Source of closed port RST responses

David Horn dhorn2000 at
Sun Dec 20 22:31:43 UTC 2009

On Sun, Dec 20, 2009 at 2:37 PM, DAve <dave.list at> wrote:

> I am routinely seeing these entries in one of my servers logs.
> Limiting closed port RST response from 373 to 200 packets/sec
> The server sits behind a PIX firewall, so I am suspicious of what is
> trying to connect to a closed port. I don't see in any other logs what
> port is being hit, or what IP is causing these log entries.
> Any way to tell what the source IP of these is?
Try using tcpdump.  You can redirect the decoded output to a log file as
well.  Make sure to replace "em0" in my example with the appropriate
interface name.  If the server is very busy, try just running it for a short
period of time to make sure that it does not interrupt operations, then
leave it running for whatever time period you want to monitor if all goes

tcpdump -np -i em0 'tcp[13] & 4 != 0'

The 'tcp[13] & 4 !=0' will cause the filter to only capture packets with the
tcp flag RST set.

man tcpdump

or google for more examples of filters.

Good Luck.

---Dave Horn

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