getaddrinfo-idn.txt   [plain text]

Libidn getaddrinfo-idn.txt -- Proposal for IDN support in POSIX getaddrinfo.
Copyright (C) 2003, 2004 Simon Josefsson
See the end for copying conditions.


Libidn is a package for internationalized string handling based on the
Stringprep, Punycode and Internationalized Domain Names in
Applications (IDNA) specifications.  It can be used by applications
directly by linking to it, as is done by, e.g., Gnus, KDE, and Mutt.

Having each and every application link with and perform its own IDN
handling is not a good idea.  It bloats the code and makes things
unnecessarily complex.  Only few applications, such as web browsers
and mail clients, will need to do this in the future, to provide good
user interfaces for internationalization.

See for more information.

Alternative Approaches

There are implementation that modify gethostbyname() to accept UTF-8
strings and perform the IDNA ToASCII operation within gethostbyname().

There are even implementations that assume gethostbyname (on the
client host) perform no validation of the string and will send UTF-8
strings out to the DNS server, and perform the IDN-conversion on the
DNS server.

Some doubts can be raised whether this is an approach that is likely
to be standardized.  It also lack in functionality: it only provide
black-box ToASCII functionality.  The application cannot extract the
output from the ToASCII operation.  More important, there is no way to
perform a ToUnicode operation that applications may want to use for
display purposes.  Furthermore, while the first can support locale
specific character sets (e.g., ISO-8859-1), the second approach is
bound to either guess the character set, or always use UTF-8.

See also the thread rooted in <>
posted to on 08 Jan 2003.

What I propose

The getaddrinfo() API should have two new flags, AI_IDN and
AI_CANONIDN.  Roughly they correspond to IDNA ToASCII and IDNA
ToUnicode, but there are several details.  Note that strings are still
'char*', i.e. it does not use the "wide" character type, and that the
encoding of non-ASCII strings are the current locale's character set
(i.e., nl_langinfo(CODESET)).

An application that uses AI_IDN signal to the getaddrinfo()
implementation that the input host name may be non-ASCII and that the
appropriate IDNA ToASCII steps should be carried out on the input, and
the output from the ToASCII operation (if any) should be used in the
lookup using the current resolver processing.

An application that uses AI_CANONIDN signal to the getaddrinfo()
implementation that the input host name should be put through the IDNA
ToUnicode steps, and the output of that placed in the 'ai_canonname'
field of the resulting structure.  Normal resolver processing applies
to the input string, of course.

Consequently, an application that uses AI_IDN|AI_CANONIDN signal to
the getaddrinfo() implementation that the input host name may be
non-ASCII and should be put through the IDNA ToASCII steps before run
through the resolver, and that the input string should also be run
through the IDNA ToUnicode steps and the output of that placed in the
'ai_canonname' field.

The semantics of AI_CANONNAME|AI_CANONIDN is that instead of running
the ToUnicode IDNA steps on the input string, the canonical host name
as returned by the resolver for the input string should be used in the
ToUnicode IDNA step.


Four new flags has been proposed; AI_IDN_ALLOW_UNASSIGNED,
AI_IDN_USE_STD3_ASCII_RULES for getaddrinfo, and
The implementation is simple, if specified those flag will set the
appropriate flag in the call to the IDNA functions.  See the RFC for
the meaning of those flags.


The AI_IDN flag has been implemented and shipped as a proof-of-concept
patch for GNU Libc with GNU Libidn since January 2003.  Binary libc
packages with the patch exists for (at least) two GNU/Linux
distributions.  The AI_CANONIDN flag is not yet implemented.

As of March 2004, Libidn has been integrated as an add-on in the GNU
Libc CVS repository.  The AI_CANONIDN flag has been implemented.  The
AllowUnassigned and UseSTD3ASCIIRules flags were added.


Allow non-ASCII in gethostname (and similar functions), if
administrator has supplied, e.g., 'option idn' in /etc/resolv.conf?


This document is a work-in-progress and the details may change.
Contact me at to discuss changes.

Permission is granted to anyone to make or distribute verbatim copies
of this document, in any medium, provided that the copyright notice
and permission notice are preserved, and that the distributor grants
the recipient permission for further redistribution as permitted by
this notice.  Modified versions may not be made.