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1. nandgator commented on Sep 27, 2025

   

   nandgator

   on Sep 27, 2025

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   This document has had many revisions. Please provide references to these requirements based on updated documents.
2. Maxk-Intezer commented on Oct 5, 2025

   

   Maxk-Intezer

   on Oct 5, 2025

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   🧩 RFC 1123 Hostname Requirements Summary

   📄 Source: RFC 1123 – Section 2.1 (Host Names and Numbers)

   “The syntax of a legal Internet host name was specified in RFC-952.
   One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit.
   Host software MUST support this more liberal syntax.
   Host software MUST handle host names of up to 63 characters and SHOULD handle host names of up to 255 characters.”
   — RFC 1123 § 2.1

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   ✅ Summary of Requirements Derived from RFC 1123

   Rule	Defined or Modified in RFC 1123	Notes
   Hostnames may begin with a letter or digit	✅ RFC 1123 § 2.1	RFC 1123 explicitly relaxes RFC 952 (which required starting with a letter).
   Hostnames use letters, digits, and hyphens only (LDH rule)	✅ Inherited from RFC 952, reaffirmed by RFC 1123	RFC 1123 didn’t change the allowed character set.
   No leading or trailing hyphen	✅ From RFC 952 § 1, still in effect	RFC 1123 doesn’t alter this restriction.
   Label length ≤ 63 characters	✅ RFC 1123 § 2.1	The RFC requires software to handle labels up to 63 characters.
   Full name ≤ 255 characters	✅ RFC 1123 § 2.1	“SHOULD handle host names of up to 255 characters.”

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3. nandgator commented on Oct 10, 2026

   

   nandgator

   on Oct 10, 2026

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   All of the requirements listed here are already enforced by hostname() and domain() on current master: letters, digits and hyphens only; no leading or trailing hyphen; labels up to 63 characters; the whole name up to 253 (#461); no underscores unless rfc_2782 is set; and a label may start with a digit, which is the relaxation RFC 1123 section 2.1 introduced. #493 adds regression tests for these limits. Please reopen with a failing example if you find a case that is accepted or rejected incorrectly.
4. nandgator

   closed this as completedon Oct 10, 2026