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> Chapter 33 - VoIP Solutions: SIP > Deep SIP message inspection > Actions taken when a malformed message line is found

Actions taken when a malformed message line is found

When a malformed message line or other error is found the SIP ALG can be configured to discard the message containing the error, pass the message without any other actions, or responding to the message with a 400 Bad Request or 413 Request entity too large client error SIP response message and then discard the message. (For information about client error SIP response messages, see Client error.)

If a message line is longer than the configured maximum, the SIP ALG sends the following message:

SIP/2.0 413 Request Entity Too Large, <optional_info>

If a message line is incorrect or in an unknown message line is found, the SIP ALG sends the following message:

SIP/2.0 400 Bad Request, <optional_info>

The <optional_info> provides more information about why the message was rejected. For example, if the SIP ALG finds a malformed Via header line, the response message may be:

SIP/2.0 400 Bad Request, malformed Via header

If the SIP ALG finds a malformed message line, and the action for this message line type is discard, the message is discarded with no further checking or responses. If the action is pass, the SIP ALG continues parsing the SIP message for more malformed message lines. If the action is respond, the SIP ALG sends the SIP response message and discards the message containing the malformed line with no further checking or response. If only malformed message line types with action set to pass are found, the SIP ALG extracts as much information as possible from the message (for example for NAT and opening pinholes, and forwards the message to its destination).

If a SIP message containing a malformed line is discarded the SIP ALG will not use the information in the message for call processing. This could result in the call being terminated. If a malformed line in a SIP message includes information required for the SIP call that the SIP ALG cannot interpret (for example, if an IP address required for SIP NAT is corrupted) the SIP ALG may not be able to continue processing the call and it could be terminated. Discarded messages are counted by SIP ALG static message counters.