I'm trying to connect to a PPTP server which is sat behind a Cisco 881 NAT router. The server is running Ubuntu Server 12.04 and is running Poptop pptpd as the PPTP daemon listening for connections.
As discussed in my other question, I'm trying to setup a Cisco 881 router to replace my old Linksys WAG320N. This same server and WAN connection worked fine with the WAG320N with no special configuration, other than allowing 1723 in through the firewall.
On the Cisco 881, I'm using the newer ip nat enable or NAT NVI to setup static routes in through the firewall for the services running behind the router. My reason being that I can't run another copy of my live DNS domains internally with local IP addresses in. For the purposes of this question, though, I have rebuilt the router with ip nat inside/outside style NAT'ing, but this issue is still apparent. HTTP/SMTP/IMAP etc. all work ok from both the WAN and LAN interfaces of the router. I'm only having issues with SIP (see other question) and PPTP.
My issue is that the GRE doesn't appear to be passing through NAT correctly and one end of the connection is not receiving GRE traffic when it should be, so the server hangs up the connection.
Here's an example of /var/log/syslog with debug enabled in /etc/pptpd.conf:
As far as Cisco are concerned, all I need is ip nat source static tcp <SERVER LAN IP> 1723 interface FastEthernet4 1723 but of course this doesn't seem to the be helping the GRE traffic through as it should.
Trying the connection to the LAN IP of the server from the same LAN as the server (behind the router), the PPTP connection works fine, so I'm confident that the server's config is ok. Furthermore, all I needed on my WAG320N was to open 1723 in the firewall.
Here's my current router config:
I appreciate that there are already a few similar questions on Server Fault: pptpd not working externally on Ubuntu Server 11.10, pptp cannot connect. They are not completely resolved and I thought it best not to hijack someone else's questions. I have tried to use the suggestions from the answers on the other questions, but I still need some help to get this working. Apologies if this wasn't the correct thing to do.
The only progress that I have been able to make on this issue is that I'm confident that the issue is caused by the GRE tunnels (which are required for the PPTP connection to complete) are being blocked. When attempting a connection, I can see in show ip nat nvi translations that both a TCP translation on 1723 is setup and also a GRE translation is setup also. I appear to be able to see GRE related packets on the LAN that the server is on, so I am lead to believe that the server is sending(?) GRE packets, however running Wireshark on a client PC when attempting a connection shows absolutely no GRE packets.
Whilst there are no configuration directives in my config posted above (that I can pin point) which would specifically block them, it would appear that the GRE packets are not being allowed in/out of the router's firewall, even though a NAT translation entry is setup to the server's LAN address. Would anyone be able to provide me with some help to ensure that GRE packets are not blocked by the router's firewall, so that this can be ruled out as a possible issue please?