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OS X: connect to a VPN using the same subnet on local and remote network

Posted on 2010-11-21
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Last Modified: 2012-05-10
Hi,

I'm trying to connect a MacBook Pro using OS X to a Windows PPTP VPN at work with the same subnet on both sides.

In XP I would just uncheck the box "use default gateway for remote network" in TCP/IP-settings to get it work.

The option "send all traffic through VPN-connection" in OS X doesn't seem to help.

I am aware that this can be avoided by using different subnets, but that is not what I'm asking for.

Any idea how I can make this work?
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Question by:alexandrus
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by:et01267
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Which aspect of the connection is failing?  There are a zillion things that can go wrong with a VPN connection, particularly between different vendors.  The subnet issue may be a red herring; there may be some protocol setting mismatch or something more fundamental.  Usually, I find that I need to use a payware VPN client, like VPN Tracker

What errors or messages do you see?  Have you checked the console logs (Applications/Utilities/Console) to see what's happening?
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by:alexandrus
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As i wrote, the problem is the same subnet, making the IPs on the remote site unreachable (i.e. I can only ping local IPs). The VPN connection is working perfectly fine (from a network with a different subnet).

It's just a routing problem. How to make this work on a Mac?
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by:alexandrus
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To make it clearer:

The behaivor I look for (and which I achieve in Windows using the setting mentioned above) is disabling the local network and routing all TCP/IP to the remote network.
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by:et01267
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And you are sure that it works on the Mac when you have different subnets?
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by:alexandrus
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Yes
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by:et01267
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What IP address is your Mac using on the local Lan?  Are you sure this address is not being used on the remote network?  
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by:alexandrus
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Local IP: 192.168.0.3
Remote IP: 192.168.0.115

both unique.

But why would the local IP matter? Only the virtual adapter IP is part of the remote network. So as long I don't want to access exactly this IP on the remote net it might also be present on both nets.

I think you didn't understand my problem.. Right now I can still ping my local network when I am connected trough VPN. If subnets are the same in local and remote net, how should the OS know how to route a packet to let's say 192.168.0.50 (which might be even present in both nets). All I want is to force routing on the mac through the VPN-adapter.
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by:njxbean
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Have you tried using another VPN client?
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by:alexandrus
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No, but I would try it if I knew one which can fix that routing problem.
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by:njxbean
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Only one way to find out.

Give this a try.  its free  Maybe there will be an equivalent setting to force it to use the remote networks default gateway.  

http://openvpn.net/
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by:njxbean
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fyi it looks like openvpn does have that option.


One interesting capability of the OpenVPN tunnel is to optionally allow the client to pass all traffic through the VPN tunnel when connected. For example, when you are using an unsecure WiFi hotspot, you might want to ensure that eavesdroppers are not able to monitor your WiFi traffic. One way to do this is to route all your WiFi traffic through the encrypted VPN tunnel, using the "redirect-gateway" option on the server or the client. When configured on the server, all clients will reroute all their traffic through the VPN. The "redirect-gateway" option requires pushing two pieces of information from the server to the client, the remote gateway and DNS IP addresses.
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by:alexandrus
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Had a short try with OpenVPN, but it seemed like to much of a hassle and I ended up changing one of the subnets.

Still, the initial question stands.

Does anyone know how to fix this issue with the OS X built-in VPN client?
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et01267 earned 500 total points
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Try this:  Set your network interface ("service order") to have the VPN connection on top, as follows:

1. Go to System Prefs -> Network and click the gear icon at lower left.
2. Select the "Set Service Order" choice from that menu, and then drag the VPN to the top.

Now experiment with the "send all traffic..." option in the vpn settings.
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by:alexandrus
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Thanks for your suggestion.

Unfortunately I don't have the MacBook anymore to try it.
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