DNS $ORIGIN dot

Good afternoon, all!

I'm going over some BIND zone files and have run across an oddity that I haven't found a good answer. Several of these zone files have an ORIGIN directive of just a dot (.). Looks weird, I know. Here's the sanitized directives:

$ORIGIN .
$TTL 600    ; 10 minutes
example.com     IN SOA  ns1.example.com. support.example.com. (
                2016010101 ; serial
                28800      ; refresh (8 hours)
                120        ; retry (2 minutes)
                1209600    ; expire (2 weeks)
                86400      ; minimum (1 day)
                )

The file seems to work OK and it passes online tests from DNSStuff. I do plan on

Any thoughts from the gurus?

Thanks to all for looking!

G

http://www.zytrax.com/books/dns/ch8/origin.html

$ORIGIN defines a base name from which 'unqualified' names (those without a terminating dot) substitutions are made when processing the zone file.

When set to only a dot . that means that only a dot is going to be added/substituted. That’s all.

Typically you would see:

$ORIGIN example.com.
@  IN NS    ns1.example.com
@  IN NS    ns2.example.com.
@  IN MX  5 mail

Which because the @ symbol is a shorthand for the $ORIGIN in zone files will be completed into:

example.com.  IN  NS   ns1.example.com.example.com.
example.com.  IN  NS   ns2.example.com.
example.com.  IN  MX 5 mail.example.com. 

Where the first line shows a typical erroneous record that will result of such substitution when the $ORIGIN gets appended to what was intended to be a FQDN which lacked the trailing . .

Setting $ORIGIN to a dot makes the use of the @ shorthand impractical but will prevent mishaps such as the top one.

$ORIGIN .
example.com   IN  NS  ns1.example.com.
example.com.  IN  NS  ns2.example.com

Will autocomplete/correct that into

example.com.  IN  NS  ns1.example.com.
example.com.  IN  NS  ns2.example.com.

And not into

example.com.example.com.  IN  NS  ns1.example.com.
example.com.              IN  NS  ns2.example.com.example.com. 

which is what would have happened if the $ORIGIN would still have been the example.com. domain.