block telnet port 25

[Hiu Yen Onn]

hi all,

i am wondering why ppl can use telnet the machine on port 25, and do
something like this.

telnet mailserver.com 25
helo mailserver.com
mail from:<***@mailserver.com>
rcpt to:<***@mailserver.com>
data

test

.

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



Mr. Hiu Yen Onn




-----------------------------------------
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[Alex Satrapa]

Post by Hiu Yen Onn
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).

Post by Hiu Yen Onn
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

[Keith Matthews]

On Wed, 28 Sep 2005 09:20:22 +0800 (SGT)

Post by Hiu Yen Onn
hi all,
i am wondering why ppl can use telnet the machine on port 25, and do
something like this.
telnet mailserver.com 25
helo mailserver.com
data

telnet from where ? if it's from a remote host then you can't stop it or
you'll stop all incoming mail, if it's from the same host then you can
stop it but not if you are using SpamAssassin or anti-virus tools.

Do you think it's a problem and if so why ?

[mouss]

Post by Hiu Yen Onn
hi all,
i am wondering why ppl can use telnet the machine on port 25, and do
something like this.
telnet mailserver.com 25
helo mailserver.com
data
test
.
the mail will be sent out through this. are there any solution in block
telnet port 25???? thanks

don't confse the telnet protocol (port 23) and the telnet client. the
latter is simply a program that can be used to establish a tcp
connection. there is no fundamental difference in sending mail using
this program or using outlook.

[/dev/rob0]

Post by mouss
connection. there is no fundamental difference in sending mail using
this program or using outlook.

telnet(1) is better at threading. ;) :)