$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

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