Man page bug - hostname.7

Man page bug - hostname.7

[Neitzert, Greg A]

Hello,

In the man-pages source, in the man.7 section, there is what I believe a bug/inaccuracy in the hostname page.
Specifically this paragraph:

"Each element of the hostname must be from 1 to 63 characters long
       and the entire hostname, including the dots, can be at most 253
       characters long.  Valid characters for hostnames are ASCII(7)
       letters from a to z, the digits from 0 to 9, and the hyphen (-).
       A hostname may not start with a hyphen."

The last sentence is the issue.
I believe it should state:

"A hostname may not start or end with a hyphen."
(suggest adding "or end")

The glibc source and kernel source themselves enforce no rules as to naming.  You can call sethostname in glibc directly and set the hostname to anything as long as it is 63 characters, violating standard rules like RFCs.
The 'hostname' program which is maintained by debian does enforce the first and last character rule.  It lives above glibc and calls glibc:

       /* Handle leading and trailing hyphen now. */
        if (!len || !isalnum(name[0]) || !isalnum(name[len-1]))
                return 0;

From maintainer:
 https://tracker.debian.org/pkg/hostname

This isn't necessarily binding on you I don't think, but it is consistent with the RFC rules.

At the bottom of your man page you do refer in the SEE ALSO to the pertinent RFCs:

     IETF RFC 1123 ⟨http://www.ietf.org/rfc/rfc1123.txt⟩
     IETF RFC 1178 ⟨http://www.ietf.org/rfc/rfc1178.txt⟩



RFC 1178 has some general best practices and is not really governing for this.
RFC 1123 however, and the RFC it refers back to, RFC-952 are directly on point.

RFC UPDATES RFC-952 and refers to it:

"The syntax of a legal Internet host name was specified in RFC-952
      [DNS:4].  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."


Referring back to RFC-952:

"1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
   to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
   sign (-), and period (.).  Note that periods are only allowed when
   they serve to delimit components of "domain style names". (See
   RFC-921, "Domain Name System Implementation Schedule", for
   background).  No blank or space characters are permitted as part of a
   name. No distinction is made between upper and lower case.  The first
   character must be an alpha character.  The last character must not be
   a minus sign or period."


Taking RFC 952, and the updates in RFC-1123 we end up with the following:
1. Length is now of 63 characters minimum (superceding 24).
2. The first character must be an alpha (or numeric - as amended by RFC 1123).
3. The last character MUST NOT be a minus sign (from RFC 952 and left unchanged).

Therefore, to be accurate with the standard you are referring to (RFC 1123 and the RFC it updates 952), I believe you should have:

"A hostname may not start or end with a hyphen."

All of your text is in harmony with RFC 952 and RFC 1123 except for the issue regarding the hyphens - at least from my reading.

Thanks.
Greg Neitzert
Greg.Neitzert@unisys.com

Re: Man page bug - hostname.7

[Alejandro Colomar]

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Hi Greg,

On 2026-04-06T19:19:36+0000, Neitzert, Greg A wrote:
> Hello,
> 
> In the man-pages source, in the man.7 section, there is what I believe a bug/inaccuracy in the hostname page.
> Specifically this paragraph:
> 
> "Each element of the hostname must be from 1 to 63 characters long
>        and the entire hostname, including the dots, can be at most 253
>        characters long.  Valid characters for hostnames are ASCII(7)
>        letters from a to z, the digits from 0 to 9, and the hyphen (-).
>        A hostname may not start with a hyphen."
> 
> The last sentence is the issue.
> I believe it should state:
> 
> "A hostname may not start or end with a hyphen."
> (suggest adding "or end")
> 
> The glibc source and kernel source themselves enforce no rules as to naming.  You can call sethostname in glibc directly and set the hostname to anything as long as it is 63 characters, violating standard rules like RFCs.
> The 'hostname' program which is maintained by debian does enforce the first and last character rule.  It lives above glibc and calls glibc:
> 
>        /* Handle leading and trailing hyphen now. */
>         if (!len || !isalnum(name[0]) || !isalnum(name[len-1]))
>                 return 0;
> 
> From maintainer:
>  https://tracker.debian.org/pkg/hostname
> 
> This isn't necessarily binding on you I don't think, but it is consistent with the RFC rules.
> 
> At the bottom of your man page you do refer in the SEE ALSO to the pertinent RFCs:
> 
>      IETF RFC 1123 ⟨http://www.ietf.org/rfc/rfc1123.txt⟩
>      IETF RFC 1178 ⟨http://www.ietf.org/rfc/rfc1178.txt⟩
> 
> 
> 
> RFC 1178 has some general best practices and is not really governing for this.
> RFC 1123 however, and the RFC it refers back to, RFC-952 are directly on point.
> 
> RFC UPDATES RFC-952 and refers to it:
> 
> "The syntax of a legal Internet host name was specified in RFC-952
>       [DNS:4].  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."
> 
> 
> Referring back to RFC-952:
> 
> "1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
>    to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
>    sign (-), and period (.).  Note that periods are only allowed when
>    they serve to delimit components of "domain style names". (See
>    RFC-921, "Domain Name System Implementation Schedule", for
>    background).  No blank or space characters are permitted as part of a
>    name. No distinction is made between upper and lower case.  The first
>    character must be an alpha character.  The last character must not be
>    a minus sign or period."
> 
> 
> Taking RFC 952, and the updates in RFC-1123 we end up with the following:
> 1. Length is now of 63 characters minimum (superceding 24).
> 2. The first character must be an alpha (or numeric - as amended by RFC 1123).
> 3. The last character MUST NOT be a minus sign (from RFC 952 and left unchanged).
> 
> Therefore, to be accurate with the standard you are referring to (RFC 1123 and the RFC it updates 952), I believe you should have:
> 
> "A hostname may not start or end with a hyphen."
> 
> All of your text is in harmony with RFC 952 and RFC 1123 except for the issue regarding the hyphens - at least from my reading.

Thanks for the very detailed report.  I've applied a patch following
your suggestion (will push tomorrow or so).


Have a lovely night!
Alex

> 
> Thanks.
> Greg Neitzert
> Greg.Neitzert@unisys.com
> 
> 
> 
> 

-- 
<https://www.alejandro-colomar.es>

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