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Seeing full 3-way handshake for connection that should be blocked

Question asked by Christopher Barnes on Dec 23, 2018
Latest reply on Dec 27, 2018 by Kishin Fatnani

Hi there,

We had someone do a port scan and packet capture against our gateways from the internet. The report came back with a bunch of ports "open" for one of our IP addresses.

 

for one of the "open" ports - FTP (tcp 21) the packet capture clearly shows the 3-way TCP handshake completes but the firewall log shows the connection is dropped. The capture desnt show any RST from the gateway. The firewall log shows no traffic actually makes it to the server behind the firewall, but the connecting host sees TCP session establishment.

 

 

My question is, is it normal for the gateway to complete the TCP build-up before dropping the connection? I was under the impression first SYN from the connecting host was evaluated against the ruleset.

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