libidn2: Invoking idn2
5 Invoking idn2
***************
‘idn2’ translates internationalized domain names to the IDNA2008 encoded
format, either for lookup or registration.
If strings are specified on the command line, they are used as input
and the computed output is printed to standard output ‘stdout’. If no
strings are specified on the command line, the program read data, line
by line, from the standard input ‘stdin’, and print the computed output
to standard output. What processing is performed (e.g., lookup or
register) is indicated by options. If any errors are encountered, the
execution of the applications is aborted.
All strings are expected to be encoded in the preferred charset used
by your locale. Use ‘--debug’ to find out what this charset is. On
POSIX systems you may use the ‘LANG’ environment variable to specify a
different locale.
To process a string that starts with ‘-’, for example ‘-foo’, use
‘--’ to signal the end of parameters, as in ‘idn2 -r -- -foo’.
5.1 Options
===========
‘idn2’ recognizes these commands:
-h, --help Print help and exit
-V, --version Print version and exit
-d, --decode Decode (punycode) domain name
-l, --lookup Lookup domain name (default)
-r, --register Register label
-T, --tr46t Enable TR46 transitional processing
-N, --tr46nt Enable TR46 non-transitional processing
--debug Print debugging information
--quiet Silent operation
5.2 Environment Variables
=========================
On POSIX systems the LANG environment variable can be used to override
the system locale for the command being invoked. The system locale may
influence what character set is used to decode data (i.e., strings on
the command line or data read from the standard input stream), and to
encode data to the standard output. If your system is set up correctly,
however, the application will use the correct locale and character set
automatically. Example usage:
$ LANG=en_US.UTF-8 idn2
...
5.3 Examples
============
Standard usage, reading input from standard input and disabling license
and usage instructions:
jas@latte:~$ idn2 --quiet
räksmörgås.se
xn--rksmrgs-5wao1o.se
...
Reading input from the command line:
jas@latte:~$ idn2 räksmörgås.se blåbærgrød.no
xn--rksmrgs-5wao1o.se
xn--blbrgrd-fxak7p.no
jas@latte:~$
Testing the IDNA2008 Register function:
jas@latte:~$ idn2 --register fußball
xn--fuball-cta
jas@latte:~$
5.4 Troubleshooting
===================
Getting character data encoded right, and making sure Libidn2 use the
same encoding, can be difficult. The reason for this is that most
systems may encode character data in more than one character encoding,
i.e., using ‘UTF-8’ together with ‘ISO-8859-1’ or ‘ISO-2022-JP’. This
problem is likely to continue to exist until only one character encoding
come out as the evolutionary winner, or (more likely, at least to some
extents) forever.
The first step to troubleshooting character encoding problems with
Libidn2 is to use the ‘--debug’ parameter to find out which character
set encoding ‘idn2’ believe your locale uses.
jas@latte:~$ idn2 --debug --quiet ""
Charset: UTF-8
jas@latte:~$
If it prints ‘ANSI_X3.4-1968’ (i.e., ‘US-ASCII’), this indicate you
have not configured your locale properly. To configure the locale, you
can, for example, use ‘LANG=sv_SE.UTF-8; export LANG’ at a ‘/bin/sh’
prompt, to set up your locale for a Swedish environment using ‘UTF-8’ as
the encoding.
Sometimes ‘idn2’ appear to be unable to translate from your system
locale into ‘UTF-8’ (which is used internally), and you will get an
error message like this:
idn2: lookup: could not convert string to UTF-8
One explanation is that you didn’t install the ‘iconv’ conversion
tools. You can find it as a standalone library in GNU Libiconv
(<https://www.gnu.org/software/libiconv/>). On many GNU/Linux systems,
this library is part of the system, but you may have to install
additional packages to be able to use it.
Another explanation is that the error is correct and you are feeding
‘idn2’ invalid data. This can happen inadvertently if you are not
careful with the character set encoding you use. For example, if your
shell run in a ‘ISO-8859-1’ environment, and you invoke ‘idn2’ with the
‘LANG’ environment variable as follows, you will feed it ‘ISO-8859-1’
characters but force it to believe they are ‘UTF-8’. Naturally this
will lead to an error, unless the byte sequences happen to be valid
‘UTF-8’. Note that even if you don’t get an error, the output may be
incorrect in this situation, because ‘ISO-8859-1’ and ‘UTF-8’ does not
in general encode the same characters as the same byte sequences.
jas@latte:~$ idn2 --quiet --debug ""
Charset: ISO-8859-1
jas@latte:~$ LANG=sv_SE.UTF-8 idn2 --debug räksmörgås
Charset: UTF-8
input[0] = 0x72
input[1] = 0xc3
input[2] = 0xa4
input[3] = 0xc3
input[4] = 0xa4
input[5] = 0x6b
input[6] = 0x73
input[7] = 0x6d
input[8] = 0xc3
input[9] = 0xb6
input[10] = 0x72
input[11] = 0x67
input[12] = 0xc3
input[13] = 0xa5
input[14] = 0x73
UCS-4 input[0] = U+0072
UCS-4 input[1] = U+00e4
UCS-4 input[2] = U+00e4
UCS-4 input[3] = U+006b
UCS-4 input[4] = U+0073
UCS-4 input[5] = U+006d
UCS-4 input[6] = U+00f6
UCS-4 input[7] = U+0072
UCS-4 input[8] = U+0067
UCS-4 input[9] = U+00e5
UCS-4 input[10] = U+0073
output[0] = 0x72
output[1] = 0xc3
output[2] = 0xa4
output[3] = 0xc3
output[4] = 0xa4
output[5] = 0x6b
output[6] = 0x73
output[7] = 0x6d
output[8] = 0xc3
output[9] = 0xb6
output[10] = 0x72
output[11] = 0x67
output[12] = 0xc3
output[13] = 0xa5
output[14] = 0x73
UCS-4 output[0] = U+0072
UCS-4 output[1] = U+00e4
UCS-4 output[2] = U+00e4
UCS-4 output[3] = U+006b
UCS-4 output[4] = U+0073
UCS-4 output[5] = U+006d
UCS-4 output[6] = U+00f6
UCS-4 output[7] = U+0072
UCS-4 output[8] = U+0067
UCS-4 output[9] = U+00e5
UCS-4 output[10] = U+0073
xn--rksmrgs-5waap8p
jas@latte:~$
The sense moral here is to forget about ‘LANG’ (instead, configure
your system locale properly) unless you know what you are doing, and if
you want to use ‘LANG’, do it carefully and after verifying with
‘--debug’ that you get the desired results.
