Action Configured in Security Rules and Seen in Traffic Log is Inconsistent
In some situations there can be a traffic log entry for a session, where the application is recognized as insufficient data, logged as "allow" while matching a drop rule.
Take FTP traffic from a zone "replay_eth2" to a zone "replay_eth3".
This FTP traffic will be incomplete; a RST packet will be sent by the client at the beginning of command session (dport 21).
This incomplete FTP transaction will allow the session to be recognized as insufficient-data.
The decoder attached to the FTP application did not receive enough data to achieve compliancy checks and validate the FTP application.
Note: FTP is not mandatory, it can happen for all applications with a decoder.
The security rulebase on the Palo Alto Networks firewall is shown in the example below:
In security rulebase, it is important to have the following:
- One rule denying traffic for 'any' application
- One rule above allowing traffic for a specific application
- Flow tuples (sip, szone, dip, dzone) for FTP traffic will have to match this rule. It will allow App-ID to kick in.
- If there is no such rule, traffic will be dropped before App-ID starts
Under these conditions, if FTP traffic is generated from zone "replay_eth2" to zone "replay_eth3", this will be blocked with the following log entry:
This behavior can be explained as the following:
- In that very specific scenario, Palo Alto Networks firewall does not have all the information needed to take final action configured in "Deny All" rule (Deny)
- The final action is taken once the application decoder will have validated the application (running some compliancy test against transaction)
- The final rule match and action will be deferred after this validation
When the RST packet is received, the session ends with these states:
FTP session is incomplete and not validated
- Matching "Deny All" rule with "allow" action
Action was temporarily set to "allow" for App-ID to be achieved (within 4 data packets or 2000 bytes of payload), and the "Deny All" rule was the best potential match.