Cisco Intrusion Prevention System Sensor CLI Configuration Guide for IPS 7.0
Defining Signatures
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Defining Signatures

Table Of Contents

Defining Signatures

Understanding Security Policies

Working With Signature Definition Policies

Understanding Signatures

Configuring Signature Variables

Understanding Signature Variables

Adding, Editing, and Deleting Signature Variables

Configuring Signatures

Signature Definition Options

Configuring Alert Frequency

Configuring Alert Severity

Configuring Event Counter

Configuring Signature Fidelity Rating

Configuring the Status of Signatures

Configuring the Vulnerable OSes for a Signature

Assigning Actions to Signatures

Configuring AIC Signatures

Understanding the AIC Engine

AIC Engine and Sensor Performance

Configuring the Application Policy

AIC Request Method Signatures

AIC MIME Define Content Type Signatures

AIC Transfer Encoding Signatures

AIC FTP Commands Signatures

Creating an AIC Signature

Configuring IP Fragment Reassembly

Understanding IP Fragment Reassembly

IP Fragment Reassembly Signatures and Configurable Parameters

Configuring IP Fragment Reassembly Parameters

Configuring the Method for IP Fragment Reassembly

Configuring TCP Stream Reassembly

Understanding TCP Stream Reassembly

TCP Stream Reassembly Signatures and Configurable Parameters

Configuring TCP Stream Reassembly Signatures

Configuring the Mode for TCP Stream Reassembly

Configuring IP Logging

Creating Custom Signatures

Sequence for Creating a Custom Signature

Example String TCP Engine Signature

Example Service HTTP Engine Signature

Example Meta Engine Signature

Example IPv6 Engine Signature


Defining Signatures


This chapter describes how to define and create signatures. It contains the following sections:

Understanding Security Policies

Working With Signature Definition Policies

Understanding Signatures

Configuring Signature Variables

Configuring Signatures

Creating Custom Signatures

Understanding Security Policies


Note The AIM IPS and the NME IPS do not support multiple policies.


You can create multiple security policies and apply them to individual virtual sensors. A security policy is made up of a signature definition policy, an event action rules policy, and an anomaly detection policy. Cisco IPS contains a default signature definition policy called sig0, a default event action rules policy called rules0, and a default anomaly detection policy called ad0. You can assign the default policies to a virtual sensor or you can create new policies.

The use of multiple security policies lets you create security policies based on different requirements and then apply these customized policies per VLAN or physical interface.

Working With Signature Definition Policies

Use the service signature-definition name command in service signature definition mode to create a signature definition policy. The values of this signature definition policy are the same as the default signature definition policy, sig0, until you edit them.

Or you can use the copy signature-definition source_destination command in privileged EXEC mode to make a copy of an existing policy and then edit the values of the new policy as needed.

Use the list signature-definition-configurations command in privileged EXEC mode to list the signature definition policies.

Use the no service signature-definition name command in global configuration mode to delete a signature definition policy. Use the default service signature-definition name command in global configuration mode to reset the signature definition policy to factory settings.

To create, copy, edit, and delete signature definition policies, follow these steps:


Step 1 Log in to the CLI using an account with administrator privileges.

Step 2 Create a signature definition policy.

sensor# configure terminal
sensor(config)# service signature-definition MySig
Editing new instance MySig.
sensor(config-sig)# exit
Apply Changes?[yes]: yes
sensor(config)# exit
 
   

Step 3 Or copy an existing signature definition policy to a new signature definition policy.

sensor# copy signature-definition sig0 sig1
sensor# 
 
   

Note You receive an error if the policy already exists or if there is not enough space available for the new policy.


Step 4 Accept the default signature definition policy values or edit the following parameters:

a. Add signature definition variables.

b. Configure the general signature options.

Step 5 To display a list of signature definition policies on the sensor.

sensor# list signature-definition-configurations
Signature Definition
   Instance   Size   Virtual Sensor
   sig0       255    vs0
   temp       707    N/A
   MySig      255    N/A
   sig1       141    vs1
sensor#
 
   

Step 6 To delete a signature definition policy:

sensor# configure terminal
sensor(config)# no service signature-definition MySig
sensor(config)# exit
sensor#
 
   

Note You cannot delete the default signature definition policy, sig0.


Step 7 Confirm the signature definition policy has been deleted.

sensor# list signature-definition-configurations
Signature Definition
   Instance   Size   Virtual Sensor
   sig0       255    vs0
   temp       707    N/A
   sig1       141    vs1
sensor#
 
   

Step 8 To reset a signature definition policy to factory settings:

sensor# configure terminal
sensor(config)# default service signature-definition sig1
sensor(config)#
 
   

For More Information

For the procedure for adding signature variables, see Configuring Signature Variables.

For the procedure for configuring the general settings, see Configuring Signatures.

Understanding Signatures

Attacks or other misuses of network resources can be defined as network intrusions. Sensors that use a signature-based technology can detect network intrusions. A signature is a set of rules that your sensor uses to detect typical intrusive activity, such as DoS attacks. As sensors scan network packets, they use signatures to detect known attacks and respond with actions that you define.

The sensor compares the list of signatures with network activity. When a match is found, the sensor takes an action, such as logging the event or sending an alert. Sensors let you modify existing signatures and define new ones.

Signature-based intrusion detection can produce false positives because certain normal network activity can be misinterpreted as malicious activity. For example, some network applications or operating systems may send out numerous ICMP messages, which a signature-based detection system might interpret as an attempt by an attacker to map out a network segment. You can minimize false positives by tuning your signatures.

To configure a sensor to monitor network traffic for a particular signature, you must enable the signature. By default, the most critical signatures are enabled when you install the signature update. When an attack is detected that matches an enabled signature, the sensor generates an alert, which is stored in the Event Store of the sensor. The alerts, as well as other events, may be retrieved from the Event Store by web-based clients. By default the sensor logs all Informational alerts or higher.

Some signatures have subsignatures, that is, the signature is divided into subcategories. When you configure a subsignature, changes made to the parameters of one subsignature apply only to that subsignature. For example, if you edit signature 3050 subsignature 1 and change the severity, the severity change applies to only subsignature 1 and not to 3050 2, 3050 3, and 3050 4.

Cisco IPS contains over 10,000 built-in default signatures. You cannot rename or delete signatures from the list of built-in signatures, but you can retire signatures to remove them from the sensing engine. You can later activate retired signatures; however, this process requires the sensing engines to rebuild their configuration, which takes time and could delay the processing of traffic. You can tune built-in signatures by adjusting several signature parameters. Built-in signatures that have been modified are called tuned signatures.


Note We recommend that you retire any signatures that you are not using. This improves sensor performance.


You can create signatures, which are called custom signatures. Custom signature IDs begin at 60000. You can configure them for several things, such as matching of strings on UDP connections, tracking of network floods, and scans. Each signature is created using a signature engine specifically designed for the type of traffic being monitored.

Configuring Signature Variables

This section describes signature variables, and contains the following topics:

Understanding Signature Variables

Adding, Editing, and Deleting Signature Variables

Understanding Signature Variables

When you want to use the same value within multiple signatures, use a variable. When you change the value of a variable, that variable is updated in all signatures in which it appears. This saves you from having to change the variable repeatedly as you configure signatures.


Note You must preface the variable with a dollar ($) sign to indicate that you are using a variable rather than a string.


Some variables cannot be deleted because they are necessary to the signature system. If a variable is protected, you cannot select it to edit it. You receive an error message if you try to delete protected variables. You can edit only one variable at a time.

Adding, Editing, and Deleting Signature Variables

Use the variables command in the signature definition submode to create signature variables.

The following options apply:

variable_name—Identifies the name assigned to this variable. A valid name can only contain numbers or letters. You can also use a hyphen (-) or underscore (_).

ip-addr-range—System-defined variable for grouping IP addresses. The valid values are: A.B.C.D-A.B.C.D[,A.B.C.D-A.B.C.D]

web-ports—System-defined variable for ports to look for HTTP traffic. To designate multiple port numbers for a single variable, place a comma between the entries. For example, 80, 3128, 8000, 8010, 8080, 8888, 24326.

To add, edit, and delete signature variables, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Create a signature variable for a group of IP addresses.

sensor(config-sig)# variables IPADD ip-addr-range 10.1.1.1-10.1.1.24
 
   

Step 4 Edit the signature variable for web ports. WEBPORTS has a predefined set of ports where web servers are running, but you can edit the value. This variable affects all signatures that have web ports. The default is 80, 3128, 8000, 8010, 8080, 8888, 24326.

sensor(config-sig)# variables WEBPORTS web-ports 80,3128,8000
 
   

Step 5 Verify the changes.

sensor(config-sig)# show settings
   variables (min: 0, max: 256, current: 2)
   -----------------------------------------------
      variable-name: IPADD
      -----------------------------------------------
         ip-addr-range: 10.1.1.1-10.1.1.24
      -----------------------------------------------
      <protected entry>
      variable-name: WEBPORTS
      -----------------------------------------------
         web-ports: 80,3128,8000 default: 80-80,3128-3128,8000-8000,8010-8010,80
80-8080,8888-8888,24326-24326
      -----------------------------------------------
 
   

Step 6 To delete a variable:

sensor(config-sig)# no variables IPADD 
 
   

Step 7 Verify the variable has been deleted.

sensor(config-sig)# show settings
   variables (min: 0, max: 256, current: 1)
   -----------------------------------------------
      <protected entry>
      variable-name: WEBPORTS
      -----------------------------------------------
         web-ports: 80,3128,8000 default: 80-80,3128-3128,8000-8000,8010-8010,80
80-8080,8888-8888,24326-24326
      -----------------------------------------------
 
   

Step 8 Exit signature definition submode.

sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 9 Press Enter to apply the changes or enter no to discard them.


Configuring Signatures

This section describes how to configure signature parameters, and contains the following topics:

Signature Definition Options

Configuring Alert Frequency

Configuring Alert Severity

Configuring Event Counter

Configuring Signature Fidelity Rating

Configuring the Status of Signatures

Configuring the Vulnerable OSes for a Signature

Assigning Actions to Signatures

Configuring AIC Signatures

Configuring IP Fragment Reassembly

Configuring TCP Stream Reassembly

Configuring IP Logging

Signature Definition Options

The following options apply to configuring the general parameters of a specific signature:

alert-frequency—Sets the summary options for grouping alerts.

alert-severity—Sets the severity of the alert.

engine—Specifies the signature engine. You can assign actions when you are in the engine submode.

event-counter—Sets the event count.

promisc-delta—The delta value used to determine the seriousness of the alert.


Caution We do not recommend that you change the promiscuous delta setting for a signature.

Promiscuous delta lowers the risk rating of certain alerts in promiscuous mode. Because the sensor does not know the attributes of the target system and in promiscuous mode cannot deny packets, it is useful to lower the prioritization of promiscuous alerts (based on the lower risk rating) so the administrator can focus on investigating higher risk rating alerts.

In inline mode, the sensor can deny the offending packets and they never reach the target host, so it does not matter if the target was vulnerable. The attack was not allowed on the network and so we do not subtract from the risk rating value.

Signatures that are not service, OS, or application specific have 0 for the promiscuous delta. If the signature is specific to an OS, service, or application, it has a promiscuous delta of 5, 10, or 15 calculated from 5 points for each category.

sig-description—Your description of the signature.

sig-fidelity-rating—Rating of the fidelity of signature.

status—Sets the status of the signature to enabled or retired.

vulnerable-os—List of OS types that are vulnerable to this attack signature.

