On Feb 6, 2018, at 12:39 AM, Lanlan Pan <abby...@gmail.com> wrote: > I mean that in 5.2. 'localhost' labels in subdomains > <https://tools.ietf.org/html/draft-ietf-dnsop-let-localhost-be-localhost-02>, > localhost.example.com <http://localhost.example.com/>. => localhost. ( > equal to ban it at dns ? )
The text says: Hosts like "localhost.example.com" and "subdomain.localhost.example.com" contain a "localhost" label, but are not themselves localhost names, as they do not fall within "localhost.". Therefore, they are not directly affected by the recommendations in this document. They have no resolution guarantees one way or another, and should not be given special treatment, either in DNS or in client software. That's pretty clear. This document is not forbidding the appearance of such names in the DNS, nor the resolution of such names. The following paragraph says: Note, however, that the admonition against searchlist usage could affect their resolution in practice, as discussed in Section 3 <https://tools.ietf.org/html/draft-ietf-dnsop-let-localhost-be-localhost-02#section-3>. For example, even with a searchlist of "example.com" in place for a given network, the name "localhost" will not be resolved as "localhost.example.com." but as "localhost.", and "subdomain.localhost" will not be resolved as "subdomain.localhost.example.com." but as "subdomain.localhost.". Admonition might be a poor choice of words here. Nonetheless, the point this text is making is simply that if you type in "localhost," you will always get 127.0.0.1, even if there is a searchlist for "example.com <http://example.com/>" and the name "localhost.example.com <http://localhost.example.com/>" exists. This is explained in the text referenced above in section 3: Application software MUST NOT use a searchlist to resolve a localhost name. That is, even if DHCP's domain search option [RFC3397 <https://tools.ietf.org/html/rfc3397>] is used to specify a searchlist of "example.com" for a given network, the name "localhost" will not be resolved as "localhost.example.com." but as "localhost.", and "subdomain.localhost" will not be resolved as "subdomain.localhost.example.com." but as "subdomain.localhost.".
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