DNS for white label authentication with SproutDNS

I wrote last year about using “stunt” nameservers for customer subdomain authentication – i.e. dynamically generating all the authentication records needed in DNS for each customer as needed.

For example, if you’re an ESP that has customers who can’t or won’t use their own domains and you still need to give them unique subdomains you can generate CNAME records to support white label DKIM authentication:

selector._domainkey.customerid.espcustomer.com CNAME \
    selector.dkim.esp.com

or generate white label DMARC with useful rua= reporting:

_dmarc.customerid.espcustomer.com TXT \
    "v=DMARC1 p=none rua=rua+customerid@esp.com"

Once you’ve set up these DNS records once they’ll work for all your customers, you just need to put the right domains in your DKIM signature and return path.

I shared some demo code to explain the concept last year, but since then we’ve developed a robust, production-ready application to dynamically serve DNS in this way.

It’s called SproutDNS – the humble brussel sprout isn’t the flashiest vegetable, but it’s robust, tasty, easy to prepare and good for you.

The product page is at sproutdns.com and there’s documentation at docs.sproutdns.com. Take a look, and sign up for our waitlist if it’s something you’d find useful. We’ll be announcing pricing in the next few weeks, but if you’d like to deploy it before then drop us a line.

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Gmail showing authentication results to endusers

A bit of older news, but worth a blog post. Early in August, Gmail announced changes to the inbox on both the web interface and the android client. They will be pushing authentication results into the interface, so end users can see which emails are authenticated.

These are not deliverability changes, the presence or absence of authentication will not affect inbox delivery. And the gmail Gmail support pages clarify that lack of authentication is not a sign that mail is spam.
This isn’t a huge change for most ESPs and most senders. In fact, Gmail has reported more than 95% of their mail is authenticated with either SPF or DKIM. Now, Gmail does a “best guess” SPF – if it looks like an IP should be authorized to send mail for a domain (like the sending IP is the same as the MX) then it’s considered authenticated.
It’s good to see authentication information being passed to the end user.

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Are they using DKIM?

It’s easy to tell if a domain is using SPF – look up the TXT record for the domain and see if any of them begin with “v=spf1”. If one does, they’re using SPF. If none do, they’re not. (If more than one does? They’re publishing invalid SPF.)
AOL are publishing SPF. Geocities aren’t.
For DKIM it’s harder, as a DKIM key isn’t published at a well-known place in DNS. Instead, each signed email includes a “selector” and you look up a record by combining that selector with the fixed string “._domainkey.” and the domain.
If you have DKIM-signed mail from them then you can find the selector (s=) in the DKIM-Signature header and look up the key. For example, Amazon are using a selector of “taugkdi5ljtmsua4uibbmo5mda3r2q3v”, so I can look up TXT records for “taugkdi5ljtmsua4uibbmo5mda3r2q3v._domainkey.amazon.com“, see that there’s a TXT record returned and know there’s a DKIM key.
That’s a particularly obscure selector, probably one they’re using to track DKIM lookups to the user the mail was sent to, but even if a company is using a selector like “jun2016” you’re unlikely to be able to guess it.
But there’s a detail in the DNS spec that says that if a hostname exists, meaning it’s in DNS, then all the hostnames “above” it in the DNS tree also exist (even if there are no DNS records for them). So if anything,_domainkey.example.com exists in DNS, so does _domainkey.example.com. And, conversely, if _domainkey.example.com doesn’t exist, no subdomain of it exists either.
What does it mean for a hostname to exist in DNS? That’s defined by the two most common responses you get to a DNS query.
One is “NOERROR” – it means that the hostname you asked about exists, even if there are no resource records returned for the particular record type you asked about.
The other is “NXDOMAIN” – it means that the hostname you asked about doesn’t exist, for any record type.
So if you look up _domainkey.aol.com you’ll see a “NOERROR” response, and know that AOL have published DKIM public keys and so are probably using DKIM.
(This is where Steve tries to find a domain that isn’t publishing DKIM keys … Ah! Al’s blog!)
If you look up _domainkey.spamresource.com you’ll see an “NXDOMAIN” response, so you know Al isn’t publishing any DKIM public keys, so isn’t sending any DKIM signed mail using that domain.
This isn’t 100% reliable, unfortunately. Some nameservers will (wrongly) return an NXDOMAIN even if there are subdomains, so you might sometimes get an NXDOMAIN even for a domain that is publishing DKIM. shrug
Sometimes you’ll see an actual TXT record in response – e.g. Yahoo or EBay – that’s detritus left over from the days of DomainKeys, a DomainKeys policy record, and it means nothing today.

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Don't just follow the HOWTO

speakingIconForBlogThere are so many moving parts to ensure good email deliverability. Email marketers need to know marketing, they need to know email and they need to know design. The technical bits of email can be a challenge to learn, and many folks who write tutorials and How-Tos write them for a different audience than marketers.
One of the things I’m trying to do is demystify the technical end of email for marketers. Today I talked about authentication in the Only Influencers newsletter. Check it out!
Understanding the technical: Authentication
Authentication in general

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