How to perform firewall flow filter debugging
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Historical ScreenOS flow-debug procedure retained for legacy NetScreen systems.
Validation status
Reviewed against current Junos tracing concepts on 27 September 2026. Modern SRX/Junos uses security flow traceoptions rather than the ScreenOS ffilter syntax below.
Tracing can affect performance and expose traffic
Use a narrow filter, reproduce briefly, stop tracing and remove the diagnostic configuration.
Original technical reference
The debug buffer is a circular buffer, once the buffer has reached the size limit, the oldest data will be overwritten.
The buffer size is configurable.
To change the size, use the following commands:
set db size 4096 set console dbuf clear db
To set-up the flow filter:
get ffilter set ffilter src-ip / dst-ip ... (see the CLI-help for more ffilter options) (same row = AND, more rows = OR) debug flow basic
Reproduce the issue. The timestamp resolution of the output is in tenths of a second. Stop the ffilter and then display the output with the following commands:
undebug all get dbuf stream or get dbuf stream > tftp <host> <name-of-file>