post will not be long, but I've spent a few hours trying to narrow down the problem so I can provide as much information without wasting anyone's time.

I started to debug why NetworkManager-ssh (which I maintain) does not allow traffic through interfaces (tun interfaces usually) and went down a deep rabbit hole, which I'm not entirely sure has got to do much with NetworkManager-ssh at the moment.

What I'm trying to do:

 * Setup a "poor man's VPN" aka SSH VPN to a remote host (on AWS)

 * On my machine I should eventually have a tun device with 172.16.40.2

 * On the server machine a tun device with 172.16.40.1

 * Those two internal addresses should be reachable from one another

The steps that are usually necessary are:

 1. SSH to remote machine (with tunnel creation parameters) + run a ifconfig command to configure the tun device

 2. Configure the tun device on the client host with ifconfig (ifconfig tun0 ...)

 3. Replacing default routes etc

So when NetworkManager-ssh does what it does, the end result is what I expect, except that things don't work. Traffic can reach 172.16.40.1, but nothing can reach 172.16.40.2. Before you point a problem with the server routing tables, please read along.

I decided to just run the SSH command from the command line, eliminating everything that NetworkManager does to the VPN connection. What happens here is very unclear to me, but someone might be able to explain it quickly. When running the steps 1 & 2 quickly one after the other - the VPN is setup properly and 172.16.40.2 is reachable from the server side. When I see quickly, I mean no delay in between, see log for that here (https://paste.fedoraproject.org/paste/QgnyCz7hEuvuLiAiOhtTfl5M1UNdIGYhyRLivL9gydE=). Then I can obviously proceed to replacing the default route and so on and so forth. VPN works.

On the other hand, if i introduce a delay of say 5 seconds, it allows NetworkManager to have enough time to do something I don't understand to the tun0 device which then renders it as unreachable, see log for that here (https://paste.fedoraproject.org/paste/0~lxbpwNuIxGUvDwjokW6F5M1UNdIGYhyRLivL9gydE=). I spaced it out a bit where the 5 seconds gap appears.

A few other things to note:

 * I'm running fedora 24 (with kernel 4.9.9-100.fc24.x86_64 and NetworkManager 1.2.6-1)

 * Selinux is disabled (both on server and client)

 * firewalld is disabled

 * When sniffing traffic, all traffic reaches the server (172.16.40.1)

 * Traffic comes back on on the SSH tunnel, but never reaches 172.16.40.2 (verified it both with strace and by sniffing)

 * Routing tables after the VPN is up are identical in both cases (ip route show table main), except for metric, but I also tried to modify it to be the same and it didn't help either

So what I'm looking for here is to understand *what* NetworkManager does to the tun0 interface that renders it unreachable? And why if I don't allow NetworkManager do its thing and configure the interface quickly - things actually work?

My end result is to fix NetworkManager-ssh to do what it should do, however at the moment I'm puzzled as to why this simple scenario doesn't even work out of NetworkManager-ssh.Any help more than appreciated.

Cheers,

Dan Fruehauf.