Incomplete application matching app-id rule

[bdugger]

I have a rule matching for ms-update app-id, but when looking at logs I see sessions hitting the rule that have an imcomplete application.

How is this possible?

[kiwi]

hi bdugger,

That behavior is explained in this article :

Not-Applicable-Incomplete-Insufficient-Data-in-the-Application

Incomplete means that either the three-way TCP handshake did not complete or the three-way TCP handshake did complete but there was no data after the handshake to identify the application.

[reaper · Accepted solution]

a single session will actually go through the security policy twice

When a new session is initiated, the SYN packet does not contain a lot of information that can be used for AppID, so it will be parsed against the security policy based on the 5 tuples: source zone, source subnet, destination zone, destination subnet, destination port

in the security policy the configured applications will be ignored in this stage (as we don't know the app yet) and sessions wil be matched based on the destination port (this is why setting 'application-default' in the service is important!)

if a rule is matched that allows the 5 tuples, the handshake is allowed through and a session is created in the session table

if then after the handshake nothing happens, the session will timeout and be discarded, the application will be set as incomplete because it never continued.

however, because the session was created, we still want to have a log entry in the traffic log to indicate the session was there, but because it never managed to reach the next stage where AppID could be applied, it was never matched against a different policy so it will be logged as a hit on the first policy that simply let the session be created

hope this makes sense

Tom

[bdugger]

Thanks for the quick response Tom!