For More Information

For the procedure for configuring alert frequency, see Configuring Alert Frequency.

For more information about signature engines, see "Signature Engines."

For the procedure for assigning actions, see Assigning Actions to Signatures.

For the procedure for configuring event counts, see Configuring Event Counter.

For the procedure for configuring the signature fidelity rating, see Configuring Signature Fidelity Rating.

For the procedure for enabling and disabling signatures, see Configuring the Status of Signatures.

For the procedure for configuring vulnerable OSes, see Configuring the Vulnerable OSes for a Signature.

Configuring Alert Frequency

Use the alert-frequency command in signature definition submode to configure the alert frequency for a signature. The alert-frequency command specifies how often the sensor alerts you when this signature is firing.

The following options apply:

sig_id—Identifies the unique numerical value assigned to this signature. This value lets the sensor identify a particular signature. The value is 1000 to 65000.

subsig_id—Identifies the unique numerical value assigned to this subsignature. A subsignature ID is used to identify a more granular version of a broad signature. The value is 0 to 255.

summary-mode—Specifies the way you want the sensor to group the alerts:

fire-all—Fires an alert on all events.

fire-once—Fires an alert only once.

global-summarize—Summarizes an alert so that it only fires once regardless of how many attackers or victims.

summarize—Summarize all the alerts.

specify-summary-threshold {yes | no}—Enables summary threshold mode:

summary-thresholdSpecifies the minimum number of hits the sensor must receive before sending a summary alert for this signature. The value is 0 to 65535.

summary-intervalSpecifies the time in seconds used in each summary alert. The value is 1 to 1000.

summary-keySpecifies the storage type on which to summarize this signature:

AxxxAttacker address.

AxxbAttacker address and victim port.

AxBx—Attacker and victim addresses.

AaBb—Attacker and victim addresses and ports.

xxBx—Victim address.

specify-global-summary-threshold {yes | no}—(Optional) Enables global summary threshold mode:

global-summary-thresholdSpecifies the threshold number of events to take alert in to global summary. The value is 1 to 65535.

To configure the alert frequency parameters of a signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify the signature you want to configure.

sensor(config-sig)# signatures 9000 0
 
   

Step 4 Enter alert frequency submode:

sensor(config-sig-sig)# alert-frequency
 
   

Step 5 Specify the alert frequency of this signature:

a. Configure the summary mode to, for example, fire once.

sensor(config-sig-sig-ale)# summary-mode fire-once
sensor(config-sig-sig-ale-fir)# specify-global-summary-threshold yes
sensor(config-sig-sig-ale-fir-yes)# global-summary-threshold 3000
sensor(config-sig-sig-ale-fir-yes)# summary-interval 5000
 
   

b. Specify the summary key.

sensor(config-sig-sig-ale-fir-yes)# exit
sensor(config-sig-sig-ale-fir)# summary-key AxBx
 
   

c. Verify the settings.

sensor(config-sig-sig-ale-fir)# show settings
   fire-once
   -----------------------------------------------
      summary-key: AxBx default: Axxx
      specify-global-summary-threshold
      -----------------------------------------------
         yes
         -----------------------------------------------
            global-summary-threshold: 3000 default: 120
            summary-interval: 5000 default: 15
         -----------------------------------------------
      -----------------------------------------------
   -----------------------------------------------
sensor(config-sig-sig-ale-fir)#
 
   

Step 6 Exit alert-frequency submode.

sensor(config-sig-sig-ale-fir)# exit
sensor(config-sig-sig-ale)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


Configuring Alert Severity

Use the alert-severity command in signature definition submode to configure the severity of a signature.

The following options apply:

sig_id—Identifies the unique numerical value assigned to this signature. This value lets the sensor identify a particular signature. The value is 1000 to 65000.

subsig_id—Identifies the unique numerical value assigned to this subsignature. A subsignature ID is used to identify a more granular version of a broad signature. The value is 0 to 255.

alert-severity—Specifies the severity of the alert:

high —Dangerous alert.

medium—Medium level alert (default).

low—Low level alert.

informational—Informational alert.

To configure the alert severity, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify the signature you want to configure.

sensor(config-sig)# signatures 9000 0
 
   

Step 4 Assign the alert severity.

sensor(config-sig-sig)# alert-severity medium
 
   

Step 5 Verify the settings.

sensor(config-sig-sig)# show settings
   <protected entry>
   sig-id: 9000
   subsig-id: 0
   -----------------------------------------------
      alert-severity: medium default: medium
      sig-fidelity-rating: 75 <defaulted>
      promisc-delta: 0 <defaulted>
      sig-description
      -----------------------------------------------
         sig-name: Back Door Probe (TCP 12345) <defaulted>
         sig-string-info: SYN to TCP 12345 <defaulted>
         sig-comment: <defaulted>
         alert-traits: 0 <defaulted>
         release: 40 <defaulted>
      -----------------------------------------------
      vulnerable-os: general-os <defaulted> 
      engine
      -----------------------------------------------
         atomic-ip
         -----------------------------------------------
            event-action: produce-alert <defaulted>
            fragment-status: any <defaulted>
            specify-l4-protocol
            -----------------------------------------------
--MORE--
 
   

Step 6 Exit signatures submode.

sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


Configuring Event Counter

Use the event-counter command in signature definition submode to configure how the sensor counts events. For example, you can specify that you want the sensor to send an alert only if the same signature fires 5 times for the same address set.

The following options apply:

event-count—Specifies the number of times an event must occur before an alert is generated. The valid range is 1 to 65535. The default is 1.

event-count-keySpecifies the storage type on which to count events for this signatures:

Axxx—Attacker address

AxBx—Attacker and victim addresses

Axxb—Attacker address and victim port

xxBx—Victim address

AaBb—Attacker and victim addresses and ports

specify-alert-interval {yes | no}Enables alert interval:

alert-interval—Specifies the time in seconds before the event count is reset. The default is 60.

To configure event counter, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify the signature for which you want to configure event counter.

sensor(config-sig)# signatures 9000 0
 
   

Step 4 Enter event counter submode.

sensor(config-sig-sig)# event-counter
 
   

Step 5 Specify how many times an event must occur before an alert is generated.

sensor(config-sig-sig-eve)# event-count 2
 
   

Step 6 Specify the storage type on which you want to count events for this signature.

sensor(config-sig-sig-eve)# event-count-key AxBx
 
   

Step 7 (Optional) Enable alert interval:

sensor(config-sig-sig-eve)# specify-alert-interval yes
 
   

Step 8 (Optional) Specify the amount of time in seconds before the event count should be reset.

sensor(config-sig-sig-eve-yes)# alert-interval 30
 
   

Step 9 Verify the settings.

sensor(config-sig-sig-eve-yes)# exit
sensor(config-sig-sig-eve)# show settings
   event-counter
   -----------------------------------------------
      event-count: 2 default: 1
      event-count-key: AxBx default: Axxx
      specify-alert-interval
      -----------------------------------------------
         yes
         -----------------------------------------------
            alert-interval: 30 default: 60
         -----------------------------------------------
      -----------------------------------------------
   -----------------------------------------------
sensor(config-sig-sig-eve)#
 
   

Step 10 Exit signatures submode.

sensor(config-sig-sig-eve)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 11 Press Enter to apply the changes or enter no to discard them.


Configuring Signature Fidelity Rating

Use the sig-fidelity-rating command in signature definition submode to configure the signature fidelity rating for a signature.

The following option applies:

sig-fidelity-rating—Identifies the weight associated with how well this signature might perform in the absence of specific knowledge of the target. The valid value is 0 to 100.

To configure the signature fidelity rating for a signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig0
 
   

Step 3 Specify the signature you want to configure.

sensor(config-sig)# signatures 12000 0
 
   

Step 4 Specify the signature fidelity rating for this signature.

sensor(config-sig-sig)# sig-fidelity-rating 50
 
   

Step 5 Verify the settings.

sensor(config-sig-sig)# show settings
   <protected entry>
   sig-id: 12000
   subsig-id: 0
   -----------------------------------------------
      alert-severity: low <defaulted>
      sig-fidelity-rating: 50 default: 85
      promisc-delta: 15 <defaulted>
      sig-description
      -----------------------------------------------
         sig-name: Gator Spyware Beacon <defaulted>
         sig-string-info: /download/ User-Agent: Gator <defaulted>
         sig-comment: <defaulted>
         alert-traits: 0 <defaulted>
         release: 71 <defaulted>
      -----------------------------------------------
 
   

Step 6 Exit signatures submode.

sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


Configuring the Status of Signatures


Caution Activating and retiring signatures can take 30 minutes or longer.

Use the status command in signature definition submode to specify the status of a specific signature.

The following options apply:

status—Identifies whether the signature is enabled, disabled, or retired:

enabled {true | false}—Enables the signature.

retired {true | false}]—Retires the signature.

obsoletes signature_ID—Shows the other signatures that have been obsoleted by this signature.

To change the status of a signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Choose the signature you want to configure.

sensor(config-sig)# signatures 12000 0
 
   

Step 4 Change the status for this signature.

sensor(config-sig-sig)# status
sensor(config-sig-sig-sta)# enabled true
 
   

Step 5 Verify the settings.

sensor(config-sig-sig-sta)# show settings
   status
   -----------------------------------------------
      enabled: true default: false
      retired: false <defaulted>
   -----------------------------------------------
sensor(config-sig-sig-sta)#
 
   

Step 6 Exit signatures submode.

sensor(config-sig-sig-sta)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


Configuring the Vulnerable OSes for a Signature

Use the vulnerable-os command in signature definition submode to configure the list of vulnerable OSes for a signature.

The following options apply:

general-os—All OS types

ios—Variants of Cisco IOS

mac-os—Variants of Macintosh OS

netware—Netware

other—Any other OS

unix—Variants of UNIX

aix—Variants of AIX

bsd—Variants of BSD

hp-ux—Variants of HP-UX

irix—Variants of IRIX

linux—Variants of Linux

solaris—Variants of Solaris

windows—Variants of Microsoft Windows

windows-nt-2k-xp—Variants of Microsoft NT, 2000, and XP

win-nt—Specific variants of Windows NT

To configure the vulnerable OSes for a signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify the signature you want to configure.

sensor(config-sig)# signatures 6000 0
 
   

Step 4 Specify the vulnerable OSes for this signature.

sensor(config-sig-sig)# vulnerable-os linux|aix
 
   

Step 5 Verify the settings.

sensor(config-sig-sig)# show settings
   sig-id: 60000
   subsig-id: 0
   -----------------------------------------------
      alert-severity: medium <defaulted>
      sig-fidelity-rating: 75 <defaulted>
      promisc-delta: 0 <defaulted>
      sig-description
      -----------------------------------------------
         sig-name: My Sig <defaulted>
         sig-string-info: My Sig Info <defaulted>
         sig-comment: Sig Comment <defaulted>
         alert-traits: 0 <defaulted>
         release: custom <defaulted>
      -----------------------------------------------
      vulnerable-os: aix|linux default: general-os
*---> engine
      -----------------------------------------------
      -----------------------------------------------
      event-counter
      -----------------------------------------------
         event-count: 1 <defaulted>
         event-count-key: Axxx <defaulted>
         specify-alert-interval
         -----------------------------------------------
--MORE--
 
   

Step 6 Exit signatures submode.

sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


Assigning Actions to Signatures

Use the event-action command in signature definition submode to configure the actions the sensor takes when the signature fires.

