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I am trying to conceptualise how networking works under the hood for linux VPNs which use TUN interfaces.

My current best guess is as follows (please correct me):

  1. Connection established to remote client/server.
  2. TUN interface created and brought up
  3. Routing table updated to set default gateway to the TUN interface

But wouldn't the packets destined for the remote client/server end up going into the TUN interface and forming a loop of sorts? How do VPN systems solve this? What is the gap in my understanding?

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  • The default gateway won't be vai TUN, I suppose Jan 11 '16 at 17:28
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You are correct that with purely destination-based routing this is a problem, if the destinations you are reaching through the tunnel overlap with routing needed for tunnel establishment, etc....

The way I have usually seen this done, and done it myself on various routers, is to use policy routing :

  • The router acting as a VPN endpoint keeps its default route pointing to the Internet through its ISP link
  • It also has a Policy Route with a source-based rule saying that traffic coming from the subnets behind it, whatever their destination, should be sent through the tunnel.
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With destination based routing it's not hard either.

The way I usually see is to load a route to the VPN server specifying the pre-existing gateway and interface and distance (really routing priority) set to 1. The VPN's default route would always have a distance of at least 2.

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  • Note: If the gateway is shared with other users, and capable of port forwarding, this leaves a security hole where connections to another user's forwarded port on the gateway won't go through the VPN.
    – user253751
    Jan 11 '16 at 23:32
  • True, but it's hard to make that mistake by accident because DNS resolution now passes through the VPN so you can only reference the gateway by IP; asking for its name yields its TUN IP.
    – joshudson
    Jan 11 '16 at 23:34
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Imagine a system with 1 physical adapter: enp2s0.

It's routing table might start out as all traffic goes out enp2s0.

Once the system connects to a VPN, a TUN interface (tun0) is initialized and the VPN updates the routing table: All traffic destined to VPN server address on port X (VPN server address and port) goes out enp2s0. All other traffic goes through tun0.

Of course a VPN doesn't have to route all traffic. For instance, I have set up VPNs before where only traffic that would go to a certain private subnet goes through the VPN so normal internet traffic would be unchanged. How you do this changes based on the VPN program.

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