AW: SPF Configurations

Andre Engel andre.engel at fhe3.com
Fri Dec 4 18:13:17 CST 2009


John ,

Nice to meet you :-)

> Right.  The only major mail system that pays attention to SPF is
> Hotmail, but there are enough small poorly run MTAs that use it that
> an SPF record which lists your outbounds and ~all (not -all) can be
> marginally useful to avoid bogus rejections of your mail.

For example :

host -t TXT hotmail.com
hotmail.com             TXT     "v=spf1 include:spf-a.hotmail.com
include:spf-b.hotmail.com include:spf-c.hotmail.com
include:spf-d.hotmail.com ~all"

host -t TXT google.com :
google.com              TXT     "v=spf1 include:_netblocks.google.com
ip4:216.73.93.70/31 ip4:216.73.93.72/31 ~all"

host -t TXT amazon.com :
amazon.com              TXT     "v=spf1 ip4:207.171.160.0/19
ip4:87.238.80.0/21 ip4:72.21.193.0/24 ip4:72.21.196.0/22 ip4:72.21.208.0/24
ip4:72.21.205.0/24 ip4:72.21.209.0/24 ip4:194.154.193.200/28
ip4:194.7.41.152/28 ip4:212.123.28.40/32 ip4:203.81.17.0/24 ~all"
amazon.com              TXT     "spf2.0/pra ip4:207.171.160.0/19
ip4:87.238.80.0/21 ip4:72.21.193.0/24 ip4:72.21.196.0/22 ip4:72.21.208.0/24
ip4:72.21.205.0/24 ip4:72.21.209.0/24 ip4:194.154.193.200/28
ip4:194.7.41.152/28 ip4:212.123.28.40/32 ip4:203.81.17.0/24 ~all"

host -t TXT ebay.de :
ebay.de                 TXT     "v=spf1 mx include:s._spf.ebay.com
include:m._spf.ebay.com include:p._spf.ebay.com include:c._spf.ebay.com
~all"
ebay.de                 TXT     "spf2.0/pra mx include:s._sid.ebay.com
include:m._sid.ebay.com include:p._sid.ebay.com include:c._sid.ebay.com
~all"

host -t TXT 1und1.de :

TXT     "v=spf1 ip4:82.165.0.0/16 ip4:195.20.224.0/19 ip4:212.227.0.0/16
ip4:87.106.0.0/16 ip4:217.160.0.0/16 ip4:213.165.64.0/19 ip4:217.72.192.0/20
ip4:74.208.0.0/17 ip4:74.208.128.0/18 ip4:66.236.18.66 ip4:67.88.206.40
ip4:67.88.206.48 ~all"

host -t TXT gmx.com :
gmx.com                 TXT     "v=spf1 ip4:213.165.64.0/23
ip4:74.208.5.64/26 ip4:74.208.122.0/26 -all"

host -t TXT enterprisemail.de :
enterprisemail.de       TXT     "v=spf1 a:mout.enterprisemail.de -all"

etc

> As everyone here should already know, the fundamental problem with SPF
> is that although it does an OK job of describing the mail sending
> patterns of dedicated bulk mail systems, it can't model the way that
> normal mail systems with human users work.  But so deep is the faith
> of the SPF cult that they blame the world for not matching SPF rather
> than the other way around, believing that it prevent forgery, having
> redefined "forgery" as whatever it is that SPF prevents.  As the
> operator of one of the world's more heavily forged domains (abuse.net)
> I can report that if you think it prevents forgery blowback, you are
> mistaken.

You do know that I love they way abuse.net flys:

In mind of the following situation for instance a infection vector around
millions of bots which are sending millions 
of forged mails within evil polymorphic files camouflage as your customers
bills you
will be glade to enforce the directive -all for a while .

Sorry Im almost german :
http://www.heise.de/security/meldung/1-1-warnt-Kunden-vor-gefaelschten-Rechn
ungen-131420.html


I know SPF is not the answer of all ....but sometimes it helps to secure a
little bit of yours "critical customers infrastructure" and sometimes it
helps to save your operative resources .


