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Suppose you have the domain yourdomain.com, and you have set the following SPF:

v=spf1 +ptr:yourdomain.com ~all

Now, I have the IP address 1.2.3.4, and I am also the administrator of 3.2.1.in-addr.arpa. I can specify

4 IN PTR fake.yourdomain.com.

and then I can send emails from my machine, pretending it's one of the yourdomain.com machines.

Is this a significant risk? Should I avoid using ptr in the SPF?

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According to wikipedia using PTR will also forward confirm the PTR (IE 1.2.3.4 might resolve to fake.yourdomain.com but fake.yourdomain.com points to 5.4.3.2)

http://en.wikipedia.org/wiki/Sender_Policy_Framework

http://en.wikipedia.org/wiki/Forward-confirmed_reverse_DNS

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