The following options apply:

event-action—Specifies the action(s) to perform when alert is triggered:

deny-attacker-inline —(Inline mode only) does not transmit this packet and future packets from the attacker address for a specified period of time.

deny-attacker-service-pair-inline(Inline mode only) Does not transmit this packet and future packets on the attacker address victim port pair for a specified period of time.

deny-attacker-victim-pair-inline(Inline mode only) Does not transmit this packet and future packets on the attacker/victim address pair for a specified period of time.

deny-connection-inline—(Inline mode only) Does not transmit this packet and future packets on the TCP Flow.

deny-packet-inline—(Inline mode only) Does not transmit this packet.

log-attacker-packets—Starts IP logging of packets containing the attacker address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-pair-packets—Starts IP logging of packets containing the attacker-victim address pair. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-victim-packets—Starts IP logging of packets containing the victim address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

produce-alert —Writes the event to Event Store as an alert.

produce-verbose-alert—Includes an encoded dump (possibly truncated) of the offending packet in the alert. This action causes an alert to be written to the Event Store, even if produce-alert is not selected.

request-block-connection—Sends a request to ARC to block this connection. You must have blocking devices configured to implement this action.

request-block-host—Sends a request to ARC to block this attacker host. You must have blocking devices configured to implement this action.

request-rate-limit—Sends a rate limit request to ARC to perform rate limiting. You must have rate limiting devices configured to implement this action.

request-snmp-trap—Sends a request to the Notification Application component of the sensor to perform SNMP notification. This action causes an alert to be written to the Event Store, even if produce-alert is not selected. You must have SNMP configured on the sensor to implement this action.

reset-tcp-connection—Sends TCP resets to hijack and terminate the TCP flow. Reset TCP Connection only works on TCP signatures that analyze a single connection. It does not work for sweeps or floods.

modify-packet-inline— Modifies packet data to remove ambiguity about what the end point might do with the packet.

event-action-settings—Lets you set the external-rate-limit-type:

none—No rate limiting configured.

percentage—Sets the rate limit by traffic percentage (external-rate-limit-percentage).

Assigning Actions to Signatures

To configure event actions and event action settings for a signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator privileges.

Step 2 Enter signature definition mode.

sensor# configure terminal
sensor(config)# service signature-definition sig0
sensor(config-sig)#
 
   

Step 3 Specify the signature you want to configure.

sensor(config-sig)# signatures 1200 0
 
   

Step 4 Specify the signature engine (for signature 1200 it is the Normalizer engine).

sensor(config-sig-sig)# engine normalizer
 
   

Step 5 Configure the event action.

sensor(config-sig-sig-nor)# event-action produce-alert|request-snmp-trap
 
   

Note Each time you configure the event actions for a signature, you overwrite the previous configuration. For example, if you always want to produce an alert when the signature is fired, you must configure it along with the other event actions you want. Use the | symbol to add more than one event action, for example, product-alert|deny-packet-inline|request-snmp-trap.


Step 6 Verify the settings.

sensor(config-sig-sig-nor)# show settings
   normalizer
   -----------------------------------------------
      event-action: produce-alert|request-snmp-trap default: 
produce-alert|deny-packet-inline
 
   

Step 7 Specify the percentage for rate limiting.

sensor(config-sig-sig-nor)# event-action-settings
sensor(config-sig-sig-nor-eve)# external-rate-limit-type percentage
sensor(config-sig-sig-nor-eve-per)# external-rate-limit-percentage 50
 
   

Step 8 Verify the settings.

sensor(config-sig-sig-nor-eve-per)# show settings
   percentage
   -----------------------------------------------
      external-rate-limit-percentage: 50 default: 100
   -----------------------------------------------
 
   

Step 9 Exit event action submode.

sensor(config-sig-sig-nor-eve-per)# exit
sensor(config-sig-sig-nor-eve)# exit
sensor(config-sig-sig-nor)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 10 Press Enter to apply the changes or enter no to discard them.


For More Information

For a detailed description of event actions, see Event Actions.

Configuring AIC Signatures

This section describes the Application Inspection and Control (AIC) signatures and how to configure them. It contains the following topics:

Understanding the AIC Engine

AIC Engine and Sensor Performance

Configuring the Application Policy

AIC Request Method Signatures

AIC MIME Define Content Type Signatures

AIC Transfer Encoding Signatures

AIC FTP Commands Signatures

Creating an AIC Signature

Understanding the AIC Engine


Note The AIC engine runs when HTTP traffic is received on AIC web ports. If traffic is web traffic, but not received on the AIC web ports, the Service HTTP engine is executed. AIC inspection can be on any port if it is configured as an AIC web port and the traffic to be inspected is HTTP traffic.


AIC provides thorough analysis of web traffic. It provides granular control over HTTP sessions to prevent abuse of the HTTP protocol. It allows administrative control over applications, such as instant messaging and gotomypc, that try to tunnel over specified ports. Inspection and policy checks for P2P and instant messaging are possible if these applications are running over HTTP.

AIC also provides a way to inspect FTP traffic and control the commands being issued. You can enable or disable the predefined signatures or you can create policies through custom signatures.

AIC has the following categories of signatures:

HTTP request method

Define request method

Recognized request methods

MIME type

Define content type

Recognized content type

Define web traffic policy

There is one predefined signature, 12674, that specifies the action to take when noncompliant HTTP traffic is seen. The parameter Alarm on Non HTTP Traffic enables the signature. By default this signature is enabled.

Transfer encodings

Associate an action with each method

List methods recognized by the sensor

Specify which actions need to be taken when a chunked encoding error is seen

FTP commands

Associates an action with an FTP command.

For More Information

For a list of signature IDs and descriptions for these signatures, see AIC Request Method Signatures, AIC MIME Define Content Type Signatures, AIC Transfer Encoding Signatures, and AIC FTP Commands Signatures.

For the procedure for creating a custom MIME signature, see Creating an AIC Signature.

AIC Engine and Sensor Performance

Application policy enforcement is a unique sensor feature. Rather than being based on traditional IPS technologies that inspect for exploits, vulnerabilities, and anomalies, AIC policy enforcement is designed to enforce HTTP and FTP service policies. The inspection work required for this policy enforcement is extreme compared with traditional IPS inspection work. A large performance penalty is associated with using this feature. When AIC is enabled, the overall bandwidth capacity of the sensor is reduced.

AIC policy enforcement is disabled in the IPS default configuration. If you want to activate AIC policy enforcement, we highly recommend that you carefully choose the exact policies of interest and disable those you do not need. Also, if your sensor is near its maximum inspection load capacity, we recommend that you not use this feature since it can oversubscribe the sensor. We recommend that you use the adaptive security appliance firewall to handle this type of policy enforcement.

Configuring the Application Policy

Use the application-policy command in signature definition submode to enable the web AIC feature. You can configure the sensor to provide Layer 4 to Layer 7 packet inspection to prevent malicious attacks related to web and FTP services.

The following options apply:

ftp-enable {true | false}—Enables protection for FTP services. Set to true to require the sensor to inspect FTP traffic. The default is false.

http-policy—Enables inspection of HTTP traffic:

aic-web-portsSpecifies the variable for ports to look for AIC traffic. The valid range is 0 to 65535. A comma-separated list of integer ranges a-b[,c-d] within 0-65535. The second number in the range must be greater than or equal to the first number. The default is 80-80,3128-3128,8000-8000,8010-8010,8080-8080,8888-8888,24326-24326.

http-enable {true | false}—Enables protection for web services. Set to true to require the sensor to inspect HTTP traffic for compliance with the RFC. The default is false.

max-outstanding-http-requests-per-connection—Specifies the maximum allowed HTTP requests per connection. The valid value is 1 to 16. The default is 10.

To configure the application policy, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter application policy submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
sensor(config-sig)# application-policy
 
   

Step 3 Enable inspection of FTP traffic.

sensor(config-sig-app)# ftp-enable true
 
   

Step 4 Configure the HTTP application policy:

a. Enter HTTP application policy submode.

sensor(config-sig-app)# http-policy
 
   

b. Enable HTTP application policy enforcement.

sensor(config-sig-app-htt)# http-enable true
 
   

c. Specify the number of outstanding HTTP requests per connection that can be outstanding without having received a response from the server.

sensor(config-sig-app-htt)# max-outstanding-http-requests-per-connection 5
 
   

d. Edit the AIC ports.

sensor(config-sig-app-htt)# aic-web-ports 80-80,3128-3128
 
   

Step 5 Verify your settings.

sensor(config-sig-app-htt)# exit
sensor(config-sig-app)# show settings
   application-policy
   -----------------------------------------------
      http-policy
      -----------------------------------------------
         http-enable: true default: false
         max-outstanding-http-requests-per-connection: 5 default: 10
         aic-web-ports: 80-80,3128-3128 default: 80-80,3128-3128,8000-8000,8010-
8010,8080-8080,8888-8888,24326-24326
      -----------------------------------------------
      ftp-enable: true default: false
   -----------------------------------------------
sensor(config-sig-app)#
 
   

Step 6 Exit signature definition submode.

sensor(config-sig-app)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


AIC Request Method Signatures

The HTTP request method has two categories of signatures:

Define request method—Allows actions to be associated with request methods. You can expand and modify the signatures (Define Request Method).

Recognized request methods—Lists methods that are recognized by the sensor (Recognized Request Methods).

Table 8-1 lists the predefined define request method signatures. Enable the signatures that have the predefined method you need.

Table 8-1 Request Method Signatures 

Signature ID
Define Request Method

12676

Request Method Not Recognized

12677

Define Request Method PUT

12678

Define Request Method CONNECT

12679

Define Request Method DELETE

12680

Define Request Method GET

12681

Define Request Method HEAD

12682

Define Request Method OPTIONS

12683

Define Request Method POST

12685

Define Request Method TRACE

12695

Define Request Method INDEX

12696

Define Request Method MOVE

12697

Define Request Method MKDIR

12698

Define Request Method COPY

12699

Define Request Method EDIT

12700

Define Request Method UNEDIT

12701

Define Request Method SAVE

12702

Define Request Method LOCK

12703

Define Request Method UNLOCK

12704

Define Request Method REVLABEL

12705

Define Request Method REVLOG

12706

Define Request Method REVADD

12707

Define Request Method REVNUM

12708

Define Request Method SETATTRIBUTE

12709

Define Request Method GETATTRIBUTENAME

12710

Define Request Method GETPROPERTIES

12711

Define Request Method STARTENV

12712

Define Request Method STOPREV


For More Information

For the procedure for enabling signatures, see Configuring the Status of Signatures.

AIC MIME Define Content Type Signatures

There are two policies associated with MIME types:

Define content type—Associates specific actions for the following cases (Define Content Type):

Deny a specific MIME type, such as an image/jpeg

Message size violation

MIME-type mentioned in header and body do not match

Recognized content type (Recognized Content Type)

Table 8-2 lists the predefined define content type signatures. Enable the signatures that have the predefined content type you need. You can also create custom define content type signatures.

