Allow underscores in DNS names

M. Warner Losh imp at
Sun Mar 30 09:50:37 PST 2003

In message: <200303301623.h2UGNHDN042824 at>
            "Louis A. Mamakos" <louie at TransSys.COM> writes:
: > In message: <xzp4r5ljitl.fsf at>
: >             des at (Dag-Erling Smørgrav) writes:
: > : "M. Warner Losh" <imp at> writes:
: > : > When this has come up in the past, it was decreed that _ is a bad bad
: > : > bad bad idea, even though people want it.  You might want to check the
: > : > ancient archives (1998?) for all the reasons why.
: > : 
: > : Arguments presented in ancient archives are not necessarily relevant
: > : five years later.
: > 
: > True.  However, they are still relevant today.  '_' is illegal in DNS
: > names, is rejected by the majority of hosts on the internet and
: > generally is a bad idea.  If you do it, make it optional.
: Strictly speaking, the '_' is illegal in HOSTNAMES.  The DNS can contains
: objects other than those used as hostnames, and the protocols support
: arbitrary strings of octets which can be used as labels in DNS names.
: It's the application of looking up host names using the DNS which is
: in question.  And if underscore characters are so toxic in hostnames,
: then why are they allowed in /etc/hosts or NIS-dervied lookups?

The question is one of standards conformance and what implementations
of DNS do when they get an illegal character.


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