Subj : Re: Email server update
To   : Finnigann
From : Angus McLeod
Date : Thu Aug 04 2005 10:16 pm

  Re: Re: Email server update
  By: Finnigann to Digital Man on Thu Aug 04 2005 19:48:00

 > Ah, to the heart of my problems.
 > 
 > Does a telnet client tell the recipient that it's a telnet session?

I don't think the client specifically identifies that it wants to use 
TELNET.  That is deduced from the port.

 > If I telnet into a mail server, (for example) why would a mail server answer
 > telnet request.

Because the mail server simply sees a connection request on the port to 
which it is listening.  *Fortunately*, both SMTP and POP servers utilize a 
text-based dialogue for communicating with mail clients.  So providing you 
can simulate the apropriate dialogue with your fingers on the keyboard, 
you can indeed talk to a mail server with a telnet client.  I do exactly 
that ALL the time!

 > I'll try it now and see.

 > telnet -f logfile.txt vert.synchro.net 25
 > 
 > C:\WINDOWS\system32>telnet -f logfile.txt vert.synchro.net 25
 > Connecting To vert.synchro.net...Could not open connection to the host, on p
 > 25:
 > Connect failed

I dunno why:

	$ telnet vert.synchro.net 25
	Trying 69.104.209.210...
	Connected to vert.synchro.net.
	Escape character is '^]'.
	220 rob.synchro.net Synchronet SMTP Server 1.365-Win32 Ready
	QUIT
	221 rob.synchro.net Service closing transmission channel
	Connection closed by foreign host.
	$

It seemed quite willing to talk to me....

 > Let's try BNB...
 > 
 > C:\WINDOWS\system32>telnet -f logfile.txt bnb.synchro.net 25
 > Connecting To bnb.synchro.net...Could not open connection to the host, on po
 > 25:
 > Connect failed

	$ telnet bnb.synchro.net 25
	Trying 66.231.38.79...
	telnet: connect to address 66.231.38.79: No route to host
	amcleod@triphod:~$ ping 66.231.38.79
	PING 66.231.38.79 (66.231.38.79) 56(84) bytes of data.

	--- 66.231.38.79 ping statistics ---
	14 packets transmitted, 0 received, 100% packet loss, time 13018ms
	
	$

 > Is this whole thing telling me that MY port 25 is closed?

It looks so.  But what exactly does "No route to host" mean, and why can't 
I even 'ping' your machine?

 > Are ports two way? Maybe closed one way but open the other way.

The ports on each end won't necessarily (or even probably) be the same, 
but the connection shopuld be bi-directional.  However, a firewall *could* 
be allowing packets in one direction and blocking those packets going back 
in the other direction.  In fact, this is one of the things you have to 
watch out for when setting up IP MASQ/NAT with a very versatile packet 
filter.



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