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If my workstation has a VPN connection to a given network (a client's, for instance) and an open connection, what determines how a request for a network resource (e.g. a web page) is routed? Moreover, given that a resource (e.g. google.com) could be available via either route (i.e. VPN or non-VPN), how is this route determined? Is there a way to 'force' the routing to use a given route or the route with lower overhead?

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This depends on two things:

  • The default gateway. Is it on the other side of the VPN tunnel, or is it on the clients local LAN.
  • The metric of each connection. This only comes into play if there is two routes to the same site. In the case of Google, it will always be your default route... unless you are google.

The only way to force a route, would be to add a static route to specific network to be through a specific router. Otherwise, your default route/gateway will be used.

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How does one determine the 'default' gateway? So, something on the workstation compares 'connection speed' of the VPN to that of the non-VPN connection? So, unless it is forced to use the 'slower' connection (probably the VPN) it will always use the non-VPN connection? I'm assuming there is also 'is the IP address on the VPN or not?' consideration as well, yes? So, theoretically, if the non-VPN connection's speed drops below the VPN's (I can't imagine this happening, btw), then all traffic would be routed over the VPN? –  craibuc Mar 12 '10 at 19:27
    
@craibuc. You can see this on a Windows machine by typing "route print". No, the OS has no knowledge (without more software) of the fastest route to it's destination. All it knows is whether the destination is local, statically routed, or it's default/last chance route, the default gateway. –  Scott Lundberg Mar 12 '10 at 19:55
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If my workstation has a VPN connection to a given network (a client's, for instance) and an open connection, what determines how a request for a network resource (e.g. a web page) is routed?

depends on the VPN Client. If your VPN client is configured to allow split tunneling then traffic destined for an internet page will go out through your local internet connection. If it's a page internal to the VPN tunnel, then it will go through the vpn adapter.

Moreover, given that a resource (e.g. google.com) could be available via either route (i.e. VPN or non-VPN), how is this route determined?

If split tunneling is allowed then only internal desitnations are going to go through the VPN adapter.

Is there a way to 'force' the routing to use a given route or the route with lower overhead?

By now you should have figured this out but yes you can choose to either allow or deny split tunneling.

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So, if no split tunneling, then all traffic is via the VPN, correct? I've seen settings that read something like 'route all traffic through the VPN'; this is essentially disabling split tunneling, right? –  craibuc Mar 12 '10 at 19:30
    
@craibuc. Yes, that is correct. non-split tunnel routes ALL traffic through the VPN. –  Scott Lundberg Mar 12 '10 at 19:56
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