Hello Tomasz:

Thank for you bringing this matter to our attention.  After reviewing the issue you are certainly correct in that our documentation is sorely lacking regarding the user-class option. ISC DHCP server and client treat the option as straight text and that does seem to defy RFC 3004.  However the implementation has been in place for a very long time and changing it now would likely upset more apple carts than it would right.

As you've already discovered one must encode the length into the string value to produce a compliant result.  In fact, you can use that technique to encode an option with multiple values:

  send user-class "\007myclass\013another-one";

Thus it is possible to send multiple length-value tuples as described in RFC 3004. I apologize for the inconvenience the lack of documentation caused you.  We will be clarifying this in our documentation as part of our next maintenance releases, 4.4.2 and 4.1-ESV-R16 (dates are TBD).

We like to thank our contributors by citing them in the release notes.  If you would like to be thanked in this manner, please reply with how you would like to be identified.  Typically it is by name and/or organization.

Thanks for taking the time and interest to report this issue to us.

Regards,

Thomas Markwalder
ISC Software Engineering