domain-name-system – 使用与主机名不匹配的CNAME替换MX记录

概述今天我们有5家公司都使用Google提供电子邮件服务 example1.com. 3w IN MX 10 mail.Google.com.example2.com. 3w IN MX 10 mail.Google.com.example3.com. 3w IN MX 10 mail.Go 今天我们有5家公司都使用Google提供电子邮件服务

example1.com.         3w   IN      MX  10   mail.Google.com.example2.com.         3w   IN      MX  10   mail.Google.com.example3.com.         3w   IN      MX  10   mail.Google.com.example4.com.         3w   IN      MX  10   mail.Google.com.example5.com.         3w   IN      MX  10   mail.Google.com.

下周我们将使用其他供应商(思科).我们可以指向MX中的A或Cname吗？

example1.com.         3w   IN      MX  10   myCnameToCisco.example.com.example2.com.         3w   IN      MX  10   myCnameToCisco.example.com.example3.com.         3w   IN      MX  10   myCnameToCisco.example.com.example4.com.         3w   IN      MX  10   myCnameToCisco.example.com.example5.com.         3w   IN      MX  10   myCnameToCisco.example.com.

我的想法是,我可以将myCnameToCisco.example.com更改为任何其他供应商.我担心的是,当客户端说helo domain.com并且220响应可能包含意外的主机或域名时,可​​能会有一些奇怪的验证.

以这种方式使用Cname或A记录与电子邮件有任何问题吗？

解决方法 如果您将MX记录指向Cname记录,那肯定会产生问题,因为它违反了标准.最清楚的解释是由 RFC2181 §10.3提供的：

10.3. MX and NS records

The domain name used as the value of a NS resource record,or part of
the value of a MX resource record must not be an alias. Not only is
the specification clear on this point,but using an alias in either
of these positions neither works as well as might be hoped,nor well
fulfills the ambition that may have led to this approach. This
domain name must have as its value one or more address records.
Currently those will be A records,however in the future other record
types giving addressing information may be acceptable. It can also
have other RRs,but never a Cname RR.

Searching for either NS or MX records causes “additional section
processing” in which address records associated with the value of the
record sought are appended to the answer. This helps avoID needless
extra querIEs that are easily anticipated when the first was made.

Additional section processing does not include Cname records,let
alone the address records that may be associated with the canonical
name derived from the alias. Thus,if an alias is used as the value
of an NS or MX record,no address will be returned with the NS or MX
value. This can cause extra querIEs,and extra network burden,on
every query. It is trivial for the DNS administrator to avoID this
by resolving the alias and placing the canonical name directly in the
affected record just once when it is updated or installed. In some
particular hard cases the lack of the additional section address
records in the results of a NS lookup can cause the request to fail.

您可以通过搜索引擎找到某些DNS和MTA软件支持这一点的轶事证据,但这应该被视为例外,而不是规则.缺乏这种支持不会被大多数软件作者视为错误.始终避免将MX记录指向Cname.

您现在面临的最大问题是,示例中MX记录的TTL都是三周,而您的更改是下周.我强烈建议您请求延迟此切换,并将TTL降低到十分钟左右的某个位置.切换完成后,您可以再次提升TTL.

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