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Re: blocking dialups

  • From: John Berthels < >
  • Date: Tue, 13 Oct 1998 13:53:50 +0100 (BST)

On Tue, 13 Oct 1998, Martin Lorensen wrote:

> On Tue, 13 Oct 1998, torben fjerdingstad wrote:
> > Can you mention just one reason for not relay through the ISP's
> > mail server (except for sending spam)?
>  - Don't known the ISP's mailserver name (Roaming/IPass)

Doesn't matter if you use the 'transparent proxying' idea to redirect port
25 traffic. No reconfiguration is necessary on the dial-up workstation.

>  - Running Linux server with sendmail.


>  - Connects to diffrent ISP's.

ditto (if these connections are all straight SMTP/ESMTP on port 25).

> Are leased lines custummers included? Dialup custummeres with static IP?

I would guess that (initially at least) the only customers it need apply
to are those whom 'demographically' are likely to be spam sources.

Experience suggests that this is customers with 'throwaway' dialups but
the same techniques could actually be applied to anyone, even leased line

Following Clive Feather's lead, I checked the SPAMTOOLS archive (but not
very thoroughly, I'll admit) and the best arguments against this I could
see (in no particular order) were:

1) time sensitive relay (e.g. stock trading)
2) "A dangerous step towards network fascism"
3) diagnostic use (EXPN, VRFY, checking Received: headers, etc)
4) testing SMTP servers to see if they are open relays
5) SMTP AUTH (don't know much about this)

I take the point that there are legitimate uses for unrediected port 25
uses, but would be interested if anyone has a view on what % of dialup
users require them.

Would it be a big trial for ISPs to maintain and allocate a pool of
'trusted dial up' IPs (which get direct access) but redirect all other
dialups port 25 traffic to a local SMTP server (which then provides
throttling, etc).

Then if a customer has specific dialup needs which would break if port 25
was redirected they could get a 'trusted IP' (allocated either by user id
or different dial-up hunt group).

So ISPs could still give away free dial-up CDs on magazines (read
'untrusted dialup IP pool') as well as support 'esoteric' SMTP use which
would require no redirection.

Is this a poor idea, or should an ISP be able to control its dial-up
customers in this way?


John Berthels
Email: j.berthels@localhost
X.400: /G=john/S=berthels/O=nexor/P=nexor/A=cwmail/C=gb/

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