> We've been using the older version (pre 9.3)
> and the underscore in the hostname in A recored
> like
> this_machine IN A 123.2.234.4


Which are not legal in hostname, see RFC 952.

> is okay.
> We have to upgrade the bind to 9.5
> The new version does not like underscore in the hostname
>
> Is there any way to workaround ?


See check-names.

> (otherwise we have to re-publish the web site URL
> with a new one
> as the user has used the underscore in its web site
> for quite some time)


You should re-publish regardless of whether you ignore the
checks as remote site do implement the checks on queries.

Mark
--
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: Mark_Andrews@isc.org