In message <20050913202040.GK16110@core.center.osis.gov>, Joseph S D Yao writes :
On Tue, Sep 13, 2005 at 04:15:29PM -0400, Mike Tancsa wrote:
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.
I don't know, myself. I said they try. Perhaps they succeed. Perhaps they check the speed of incoming queries. Perhaps they try to use a Telnet OPTION. I don't know. Perhaps it's a sales gag. [I think it was a telnet OPTION, actually.]
Telnet options, and for that matter speed, happen after the 3-way handshake. We're not getting that far. --Steven M. Bellovin, http://www.cs.columbia.edu/~smb