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I have set up a PPTP VPN on Ubuntu 12.04, and it seems to be working. I am able to connect to it from a remote device, and once connected, I can see the VPN Server, SSH into it, and so on, all without any problem. So clearly it is working.

However, I cannot access any other devices on the same network. I have set up IP forwarding by uncommenting the line net.ipv4.ip_forwarding=1 and restarting syctl. I thought that I had opened the firewall to allow it, but something is not working, and the only device I can access on the remote network is the VPN server itself. I actually have another VPN Server on a different network set up similarly, except it is working, so clearly something is different.

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Could be as simple as a routing issue. Check the netmask and the routes on the client ("ifconfig pppX; route" for a Unix-y machine, probably ipconfig /something on Windows) to make sure packets from the client from the wider network would be sent down the link. Then check the other way around - would the routing table on the network devices forward packets for the client's IP address to the VPN server ..

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  • That helped me find the correct source of the problem, thank you. My routing in the client was set (correctly) to forward everything on the vpn network through the appropriate router, so it wasn't that. When it picks up an ip address for the vpn, the ip address is correct, but the net mask is 255.255.255.255, which explains why I can only access the vpn server. The ip address is set automatically by DHCP. Any idea on how I can use DHCP for the clients, but still get a correct net mask from the vpn server? Jan 28, 2014 at 23:51
  • Hmm, which PPTP server software are you using? IP address allocation for PPTP clients is normally done by pptpd or pppd, not DHCP .. can you explain your network architecture in a little more detail?
    – Steve Dee
    Jan 29, 2014 at 11:05
  • Sorry, the client accepts the IP address using DHCP, but it is set by pptpd. The standard DHCP server on the network is not involved at all. The IP addresses cannot be set statically on the clients, because then they would need to manually co-ordinate IP addresses. Does that make more sense? Jan 29, 2014 at 22:46
  • Do you really mean "DHCP" (i.e. the specific protocol), or do you just mean dynamically? IP address negotation is part of the PPP protocol, on which PPTP is based. Try setting the netmask option in /etc/ppp/options, or whatever other option file is being passed to ppp (with the -o option to pptpd or the option directive in pptpd.conf.) If the addresses you are assigning are on the the same subnet as local network, I guess you'll need the proxyarp option as well..
    – Steve Dee
    Jan 29, 2014 at 23:20
  • It's supposed to be set dynamically. The client (Ubuntu) is set to Automatic. I have set proxyarp, and the last line of the options file is netmask 255.255.255.0, but the netmask is still showing up as 255.255.255.255 for the client. Thanks for the help, but for some reason it still is not working. Any idea why? Feb 19, 2014 at 19:32

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