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Cisco Intrusion Prevention System Sensor CLI Configuration Guide for IPS 7.2
OL-29168-01
Chapter7 Defining Signatures
Configuring Signatures
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.