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$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:

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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Pro DNS and BIND by Ron Aitchison

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