Re: [spf-discuss] How does SPF prevent forgery?

Wally Winchester wrote:
> Ah, that clears that up then - but leaves me confused, why (or when) is
> it important to check 2821 addresses?
The truth is, there is only 1 2821 identity (well, or none, but then you 
can check the HELO of the sender)
Whereas there are MANY 2822 candidate fields (from, reply to, bounces 
to, list fields, etc etc etc).  Hence the PRA "algorithm".
In order to apply whitelisting or blacklisting on something (usually a 
domain name), you need to know that the domain name is not forged.
The ideal place to do that is right at the beginning, at 2821 time, at 
the beginning of the SMTP transfer (so you don't waste all that 
bandwidth accepting something you are going to reject as forged).  Note 
that you have to go into DATA to get the 2822 headers, and once in DATA 
you cannot reject the email until you have received ALL of it (well, 
mostly true, you can reject it if its too large, but that's about it). 
Point being, you cannot accept the 2822 headers, and then reject right 
away, you have to wait for the entire email (upto your size cap), and 
then reject.
SPF is *not* a spam solution, it is a forgery solution.  Once you can be 
sure you have some "identity" aka domain name that is verified, you can 
then check whitelists/blacklists/etc to see if you want to accept email 
from the (verified) identity.
Terry

> WW
>
> On Thu, 16 Feb 2006 17:31:54 -0500, "Terry Fielder"
> <terry(_at_)ashtonwoodshomes(_dot_)com> said:
>
> > SPF checks 2821 addresses.  It makes no assertion about 2822 addresses.
> >
> > Some would call 2822 address fakes "phishing", not forgery.
> >
> > For 2822 checking, you should look at PRA (aka sender id).  But there 
> > are serious caveats with using it, so be careful.
> > Terry
> >
> > Wally Winchester wrote:
> >
> > > Hello.
> > >
> > > If SPF only checks the envelope from address, how does it prevent
> > > forgery?
> > >
> > > i.e. how does it prevent a user getting an email From: 
> > > person(_at_)bank(_dot_)com?
> > >
> > > WW
> > --
> > Terry Fielder
> > terry(_at_)greatgulfhomes(_dot_)com
> > Associate Director Software Development and Deployment
> > Great Gulf Homes / Ashton Woods Homes
> > Fax: (416) 441-9085
> >
> > -------
> > Sender Policy Framework: http://www.openspf.org/
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--
Terry Fielder
terry(_at_)greatgulfhomes(_dot_)com
Associate Director Software Development and Deployment
Great Gulf Homes / Ashton Woods Homes
Fax: (416) 441-9085

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