Pptp Inspection; Radius Accounting Inspection - Cisco PIX 500 Series Configuration Manual

PPTP Inspection

PPTP Inspection
PPTP is a protocol for tunneling PPP traffic. A PPTP session is composed of one TCP channel and
usually two PPTP GRE tunnels. The TCP channel is the control channel used for negotiating and
managing the PPTP GRE tunnels. The GRE tunnels carries PPP sessions between the two hosts.
When enabled, PPTP application inspection inspects PPTP protocol packets and dynamically creates the
GRE connections and xlates necessary to permit PPTP traffic. Only Version 1, as defined in RFC 2637,
is supported.
PAT is only performed for the modified version of GRE [RFC 2637] when negotiated over the PPTP TCP
control channel. Port Address Translation is not performed for the unmodified version of GRE [RFC
1701, RFC 1702].
Specifically, the security appliance inspects the PPTP version announcements and the outgoing call
request/response sequence. Only PPTP Version 1, as defined in RFC 2637, is inspected. Further
inspection on the TCP control channel is disabled if the version announced by either side is not Version
1. In addition, the outgoing-call request and reply sequence are tracked. Connections and xlates are
dynamic allocated as necessary to permit subsequent secondary GRE data traffic.
The PPTP inspection engine must be enabled for PPTP traffic to be translated by PAT. Additionally, PAT
is only performed for a modified version of GRE (RFC2637) and only if it is negotiated over the PPTP
TCP control channel. PAT is not performed for the unmodified version of GRE (RFC 1701 and
RFC 1702).
As described in RFC 2637, the PPTP protocol is mainly used for the tunneling of PPP sessions initiated
from a modem bank PAC (PPTP Access Concentrator) to the headend PNS (PPTP Network Server).
When used this way, the PAC is the remote client and the PNS is the server.
However, when used for VPN by Windows, the interaction is inverted. The PNS is a remote single-user
PC that initiates connection to the head-end PAC to gain access to a central network.

RADIUS Accounting Inspection

One of the well known problems is the over-billing attack in GPRS networks. The over-billing attack
can cause consumers anger and frustration by being billed for services that they have not used. In this
case, a malicious attacker sets up a connection to a server and obtains an IP address from the SGSN.
When the attacker ends the call, the malicious server will still send packets to it, which gets dropped by
the GGSN, but the connection from the server remains active. The IP address assigned to the malicious
attacker gets released and reassigned to a legitimate user who will then get billed for services that the
attacker will use.
RADIUS accounting inspection prevents this type of attack using by ensuring the traffic seen by the
GGSN is legitimate. With the RADIUS accounting feature properly configured, the security appliance
tears down a connection based on matching the Framed IP attribute in the Radius Accounting Request
Start message with the Radius Accounting Request Stop message. When the Stop message is seen with
the matching IP address in the Framed IP attribute, the security appliance looks for all connections with
the source matching the IP address.
You have the option to configure a secret pre-shared key with the RADIUS server so the security
appliance can validate the message. If the shared secret is not configured, the security appliance does
not need to validate the source of the message and will only check that the source IP address is one of
the configured addresses allowed to send the RADIUS messages.
Cisco Security Appliance Command Line Configuration Guide
25-60
Chapter 25 Configuring Application Layer Protocol Inspection
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