Ahem..
Forgive me for breaking into the topic here and possibly arriving from
left field, but:
Table 4.1 of "Internet Firewalls and Network Security" (Sijan/Hare, 1995)
shows this:
Port Number Description
23 Telnet
25 SMTP(mail)
So, why would you not simply use the sepaerate port numbers for the
seperate applications; and why would a firewall not know the difference
between these ports?
(Please forgive this newbie, O Great Firewall Wizards)
Am I missing a huge foundation of knowledge here?
On Wed, 23 Aug 2000 22:47:01 +0200 mouss <[EMAIL PROTECTED]> writes:
> There is nothing that a firewall can use to distinguish a
> "normal"client
> from a guy who telnet to the
> port, for the simple reason that both are exactly the same thing.
>
> an smtp connection consists of a client connectiing to port 25 of a
> host
> sending commands and
> reading responses. This client may be anything that is capable of
> handling
> a TCP connection.
> The telnet program may be used for this.
>
> so a firewall that rejects an SMTP connection just because it thinks
> it is
> coming from a "user doing
> telnet" is not only stupid, but it is not serving SMTP. relying on
> headers
> is not only useless, it is a
> loss of developpers energy, of the host CPU and yet another source
> of bugs.
>
> The problem of forged SMTP connections is the same as that of forged
>
> letters. the only way to guard against
> is to add a verification process which makes it harder to
> communicate by
> email. you can use encryption to make
> sure who sent what, but only if you know who can send you mail. if
> you
> accept to receive email from anybody,
> then that's it, you choosed to get served, accept the risks....
>
>
> mouss
>
>
> At 10:43 23/08/00 -0700, [EMAIL PROTECTED] wrote:
>
> >The characteristics of a Telnet connection are significantly
> different
> >from a standard SMTP connection and some firewall proxies can
> recognize
> >and drop Telnet connections but what's the point? It's trivial to
> create
> >an interactive SMTP emulation that would bypass this check. The
> port is
> >designed to for TCP connections that passes ASCII text characters.
> What
> >would you be accomplishing by preventing/dropping Telnet
> connections?
>
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