• Intro
• Blank Substitutins

$ORIGIN, @ and blank Substitution
The symbol @ is used in BIND to denote zone root (which is sometimes also
called the zone apex. The value substituted for @ is either:

The last $ORIGIN directive encountered in the file. To illustrate possible
uses:
; example.com zone file fragment 
 $ORIGIN example.com.
 @ IN SOA ns1.example.com. hostmaster.example.com. (
 2003080800 ; se = serial number
 172800 ; ref = refresh = 2d
 900 ; ret = update retry = 15m
 1209600 ; ex = expiry = 2w
 3600 ; min = minimum = 1h
 )
 ....
 @ IN NS ns1.example.com. 
 ; ns1.example.com. is the NS for example.com
 ; and could have written as 
 example.com. IN NS ns1.example.com.
 ; OR using blank substitution
 IN NS ns1.example.com.
 ; MX traditionally written using blank substitution
 IN MX 10 mail.example.com.
 ; but could have been written as
 @ IN MX 10 mail.example.com.
 ...
 ; @ works anywhere
 www IN A 192.168.2.3
 ; this RR allows use of http://example.com
 @ IN A 192.168.2.3
 ; and could have been written as 
 example.com. IN A 192.168.2.3
 ...
 $ORIGIN uk.example.com.
 ; @ reference last $ORIGIN
 @ IN NS ns2.example.com. 
 ; ns2.example.com. is name server for uk.example.com subdomain
 ; and could have been written as 
 uk.example.com. IN NS ns2.example.com.

If no $ORIGIN directive is present - BIND synthesizes one from the value of
the zone name in the named.conf file , for example:

// named.conf file fragment

 zone "example.com" in{
 	type master;
 	file "pri.example.com";
 };

 example.com. is synthesized as the zone $ORIGIN and will replace @ in the
zone file.
; example.com zone file fragment 
 ; no ORIGIN directive
 @ IN SOA ns1.example.com. hostmaster.example.com. (
 2003080800 ; se = serial number
 172800 ; ref = refresh = 2d
 900 ; ret = update retry = 15m
 1209600 ; ex = expiry = 2w
 3600 ; min = minimum = 1h
 )
 ....
 @ IN NS ns1.example.com. 
 ; ns1.example.com. is the NS for example.com
 ; and could have written as 
 example.com. IN NS ns1.example.com.
 ; OR using blank substitution
 IN NS ns1.example.com.
 ....



Blank Substitution
Blank labels are a little messy and the substitution here is the last valid
name (or label) or $ORIGIN if there are no previous names (labels). To
illustrate blank name substitution:
; example.com zone file fragment 
; no ORIGIN directive
@ IN SOA ns1.example.com. hostmaster.example.com. (
 2003080800 ; se = serial number
 172800 ; ref = refresh = 2d
 900 ; ret = update retry = 15m
 1209600 ; ex = expiry = 2w
 3600 ; min = minimum = 1h
 )
....
 IN NS ns1.example.com. 
; the blank label in the NS substitutes example.com.
; Confusingly the whole definition could use blank substitution 
; as shown below which is functionally equivalent to above
; example.com zone file fragment 
; no ORIGIN directive
 IN SOA ns1.example.com. hostmaster.example.com. (
 2003080800 ; se = serial number
 172800 ; ref = refresh = 2d
 900 ; ret = update retry = 15m
 1209600 ; ex = expiry = 2w
 3600 ; min = minimum = 1h
 )
....
 IN NS ns1.example.com.

Finally, to illustrate other uses of blank substitution see the following:

; zone file fragment for example.com.
...
; following two A RRs both have a label of www.example.com.
www IN A 192.168.2.3
 IN A 192.168.2.4
; this could have also been written as shown if less confusing
www IN A 192.168.2.3
www IN A 192.168.2.4