Point-to-Point on PIX

In this segment from the Cisco Press book, Cisco Secure Internet Security Solutions, you'll learn commands and parameters for configuring your VPN with Point-to-Point Tunneling Protocol for a PIX Firewall.

 By Cisco Press
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Cisco Secure Internet Security Solutions - Chapter 4
by Andrew Mason, Mark Newcomb

Cisco Secure PIX Firewall - Part 7
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VPN with Point-to-Point Tunneling Protocol (PPTP)
Starting with Version 5.1 of the PIX IOS, Cisco provides support for Microsoft PPTP VPN clients as an alternative to IPSec. Although PPTP is a less secure technology than IPSec, PPTP is easier to configure and maintain. PPTP also enjoys a great deal of support, especially from Microsoft clients. The PPTP is an OSI Layer 2 tunneling protocol that allows a remote client to communicate securely through the Internet. PPTP is described by RFC 2637. The PIX Firewall only supports inbound PPTP, and only a single interface can have PPTP enabled at any given time. PPTP through the PIX has been tested with Windows 95 using DUN1.3, Windows 98, Windows NT 4.0 with SP6, and Windows 2000.

The PIX Firewall supports Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), and Microsoft Challenge Handshake Authentication Protocol (MS-CHAP), using an external AAA server or the PIX local username and password database. Point-to-Point Protocol (PPP) with Combined Packet Protocol (CCP) negotiations with Microsoft Point-To-Point Encryption (MPPE) extensions using the RSA/RC4 algorithm and either 40- or 128-bit encryption is also supported. The compression features of MPPE are not currently supported.

To enable PPTP support, you first need to have the PIX configured to allow and deny packets in the normal fashion. The interfaces must be configured and the passwords set. After this is accomplished, you can add additional features. The sections regarding VPN in this chapter do not show all of the commands necessary to configure the PIX. Instead, this section concentrates on those commands that require configuration changes from previously shown examples or that are new commands.

Take a moment to look at Figure 4-10. Notice that the VPN tunnel is terminated on the outside interface of the PIX. Although you could terminate the VPN on the perimeter router, there are a few reasons why terminating at the PIX is preferred. The first reason is that the PIX is optimized for security operations, including VPN termination. The PIX is able to handle a much larger number of VPN terminations than most routers. The second reason is that if you terminate on the perimeter router, then only the perimeter router ensures security on the packets after the VPN tunnel has been decrypted. Because the PIX is considered the primary defense, it makes logical sense to keep packets encrypted all the way to the PIX, even if the perimeter router is running the PIX Firewall IOS.

Figure 4-10: PIX PPTP VPN

(Click image for larger view in a new window)

This article was originally published on Oct 17, 2001
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