Table 8-2 Define Content Type Signatures 

Signature ID
Signature Description

12621

Content Type image/gif Invalid Message Length

12622 2

Content Type image/png Verification Failed

12623 0
12623 1
12623 2

Content Type image/tiff Header Check
Content Type image/tiff Invalid Message Length
Content Type image/tiff Verification Failed

12624 0
12624 1
12624 2

Content Type image/x-3ds Header Check
Content Type image/x-3ds Invalid Message Length
Content Type image/x-3ds Verification Failed

12626 0
12626 1
12626 2

Content Type image/x-portable-bitmap Header Check
Content Type image/x-portable-bitmap Invalid Message Length
Content Type image/x-portable-bitmap Verification Failed

12627 0
12627 1
12627 2

Content Type image/x-portable-graymap Header Check
Content Type image/x-portable-graymap Invalid Message Length
Content Type image/x-portable-graymap Verification Failed

12628 0
12628 1
12628 2

Content Type image/jpeg Header Check
Content Type image/jpeg Invalid Message Length
Content Type image/jpeg Verification Failed

12629 0
12629 1

Content Type image/cgf Header Check
Content Type image/cgf Invalid Message Length

12631 0
12631 1

Content Type image/x-xpm Header Check
Content Type image/x-xpm Invalid Message Length

12633 0
12633 1
12633 2

Content Type audio/midi Header Check
Content Type audio/midi Invalid Message Length
Content Type audio/midi Verification Failed

12634 0
12634 1
12634 2

Content Type audio/basic Header Check
Content Type audio/basic Invalid Message Length
Content Type audio/basic Verification Failed

12635 0
12635 1
12635 2

Content Type audio/mpeg Header Check
Content Type audio/mpeg Invalid Message Length
Content Type audio/mpeg Verification Failed

12636 0
12636 1
12636 2

Content Type audio/x-adpcm Header Check
Content Type audio/x-adpcm Invalid Message Length
Content Type audio/x-adpcm Verification Failed

12637 0
12637 1
12637 2

Content Type audio/x-aiff Header Check
Content Type audio/x-aiff Invalid Message Length
Content Type audio/x-aiff Verification Failed

12638 0
12638 1
12638 2

Content Type audio/x-ogg Header Check
Content Type audio/x-ogg Invalid Message Length
Content Type audio/x-ogg Verification Failed

12639 0
12639 1
12639 2

Content Type audio/x-wav Header Check
Content Type audio/x-wav Invalid Message Length
Content Type audio/x-wav Verification Failed

12641 0
12641 1
12641 2

Content Type text/html Header Check
Content Type text/html Invalid Message Length
Content Type text/html Verification Failed

12642 0
12642 1

Content Type text/css Header Check
Content Type text/css Invalid Message Length

12643 0
12643 1

Content Type text/plain Header Check
Content Type text/plain Invalid Message Length

12644 0
12644 1

Content Type text/richtext Header Check
Content Type text/richtext Invalid Message Length

12645 0
12645 1
12645 2

Content Type text/sgml Header Check
Content Type text/sgml Invalid Message Length
Content Type text/sgml Verification Failed

12646 0
12646 1
12646 2

Content Type text/xml Header Check
Content Type text/xml Invalid Message Length
Content Type text/xml Verification Failed

12648 0
12648 1
12648 2

Content Type video/flc Header Check
Content Type video/flc Invalid Message Length
Content Type video/flc Verification Failed

12649 0
12649 1
12649 2

Content Type video/mpeg Header Check
Content Type video/mpeg Invalid Message Length
Content Type video/mpeg Verification Failed

12650 0
12650 1

Content Type text/xmcd Header Check
Content Type text/xmcd Invalid Message Length

12651 0
12651 1
12651 2

Content Type video/quicktime Header Check
Content Type video/quicktime Invalid Message Length
Content Type video/quicktime Verification Failed

12652 0
12652 1

Content Type video/sgi Header Check
Content Type video/sgi Verification Failed

12653 0
12653 1

Content Type video/x-avi Header Check
Content Type video/x-avi Invalid Message Length

12654 0
12654 1
12654 2

Content Type video/x-fli Header Check
Content Type video/x-fli Invalid Message Length
Content Type video/x-fli Verification Failed

12655 0
12655 1
12655 2

Content Type video/x-mng Header Check
Content Type video/x-mng Invalid Message Length
Content Type video/x-mng Verification Failed

12656 0
12656 1
12656 2

Content Type application/x-msvideo Header Check
Content Type application/x-msvideo Invalid Message Length
Content Type application/x-msvideo Verification Failed

12658 0
12658 1

Content Type application/ms-word Header Check
Content Type application/ms-word Invalid Message Length

12659 0
12659 1

Content Type application/octet-stream Header Check
Content Type application/octet-stream Invalid Message Length

12660 0
12660 1
12660 2

Content Type application/postscript Header Check
Content Type application/postscript Invalid Message Length
Content Type application/postscript Verification Failed

12661 0
12661 1

Content Type application/vnd.ms-excel Header Check
Content Type application/vnd.ms-excel Invalid Message Length

12662 0
12662 1

Content Type application/vnd.ms-powerpoint Header Check
Content Type application/vnd.ms-powerpoint Invalid Message Length

12663 0
12663 1
12663 2

Content Type application/zip Header Check
Content Type application/zip Invalid Message Length
Content Type application/zip Verification Failed

12664 0
12664 1
12664 2

Content Type application/x-gzip Header Check
Content Type application/x-gzip Invalid Message Length
Content Type application/x-gzip Verification Failed

12665 0
12665 1

Content Type application/x-java-archive Header Check
Content Type application/x-java-archive Invalid Message Length

12666 0
12666 1

Content Type application/x-java-vm Header Check
Content Type application/x-java-vm Invalid Message Length

12667 0
12667 1
12667 2

Content Type application/pdf Header Check
Content Type application/pdf Invalid Message Length
Content Type application/pdf Verification Failed

12668 0
12668 1

Content Type unknown Header Check
Content Type unknown Invalid Message Length

12669 0
12669 1

Content Type image/x-bitmap Header Check
Content Type image/x-bitmap Invalid Message Length

12673 0

Recognized content type


For More Information

For the procedure for enabling signatures, see Configuring the Status of Signatures.

For the procedure for creating an AIC signature, see Creating an AIC Signature.

AIC Transfer Encoding Signatures

There are three policies associated with transfer encoding:

Associate an action with each method (Define Transfer Encoding)

List methods recognized by the sensor (Recognized Transfer Encodings)

Specify which actions need to be taken when a chunked encoding error is seen (Chunked Transfer Encoding Error)

Table 8-3 lists the predefined transfer encoding signatures. Enable the signatures that have the predefined transfer encoding method you need.

Table 8-3 Transfer Encoding Signatures 

Signature ID
Transfer Encoding Method

12686

Recognized Transfer Encoding

12687

Define Transfer Encoding Deflate

12688

Define Transfer Encoding Identity

12689

Define Transfer Encoding Compress

12690

Define Transfer Encoding GZIP

12693

Define Transfer Encoding Chunked

12694

Chunked Transfer Encoding Error


For More Information

For the procedure for enabling signatures, see Configuring the Status of Signatures.

AIC FTP Commands Signatures

Table 8-4 lists the predefined FTP commands signatures. Enable the signatures that have the predefined FTP command you need.

Table 8-4 FTP Commands Signatures 

Signature ID
FTP Command

12900

Unrecognized FTP command

12901

Define FTP command abor

12902

Define FTP command acct

12903

Define FTP command allo

12904

Define FTP command appe

12905

Define FTP command cdup

12906

Define FTP command cwd

12907

Define FTP command dele

12908

Define FTP command help

12909

Define FTP command list

12910

Define FTP command mkd

12911

Define FTP command mode

12912

Define FTP command nlst

12913

Define FTP command noop

12914

Define FTP command pass

12915

Define FTP command pasv

12916

Define FTP command port

12917

Define FTP command pwd

12918

Define FTP command quit

12919

Define FTP command rein

12920

Define FTP command rest

12921

Define FTP command retr

12922

Define FTP command rmd

12923

Define FTP command rnfr

12924

Define FTP command rnto

12925

Define FTP command site

12926

Define FTP command smnt

12927

Define FTP command stat

12928

Define FTP command stor

12929

Define FTP command stou

12930

Define FTP command stru

12931

Define FTP command syst

12932

Define FTP command type

12933

Define FTP command user


For More Information

For the procedure for enabling signatures, see Configuring the Status of Signatures.

Creating an AIC Signature

The following example demonstrates how to create a MIME-type signature based on the AIC engine.

The following options apply:

event-action—Specifies the action(s) to perform when alert is triggered:

deny-attacker-inline —(Inline only) does not transmit this packet and future packets from the attacker address for a specified period of time.

deny-attacker-service-pair-inline(Inline only) Does not transmit this packet and future packets on the attacker address victim port pair for a specified period of time.

deny-attacker-victim-pair-inline(Inline only) Does not transmit this packet and future packets on the attacker/victim address pair for a specified period of time.

deny-connection-inline—(Inline only) Does not transmit this packet and future packets on the TCP Flow.

deny-packet-inline—(Inline only) Does not transmit this packet.

log-attacker-packets—Starts IP logging of packets containing the attacker address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-pair-packets—Starts IP logging of packets containing the attacker-victim address pair. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-victim-packets—Starts IP logging of packets containing the victim address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

produce-alert —Writes the event to Event Store as an alert.

produce-verbose-alert—Includes an encoded dump (possibly truncated) of the offending packet in the alert. This action causes an alert to be written to the Event Store, even if produce-alert is not selected.

request-block-connection—Sends a request to ARC to block this connection. You must have blocking devices configured to implement this action.

request-block-host—Sends a request to ARC to block this attacker host. You must have blocking devices configured to implement this action.

request-rate-limit—Sends a rate limit request to ARC to perform rate limiting. You must have rate limiting devices configured to implement this action.

request-snmp-trap—Sends a request to the Notification Application component of the sensor to perform SNMP notification. This action causes an alert to be written to the Event Store, even if produce-alert is not selected. You must have SNMP configured on the sensor to implement this action.

reset-tcp-connection—Sends TCP resets to hijack and terminate the TCP flow. Reset TCP Connection only works on TCP signatures that analyze a single connection. It does not work for sweeps or floods.

modify-packet-inline— Modifies packet data to remove ambiguity about what the end point might do with the packet.

no—Removes an entry or selection setting.

signature-type—Specifies the type of signature desired:

content-types—Content-types

define-web-traffic-policy—Defines web traffic policy

max-outstanding-requests-overrun—Inspects for large number of outstanding HTTP requests

msg-body-pattern—Message body pattern

request-methods—Signature types that deal with request methods

transfer-encodings—Signature types that deal with transfer encodings

To define a MIME-type policy signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter application policy enforcement submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
sensor(config-sig)# signatures 60001 0
sensor(config-sig-sig)# engine application-policy-enforcement-http
 
   

Step 3 Specify the event action.

sensor(config-sig-sig-app)# event-action produce-alert|log-pair-packets
 
   

Step 4 Define the signature type.

sensor(config-sig-sig-app)# signature-type content-type define-content-type
 
   

Step 5 Define the content type.

sensor(config-sig-sig-app-def)# name MyContent
 
   

Step 6 Verify your settings.

sensor(config-sig-sig-app-def)# show settings
-> define-content-type
   -----------------------------------------------
      name: MyContent
*---> content-type-details
      -----------------------------------------------
      -----------------------------------------------
   -----------------------------------------------
sensor(config-sig-sig-app-def)#
 
   

Step 7 Exit signatures submode.

sensor(config-sig-sig-app-def)# exit
sensor(config-sig-sig-app)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 8 Press Enter to apply the changes or enter no to discard them.


