mime - 在字段正文的开头折叠 MIME header 是否有效？

Question

我正在调试一个 MIME 解析器，它无法解析来自一个电子邮件服务的 header 。电子邮件服务包含一个 header ，该 header 由完全位于下一行的字段主体组成，如下所示:

Message-Id:
  <12345.67890.abcdef@example.com>

这合法吗？

RFC-822 为有效 header 指定以下语法:

3.2.  HEADER FIELD DEFINITIONS

      These rules show a field meta-syntax, without regard for the
 particular  type  or internal syntax.  Their purpose is to permit
 detection of fields; also, they present to  higher-level  parsers
 an image of each field as fitting on one line.

 field       =  field-name ":" [ field-body ] CRLF

 field-name  =  1*<any CHAR, excluding CTLs, SPACE, and ":">

 field-body  =  field-body-contents
                [CRLF LWSP-char field-body]

 field-body-contents =
               <the ASCII characters making up the field-body, as
                defined in the following sections, and consisting
                of combinations of atom, quoted-string, and
                specials tokens, or else consisting of texts>

空字符串是否满足 field-body-contents 以允许 field-body 的 [CRLF LWSP field-body] 部分？

Answer 1

是的，完全允许在空白处折叠。通常，只有当线路太长时才会这样做，但无论如何它都是完全可选的；没有要求折叠或避免折叠。 (当然，如果一行太长而无法满足 RFC5321 的要求，则必须将其折叠，或者以其他方式缩短。如今，RFC2047 比传统的空白折叠更加通用和透明。)

如 https://www.rfc-editor.org/rfc/rfc5322#section-2.2.3 中所述折叠线是一条“逻辑线”。 (请注意，旧的 822 早已被更新的标准取代；5322 是当前的替代品。)