在正向区域中插入 mx 记录

Question

使用 bind9 将 MX 记录添加到转发区域的正确方法（以及为什么）是什么？

选项 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

选项 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

或者还有其他选择吗？

Answer 1

$ORIGIN 定义了一个基值，在处理区域文件时，从该基值中进行“非限定”名称（没有终止点的名称）替换。该@符号是区域文件中 $ORIGIN 的简写。

因此，使用简写@或全名example.com.是等效的。

$ORIGIN 可以在区域文件中明确指定，但通常从绑定配置文件中指定的区域名称中隐含：

// named.conf file fragment

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

和

; 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.

第三种选择是，区域文件中的一行既不以主机名、区域名称或@区域来源的简写开头，也可以成为上述记录的延续。

@   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

也相当于：

@              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.