Configuring IP Fragment Reassembly

This section describes IP fragment reassembly, lists the IP fragment reassembly signatures with the configurable parameters, describes how to configure these parameters, and how to configure the method for IP fragment reassembly. It contains the following topics:

Understanding IP Fragment Reassembly

IP Fragment Reassembly Signatures and Configurable Parameters

Configuring IP Fragment Reassembly Parameters

Configuring the Method for IP Fragment Reassembly

Understanding IP Fragment Reassembly


Note You configure the IP fragment reassembly per signature.


You can configure the sensor to reassemble a datagram that has been fragmented over multiple packets. You can specify boundaries that the sensor uses to determine how many datagram fragments it reassembles and how long to wait for more fragments of a datagram. The goal is to ensure that the sensor does not allocate all its resources to datagrams that cannot be completely reassembled, either because the sensor missed some frame transmissions or because an attack has been launched that is based on generating random fragmented datagrams.

IP Fragment Reassembly Signatures and Configurable Parameters

Table 8-5 lists IP fragment reassembly signatures with the parameters that you can configure for IP fragment reassembly. The IP fragment reassembly signatures are part of the Normalizer engine.

Table 8-5 IP Fragment Reassembly Signatures 

Signature ID and Name
Description
Parameter With Default Value and Range
Default Action

1200 IP Fragmentation Buffer Full

Fires when the total number of fragments in the system exceeds the threshold set by Max Fragments.

Specify Max Fragments 10000
(0-42000)

Deny Packet Inline
Produce Alert1

1201 Fragment Overlap

Fires when the fragments queued for a datagram overlap each other.

None2

 

1202 Datagram Too Long

Fires when the fragment data (offset and size) exceeds the threshold set with Max Datagram Size.

Specify Max Datagram Size 65536 (2000-65536)

Deny Packet Inline
Produce Alert3

1203 Fragment Overwrite

Fires when the fragments queued for a datagram overlap each other and the overlapping data is different.4

None

Deny Packet Inline
Produce Alert5

1204 No Initial Fragment

Fires when the datagram is incomplete and missing the initial fragment.

None

Deny Packet Inline
Produce Alert6

1205 Too Many Datagrams

Fires when the total number of partial datagrams in the system exceeds the threshold set by Max Partial Datagrams.

Specify Max Partial Datagrams 1000 (0-10000)

Deny Packet Inline
Produce Alert7

1206 Fragment Too Small

Fires when there are more than Max Small Frags of a size less than Min Fragment Size in one datagram.8

Specify Max Small Frags 2 (8-1500)
Specify Min Fragment Size 400 (1-8)

Deny Packet Inline
Produce Alert9

1207 Too Many Fragments

Fires when there are more than Max Fragments per Datagram in one datagram.

Specify Max Fragments per Datagram 170 (0-8192)

Deny Packet Inline
Produce Alert10

1208 Incomplete Datagram

Fires when all of the fragments for a datagram have not arrived during the Fragment Reassembly Timeout.11

Specify Fragment Reassembly Timeout 60 (0-360)

Deny Packet Inline
Produce Alert12

1220 Jolt2 Fragment Reassembly DoS attack

Fires when multiple fragments are received all claiming to be the last fragment of an IP datagram.

Specify Max Last Fragments 4 (1-50)

Deny Packet Inline
Produce Alert13

1225 Fragment Flags Invalid

Fires when a bad combination of fragment flags is detected.

None14

 

1 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packets and all associated fragments for this datagram. If you disable this signature, the default values are still used and packets are dropped (inline mode) or not analyzed (promiscuous mode) and no alert is sent.

2 This signature does not fire when the datagram is an exact duplicate. Exact duplicates are dropped in inline mode regardless of the settings. Modify Packet Inline removes the overlapped data from all but one fragment so there is no ambiguity about how the endpoint treats the datagram. Deny Connection Inline has no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

3 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram. Regardless of the actions set the datagram is not processed by the IPS if the datagram is larger than the Max Datagram size.

4 This is a very unusual event.

5 Modify Packet Inline removes the overlapped data from all but one fragment so there is no ambiguity about how the endpoint treats the datagram. Deny Connection Inline has no effect on this signature. Deny Packet Inline drops the packets and all associated fragments for this datagram.

6 IPS does not inspect a datagram missing the first fragments regardless of the settings. Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

7 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

8 IPS does not inspect the datagram if this signature is on and the number of small fragments is exceeded.

9 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

10 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

11 The timer starts when the packet for the datagram arrives.

12 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

13 Modify Packet Inline and Deny Connection Inline have no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.

14 Modify Packet Inline modifies the flags to a valid combination. Deny Connection Inline has no effect on this signature. Deny Packet Inline drops the packet and all associated fragments for this datagram.


For More Information

For more information about the Normalizer engine, see Normalizer Engine.

Configuring IP Fragment Reassembly Parameters

To configure IP fragment reassembly parameters for a specific signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify the IP fragment reassembly signature ID and subsignature ID.

sensor(config-sig)# signatures 1200 0
 
   

Step 4 Specify the engine.

sensor(config-sig-sig)# engine normalizer
 
   

Step 5 Enter edit default signatures submode.

sensor(config-sig-sig-nor)# edit-default-sigs-only default-signatures-only
 
   

Step 6 Enable and change the default setting (if desired) of any of the IP fragment reassembly parameter for signature 1200 for example, specifying the maximum fragments.

sensor(config-sig-sig-nor-def)# specify-max-fragments yes
sensor(config-sig-sig-nor-def-yes)# max-fragments 20000
 
   

Step 7 Verify the settings.

sensor(config-sig-sig-nor-def-yes)# show settings
   yes
   -----------------------------------------------
      max-fragments: 20000 default: 10000
   -----------------------------------------------
sensor(config-sig-sig-nor-def-yes)#
 
   

Step 8 Exit signature definition submode.

sensor(config-sig-sig-nor-def-yes)# exit
sensor(config-sig-sig-nor-def)# exit
sensor(config-sig-sig-nor)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 9 Press Enter for apply the changes or enter no to discard them.


Configuring the Method for IP Fragment Reassembly

Use the fragment-reassembly command in the signature definition submode to configure the method the sensor will use to reassemble fragments. You can configure this option if your sensor is operating in promiscuous mode. If your sensor is operating in inline mode, the method is NT only.

The following options apply:

ip-reassemble-mode—Identifies the method the sensor uses to reassemble the fragments based on the operating system:

nt—Windows systems (default).

solaris—Solaris systems.

linux—GNU/Linux systems.

bsd—BSD UNIX systems.

To configure the method for IP fragment reassembly, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter fragment reassembly submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
sensor(config-sig)# fragment-reassembly
 
   

Step 3 Configure the operating system you want the sensor to use to reassemble IP fragments.

sensor(config-sig-fra)# ip-reassemble-mode linux
 
   

Step 4 Verify the setting.

sensor(config-sig-fra)# show settings
   fragment-reassembly
   -----------------------------------------------
      ip-reassemble-mode: linux default: nt
   -----------------------------------------------
sensor(config-sig-fra)#
 
   

Step 5 Exit signature definition submode.

sensor(config-sig-fra)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 6 Press Enter to apply the changes or enter no to discard them.


Configuring TCP Stream Reassembly

This section describes TCP stream reassembly, lists the TCP stream reassembly signatures with the configurable parameters, describes how to configure TCP stream signatures, and how to configure the mode for TCP stream reassembly. It contains the following topics:

Understanding TCP Stream Reassembly

TCP Stream Reassembly Signatures and Configurable Parameters

Configuring TCP Stream Reassembly Signatures

Configuring the Mode for TCP Stream Reassembly

Understanding TCP Stream Reassembly

You can configure the sensor to monitor only TCP sessions that have been established by a complete three-way handshake. You can also configure how long to wait for the handshake to complete, and how long to keep monitoring a connection where no more packets have been seen. The goal is to prevent the sensor from creating alerts where a valid TCP session has not been established. There are known attacks against sensors that try to get the sensor to generate alerts by simply replaying pieces of an attack. The TCP session reassembly feature helps to mitigate these types of attacks against the sensor.

You configure TCP stream reassembly parameters per signature. You can configure the mode for TCP stream reassembly.

TCP Stream Reassembly Signatures and Configurable Parameters

Table 8-6 lists TCP stream reassembly signatures with the parameters that you can configure for TCP stream reassembly. TCP stream reassembly signatures are part of the Normalizer engine.

Table 8-6 TCP Stream Reassembly Signatures 

Signature ID and Name
Description
Parameter With Default Value and Range
Default Actions

1301 TCP Session Inactivity Timeout1

Fires when a TCP session has been idle for a TCP Idle Timeout.

TCP Idle Timeout 3600 (15-3600)

2

1302 TCP Session Embryonic Timeout3

Fires when a TCP session has not completes the three-way handshake in TCP embryonic timeout seconds.

TCP Embryonic Timeout 15
(3-300)

4

1303 TCP Session Closing Timeout5

Fires when a TCP session has not closed completely in TCP Closed Timeout seconds after the first FIN.

TCP Closed Timeout 5 (1-60)

6

1304 TCP Session Packet Queue Overflow

This signature allows for setting the internal TCP Max Queue size value for the Normalizer engine. As a result it does not function in promiscuous mode. By default this signature does not fire an alert. If a custom alert event is associated with this signature and if the queue size is exceeded, an alert fires.

Note The IPS signature team discourages modifying this value.

TCP Max Queue 32
(0-128)
TCP Idle Timeout 3600

7

1305 TCP Urg Flag Set8

Fires when the TCP urgent flag is seen

TCP Idle Timeout 3600

Modify Packet Inline9

1306 0 TCP Option Other

Fires when a TCP option in the range of TCP Option Number is seen. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Option Number 6-7,9-255
(Integer Range Allow Multiple 0-255 constraints)
TCP Idle Timeout 3600

Modify Packet Inline
Produce Alert10

1306 1 TCP SACK Allowed Option

Fires when a TCP selective ACK allowed option is seen. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Idle Timeout 3600

Modify Packet Inline11

1306 2 TCP SACK Data Option

Fires when a TCP selective ACK data option is seen. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Idle Timeout 3600

Modify Packet Inline12

1306 3 TCP Timestamp Option

Fires when a TCP timestamp option is seen. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Idle Timeout 3600

Modify Packet Inline13

1306 4 TCP Window Scale Option

Fires when a TCP window scale option is seen. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Idle Timeout 3600

Modify Packet Inline14

1306 5 TCP MSS Option

Fires when a TCP MSS option is detected. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Idle Timeout 3600

Modify Packet Inline

1306 6 TCP option data after EOL option

Fires when the TCP option list has data after the EOL option. All 1306 signatures fire an alert and do not function in promiscuous mode.

TCP Idle Timeout 3600

Modify Packet Inline

1307 TCP Window Variation

Fires when the right edge of the recv window for TCP moves to the right (decreases).

TCP Idle Timeout 3600

Deny Connection Inline
Produce Alert15

1308 TTL Evasion16

Fires when the TTL seen on one direction of a session is higher than the minimum that has been observed.

TCP Idle Timeout 3600

Modify Packet Inline17

1309 TCP Reserved Flags Set

Fires when the reserved bits (including bits used for ECN) are set on the TCP header.

TCP Idle Timeout 3600

Modify Packet Inline
Produce Alert18

1311 TCP Packet Exceeds MSS

Fires when a packet exceeds the MSS that was exchanged during the three-way handshake.

