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24-15
Cisco ASDM User Guide
OL-16647-01
Chapter 24 Configuring Application Layer Protocol Inspection
MGCP Inspection
For search responses, when the LDAP server is located outside, NAT should be considered to allow
internal peers to communicate locally while registered to external LDAP servers. For such search
responses, xlates are searched first, and then DNAT entries to obtain the correct address. If both of these
searches fail, then the address is not changed. For sites using NAT 0 (no NAT) and not expecting DNAT
interaction, we recommend that the inspection engine be turned off to provide better performance.
Additional configuration may be necessary when the ILS server is located inside the security appliance
border. This would require a hole for outside clients to access the LDAP server on the specified port,
typically TCP 389.
Because ILS traffic only occurs on the secondary UDP channel, the TCP connection is disconnected after
the TCP inactivity interval. By default, this interval is 60 minutes and can be adjusted using the
Configuration > Firewall > Advanced > Global Timeouts pane.
ILS/LDAP follows a client/server model with sessions handled over a single TCP connection.
Depending on the client's actions, several of these sessions may be created.
During connection negotiation time, a BIND PDU is sent from the client to the server. Once a successful
BIND RESPONSE from the server is received, other operational messages may be exchanged (such as
ADD, DEL, SEARCH, or MODIFY) to perform operations on the ILS Directory. The ADD REQUEST
and SEARCH RESPONSE PDUs may contain IP addresses of NetMeeting peers, used by H.323 (SETUP
and CONNECT messages) to establish the NetMeeting sessions. Microsoft NetMeeting v2.X and v3.X
provides ILS support.
The ILS inspection performs the following operations:
• Decodes the LDAP REQUEST/RESPONSE PDUs using the BER decode functions
• Parses the LDAP packet
• Extracts IP addresses
• Translates IP addresses as necessary
• Encodes the PDU with translated addresses using BER encode functions
• Copies the newly encoded PDU back to the TCP packet
• Performs incremental TCP checksum and sequence number adjustment
ILS inspection has the following limitations:
• Referral requests and responses are not supported
• Users in multiple directories are not unified
• Single users having multiple identities in multiple directories cannot be recognized by NAT
Note Because H225 call signalling traffic only occurs on the secondary UDP channel, the TCP connection is
disconnected after the interval specified by the TCP option on the Configuration > Firewall > Advanced
> Global Timeouts pane. By default, this interval is set at 60 minutes.
MGCP Inspection
MGCP is a master/slave protocol used to control media gateways from external call control elements
called media gateway controllers or call agents. A media gateway is typically a network element that
provides conversion between the audio signals carried on telephone circuits and data packets carried over