Tools!

I just added a DMARC validation tool over on tools.wordtothewise.com.

You can give it a domain – such as ebay.com – and it will fetch the DMARC record, then explain and validate it. Or you can paste the DMARC record you’re planning to publish into it,  to validate it before you go live.
If you’ve not seen our tools page before, take a look. As well as DMARC we have a DKIM validator, SPF expander and optimizer, general DNS lookup tools, a bunch of RFCs covering all sorts of protocols, and base64 and quoted-printable decoders.
There’s also a widget that lets you add those little unicode pictures to your subject lines, whether you need a snowman ⛄, a forest ????, or a pig getting closer ???.
The results pages all have easily copyable URLs so they’re pretty good for sharing with co-workers or customers if you need that sort of thing.
(And if you need a cidr calculator, whois, or easy access to abuse.net & Microsoft SNDS check out Al’s xnnd.com.)

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Authentication and Repudiation

Email Authentication lets you demonstrate that you sent a particular email.
Email Repudiation is a claim that you didn’t send a particular email.
 
SPF is only for email authentication1
DKIM is only for email authentication
DMARC is only for email repudiation
 
1 SPF was originally intended to provide repudiation, but it didn’t work reliably enough to be useful. Nobody uses it for that now.

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A brief history of TXT Records

txt
When the Domain Name System was designed thirty years ago the concept behind it was pretty simple. It’s mostly just a distributed database that lets you map hostname / query-type pairs to values.
If you want to know the IP address of cnn.com, you look up {cnn.com, A} and get back a couple of IP addresses. If you want to know where to send mail for aol.com users, you look up {aol.com, MX} and you get a set of four hostname / preference pairs back. If you want to know the hostname for the IP address 206.190.36.45 you look up {45.36.190.206.in-addr.arpa, PTR} and get a hostname back.
There’s a well-defined meaning to each of those query types  – A is for IP addresses, MX is for mailservers, PTR is for hostnames – and that was always the intent for how DNS should work.
When DNS was first standardized, though, there was one query type that didn’t really have any semantic meaning:

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