From: Alex Satrapa (alex.satrapaapf.edu.au)
Date: Tue Sep 27 2005 - 21:19:08 CDT

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On 28 Sep 2005, at 11:20, Hiu Yen Onn wrote:

> i am wondering why ppl can use telnet the machine on port 25, and
> [send mail]

Port 25 is the registered port for the Simple Mail Transfer Protocol
(SMTP). This protocol is described in RFC 2821:
http://www.ietf.org/rfc/rfc2821.txt

The text being typed are SMTP commands, so what is happening is that
the user is performing an SMTP transaction by hand - many Internet
protocols are designed to be easy to test by hand, since this makes
diagnosing problems easier (you don't need special tools to find
problems).

> the mail will be sent out through this. are there any solution in block
> telnet port 25?

Blocking TCP port 25 ingoing or outgoing will prevent all email being
sent to or from that host.

Some ISPs block TCP port 25 outgoing in order to prevent their client's
computers sending spam or viruses through mail. The catch here is that
you will need to provide some means of sending legitimate email out
from your network - this is usually done by allowing SMTP connections
to the ISP's mail relay, and blocking all outgoing STMP traffic that
doesn't originate from that relay.

There's nothing special about telnet - any program can send email by
making a connection to TCP port 25 on the receiving host. If you look
at your firewall tools, there should be a means available for blocking
TCP port 25. Just be aware that when you block that port, you must
provide some means for legitimate email to be transmitted.

Alex Satrapa
IT Support
Australian Phenomics Facility
Building 117, Garran Road
Australian National University

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