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  • DNS域名规则

    例子:***.***.com

    对于每个***,由字母起始,字母或数字结尾,仅包含字母、数字或者横杠(-)

    ***的长度不应超过63

    总长度不应超过255

    Ref: RFC1035

    2.3.1
    The labels must follow the rules for ARPANET host names.  They must
    start with a letter, end with a letter or digit, and have as interior
    characters only letters, digits, and hyphen.  There are also some
    restrictions on the length.  Labels must be 63 characters or less.

     2.3.4. Size limits

     Various objects and parameters in the DNS have size limits.  They are
    listed below.  Some could be easily changed, others are more
    fundamental.
    labels          63 octets or less
    names           255 octets or less

     3.1. Name space definitions

    Domain names in messages are expressed in terms of a sequence of labels.
    Each label is represented as a one octet length field followed by that
    number of octets.  Since every domain name ends with the null label of
    the root, a domain name is terminated by a length byte of zero.  The
    high order two bits of every length octet must be zero, and the
    remaining six bits of the length field limit the label to 63 octets or
    less.

    To simplify implementations, the total length of a domain name (i.e.,
    label octets and label length octets) is restricted to 255 octets or
    less.

    Although labels can contain any 8 bit values in octets that make up a
    label, it is strongly recommended that labels follow the preferred
    syntax described elsewhere in this memo, which is compatible with
    existing host naming conventions.  Name servers and resolvers must
    compare labels in a case-insensitive manner (i.e., A=a), assuming ASCII
    with zero parity.  Non-alphabetic codes must match exactly.

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  • 原文地址:https://www.cnblogs.com/cshao/p/1640397.html
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