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[ScreenOS] When to use 'snoop' and 'debug flow'



Article ID: KB5967 KB Last Updated: 29 May 2019Version: 7.0
This article provides information on when to use the snoop and debug flow commands.
  • Snoop
  • Debug Flow
  • Troubleshooting tools

Symptoms and errors:

Troubleshooting how packet flows through the firewall.


Snoop provides a layer 2 through layer 4 view of the packet, as it comes in and out of the Juniper firewall interface. You can filter a packet, based on the source and/or destination IP, source and/or destination port, protocol, and Ethernet type; including VLAN tags and ARPs. Snoop can capture information, such as frag flags, sequence number, acknowledgment number, TTL, and TOS bit, source and destination IP and port information, as well as ARP request and reply information.

For more information on using Snoop, refer to KB5411 - How do you use Snoop for troubleshooting?


On the other hand, Debug Flow provides information about the packet as it traverses through the interfaces of the firewall device. This will take you through the entire flow of the packet inside the firewall.  It also provides policy id, session id, source and destination IP and port information, and next hop routes; or where the packet actually came from.

For more information on using Debug, refer to KB5536 - How do I capture debugging (debug flow) information?
Modification History:
2019-05-22: Content reviewed for accuracy

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