TCP Idle Timeout 3600

Produce Alert19

1312 TCP MSS Below Minimum

Fires when the MSS value in a packet containing a SYN flag is less that TCP Min MSS.

TCP Min MSS 400
(0-16000)
TCP Idle Timeout 3600

Modify Packet Inline20

1313 TCP Max MSS

Fires when the MSS value in a packet containing a SYN flag exceed TCP Max MSS

TCP Max MSS1460
(0-16000)

Modify Packet Inline disabled21

1314 TCP Data SYN

Fires when TCP payload is sent in the SYN packet.

Deny Packet Inline disabled22

1315 ACK Without TCP Stream

Fires when an ACK packet is sent that does not belong to a stream.

Produce Alert disabled23

1317 Zero Window Probe

Fires when a zero window probe packet is detected.

Modify Packet Inline removes data from the Zero Window Probe packet.

Modify Packet Inline

133024 0 TCP Drop - Bad Checksum

Fires when TCP packet has bad checksum.

Modify Packet Inline corrects the checksum.

Deny Packet Inline

1330 1 TCP Drop - Bad TCP Flags

Fires when TCP packet has bad flag combination.

Deny Packet Inline

1330 2 TCP Drop - Urgent Pointer With No Flag

Fires when TCP packet has a URG pointer and no URG flag.

Modify Packet Inline clears the pointer.

Modify Packet Inline disabled

1330 3 TCP Drop - Bad Option List

Fires when TCP packet has a bad option list.

Deny Packet Inline

1330 4 TCP Drop - Bad Option Length

Fires when TCP packet has a bad option length.

Deny Packet Inline

1330 5 TCP Drop - MSS Option Without SYN

Fires when TCP MSS option is seen in packet without the SYN flag set.

Modify Packet Inline clears the MSS option.

Modify Packet Inline

1330 6 TCP Drop - WinScale Option Without SYN

Fires when TCP window scale option is seen in packet without the SYN flag set.

Modify Packet Inline clears the window scale option.

Modify Packet Inline

1330 7 TCP Drop - Bad WinScale Option Value

Fires when a TCP packet has a bad window scale value.

Modify Packet Inline sets the value to the closest constraint value.

Modify Packet Inline

1330 8 TCP Drop - SACK Allow Without SYN

Fires when the TCP SACK allowed option is seen in a packet without the SYN flags set.

Modify Packet Inline clears the SACK allowed option.

Modify Packet Inline

1330 9 TCP Drop - Data in SYN|ACK

Fires when TCP packet with SYN and ACK flags set also contains data.

Deny Packet Inline

1330 10 TCP Drop - Data Past FIN

Fires when TCP data is sequenced after FIN.

Deny Packet Inline

1330 11 TCP Drop - Timestamp not Allowed

Fires when TCP packet has timestamp option when timestamp option is not allowed.

Deny Packet Inline

1330 12 TCP Drop - Segment Out of Order

Fires when TCP segment is out of order and cannot be queued.

Deny Packet Inline

1330 13 TCP Drop - Invalid TCP Packet

Fires when TCP packet has invalid header.

Deny Packet Inline

1330 14 TCP Drop - RST or SYN in window

Fires when TCP packet with RST or SYN flag was sent in the sequence window but was not the next sequence.

Deny Packet Inline

1330 15 TCP Drop - Segment Already ACKed

Fires when TCP packet sequence is already ACKed by peer (excluding keepalives).

Deny Packet Inline

1330 16 TCP Drop - PAWS Failed

Fires when TCP packet fails PAWS check.

Deny Packet Inline

1330 17 TCP Drop - Segment out of State Order

Fires when TCP packet is not proper for the TCP session state.

Deny Packet Inline

1330 18 TCP Drop - Segment out of Window

Fires when TCP packet sequence number is outside of allowed window.

Deny Packet Inline

3050 Half Open SYN Attack

 

syn-flood-max-embryonic 5000

 

3250 TCP Hijack

 

max-old-ack 200

 

3251 TCP Hijack Simplex Mode

 

max-old-ack 100

 

1 The timer is reset to 0 after each packet on the TCP session. by default, this signature does not produce an alert. You can choose to produce alerts for expiring TCP connections if desired. A statistic of total number of expired flows is updated any time a flow expires.

2 Modify Packet Inline, Deny Connection Inline, and Deny Packet Inline have no effect on this signature.

3 The timer starts with the first SYN packet and is not reset. State for the session is reset and any subsequent packets for this flow appear to be out of order (unless it is a SYN).

4 Modify Packet Inline, Deny Connection Inline, and Deny Packet Inline have no effect on this signature.

5 The timer starts with the first FIN packet and is not reset. State for the session is reset and any subsequent packets for this flow appear to be out of order (unless it is a SYN).

6 Modify Packet Inline, Deny Connection Inline, and Deny Packet Inline have no effect on this signature.

7 Modify Packet Inline and Deny Packet Inline have no effect on this signature. Deny Connection Inline drops the current packet and the TCP session.

8 Phrak 57 describes a way to evade security policy using URG pointers. You can normalize the packet when it is in inline mode with this signature.

9 Modify Packet Inline strips the URG flag and zeros the URG pointer from the packet. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

10 Modify Packet Inline strips the selected option(s) from the packet. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

11 Modify Packet Inline strips the selected ACK allowed option from the packet. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

12 Modify Packet Inline strips the selected ACK allowed option from the packet. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

13 Modify Packet Inline strips the timestamp option from the packet. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

14 Modify Packet Inline strips the window scale option from the packet. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

15 Modify Packet Inline has no effect on this signature. Deny Connection Inline drops the current packet and the TCP connection. Deny Packet Inline drops the packet.

16 This signature is used to cause TTLs to monotonically decrease for each direction on a session. For example, if TTL 45 is the lowest TTL seen from A to B, then all future packets from A to B will have a maximum of 45 if Modify Packet Inline is set. Each new low TTL becomes the new maximum for packets on that session.

17 Modify Packet Inline ensures that the IP TTL monotonically decreases. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

18 Modify Packet Inline clears all reserved TCP flags. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

19 Modify Packet Inline has no effect on this signature. Deny Connection Inline drops the current packet and the TCP connection. Deny Packet Inline drops the packet.

20 2.4.21-15.EL.cisco.1 Modify Packet Inline raises the MSS value to TCP Min MSS. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet 2.4.21-15.EL.cisco.1.

21 Modify Packet Inline lowers the MSS value to TCP Max MSS. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet 2.4.21-15.EL.cisco.1.

22 Modify Packet Inline has no effect on this signature. Deny Connection Inline drops the current packet and the TCP session. Deny Packet Inline drops the packet.

23 Modify Packet Inline, Deny Connection Inline, and Deny Packet Inline have no effect on this signature. By default, the 1330 signatures drop packets for which this signature sends alerts.

24 These subsignatures represent the reasons why the Normalizer might drop a TCP packet. By default these subsignatures drop packets. These subsignatures let you permit packets that fail the checks in the Normalizer through the IPS. The drop reasons have an entry in the TCP statistics. By default these subsignatures do not produce an alert.


For More Information

For more information about the Normalizer engine, see Normalizer Engine.

Configuring TCP Stream Reassembly Signatures

To configure TCP stream reassembly for a specific signature, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify the TCP stream reassembly signature ID and subsignature ID.

sensor(config-sig)# signatures 1313 0
 
   

Step 4 Specify the engine.

sensor(config-sig-sig)# engine normalizer
 
   

Step 5 Enter edit default signatures submode.

sensor(config-sig-sig-nor)# edit-default-sigs-only default-signatures-only
 
   

Step 6 Enable and change the default setting (if desired) of the maximum MSS parameter for signature 1313.

sensor(config-sig-sig-nor-def)# specify-tcp-max-mss yes
sensor(config-sig-sig-nor-def-yes)# tcp-max-mss 1380
 
   

Note Changing this parameter from the default of 1460 to 1380 helps prevent fragmentation of traffic going through a VPN tunnel.


Step 7 Verify the settings.

sensor(config-sig-sig-nor-def-yes)# show settings
   yes
   -----------------------------------------------
      tcp-max-mss: 1380 default: 1460
   -----------------------------------------------
sensor(config-sig-sig-nor-def-yes)#
 
   

Step 8 Exit signature definition submode.

sensor(config-sig-sig-nor-def-yes)# exit
sensor(config-sig-sig-nor-def)# exit
sensor(config-sig-sig-nor)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 9 Press Enter for apply the changes or enter no to discard them.


Configuring the Mode for TCP Stream Reassembly

Use the stream-reassembly command in the signature definition submode to configure the mode that the sensor will use to reassemble TCP sessions.


Note The parameters tcp-3-way-handshake-required and tcp-reassembly-mode only impact sensors inspecting traffic in promiscuous mode, not inline mode. To configure asymmetric options for sensors inspecting inline traffic, use the inline-TCP-evasion-protection-mode parameter.


The following options apply:

tcp-3-way-handshake-required {true | false}—Specifies that the sensor should only track sessions for which the 3-way handshake is completed. The default is true.

tcp-reassembly-mode—Specifies the mode the sensor should use to reassemble TCP sessions.

strict—Only allows the next expected in the sequence (default).

loose—Allows gaps in the sequence.

asym—Allows asymmetric traffic to be reassembled.


Caution The asymmetric option disables TCP window evasion checking.

To configure the TCP stream reassembly parameters, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter TCP stream reassembly submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
sensor(config-sig)# stream-reassembly
 
   

Step 3 Specify that the sensor should only track session for which the 3-way handshake is completed.

sensor(config-sig-str)# tcp-3-way-handshake-required true
 
   

Step 4 Specify the mode the sensor should use to reassemble TCP sessions.

sensor(config-sig-str)# tcp-reassembly-mode strict
 
   

Step 5 Verify the settings.

sensor(config-sig-str)# show settings
   stream-reassembly
   -----------------------------------------------
      tcp-3-way-handshake-required: true default: true
      tcp-reassembly-mode: strict default: strict
   -----------------------------------------------
sensor(config-sig-str)#
 
   

Step 6 Exit signature definition submode.

sensor(config-sig-str)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 7 Press Enter to apply the changes or enter no to discard them.


For More Information

For information on asymmetric inspection options for sensors configured in inline mode, see Inline TCP Session Tracking Mode and Adding, Editing, and Deleting Virtual Sensors.

Configuring IP Logging

You can configure a sensor to generate an IP session log when the sensor detects an attack. When IP logging is configured as a response action for a signature and the signature is triggered, all packets to and from the source address of the alert are logged for a specified period of time.


Note IP logging allows a maximum limit of 20 concurrent IP log files. Once the limit of 20 is reached, you receive the following message in main.log: Cid/W errWarnIpLogProcessor::addIpLog: Ran out of file descriptors.


Use the ip-log command in the signature definition submode to configure IP logging.

The following options apply:

ip-log-bytes—Identifies the maximum number of bytes you want logged. The valid value is 0 to 2147483647. The default is 0.

ip-log-packets—Identifies the number of packets you want logged. The valid value is 0 to 65535. The default is 0.

ip-log-time—Identifies the duration you want the sensor to log. The valid value is 30 to 300 seconds. The default is 30 seconds.


Note When the sensor meets any one of the IP logging conditions, it stops IP logging.


