I have a question regardin this Zone file from wikipedia.com "

$ORIGIN example.com.     ; designates the start of this zone file in the namespace
$TTL 1h                  ; default expiration time of all resource records without their own TTL value
example.com.  IN  SOA  ns.example.com. username.example.com. (
              2007120710 ; serial number of this zone file
              1d         ; slave refresh (1 day)
              2h         ; slave retry time in case of a problem (2 hours)
              4w         ; slave expiration time (4 weeks)
              1h         ; maximum caching time in case of failed lookups (1 hour)
example.com.  NS    ns                    ; ns.example.com is a nameserver for example.com
example.com.  NS    ns.somewhere.example. ; ns.somewhere.example is a backup nameserver for example.com
example.com.  MX    10 mail.example.com.  ; mail.example.com is the mailserver for example.com
@             MX    20 mail2.example.com. ; equivalent to above line, "@" represents zone origin
@             MX    50 mail3              ; equivalent to above line, but using a relative host name
example.com.  A             ; IPv4 address for example.com
              AAAA  2001:db8:10::1        ; IPv6 address for example.com
ns            A             ; IPv4 address for ns.example.com
              AAAA  2001:db8:10::2        ; IPv6 address for ns.example.com
www           CNAME example.com.          ; www.example.com is an alias for example.com
wwwtest       CNAME www                   ; wwwtest.example.com is another alias for www.example.com
mail          A             ; IPv4 address for mail.example.com,
                                          ;  any MX record host must be an address record
                                          ; as explained in RFC 2181 (section 10.3)
mail2         A             ; IPv4 address for mail2.example.com
mail3         A             ; IPv4 address for mail3.example.com

Why is these lines necessary?: 
example.com.  NS    ns                    ; ns.example.com is a nameserver for example.com
ns            A             ; IPv4 address for ns.example.com

We already know that the ns.example.com is the default name server, right? Then why should we specify its IP-address? Isn't this zone file in the ns.example.com name server, I mean if we are looking at it why do we want the ip-address of the server that has the file we are already looking at?


The information presented by the authoritative name server must be consistent with information presented elsewhere.

In this case, a resolver has used a glue record to find this name server. The delegation and name information found here must match the delegation from the TLD (the NS record in the example), and the host information found here must match what was provided as extra information by the TLD, the glue record (the A record in the example).

  • Let's say ns.example.com is the only name server for this Zone. Does that mean that the only place we can find this zone file is in that name server? If yes, do we still need the A record for it? – user951732 Sep 20 '11 at 19:42
  • Yes, when you only have one NS record, you're telling clients that this is the only place to find the zone's data. And yes, you would still need an A record; a DNS server must present consistent data. Yes, they know your IP if they're talking to it, but DNS caches heavily; it's important that your server confirm that it is where a client was told that it was. – Shane Madden Sep 20 '11 at 19:55

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