• Intro
• With Implicit $ORIGIN
• With Implicit $ORIGIN
• Dots Before The Eyes Examples
• Reverse Mapping Zone Files

The DOT in a Zone File
Sometimes you need it, sometimes you don't. At first glance, and even at the
fourth glance, it seems confusing.

It is not. The rule is simple and we call it the ORIGIN substitution rule.

If there is a dot at the end of a name in a resource record or directive, the 
name is qualified and if it contains the whole name including the host then it
is a Fully Qualified Domain Name - FQDN. In this case the name as it appears in
the RR is used unchanged.

If there is NO dot at the end of the name (a.k.a. label(s) in DNS jargon), the 
name is unqualified and DNS software adds the value of the last or only $ORIGIN
directive. Unqualified names can appear on left-hand names, right-hand names or
both. In the absence of an $ORIGIN directive the zone name from the named.conf
file for this zone is used to synthesize an $ORIGIN directive. The fragment 
below illustrates this using A records and CNAME records.



With implicit $ORIGIN statement:
In the zone fragment below an implicit $ORIGIN directive is created from the 
zone file name as noted. In general this is bad practise for two reasons. First,
the file is not self-referencing - you need to know information from another 
source - in this case the zone name for which this file is being used which is
defined in the named.conf file (for BIND). Second, it is not readily apparent
what value is being substituted. At 3 A.M. when working under stress to bring
back service it may not be so apparent. All to save typing a few characters.
Just not worth it.
; zone file fragment for example.com
; the named.conf file contains 'zone "example.com"'
; there is no $ORIGIN statement and therefore one is synthesized
; name in the line below is expanded to joe.example.com.
joe IN A 192.168.254.3 
; line below - www.example.com. aliased to joe.example.com.
www IN CNAME joe 
; next line is functionally the same as line above
www.example.com. IN CNAME joe.example.com.
; and so is this line
www IN CNAME joe.example.com.
; the name in this record defaults to example.com
; assuming it was placed at the zone apex 
 IN A 192.168.254.3
; and could have been written as 
example.com. IN A 192.168.254.3
; OR
@ IN A 192.168.254.3

Easy really!



With explicit $ORIGIN statement:
In general, we always suggest that you use an $ORIGIN directive in every zone
file because it makes understanding the effects of the above substitution rule
much simpler. Here are the same examples as above but with an explicit $ORIGIN
directive in the zone file.
; zone file fragment for example.com
$ORIGIN example.com.
....
; name in the line below is expanded to joe.example.com. by adding $ORIGIN
joe IN A 192.168.254.3 
; line below - www.example.com. aliased to joe.example.com.
www IN CNAME joe 
; next line is functionally the same as line above
www.example.com. IN CNAME joe.example.com.
; and so is this line
www IN CNAME joe.example.com.
; the name in this is record defaults to example.com
; assuming it was placed at the zone apex 
 IN A 192.168.254.3
; and could have been written as 
example.com. IN A 192.168.254.3
; OR
@ IN A 192.168.254.3



Dots before the eyes: Some more examples:
; zone file fragment for example.com 

$ $ORIGIN must terminate with a DOT ALWAYS
$ORIGIN example.com
; in the above case the dot is missing from the $ORIGIN
; using the substitution rule this will give a substitution value of
; example.com.example.com.

; In A RR's the right hand expression is an address not a label
; thus expansion will be applied to the left hand label but not
; the right hand side
joe IN A 192.168.254.3 
; line above expands to - joe.example.com. IN A 192.168.254.3



Reverse Mapping Zone Files: A tad messier:
; zone file fragment for 2.168.192.IN-ADDR.ARPA
$ORIGIN 2.168.192.IN-ADDR.ARPA.
....
1 IN PTR joe.example.com.
; In the above case the left hand value is a name (or label) not an address
; and therefore subsitution is applied giving
; 1.2.168.192.IN-ADDR.ARPA. IN PTR joe.example.com.
; which maps 192.168.2.1 to the host name joe.example.com

; and if you wrote the above PTR like this
1 IN PTR joe
; it will expand to 
; 1.2.168.192.IN-ADDR.ARPA. IN PTR joe.2.168.192.IN-ADDR.ARPA.
; which is probably not what you intended