Include support for exists element in SPF.

[kjacobsen]

Hi,

Related to issues #355, I suspect it would only take a simple change of the handling for the exists: element to work, however I could be very wrong (and totally happy for you to tell me so).

[kjacobsen]

What's the best way to test these changes?

[captncraig]

Awesome to hear that somebody is using this kinda fringe feature!

I had to dig into the spec on this one. exists is a part of spf I was hoping wasn't widely used. Apparently I was wrong.

Apparently exists:%{i}._spf.sparkpostmail.com means "look for an A record at 1.2.3.4._spf.sparkpostmail.com" where the IP is the sender IP. Not sure exactly what cases this would be preferable to just listing ip4 records (maybe to not publish IPs so easily?), but seems potentially useful.

So my question is: are there any situations where a macro will expand to cause multiple lookups, or will it always be one? Reading the spec I suspect not, but my eyes kinda glazed over after a while.

If that is the case, I have no reason not to include this. Thank you!

[captncraig]

With regards to testing, the tests in that package are not very robust. You could probably add an exists clause to the test here I'd love to hear ideas if you have a good way to test all of this better. But I wouldn't block this for that.

[kjacobsen]

Thanks for the fast response.

My hope is that the SPF support simplifies the management of our SPF records in the long run.

From my reading of the RFC, I can't see a situation where the expansion would cause multiple lookups. Section 5.7 says

The domain-spec is expanded as per Section 8. The resulting domain name is used for a DNS A RR lookup. If any A record is returned, this mechanism matches. The lookup type is A even when the connection type is IPv6.

In terms of testing, we could include an entry using exists: to test if the code correctly produces the SPF we are expecting.

[tlimoncelli]

The change looks good. Please add an "exists:" example to dnscontrol/pkg/spflib/parse_test.go then we'll merge the PR.

[kjacobsen]

Forgot to let you know that I have added the example into the tests.