這就好像以前的 mail server,規定一定要讀到有 MX 的 a record 才可以送信,
現在的有些 mail server 比較隨便,讀到 a record 有目標 hostname, try port 25 又有通,
不管三七二十一就把信丟過去了.
這並不表示你就可以不管原本人家怎麼規定,甚至牽拖沒看到 MX 就不寄信的 mail server 是有 bug.
不懂又隨便說人家錯,有 bug.
那標準都給你說了算就好了.
指個 A recode 會多花你的錢?那麼懶幹嘛?
The domain system provides such a feature using the canonical name
(CNAME) RR. A CNAME RR identifies its owner name as an alias, and
specifies the corresponding canonical name in the RDATA section of the
RR. If a CNAME RR is present at a node, no other data should be
present; this ensures that the data for a canonical name and its aliases
cannot be different. This rule also insures that a cached CNAME can be
used without checking with an authoritative server for other RR types.