        
I know there is a problem so far with forwarded emails but there is  also a
solution :


The solution could be to rewrite the envelope from of all forwarded mail so
that the given domain is a local domain with matching SPF records to the
originating mail server (or no SPF records at all). You have to transform
the original envelope from into a localpart and add some special local SRS
domain to it.

Find http://spf.pobox.com/srs.html <http://spf.pobox.com/srs.html>  and
http://www.libsrs2.org/ <http://www.libsrs2.org/>  for a full description of
SRS.

In practice

andre.engel at fhe3.com could receiving an email from misterX at google.com where
andre.engel at fhe3.com could be forwarded to andre.engel at hotmail.de. Before
forwarding the email to the hotmail server I could rewrite the envelope-from
from misterX at google.com <mailto:misterX at google.com>  to
google.com=misterX at srs.enterprisemail.de srs.enterprisemail.de could be a
valid domain for mails originating from our main mail
clusters(enterprisemail) so possible SPF checks at hotmail would not bother.

In case a bounce is generated at hotmail it could  be delivered back to the
SRS address, thus to our enterprisemail main mail cluster, where we would
recognise the SRS scheme and "un-rewrite" it back to misterX at google.com and
deliver the mail onward to the misterX at google.com mail system.

But in the real world the rewriting isn't that simple as stated in the
previous section. In fact you have to add some kind of checksum where the
original mail address is mangled with a secret password, and a time stamp
that makes the SRS address valid for some period of time.

The mail address from above could look more like this:

<SRS38=ldl23v=tz=google.com=MisterX at srs.enterprisemail.de>

Every time a mail arrives that is an SRS address the password and timestamp
could be checked, and faked or outdated recipients could be rejected.

If you asked around drawbacks your right :

SRS generates very long localparts. Mail servers should according to the RFC
accept local parts with at least 63 characters. Most mail servers accept
longer local parts, but unfortunately some won't. For those rare cases it is
possible to configure a list of mail servers for which SRS won't be
accomplished.

> For rants about how badly the world and/or SPF stink, followups to
> Spam-L.  For proposals about other anti-spam magic bullets, followups
> to ASRG.

Indeed Spam-L is the best place to talk about anti-spam . Indeed CII is the
best place to talk about critical infrastructures ,indeed nanog is the best
place to talk about networkstuff but we are mostly operators looking for
a valuable , comfortable solution to protect and share information .

I do not really know if this will be a little off topic .


Cheers

Andre 



 --
Andre Engel

Consulting Program Director, 
Email and Cyber Intelligence Services            "..ehy my friend we seek
the Grail!"



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andre.engel at fhe3.com
http://www.fhe3.com/

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> -----Ursprüngliche Nachricht-----
> Von: John Levine [mailto:johnl at iecc.com]
> Gesendet: Freitag, 4. Dezember 2009 18:25
> An: nanog at nanog.org
> Betreff: Re: SPF Configurations
> 
> >> If the customer insist on using their domain, then you would have to
> have
> >> the customer setup an SPF record within their domain that points to
> your
> >> email server IP blocks.
> 
> Right.  The only major mail system that pays attention to SPF is
> Hotmail, but there are enough small poorly run MTAs that use it that
> an SPF record which lists your outbounds and ~all (not -all) can be
> marginally useful to avoid bogus rejections of your mail.

> As everyone here should already know, the fundamental problem with SPF
> is that although it does an OK job of describing the mail sending
> patterns of dedicated bulk mail systems, it can't model the way that
> normal mail systems with human users work.  But so deep is the faith
> of the SPF cult that they blame the world for not matching SPF rather
> than the other way around, believing that it prevent forgery, having
> redefined "forgery" as whatever it is that SPF prevents.  As the
> operator of one of the world's more heavily forged domains (abuse.net)
> I can report that if you think it prevents forgery blowback, you are
> mistaken.
 




> For rants about how badly the world and/or SPF stink, followups to
> Spam-L.  For proposals about other anti-spam magic bullets, followups
> to ASRG.
> 
> R's,
> John
> 
> 






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