Response traffic not going out on local interface with Local Network Sharing enabled [Linux]

[nicolaipre]

Is it a bug?

• [X] I know this is an issue with the app, and contacting Mullvad support is not relevant.

I have checked if others have reported this already

• [X] I have checked the issue tracker to see if others have reported similar issues.

Current Behavior

I have a VM in a certain VLAN which is running Mullvad VPN. To reach this VM I use a jump host (which sits in a different VLAN) and SSH into it. To be able to use Mullvad VPN while also being able to SSH into the VM, I enabled Local Network Sharing (10.0.0.0/8 covers both my VLANs).

I am using this exact same setup for a Windows VM and it works excellent. For my Linux VM with the same Mullvad VPN app config however, it does not work. While debugging the problem I discovered from tcpdumping that my SSH requests from the jump host actually reach the VM in question on the local interface (eth0), but the SSH response traffic is going out on the Mullvad VPN interface (tun0).

I have confirmed that Local Network Sharing is enabled which should allow this response traffic to flow over the local inteface (eth0), but my suspicion is that this is not respected by Mullvad. I have also tried adding routes manually to resolve the problem without much luck so far.

Expected Behavior

Allow response traffic for local subnets if Local Network Sharing is enabled.

Steps to Reproduce

1. SSH from jump host in separate VLAN to VM without Mullvad VPN running to confirm you are able to reach the VM.
2. Ensure Local Network Sharing is enabled in VPN Settings and connect to Mullvad VPN.
3. Run a tcpdump on eth0: $ tcpdump port 22 -i eth0. You will then see incoming SSH traffic, but no responses.
4. With Mullvad VPN still connected from previous steps, run a tcpdump on tun0: $ tcpdump port 22 -i tun0. You will now see the response traffic being sent over the Mullvad VPN link instead of the local interface.

Failure Logs

Not necessarily relevant, but can be provided upon request.

Operating system version

Kali Linux 2024.3 (Debian based)

Mullvad VPN app version

2024.5

Additional Information

No response