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# Why CNAME/MX/NS targets require a "dot"

People are often confused about this error message:

```
 1: ERROR: target (ghs.googlehosted.com) includes a (.), must end with a (.)
```

What this means is that CNAME/MX/NS records (anything where
the "target" is a hostname) must either contain no dots or
must end in a dot.

Here are four examples:

```
    CNAME("foo", "bar)        // Permitted. (expands to bar.$DOMAIN)
    CNAME("foo", "bar.com.")  // Permitted.
    CNAME("foo", "bar.com")   // ERROR
    CNAME("foo", "meta.xyz")  // ERROR

```

The first 2 examples are permitted.  The last 2 examples are
ambiguous and are therefore are considered errors.

How are they ambiguous?

  * Should $DOMAIN be added to "bar.com"?  Well, obviously not, because it already ends with ".com" and we all know that "bar.com.bar.com" is probably not what they want. Or is it?  
  * Should $DOMAIN be added to "meta.xyz"?  Everyone knows that ".xyz" isn't a TLD. Obviously, yes, $DOMAIN should be appended. However, wait...  ".xyz" became a TLD in June 2014.  We don't want to be surprised by changes like that.  Also, users should not be required to memorize all the TLDs. (When there was just "gov/edu/com/mil/org/net " that was a reasonable expectation, but that hasn't been true since around 2000. By the way, we forgot to include "int" and you didn't notice.)
  * What if $DOMAIN is "bar.com"? Shouldn't that be enough to know that "x.bar.com" is an FQDN and should not be turned into "x.bar.com.bar.com"? Maybe. What if we are copying 100 lines of dnsconfig.js from one `D()` to another. Buried in the middle is this one CNAME that means something entirely different when in a new $DOMAIN. We've seen similar mistakes and want to prevent them.

Yes, we could layer rule upon rule upon rule.  Eventually we'd get
all the rules right.  However, now a user would have to know all the
rules to be able to use Dnscontrol.  The point of the Dnscontrol DSL
is to enable the casual user to be able to make DNS updates. By
"casual user" we do not mean someone someone that lives and breathes DNS
like you and I do.  In fact, we mean someone that hasn't memorized
the list of rules.

We know of no time where a human intentionally wanted
"foo.example.com.domain.com" as the target of an MX record.
In fact, the opposite is true. StackExchange.com had
a big email outage in 2013 because MX records were updated and the
"trailing dot" was forgotten. Our MX records became
"aspmx.l.google.com.stackexchange.com" and due to a high TTL we
lost email for a few hours.  Recently (2017) we had a similar problem
and it delayed a new service from working. Luckily this was a new
service and didn't have existing users so the problem was unnoticed
except for the fact that a project schedule slipped by 3 days.

Therefore, we prefer the rule to be "when in doubt, error out". It
is less to remember and catches errors. It also doesn't remove
the expressiveness of the language.  One dot is better than 100 rules.


## Simple mental models are better

SRE ... the R stands for reliability.

A big source of human error is mental-model mismatch. That is, when
operating a complex system, the user has a mental model of
what is going on in the system. They are, essentially, emulating
the software in their head to predict that the change they are
making will have the result they seek. The more complex the
system the less likely the mental model will match reality.

A mental model mismatch leads to confusion, frustration, and
more importantly it increases the risk of operating error the creates
production problems.

If the rules are simple, the mental model will be more accurate
than if it is complex.  "If something is ambiguous, we give an error
and tell you to add a dot to the end" is *simple.*  "If something
is ambiguous, we follow this list of 100 rules that decide what
the user had intended" is *complex.*

One could argue that your users are very smart and can memorize
all the rules. Why should they have to?  It's just a single keystroke!


## Future

We welcome proposals for how to resolve this ambiguity.

["Future proofing is not adding stuff. Future proofing is making sure you can easily add code/features without breaking existing functionality."](http://softwareengineering.stackexchange.com/a/79591/116123)
By not solving the problem now, we open the door to upwards compatible
solutions.  If we created a partial solution now, we might prevent
future solutions from being upward compatible. By simply giving an
error we open the door to new solutions.

We should warn you, however, that any new proposals should be
simpler than "add a dot".
