A network security engineer is troubleshooting an issue where internal users cannot access a newly deployed internal web application. A security policy rule has been created to allow access, but traffic is still being blocked. Upon reviewing the traffic logs, the engineer sees 'deny' actions with a 'default-deny' rule and the application identified as 'incomplete'. What is the most likely cause of this issue?
- AThe App-ID for the web application is not correctly recognized.
- BThe security policy rule is placed too low in the rule order.
- CThe server-side application is not responding to SYN packets, causing the TCP handshake to fail.
- DA NAT policy is misconfigured, causing the destination IP to be incorrect.
Show answer & explanationAnswer & explanation
Correct answer: C. The server-side application is not responding to SYN packets, causing the TCP handshake to fail.
When an application shows as 'incomplete' in the traffic logs and the action is 'default-deny', it indicates that the Palo Alto Networks firewall could not establish the initial TCP handshake (SYN, SYN-ACK, ACK). This usually means the server is not responding to the firewall's SYN, or the return SYN-ACK from the server is not reaching the firewall, preventing App-ID from identifying the application.
Why the other options are wrong
- A. If App-ID was not correctly recognized, it would likely be identified as 'unknown-tcp' or 'unknown-udp', not 'incomplete'. 'Incomplete' specifically points to a failed handshake.
- B. While rule order can cause blocks, it typically results in a 'deny' action from an explicit deny rule or a 'default-deny' with a different application, not 'incomplete'.
- D. A NAT misconfiguration could prevent traffic from reaching the server, but the firewall itself would still attempt the handshake if the initial packet was routed to it. If the server isn't receiving the SYN, it could be a routing or server issue, leading to 'incomplete'.
App-ID 'incomplete' state
The 'incomplete' application state in Palo Alto Networks firewalls indicates that the firewall could not establish the initial TCP 3-way handshake, preventing App-ID from identifying the application.
- Occurs when the firewall doesn't see a successful TCP handshake (SYN, SYN-ACK, ACK).
- Common causes include server not listening, routing issues, or upstream firewall blocking.
- Prevents App-ID from accurately identifying the application.
- Often results in a 'default-deny' action if no other rule matches.
Memory trick: App-ID: If it's incomplete, the handshake's weak, or the path's a leak.