SPF Configuration – How to Set Up for Multiple Domains and Allow Gmail

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SPF (Sender Policy Framework) seems like a good way to combat spammers/spoofing.

However, despite reading the explanations several times, I'm not quite understanding how to configure it correctly.

Let's say I have my server at a.x.com which hosts www.x.com and b.x.com and c.x.com and so on.

I also have a.co.uk b.net c.info and so on, each of these with an assortment of sub-domains, all hosted on x.com

For all of these domains and sub-domains, I want to permit mail to be sent from a.x.com

I would also like them all to permit mail sent from Gmail for all these domains.

How do I set this up with SPF?

Can I set one SPF record for x.com (or a.x.com) and then for everything else just have a simple include/pointer to x.com's record, or would it need to be done differently?

Can anyone provide some SPF records for the above example?

Note: The second part of my question has been answered (use "v=spf1 include:x.com -all" to include/point at x.com's record), but the key part of what to set on x.com remains unanswered…

Best Answer

You can't avoid having to alter the zone files for the domains other than x.com, but you can save yourself a lot of trouble by defining common policies hosted on one domain and using the redirect SPF keyword on the other domains. Example:

  • In the zonefile for the x.com domain:
_policy1    IN    TXT     "v=spf1 a:a.x.com -all"
_policy2    IN    TXT     "v=spf1 include:_spf.google.com a:a.x.com -all"

_spf.google.com is the record holding the Gmail SPF record. Not sure whether it's documented. Theoretically you should include:gmail.com but that's a redirect to _spf.google.com and there has been at least one widely used SPF patch for qmail which didn't follow it properly (got fixed in August 2008 but might still be deployed.) The two policies are examples, of course - having more than one with various levels of strictness is extremely useful when debugging since you only have to alter a short name in the target domain instead of error-prone copypasting.

  • In the zonefiles for the other domains:
@                       IN    TXT    "v=spf1 redirect=_policy1.x.com"

or

@                       IN    TXT    "v=spf1 redirect=_policy2.x.com"

etc. I'm using redirect, not include, to cause the SPF check to completely replace the currently evaluated record with the one I'm redirecting to. include does not do that - for example, an -all at the end of an include does not cause evaluation to stop (include is a big misnomer.) You should avoid using include when you want to "alias" a SPF record from another domain, since it's quite brittle - if you accidentally forget the trailing -all you might render your entire SPF on that domain ineffective.

Edit: Please note, though, that you need to be on guard if you want to allow Gmail's servers as senders. The Gmail chaptcha has been cracked, which means that it's possible to automate account signups, which means Gmail can be (indirectly) used as an open relay (I'm getting tens of spambot signup requests per week for my company discussion forum, all using gmail.com email addresses - and those addresses are live, I've allowed a few to go through for checking purposes.) Additionally, anyone with a Gmail account can bypass SPF checking if familiar with the uwsername parts of the email addresses at your domains.

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