North American Network Operators Group
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Re: Computer systems blamed for feeble hurricane response?

  • From: Mike Tancsa
  • Date: Tue Sep 13 16:20:33 2005 
At 03:50 PM 13/09/2005, Joseph S D Yao wrote:


Oh, and also ... please consider that some firewalls try to discern
whether the connection on port 25 is from a mail server or from Telnet.
While I mourn the simplicity of manual debugging of such sites, it
remains that: the fact that you can't TELNET HOST.DOMAIN 25 doesn't mean
that there's no mail service there.
Making a network connection using the application "telnet" vs the application "sendmail" (or whatever MTA one uses) seems to be the same when doing a tcpdump on the data. I am not sure how a firewall would know -- purely at the network layer -- what the other side's application was/is that initiated the connection. Yes, the other end could try and connect back to the host, but there is no 2 way traffic as the 3way handshake is not completing and I dont see any other traffic coming back from that host attempting to discern any info.

---Mike