Intrusion Detection

Intrusion Detection

The Cisco Secure PIX Firewall, like the Cisco Secure IOS Firewall covered in Chapter 7, added intrusion-detection technology to extend the Cisco Secure IDS technology. IDS sensor incorporation into the firewall is ideal for locations requiring additional security between network segments. It can also provide enhanced visibility at intranet, extranet, and branch-office Internet perimeters.

PIX Firewall IDS v6.2 audits (monitors) 53 attack signatures, representing a broad cross section of severe security breaches and the most common information-gathering scans. The PIX Firewall IDS technology auditing is performed by looking at the IP packets as they arrive at an input interface. If a packet matches an active signature, the IDS can perform any or all of the following actions based on the predefined router configuration:

  • Alarm Sends an alarm to a Syslog server and/or a Cisco Secure IDS Director

  • Drop Discards the packet

  • Reset Resets the questionable TCP connection

Any packet that triggers a signature for which the configured action doesn’t drop the packet, can then trigger additional signatures.

PIX Firewall IDS supports both inbound and outbound auditing, as well as interface specific auditing.

Define Default Audit Actions

Use the global configuration mode ip audit command to specify the default audit action(s). Use the no form of this command to set the default action for info signatures. The syntax is the following:

Pix(config)# ip audit {info | attack} {action [alarm] [drop] [reset]}
Pix(config)# no ip audit {info | attack}


For info-type signatures (reconnaissance attacks).


For attack-type signatures.


Sets an action for the info signature to take when a match occurs.


Sends an alarm to all configured Syslog servers. The default option.


Drops the packet.


Drops the packet and closes any related TCP session.0.+.

In the following example, the default action for info and attack signatures is set:

Pix(config)# ip audit info action alarm
Pix(config)# ip audit attack action reset

Disabling Individual Signatures

Use the global configuration mode ip audit signature command to attach a policy to a signature and disable the signature. Use the no form of this command to remove the policy and reenable the signature. The syntax is as follows:

Pix(config)# ip audit signature signature-id disable Pix(config)# no ip audit signature signature-id


Unique integer specifying a signature in the Director Network Security Database.

In this example, the two signatures are disabled. The show ip audit signature command is used to display the disabled signatures, and then one is reenabled.

Pix(config)# ip audit signature 1001 disable
Pix(config)# ip audit signature 1004 disable
Pix(config)# show ip audit signature
ip audit signature 1001 disable
ip audit signature 1004 disable 
Pix(config)# no ip audit signature 1001 disable

The show ip audit count command lists the active signatures:

Pix(config)# show ip audit count
Signature ???? ? ??? ?? ? ? ???? ??? ?? Global
1000 I Bad IP Options List ? ? ?? ?? ?? 0
1001 I Record Packet Route ? ??? ??? ? ?0
1002 I Timestamp ????? ?? ??? ?????? ? ?0
1003 I Provide s,c,h,tcc ??? ???? ?? ?? 0
1005 I SATNET ID ? ? ?? ??? ? ? ??? ? ??0
1006 I Strict Source Route ? ?? ? ???? ?0

Create Named Audit Rules

An audit rule (audit policy) defines the actions for all active signatures that can be applied to an interface. Each audit rule/policy is identified by a unique user-defined case-sensitive name. Each interface can have two policies: one each for informational and attack signatures. If a policy is defined without actions, then the defined default actions are used. Each policy requires a different name.

Use the global configuration mode command ip audit name to create audit rules for info and attack signature types. Any signatures disabled with the ip audit signature command don’t become part of the audit rule created with the ip audit name command. Use the no form of this command to delete an audit rule. The syntax is

Pix(config)ip audit name audit-name {info | attack} [action [alarm] [drop] [reset]]
Pix(config)no ip audit name audit-name {info | attack}

The following example shows creating an audit policy—Audit.99—and then using the show ip audit name command to display the named policies.

Pix(config)# ip audit name Audit.99 info action alarm drop reset
Pix(config)# show ip audit name
ip audit name Audit.99 info action alarm drop reset

Apply the Audit Rule to the Interface(s)

The audit rule is applied to an interface on the PIX Firewall using the ip audit interface command. The no form of the command removes a policy from the interface. The syntax is

Pix(config)# ip audit interface int_name audit-name
Pix(config)# no ip audit interface int_name

The following example shows an attempt to assign the policy to the outside interface with a case error on the name. This is followed by a correct entry, and then the show ip audit interface command verifying the interface assignments.

Pix(config)# ip audit interface outside audit.99
Could not locate an IDS policy with name audit.99
Pix(config)# ip audit interface outside Audit.99
Pix(config)# show ip audit interface
ip audit interface outside Audit.99

PIX Firewall IDS Syslog Messages

PIX Firewall IDS Syslog messages all start with %PIX-4-4000nn IDS:signature_id . . . . The PIX-4 indicates trapping level 4 at least would be needed to capture these messages. The following sample messages include an info (2003) and attack (4051) message:

%PIX-4-400013 IDS:2003 ICMP redirect from to on interface dmz
%PIX-4-400032 IDS:4051 UDP Snork attack from to on interface outside

Part III: Virtual Private Networks (VPNs)