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区域文件解释，DNS？

tag-icon • tag-icon domain-name-system dns-zone
区域文件解释，DNS？

我对 wikipedia.com 上的这个区域文件有疑问”

$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     192.0.2.1             ; IPv4 address for example.com
              AAAA  2001:db8:10::1        ; IPv6 address for example.com
ns            A     192.0.2.2             ; 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     192.0.2.3             ; 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     192.0.2.4             ; IPv4 address for mail2.example.com
mail3         A     192.0.2.5             ; 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     192.0.2.2             ; IPv4 address for ns.example.com

我们已经知道 ns.example.com 是默认名称服务器，对吧？那么我们为什么要指定它的 IP 地址？这个区域文件不是在 ns.example.com 名称服务器中吗？我的意思是，如果我们正在查看它，为什么我们需要包含我们正在查看的文件的服务器的 IP 地址？

答案1

权威名称服务器提供的信息必须与其他地方提供的信息一致。

在这种情况下，解析器已使用粘合记录来查找此名称服务器。此处找到的授权和名称信息必须与 TLD 的授权（NS示例中的记录）相匹配，而此处找到的主机信息必须与 TLD 提供的额外信息（粘合记录（A示例中的记录））相匹配。

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