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Re: All tcp ports open?
From: nathan () ccc-ltd com
Date: Wed, 01 Sep 2004 14:44:15 +0000

Looks suspisiously like a FW1 syndefender in relay mode, here are some
(brief and probably inaccurate) notes that I made a while back. It all boils 
to TTLs if you want to scan hosts behind it.

Hope this points you in the right direction;


you will always get a spoofed syn/ack from the firewall (when unfiltered).

If the firewall is rejecting the port you will get a reset without a synack.

If the machine is not there, rst will appear (after arp? timeout) to have the 
ttl as the firewall. (i.e. the same as the spoofed syn/ack sent back to the 

A reset should happen if the firewall is Rejecting the connection (i.e. 
resetting it).

If the machine is there but port closed, rst will have a different ttl as the 
initial syn/ack of the firewal.

If the machine is there but port open:
        If there is no data waiting, FIN the server and look for ACK FIN.
        If there is data waiting for you, you will get an ack + data, it might be
        prudent to fin the connection.

If the machine is there but filtered by the firewall, the firewall will 
syn/ack from server on it's behalf, the ack comming back from the server 
however will
be blocked and the client will be stuck retransmitting ack until client 

More detailed information:




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