Re: What NetworkManager does to VPN devices?

[Dan Fruehauf <malkodan gmail com>]

Thanks Thomas for the reply.

I'm en route to fix the problems with NetworkManager-ssh, yet I'm not quite sure what it doesn't do right. Or in other words - what changed in nm that makes the whole thing not work properly.

Previously it would use ifconfig to configure the interface after the VPN is up. Now NetworkManager does that. I've been looking also through the openvpn plugin to see what has changed recently. Generally, the routes and interfaces are setup correctly, yet, as I said, there's no connectivity between the 2 VPN interfaces (client and server).

If you can come up with any useful advice, it'd be more than welcome. I can also find you on IRC if you think that might be easier.

Cheers,

Dan.

On Thu, Apr 13, 2017 at 4:32 PM, Thomas Haller <thaller redhat com> wrote:

On Tue, 2017-04-11 at 09:17 +1000, Dan Fruehauf wrote:
> 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.fedora
> project.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~lxbpwNuIxGUvDwjokW
> 6F5M1UNdIGYhyRLivL9gydE=). 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.

Hi Dan,


NM notices that a device tun0 appears and "assumes" the connection.
That process should not modify the interface, in order not to interfere
with whoever created and manages the device. It seems that doesn't work
well here.


It's not clear to me what NM does to interfere with the tun device. But
it would be interesting to see that setting tun0 as unmanaged
avoids the problem.
like:
  [keyfile]
  unmanaged-device=interface-name:tun0
and `killall -SIGHUP NetworkManager`, and reactivate the SSH VPN, and
notice tun0 as unmanaged in `nmcli device`.


With upcoming 1.8, NM was changed to improve the situation here
(https://bugzilla.gnome.org/show_bug.cgi?id=746440).
It would also be interesting to see how 1.8 works there.
It's actually very simple to build a RPM for Fedora of upstream NM, see
  https://wiki.gnome.org/Projects/NetworkManager/Hacking


best,
Thomas