Linux Networking Cookbook

10.10. Troubleshooting PPTP

Problem

You're having trouble establishing a connection from a Windows client to your Linux Poptop server. What do you do?

Solution

First, make sure your pptp server is running with the netstat command:

	# netstat -untap | grep pptp
	tcp 0 0    0.0.0.0:1723   0.0.0.0:*   LISTEN   12893/pptpd

Then, use the good old ping command to test connectivity. When that's established, your Windows client error messages can be helpful. Figure 10-2 shows what it looks like on Windows XP when the server is unreachable.

You can take the number of the error message and look it up online, because Windows uses the standard Remote Access Server (RAS) error codes.

Next, make sure your firewall isn't blocking your VPN. The easy but scary way is to turn it off. Another way to do this for an iptables firewall is to run the fw_status script (see Chapter 3), and look for lines like these:

	Chain PREROUTING (policy ACCEPT 74530 packets, 7108K bytes)
	num   pkts bytes target     prot opt in     out     source        destination
	1  0  0 DNAT   tcp  --  eth1 any  anywhere   foo.net tcp dpt:1723 to:192.168.1.10
	2  0  0 DNAT   gre  --  eth1 any  anywhere   foo.net to:192.168.1.10
	7  0  0 ACCEPT tcp  --  eth1 eth0 anywhere   xena.alrac.net  tcp dpt:1723 state
	NEW,RELATED,ESTABLISHED
	8  0  0 ACCEPT gre  --  eth1 eth0 anywhere   xena.alrac.net  state
	NEW,RELATED,ESTABLISHED

You can check your destination address, state matches, interfaces name, and protocol matches.

Enabling the dump and debug options in /etc/pptpd.conf generates bales of helpful output in ...