To configure the IP logging parameters, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter IP log submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
sensor(config-sig)# ip-log
 
   

Step 3 Specify the IP logging parameters:

a. Specify the maximum number of bytes you want logged.

sensor(config-sig-ip)# ip-log-bytes 200000
 
   

b. Specify the number of packets you want logged.

sensor(config-sig-ip)# ip-log-packets 150
 
   

c. Specify the length of time you want the sensor to log.

sensor(config-sig-ip)# ip-log-time 60
 
   

Step 4 Verify the settings.

sensor(config-sig-ip)# show settings
   ip-log
   -----------------------------------------------
      ip-log-packets: 150 default: 0
      ip-log-time: 60 default: 30
      ip-log-bytes: 200000 default: 0
   -----------------------------------------------
sensor(config-sig-ip)#
 
   

Step 5 Exit signature definition submode.

sensor(config-sig-ip)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 6 Press Enter to apply the changes or enter no to discard them.


Creating Custom Signatures

This section describes how to create custom signatures and contains the following topics:

Sequence for Creating a Custom Signature

Example String TCP Engine Signature

Example Service HTTP Engine Signature

Example Meta Engine Signature

Example IPv6 Engine Signature

Sequence for Creating a Custom Signature

Use the following sequence when you create a custom signature:


Step 1 Select a signature engine.

Step 2 Assign the signature identifiers:

Signature ID

SubSignature ID

Signature name

Alert notes (optional)

User comments (optional)

Step 3 Assign the engine-specific parameters. The parameters differ for each signature engine, although there is a group of master parameters that applies to each engine.

Step 4 Assign the alert response:

Signature fidelity rating

Severity of the alert

Step 5 Assign the alert behavior.

Step 6 Apply the changes.


Example String TCP Engine Signature


Note This procedure also applies to String UDP and ICMP signatures.


The String engine is a generic-based pattern-matching inspection engine for ICMP, TCP, and UDP protocols. The String engine uses a regular expression engine that can combine multiple patterns into a single pattern-matching table allowing for a single search through the data. There are three String engines: String ICMP, String TCP, and String UDP. The following example demonstrates how to create a custom String TCP signature.

The following options apply:

default—Sets the value back to the system default setting.

direction—Specifies the direction of the traffic:

from-service—Traffic from service port destined to client port.

to-service—Traffic from client port destined to service port.

event-action—Specifies the action(s) to perform when alert is triggered:

deny-attacker-inline —(Inline only) does not transmit this packet and future packets from the attacker address for a specified period of time.

deny-attacker-service-pair-inline(Inline only) Does not transmit this packet and future packets on the attacker address victim port pair for a specified period of time.

deny-attacker-victim-pair-inline(Inline only) Does not transmit this packet and future packets on the attacker/victim address pair for a specified period of time.

deny-connection-inline—(Inline only) Does not transmit this packet and future packets on the TCP Flow.

deny-packet-inline—(Inline only) Does not transmit this packet.

log-attacker-packets—Starts IP logging of packets containing the attacker address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-pair-packets—Starts IP logging of packets containing the attacker-victim address pair. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-victim-packets—Starts IP logging of packets containing the victim address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

produce-alert —Writes the event to Event Store as an alert.

produce-verbose-alert—Includes an encoded dump (possibly truncated) of the offending packet in the alert. This action causes an alert to be written to the Event Store, even if produce-alert is not selected.

request-block-connection—Sends a request to ARC to block this connection. You must have blocking devices configured to implement this action.

request-block-host—Sends a request to ARC to block this attacker host. You must have blocking devices configured to implement this action.

request-rate-limit—Sends a rate limit request to ARC to perform rate limiting. You must have rate limiting devices configured to implement this action.

request-snmp-trap—Sends a request to the Notification Application component of the sensor to perform SNMP notification. This action causes an alert to be written to the Event Store, even if produce-alert is not selected. You must have SNMP configured on the sensor to implement this action.

reset-tcp-connection—Sends TCP resets to hijack and terminate the TCP flow. Reset TCP Connection only works on TCP signatures that analyze a single connection. It does not work for sweeps or floods.

modify-packet-inline— Modifies packet data to remove ambiguity about what the end point might do with the packet.

no—Removes an entry or selection setting.

regex-string —Specifies a regular expression to search for in a single TCP packet.

service-ports—Specifies the ports or port ranges where the target service may reside. The valid range is 0 to 65535. It is a separated list of integer ranges a-b[,c-d] within 0 to 65535. The second number in the range must be greater than or equal to the first number.

specify-exact-match-offset {yes | no}—(Optional) Enables exact-match-offset:

exact-match-offset—Specifies the exact stream offset the regular expression string must report for a match to be valid. The valid range is 0 to 65535.

specify-min-match-length {yes | no}—(Optional) Enables min-match-length:

min-match-length—Specifies the minimum number of bytes the regular expression string must match. The valid range is 0 to 65535.

strip-telnet-options {true | false}—Strips Telnet option characters from data before searching.

swap-attacker-victim {true | false}—Swaps the attacker and victim addresses and ports (source and destination) in the alarm message and for any actions taken. The default is false for no swapping.

To create a signature based on the String TCP engine, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify a signature ID and subsignature ID for the signature. Custom signatures are in the range of 60000 to 65000.

sensor(config-sig)# signatures 60025 0
 
   

Step 4 Enter signature description submode.

sensor(config-sig-sig)# sig-description
 
   

Step 5 Specify a name for the new signature. You can also specify a additional comments about the sig using the sig-comment command or additional information about the signature using the sig-string-info command.

sensor(config-sig-sig-sig)# sig-name This is my new name
 
   

Step 6 Exit signature description submode.

sensor(config-sig-sig-sig)# exit
 
   

Step 7 Specify the string TCP engine.

sensor(config-sig-sig)# engine string-tcp
 
   

Step 8 Specify the service ports.

sensor(config-sig-sig-str)# service-ports 23
 
   

Step 9 Specify the direction.

sensor(config-sig-sig-str)# direction to-service
 
   

Step 10 Specify the regex string to search for in the TCP packet. You can change the event actions if needed according to your security policy using the event-action command. The default event action is produce-alert.

sensor(config-sig-sig-str)# regex-string This-is-my-new-Sig-regex
 
   

Step 11 You can also modify the following optional parameters for this custom String TCP signature:

specify-exact-match-offset

specify-min-match-length

strip-telnet-options

swap-attacker-victim.

Step 12 Verify the settings.

sensor(config-sig-sig-str)# show settings
   string-tcp
   -----------------------------------------------
      event-action: produce-alert <defaulted>
      strip-telnet-options: false <defaulted>
      specify-min-match-length
      -----------------------------------------------
         no
         -----------------------------------------------
         -----------------------------------------------
      -----------------------------------------------
      regex-string: This-is-my-new-Sig-regex
      service-ports: 23
      direction: to-service default: to-service
      specify-exact-match-offset
      -----------------------------------------------
         no
         -----------------------------------------------
            specify-max-match-offset
            -----------------------------------------------
               no
               -----------------------------------------------
               -----------------------------------------------
            -----------------------------------------------
            specify-min-match-offset
            -----------------------------------------------
               no
               -----------------------------------------------
               -----------------------------------------------
            -----------------------------------------------
         -----------------------------------------------
      -----------------------------------------------
      swap-attacker-victim: false <defaulted>
   -----------------------------------------------
sensor(config-sig-sig-str)#
 
   

Step 13 Exit signature definition submode.

sensor(config-sig-sig-str)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 14 Press Enter to apply the changes or enter no to discard them.


Example Service HTTP Engine Signature

The Service HTTP engine is a service-specific string-based pattern-matching inspection engine. The HTTP protocol is one of the most commonly used in networks of today. In addition, it requires the most amount of preprocessing time and has the most number of signatures requiring inspection making it critical to the overall performance of the system.

The Service HTTP engine uses a Regex library that can combine multiple patterns into a single pattern-matching table allowing a single search through the data. This engine searches traffic directed to web services only to web services, or HTTP requests. You cannot inspect return traffic with this engine. You can specify separate web ports of interest in each signature in this engine.

HTTP deobfuscation is the process of decoding an HTTP message by normalizing encoded characters to ASCII equivalent characters. It is also known as ASCII normalization.

Before an HTTP packet can be inspected, the data must be deobfuscated or normalized to the same representation that the target system sees when it processes the data. It is ideal to have a customized decoding technique for each host target type, which involves knowing what operating system and web server version is running on the target. The Service HTTP engine has default deobfuscation behavior for the Microsoft IIS web server.

The following options apply:

de-obfuscate {true | false}—Applies anti-evasive deobfuscation before searching.

default—Sets the value back to the system default setting.

event-action —Specifies the action(s) to perform when alert is triggered:

deny-attacker-inline —(Inline only) does not transmit this packet and future packets from the attacker address for a specified period of time.

deny-attacker-service-pair-inline(Inline only) Does not transmit this packet and future packets on the attacker address victim port pair for a specified period of time.

deny-attacker-victim-pair-inline(Inline only) Does not transmit this packet and future packets on the attacker/victim address pair for a specified period of time.

deny-connection-inline—(Inline only) Does not transmit this packet and future packets on the TCP Flow.

deny-packet-inline—(Inline only) Does not transmit this packet.

log-attacker-packets—Starts IP logging of packets containing the attacker address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-pair-packets—Starts IP logging of packets containing the attacker-victim address pair. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

log-victim-packets—Starts IP logging of packets containing the victim address. This action causes an alert to be written to Event Store, even if produce-alert is not selected.

produce-alert —Writes the event to Event Store as an alert.

produce-verbose-alert—Includes an encoded dump (possibly truncated) of the offending packet in the alert. This action causes an alert to be written to the Event Store, even if produce-alert is not selected.

request-block-connection—Sends a request to ARC to block this connection. You must have blocking devices configured to implement this action.

request-block-host—Sends a request to ARC to block this attacker host. You must have blocking devices configured to implement this action.

request-rate-limit—Sends a rate limit request to ARC to perform rate limiting. You must have rate limiting devices configured to implement this action.

request-snmp-trap—Sends a request to the Notification Application component of the sensor to perform SNMP notification. This action causes an alert to be written to the Event Store, even if produce-alert is not selected. You must have SNMP configured on the sensor to implement this action.

reset-tcp-connection—Sends TCP resets to hijack and terminate the TCP flow. Reset TCP Connection only works on TCP signatures that analyze a single connection. It does not work for sweeps or floods.

modify-packet-inline— Modifies packet data to remove ambiguity about what the end point might do with the packet.

max-field-sizes —Enables grouping for maximum field sizes:

specify-max-arg-field-length {yes | no}—Enables max-arg-field-length (optional).

specify-max-header-field-length {yes | no}—Enables max-header-field-length (optional).

specify-max-request-length {yes | no}—Enables max-request-length (optional).

specify-max-uri-field-length {yes | no}—Enables max-uri-field-length (optional).

no—Removes an entry or selection setting.

regex—Enables regular expression grouping:

specify-arg-name-regex—(Optional) Enables arg-name-regex.

specify-header-regex—(Optional) Enables header-regex.

specify-request-regex—(Optional) Enables request-regex.

specify-uri-regex—(Optional) Enables uri-regex.

service-ports —Specifies a comma-separated list of ports or port ranges where the target service may reside.

swap-attacker-victim {true | false}—Swaps the attacker and victim addresses and ports (source and destination) in the alarm message and for any actions taken. The default is false for no swapping.

