The "Log at Session Start" option is primarily a troubleshooting tool. It generates a log entry as soon as the firewall receives the first packet of a session, providing immediate feedback on whether traffic is matching a security rule. This is invaluable for diagnosing connectivity issues in real-time. However, enabling this feature permanently, especially on high-traffic rules, generates two logs per session (one at the start, one at the end), which can significantly increase log volume and consume system resources on both the firewall and the log collector. Therefore, the best practice is to use it temporarily for specific diagnostic purposes.

B: Deny actions are typically stateless and log immediately upon packet drop, making the "session start" concept less relevant than for stateful "allow" rules.

C: Enabling this on all "allow" rules is not a best practice as it would cause excessive logging and performance degradation on the firewall and log collectors.

D: "Log at Session Start" is an independent setting; it can be enabled without "Log at Session End" and is often used alone during troubleshooting.

1. Palo Alto Networks PAN-OS® Administrator’s Guide 10.2

"Security Policy Rule Actions": In the section describing policy rule actions

the guide states

"Select Log at Session Start to generate a log entry at the start of a session. This option is useful for troubleshooting." It also warns

"...it can cause a significant increase in the volume of logs." This directly supports its use for troubleshooting (A) and advises against broad application (C).

2. Palo Alto Networks

"Best Practice Assessment (BPA) Checks - PAN-OS 10.2"

Document Version 20220620: The BPA check security-policy-log-start explicitly states

"Enabling 'Log at Session Start' is a useful troubleshooting tool

but it can have a significant performance impact on the firewall and any downstream log collectors. It is recommended to only enable this setting for troubleshooting purposes." This reinforces that the primary

recommended use case is troubleshooting.

3. Palo Alto Networks Knowledge Base

Article #2136

"How to Troubleshoot Traffic Not Passing the Firewall": This article recommends as a troubleshooting step to "Enable 'Log at Session Start' on the security policy rule that should be matching the traffic." This demonstrates its practical application as a diagnostic tool.