Re: Blocking port 25 telnet access on firewall & still receiving email

From: RT (russandsandy_at_no.slimey.spammers.attbi.noteven.com)
Date: 05/03/03 

Date: Sat, 03 May 2003 15:37:53 GMT

BS.

I have telnet blocked on the incoming IP address of my mail server and every
other firewall I've ever installed.

You're all clueless.

RT

"Maxime Ducharme" <maxime@pandore-design.com> wrote in message
news:WwXpa.3647$Zj2.744983@news20.bellglobal.com...
>
> Hi,
> there is no way to block "telnet" on port 25 and let SMTP
> pass trought.
>
> A simple command "telnet domainname.com 25" opens a pure
> TCP connection to the server without sending anything until you hit
> a button on your keyboard.
>
> Others SMTP servers opens a pure TCP connection too, and then
> "talk" to each other on this port.
>
> For the firewall, there is no way at this point to determine if the
> TCP connection is from a telnet program or SMTP server or anything
> that is able to open a TCP link.
>
> There is one way I tought they could get mail and block inbound
> SMTP (TCP on 25), is allowing specific foreign SMTP servers
> IPs, and block every others IPs.
>
> This will cause problem to mail transfert, since it is impossible
> to determine where the foreign SMTPs will be from, unless
> they accept mail from specific domains and not the entire world.
>
> Some mail will get in, some not.
>
> So as long as you can't open a telnet link to the port 25, you
> can say that their firewall is not configured correctly.
>
> Ensure that this problem occurs from different source IPs,
> so try from job, from home, ask friends to try too.
>
> If everyone have the telnet problem, you found the problem :).
>
> Hope it helps
>
> ---------------------------------------------------------------
> Maxime Ducharme
> Administrateur reseau, Programmeur
> E-Mail : maxime@pandore-design.com
> Pandore-Design [http://www.pandore-design.com]
>
>
> "Claudia" <spam.free-AT-integraonline.com> wrote in message
> news:vag7i6gdnjei12@corp.supernews.com...
> > OK, someone a little smarter than I please help me out here. I work for
> an
> > ISP and have a customer whose domain name we host. In the last few
weeks,
> > we have received a few calls from non-customers asking if they have any
> idea
> > why they cannot get email to this customer. Apparently no immediate
> reject
> > comes back, but the email never arrives.
> >
> > I have been completely unable to telnet to port 25 on their mail server,
> so
> > assume that is why. Foolish me. I spoke with the customer's consultant
> who
> > states that they are receiving plenty of email, but have their firewall
> > configured to not allow specifically *telnet* access on port 25, so that
> no
> > one can see what kind of email server they have.
> >
> > How is it possible to block telnet access to port 25 at the firewall,
> while
> > still allowing email to get through? I'm obviously missing something
> here.
> >
> > Unfortunately, I know nothing as to what kind of firewall they have set
> up.
> > Any help in understanding this would be greatly appreciated.
> >
> > Thanks.
> > Claudia
> >
> >
>
>