[Bug 1916325] Re: Overly strict NAME_REGEX rejects valid hostnames

[Ryan Harper]

Hi,

Thank you for filing a bug and helping make Ubuntu better.  I believe
the regex restriction is happening in the Ubuntu Server Live Installer
(subiquity).  I've added that package to this bug.

Here's the source code in question you're hitting.
https://github.com/canonical/subiquity/blob/a76581cd2b973b55e55c6ac05b5bf47168493140/subiquity/ui/views/identity.py#L110

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https://bugs.launchpad.net/bugs/1916325

Title:
  Overly strict NAME_REGEX rejects valid hostnames

Status in curtin:
  New
Status in curtin package in Ubuntu:
  New
Status in subiquity package in Ubuntu:
  New

Bug description:
  All our servers' hostnames start with a location identifier, which is
  mostly numeric e.g. 123, 124 or sometimes 12A. A full hostname might
  be 12a-proxy-01.mydomain.co.uk, for example.

  We are switching to Ubuntu but have hit this bug in the 20.04 curtin
  installer, which prevents us setting the hostname during installation.
  (Tested on plain 20.04 and on 20.04.02)

  Our workaround has been to set a dummy hostname on installation, and
  then change it back once curtin has finished - but that gets messy
  especially as we are now looking into cloud-init and automation, where
  this won't work.

  The NAME_REGEX quoted by curtin is "[a-z_][a-z0-9_-]*" which correctly
  prevents hostnames starting with a hyphen.. but it also prevents
  otherwise perfectly valid hostnames that start with a number.
  (Interestingly, an underscore is not defined as a valid hostname
  character, but is included as valid in the regex above)

  I believe the correct NAME_REGEX would be "[a-z0-9_][a-z0-9_-]*" -
  i.e. adding 0-9 as valid characters at the start. (I've left the
  underscore character in, also)

  Thanks

  Jonathan


  Ref:
  https://man7.org/linux/man-pages/man7/hostname.7.html#:~:text=Valid%20characters%20for%20hostnames%20are,to%20an%20address%20for%20use.
         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.

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