[ietf-dkim] New Issue: Applicability of SSP to subdomains

[Jim Fenton]

In the process of preparing my slides for the recent WG meeting, it 
occurred to me that there is no requirement in the SSP requirements doc 
for SSP to apply to subdomains of a given domain.

The issue is this:  If an SSP record exists for example.com saying, for 
example, "I sign everything", it's probably not a good idea if an 
attacker can avoid that policy by sending mail from (for example) 
mail.example.com.  The recipient is still likely to associate the 
message with the example.com domain.

This can occur whether or not there actually is a mail.example.com 
subdomain, or some other sort of record (such as an A record) for 
mail.example.com.

It's also probably a good idea to require a flag in SSP that indicates 
whether the policy published there is intended to apply to subdomains.  
This would be used when the subdomains are under separate administrative 
control, and there is a desire to avoid having a parent's SSP "bleed 
through" to subdomains.

This also needs to be done to (sub-)*domains, e.g., q.w.e.r.t.y.example.com.

-Jim