[dns-operations] about the underline in hostname

Bob Harold rharolde at umich.edu
Thu May 29 17:20:35 UTC 2014

If I (reluctantly) accept that DNS names that are not hostnames can have
underscores in them, why does BIND not have an option to allow that, while
still rejecting invalid hostnames?  Or have I missed something?

If someone decides to add that feature, I would really like the option to
only add the underscore to the allowed list, and not any other special
characters, which would break all kinds of things, without adding any real

Bob Harold
Information and Technology Services (ITS)
University of Michigan

> On May 29, 2014, at 04:24 , hua peng <huapeng at arcor.de> wrote:
> > I found this is a valid RR:
> >
> > _spf.yandex.ru.         2768    IN      TXT     "v=spf1
> > include:_spf-ipv4.yandex.ru include:_spf-ipv6.yandex.ru ~all"
> >
> >
> > But for A, CNAME, AAAA etc, the underline in hostname is invalid.
> > Does this make a confusion?
> There is a distinction between domain names and host names.  Host names
> are a subset of domain names; a host name is any domain name that points to
> an actual host (i.e. has an address record).  The rules for the characters
> in domain names and host names are slightly different.
> Host names are only allowed the characters [a-z] (case insensitive),
> [0-9], and [-].  See RFCs 952 and 1123.
> Domain names may use any string as a label, so for example the underscore
> is perfectly legal.  See RFC 2181.
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