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What is the correct way (and why) to add MX records to a forward zone using bind9?

Option A

@   IN MX 10 mx1.example.com.
@   IN MX 20 mx2.example.com.
mx1.example.com. IN A a.b.c.d
mx2.example.com. IN A a.b.c.d

Option B

example.com.    IN MX 10 mx1.example.com.
example.com.    IN MX 20 mx2.example.com.
mx1.example.com. IN A a.b.c.d
mx2.example.com. IN A a.b.c.d

Or is there another alternative?

HBruijn
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linguru771x
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1 Answers1

5

$ORIGIN defines a base value from which 'unqualified' name (those without a terminating dot) substitutions are made when processing the zone file. The @ symbol is a short-hand for the $ORIGIN in zone files.

Using the short-hand @ or the full name example.com. is therefore equivalent.

$ORIGIN can be specified explicitly in the zone file, but is often implied from the zone name as specified in the Bind configuration file:

// named.conf file fragment

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

and 

; example.com.zone file fragment 
; no $ORIGIN present and is synthesized from the zone name in named.conf ==> example.com
....
@          IN      NS     ns1.example.com. 
; ns1.example.com. is the name server for example.com.
@          IN      MX 10  mx1.example.com. 
; mx1.example.com. is the primary mailserver name for example.com.    
....
$ORIGIN uk.example.com.
; explicitly define or "reset" $ORIGIN to uk.example.com.
; doing this mid-way in a zone-file is not common practice anymore (if it ever was) 
; but it will keep a large single zone-file somewhat more readable.
@          IN      NS     ns2.example.com. 
; functionally identical to
; uk.example.com. IN NS ns2.example.com
; ns2.example.com. is the name server for uk.example.com.

A third option, is that a line in a zone file that starts with neither a hostname, the zone name or the @ shorthand for the zone origin, becomes a continuation of the record above.

@   IN MX 10 mx1.example.com.
    IN MX 20 mx2.example.com.  ; another record for the @ record above
    IN NS    ns1.example.com.  ; yet another record for the @ record above
    IN NS    ns2.example.com.  ; and a 3rd continuation for the @ record above

is also equivalent to:

@              IN MX 10 mx1.example.com.
example.com.   IN MX 20 mx2.example.com.
               IN NS    ns1.example.com. ; NS record for the example.com. record above
                                         ; the continuation can be to set different 
                                         ; record types for the same resource record name
example.com.   IN NS    ns2.example.com.

www            IN A     10.9.8.7
               IN A     192.168.0.1      ; or set multiple (round-robin) records when 
                                         ; continuing with the same record type.
HBruijn
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