reverse DNS (was: Provo Internet options)

Corey Edwards tensai at
Tue Nov 28 14:20:55 MST 2006

On Tue, 2006-11-28 at 13:48 -0700, Wade Preston Shearer wrote:
> [1] From
> "Reverse DNS was designed to be primarily a tool for network  
> troubleshooting. However, it is sometimes used as a poor defence  
> against spam. When an Internet mail server receives incoming mail  
> from an external machine, it may check that the reverse DNS record  
> for the IP address of the originating server matches up with name by  
> which the originating server identifies itself during the SMTP  
> greeting. […] This is not a good defence against spam for several  
> reasons.

Using it in that manner is not a good idea, and in fact is not how AOL
does it (nor I). Simply having reverse DNS set up to resolve to
*something* is all that's required. It works because there are quite a
few IPs that don't resolve and the majority of them generate spam.

There is also a push to have ISPs generate reverse dns for their dynamic
pools in a machine readable fashion, ie. Some
providers do this and some don't. I used to oppose that but given the
number of compromised machines on users' desktops, I think I've been


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