Security Basics mailing list archives
FW: cisco IOS firewall terminating pptp
From: "Mark Lewis" <mark () mjlnet com>
Date: Wed, 24 Nov 2004 22:44:20 -0000
Andrew, *From your brief description*, it seems likely that you are running into an issue with the PPTP data tunnel (PPTP, as you may know consists of a control channel which uses TCP port 1723, and a data tunnel that uses Enhanced GRE). The issue is as follows: the remote access client (say an XP box) and your IOS box negotiate PPTP tunnel setup on the control channel (using PPTP SCCRQ, SCCTP, OCRQ, and OCRP messages). Because the control channel runs over TCP, NAT/PAT boxes typically don't have a problem with it. But because the data tunnel (which transports end user traffic over PPP) runs over GRE (IP port 47), NAT/*PAT* boxes may have problems translating data tunnel packets. The upshot is that the control channel sets up the PPTP tunnel, but then data tunnel transport fails, and the whole PPTP tunnel goes down. You can verify if this is happening in your case by using the 'debug vpdn l2x-packets'/'debug vpdn l2x-events' and 'debug ppp negotiation' on your ios box [but check cpu load 1st using 'show proc cpu'!]. If you see the SCCRQ/SCCRP/OCRQ/OCRP control channel messages, but PPP negotiation fails then the issue described here is likely the one you are running into. PPP messages are the first traffic frames sent over the data tunnel, so if you don't see them (or just one or two), then it's *likely* that there is indeed a problem translating data tunnel messages (though it could also *possibly* be a simple PPP negotiation/ios virtual template issue). If you are really curious, you can also watch PPP negotiation from the Microsoft client side by enabling PPP logging (see Microsoft KB article 234014 at www.microsoft.com). Anyway, Cisco IOS supports 'regular' 1-1 NAT, but support for PAT with PPTP was only added in IOS 12.1(4)T. So, double check that you have a version of IOS that supports PPTP & PAT (no explicit command is necessary to enable support). See the following website for a Cisco explanation: http://www.cisco.com/en/US/tech/tk827/tk369/technologies_configuration_examp le09186a00800949c0.shtml Phew! Hope that helps... Mark Author: http://www.amazon.com/exec/obidos/tg/detail/-/1587051044/
-- Original Message -- Subject: cisco IOS firewall terminating pptp Date: Mon, 22 Nov 2004 16:44:08 -0000 From: "Andrew Shore" <andrew.shore () holistecs com> To: <firewalls () securityfocus com> Guys, I have a cisco ISO firewall router terminating pptp vpn for remote access. This works fine for dial-up users and users using adsl modems as the source address is not natted. However, if the source address is natted the VPN
fails
to connect. I know that on the PIX there is an IP NAT TRANSLATE command with gets over this problem but I can not find an equivalent command for IOS. Any help greatfully received. Andy
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Current thread:
- FW: cisco IOS firewall terminating pptp Mark Lewis (Nov 24)
- Re: FW: cisco IOS firewall terminating pptp Jamie Schmidt (Nov 27)
- <Possible follow-ups>
- RE: cisco IOS firewall terminating pptp Andrew Shore (Nov 25)