PPTP Vpn through a Linux IpTables/Netfilter firewall

I spent some time configuring a pptp vpn for one of our offices yesterday, but it kept failing to connect during testing. Each failure generated these errors in the firewall (the one I was Vpn-ing too) log.

pptpd[9078]: GRE: Bad checksum from pppd.
pptpd[9078]: CTRL: Received PPTP Control Message (type: 15)
pptpd[9078]: CTRL: Got a SET LINK INFO packet with standard ACCMs
pptpd[9078]: GRE: read(fd=7,buffer=80505a0,len=8260)
 from network failed: status = -1 error = Protocol not available
pptpd[9078]: CTRL: GRE read or PTY write failed (gre,pty)=(7,6)

As my comp.sci instructor would say, "Bad Times".

The vpn connection worked from a direct-internet-connected host, so logically the VPN setup was probably right. Here in the office I am behind a Linux IpTables firewall, and I deduced that it was probably the issue. Nothing was set to block the GRE protocol though, so I was a little puzzled.

I found this TLDP post (Do not bother reading it...) on how to make pptp work through iptables. Ah-ha! It was the firewall. Then I realized that documentation was ancient. By Ancient I mean it refers to the 2.0 and "new" 2.2 kernel. The current Linux kernel is > 2.6! eep! It was probably translated from some obscure dead language it is so old.

Anyway, the solution is much simpler... Iptables has a module that allows pptp to pass through NAT. My tale of woe happily resolved with this command.

modprobe ip_nat_pptp

One more useful tidbit. You can see a list of available IpTables modules with this command..

locate netfilter | grep .ko

Good Luck,
-Ellie