To create a custom signature based on the Service HTTP engine, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify a signature ID and a subsignature ID for the signature.

sensor(config-sig)# signatures 63000 0 
 
   

Custom signatures are in the range of 60000 to 65000.

Step 4 Enter signature description mode.

sensor(config-sig-sig)# sig-description
 
   

Step 5 Specify a signature name.

sensor(config-sig-sig-sig)# sig-name myWebSig
 
   

Step 6 Specify the alert traits. The valid range is from 0 to 65535.

sensor(config-sig-sig-sig)# alert-traits 2
 
   

Step 7 Exit signature description submode.

sensor(config-sig-sig-sig)# exit
 
   

Step 8 Specify the alert frequency.

sensor(config-sig-sig)# alert-frequency
sensor(config-sig-sig-ale)# summary-mode fire-all
sensor(config-sig-sig-ale-fir)# summary-key Axxx
sensor(config-sig-sig-ale-fir)# specify-summary-threshold yes
sensor(config-sig-sig-ale-fir-yes)# summary-threshold 200
 
   

Step 9 Exit alert frequency submode.

sensor(config-sig-sig-ale-fir-yes)# exit
sensor(config-sig-sig-ale-fir)# exit
sensor(config-sig-sig-ale)# exit
 
   

Step 10 Configure the signature to apply anti-evasive deobfuscation before searching:

sensor(config-sig-sig)# engine service-http
sensor(config-sig-sig-ser)# de-obfuscate true
 
   

Step 11 Configure the Regex parameters.

sensor(config-sig-sig)# engine service-http
sensor(config-sig-sig-ser)# regex
sensor(config-sig-sig-ser-reg)# specify-uri-regex yes
sensor(config-sig-sig-ser-reg-yes)# uri-regex [Mm][Yy][Ff][Oo][Oo]
 
   

Step 12 Exit Regex submode.

sensor(config-sig-sig-ser-reg-yes)# exit
sensor(config-sig-sig-ser-reg-)# exit
 
   

Step 13 Configure the service ports using the signature variable WEBPORTS:

sensor(config-sig-sig-ser)# service-ports $WEBPORTS
 
   

Step 14 Exit signature definition submode.

sensor(config-sig-sig-ser)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes:?[yes]:
 
   

Step 15 Press Enter to apply the changes or enter no to discard them.


Example Meta Engine Signature


Caution A large number of Meta signatures could adversely affect overall sensor performance.


Note The Meta engine is different from other engines in that it takes alerts as input where most engines take packets as input.



Note The Meta engine enhancement is available in IPS 7.0(2)E4 and later.


The Meta engine defines events that occur in a related manner within a sliding time interval. This engine processes events rather than packets. As signature events are generated, the Meta engine inspects them to determine if they match any or several Meta definitions. The Meta engine generates a signature event after all requirements for the event are met.

All signature events are handed off to the Meta engine by the Signature Event Action Processor. The Signature Event Action Processor hands off the event after processing the minimum hits option. Summarization and event action are processed after the Meta engine has processed the component events.

The following options apply:

component-list name1—Specifies the list of Meta components:

edit—Edits an existing entry in the list.

insert —Inserts a new entry into the list.

move—Moves an entry in the list.

begin—Places the entry at the beginning of the active list.

end—Places the entry at the end of the active list.

inactive—Places the entry into the inactive list.

before—Places the entry before the specified entry.

after—Places the entry after the specified entry.

component-count—Specifies the number of times component must fire before this component is satisfied.

component-sig-id—Specifies the signature ID of the signature to match this component on.

component-subsig-id—Specifies the subsignature ID of the signature on which to match this component.

is-not-component {true | false}—Specifies that the component is a NOT component.

component-list-in-order {true | false}—Specifies whether to have the component list fire in order. For example, if signature 1001 in the m2 component fires before signature 1000 in the m1 component, the Meta signature will not fire.

all-components-required {true | false}—Specifies to use all components. This option works with the all-not-components-required option, if you have NOT components configured as required, the Meta signature will not fire.

all-not-components-required {true | false}—Specifies to use all of the NOT components.

swap-attacker-victim {true | false}—Swaps the attacker and victim addresses and ports (source and destination) in the alert message and in any actions taken.

meta-reset-interval—Specifies the time in seconds to reset the Meta signature. The valid range is 0 to 3600 seconds. The default is 60 seconds.

meta-key—Specifies the storage type for the Meta signature:

AaBb—Attacker and victim addresses and ports.

AxBx—Attacker and victim addresses.

Axxx—Attacker address.

xxBx—Victim address.

unique-victim-ports—Specifies the number of unique victims ports required per Meta signature. The valid range is 1 to 256.

event-action—Specifies the action(s) to perform when an alert is triggered:

deny-attacker-inline (inline only)—Does not transmit this packet and future packets from the attacker address for a specified period of time.

deny-attacker-service-pair-inline (inline only)—Does not transmit this packet and future packets on the attacker address victim port pair for a specified period of time.

deny-attacker-victim-pair-inline (inline only)—Does not transmit this packet and future packets on the attacker/victim address pair for a specified period of time.

deny-connection-inline (inline only)—Does not transmit this packet and future packets on the TCP flow.

deny-packet-inline (inline only)—Does not transmit this packet.

log-attacker-packets—Starts IP logging of packets containing the attacker address.

log-pair-packets—Starts IP logging of packets containing the attacker-victim address pair.

log-victim-packets—Starts IP logging of packets containing the victim address.

produce-alert —Writes the event to the Event Store as an alert.

produce-verbose-alert—Includes an encoded dump (possibly truncated) of the offending packet in the alert.

request-block-connection—Sends a request to the ARC to block this connection.

request-block-host—Sends a request to the ARC to block this attacker host.

request-rate-limit—Sends a rate limit request to the ARC to perform rate limiting.

request-snmp-trap—Sends a request to the Notification Application component of the sensor to perform SNMP notification.

reset-tcp-connection—Sends TCP resets to hijack and terminate the TCP flow.

modify-packet-inline— Modifies packet data to remove ambiguity about what the end point might do with the packet.


Note Signature 64000 subsignature 0 will fire when it sees the alerts from signature 1000 subsignature 0 and signature 1001 subsignature 0 on the same source address. The source address selection is a result of the meta key default value of Axxx. You can change the behavior by changing the meta key setting to xxBx (destination address) for example.


Creating a Meta Engine Signature

To create a signature based on the Meta engine, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig1
 
   

Step 3 Specify a signature ID and a subsignature ID for the signature. Custom signatures are in the range of 60000 to 65000.

sensor(config-sig)# signatures 64000 0 
 
   

Step 4 Specify the signature engine.

sensor(config-sig-sig)# engine meta
 
   

Step 5 Insert a signature (named m1) at the beginning of the list.

sensor(config-sig-sig-met)# component-list insert m1 begin
 
   

Step 6 Specify the signature ID of the signature on which to match this component.

sensor(config-sig-sig-met-com)# component-sig-id 1000
 
   

Step 7 Exit component list submode.

sensor(config-sig-sig-met-com)# exit
 
   

Step 8 Insert another signature (named m2) at the end of the list.

sensor(config-sig-sig-met)# component-list insert m2 end 
 
   

Step 9 Specify the signature ID of the signature on which to match this component.

sensor(config-sig-sig-met-com)# component-sig-id 1001
 
   

Step 10 Configure the component list not to fire in order.

sensor(config-sig-sig-met-com)# component-list-in-order false
 
   

Step 11 Specify to use all components you have created.

sensor(config-sig-sig-met-com)# all-components-required true
 
   

Step 12 Specify not to use all of the NOT components.

sensor(config-sig-sig-met-com)# all-not-components-required false 
 
   

Step 13 Verify the settings.

sensor(config-sig-sig-met-com)# exit
sensor-128(config-sig-sig-met)# show settings
   meta
   -----------------------------------------------
      event-action: produce-alert <defaulted>
      swap-attacker-victim: false <defaulted>
      meta-reset-interval: 60 <defaulted>
      component-list (ordered min: 1, max: 32, current: 2 - 2 active, 0 inactive)
      -----------------------------------------------
      ACTIVE list-contents
      -----------------------------------------------
         NAME: m1
         -----------------------------------------------
            component-sig-id: 1000
            component-subsig-id: 0 default: 0
            component-count: 1 default: 1
            is-not-component: false <defaulted>
         -----------------------------------------------
         -----------------------------------------------
         NAME: m2
         -----------------------------------------------
            component-sig-id: 1001
            component-subsig-id: 0 <defaulted>
            component-count: 1 <defaulted>
            is-not-component: true default: false
         -----------------------------------------------
         -----------------------------------------------
      -----------------------------------------------
      meta-key
      -----------------------------------------------
         Axxx
         -----------------------------------------------
            unique-victims: 1 <defaulted>
         -----------------------------------------------
      -----------------------------------------------
      component-list-in-order: false default: false
      all-components-required: true default: true
      all-nots-required: false default: false
   -----------------------------------------------
sensor(config-sig-sig-met)#
 
   

Step 14 Exit signature definition submode.

sensor(config-sig-sig-met)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit 
Apply Changes:?[yes]:
 
   

Step 15 Press Enter to apply the changes or enter no to discard them.


For More Information

For more information on the Signature Event Action Processor, see Signature Event Action Processor.

For more information on the Meta engine and the Meta engine enhancement, see Meta Engine.

Example IPv6 Engine Signature

The following example Atomic IP Advanced custom signature prohibits Protocol ID 88 over IPv6.

To create a signature based on the Atomic IP Advanced signature engine, follow these steps:


Step 1 Log in to the CLI using an account with administrator or operator privileges.

Step 2 Enter signature definition submode.

sensor# configure terminal
sensor(config)# service signature-definition sig0
 
   

Step 3 Specify a signature ID and a subsignature ID for the signature. Custom signatures are in the range of 60000 to 65000.

sensor(config-sig)# signatures 60000 0 
 
   

Step 4 Specify the signature engine.

sensor(config-sig-sig)# engine atomic-ip-advanced
 
   

Step 5 Specify the IP version.

sensor(config-sig-sig-ato)# specify-ip-version yes
 
   

Step 6 Specify IPv6.

sensor(config-sig-sig-ato-yes)# version ipv6
 
   

Step 7 Specify the L4 protocol.

sensor(config-sig-sig-ato-yes-ipv)# exit
sensor(config-sig-sig-ato-yes)# exit
sensor(config-sig-sig-ato)# specify-l4-protocol yes
 
   

Step 8 Specify protocol ID 88.

sensor(config-sig-sig-ato-yes)# l4-protocol other-protocol
sensor(config-sig-sig-ato-yes-oth)# other-ip-protocol-id 88
 
   

Step 9 Verify the settings.

sensor(config-sig-sig-ato-yes-oth)# show settings
   other-protocol
   -----------------------------------------------
      other-ip-protocol-id: 88
   -----------------------------------------------
sensor(config-sig-sig-ato-yes-oth)#
 
   

Step 10 Exit signature definition submode.

sensor(config-sig-sig-ato-yes-oth)# exit
sensor(config-sig-sig-ato-yes)# exit
sensor(config-sig-sig-ato)# exit
sensor(config-sig-sig)# exit
sensor(config-sig)# exit
Apply Changes?[yes]:
 
   

Step 11 Press Enter to apply the changes or enter no to discard them.


For More Information

For more information about the Atomic IP Advanced engine and a list of the parameters, see Atomic IP Advanced Engine.

For more information on the Atomic engines, see Atomic Engine.