[CentOS] SPAM on the List

[Stephen Harris]

On Sun, Jul 17, 2011 at 11:36:49PM +0200, Patrick Lists wrote:
> On 07/17/2011 11:24 PM, Always Learning wrote:
> > *almost* correct. In Linux, like Unix and the pre-Microsoft days,
> > uppercase letters have a different numerical value to lowercase letters.
> >
> > Uppercase 'COM' is definitely not the same as lowercase 'com'.
> 
> Please correct me if I am wrong but afaik upper-/lowercase does not 
> matter in DNS. Also, I am not aware of e.g. Postfix actually rejecting 
> (with reject_unknown_client_hostname) a FQDN with capitals when a FQDN 
> in lowercase was expected.

Postfix HELO verification simply does the relevant DNS lookups; if they
succeed then the HELO is OK.

Postfix IP verification does the IP rDNS lookup, then a forward lookup
of the result; if the result set includes the original IP then it
succeeds.

Case doesn't matter unless the underlying DNS libraries somehow break
on case.  Which they shouldn't :-)

In the example given earlier:

   HELO / EHLO:     smtpe1.intersmtp.com
   HELO IP:         62.239.224.89
   MX IP:           62.239.224.234
   MX DNS A record: smtp61.intersmtp.com

The HELO name successfully resolves to 62.239.224.89, so passes.

Now the source IP address isn't given but if it was 62.239.224.89 then
postfix would have done
  62.239.224.89 -> smtpe1.intersmtp.COM.
and then
  smtpe1.intersmtp.COM. -> 62.239.224.89
Since the final IP address matches the source IP address then the connecting
IP address check would also have passed.

You'll note the MX IP and A records aren't actually involved, in this
case!

After 5+ years of running these checks myself, I finally got fed up with
all the stupid companies who had broken DNS (including banks and ISPs
and Fortune 500 companies - my "white list" made 99 entries!) that
I eventually turned it off and just use the Zen RBL.  It lets through
spam that the stricter checks would reject, but it's good enough.

-- 

rgds
Stephen