Libidn2 0.9

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Libidn2

This manual is for Libidn2 (version 0.9, 23 July 2013), an implementation of IDNA2008 internationalized domain names.

Copyright © 2011-2013 Simon Josefsson


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1 Introduction

Libidn2 is a free software implementation of IDNA2008.


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2 Library Functions

Below are the interfaces of the Libidn2 library documented.

2.1 Header file idn2.h

To use the functions documented in this chapter, you need to include the file idn2.h like this:

     #include <idn2.h>

2.2 Core Functions

When you have the data encoded in UTF-8 form the direct interfaces to the library are as follows.

idn2_lookup_u8

— Function: int idn2_lookup_u8 (const uint8_t * src, uint8_t ** lookupname, int flags)

src: input zero-terminated UTF-8 string in Unicode NFC normalized form.

lookupname: newly allocated output variable with name to lookup in DNS.

flags: optional idn2_flags to modify behaviour.

Perform IDNA2008 lookup string conversion on domain name src, as described in section 5 of RFC 5891. Note that the input string must be encoded in UTF-8 and be in Unicode NFC form.

Pass IDN2_NFC_INPUT in flags to convert input to NFC form before further processing. Pass IDN2_ALABEL_ROUNDTRIP in flags to convert any input A-labels to U-labels and perform additional testing. Multiple flags may be specified by binary or:ing them together, for example IDN2_NFC_INPUT | IDN2_ALABEL_ROUNDTRIP.

Returns: On successful conversion IDN2_OK is returned, if the output domain or any label would have been too long IDN2_TOO_BIG_DOMAIN or IDN2_TOO_BIG_LABEL is returned, or another error code is returned.

idn2_register_u8

— Function: int idn2_register_u8 (const uint8_t * ulabel, const uint8_t * alabel, uint8_t ** insertname, int flags)

ulabel: input zero-terminated UTF-8 and Unicode NFC string, or NULL.

alabel: input zero-terminated ACE encoded string (xn–), or NULL.

insertname: newly allocated output variable with name to register in DNS.

flags: optional idn2_flags to modify behaviour.

Perform IDNA2008 register string conversion on domain label ulabel and alabel, as described in section 4 of RFC 5891. Note that the input ulabel must be encoded in UTF-8 and be in Unicode NFC form.

Pass IDN2_NFC_INPUT in flags to convert input ulabel to NFC form before further processing.

It is recommended to supply both ulabel and alabel for better error checking, but supplying just one of them will work. Passing in only alabel is better than only ulabel. See RFC 5891 section 4 for more information.

Returns: On successful conversion IDN2_OK is returned, when the given ulabel and alabel does not match each other IDN2_UALABEL_MISMATCH is returned, when either of the input labels are too long IDN2_TOO_BIG_LABEL is returned, when alabel does does not appear to be a proper A-label IDN2_INVALID_ALABEL is returned, or another error code is returned.

2.3 Locale Functions

As a convenience, the following functions are provided that will convert the input from the locale encoding format to UTF-8 and normalize the string using NFC, and then apply the core functions described earlier.

idn2_lookup_ul

— Function: int idn2_lookup_ul (const char * src, char ** lookupname, int flags)

src: input zero-terminated locale encoded string.

lookupname: newly allocated output variable with name to lookup in DNS.

flags: optional idn2_flags to modify behaviour.

Perform IDNA2008 lookup string conversion on domain name src, as described in section 5 of RFC 5891. Note that the input is assumed to be encoded in the locale's default coding system, and will be transcoded to UTF-8 and NFC normalized by this function.

Pass IDN2_ALABEL_ROUNDTRIP in flags to convert any input A-labels to U-labels and perform additional testing.

Returns: On successful conversion IDN2_OK is returned, if conversion from locale to UTF-8 fails then IDN2_ICONV_FAIL is returned, if the output domain or any label would have been too long IDN2_TOO_BIG_DOMAIN or IDN2_TOO_BIG_LABEL is returned, or another error code is returned.

idn2_register_ul

— Function: int idn2_register_ul (const char * ulabel, const char * alabel, char ** insertname, int flags)

ulabel: input zero-terminated locale encoded string, or NULL.

alabel: input zero-terminated ACE encoded string (xn–), or NULL.

insertname: newly allocated output variable with name to register in DNS.

flags: optional idn2_flags to modify behaviour.

Perform IDNA2008 register string conversion on domain label ulabel and alabel, as described in section 4 of RFC 5891. Note that the input ulabel is assumed to be encoded in the locale's default coding system, and will be transcoded to UTF-8 and NFC normalized by this function.

It is recommended to supply both ulabel and alabel for better error checking, but supplying just one of them will work. Passing in only alabel is better than only ulabel. See RFC 5891 section 4 for more information.

Returns: On successful conversion IDN2_OK is returned, when the given ulabel and alabel does not match each other IDN2_UALABEL_MISMATCH is returned, when either of the input labels are too long IDN2_TOO_BIG_LABEL is returned, when alabel does does not appear to be a proper A-label IDN2_INVALID_ALABEL is returned, or another error code is returned.

2.4 Control Flags

The flags parameter can take on the following values, or a bit-wise inclusive or of any subset of the parameters:

— Global flag: idn2_flags IDN2_NFC_INPUT

Apply NFC normalization on input.

— Global flag: idn2_flags IDN2_ALABEL_ROUNDTRIP

Apply additional round-trip conversion of A-label inputs.

2.5 Error Handling

idn2_strerror

— Function: const char * idn2_strerror (int rc)

rc: return code from another libidn2 function.

Convert internal libidn2 error code to a humanly readable string. The returned pointer must not be de-allocated by the caller.

Return value: A humanly readable string describing error.

idn2_strerror_name

— Function: const char * idn2_strerror_name (int rc)

rc: return code from another libidn2 function.

Convert internal libidn2 error code to a string corresponding to internal header file symbols. For example, idn2_strerror_name(IDN2_MALLOC) will return the string "IDN2_MALLOC".

The caller must not attempt to de-allocate the returned string.

Return value: A string corresponding to error code symbol.

2.6 Return Codes

The functions normally return 0 on sucess or a negative error code.

— Return code: idn2_rc IDN2_OK

Successful return.

— Return code: idn2_rc IDN2_MALLOC

Memory allocation error.

— Return code: idn2_rc IDN2_NO_CODESET

Could not determine locale string encoding format.

— Return code: idn2_rc IDN2_ICONV_FAIL

Could not transcode locale string to UTF-8.

— Return code: idn2_rc IDN2_ENCODING_ERROR

Unicode data encoding error.

— Return code: idn2_rc IDN2_NFC

Error normalizing string.

— Return code: idn2_rc IDN2_PUNYCODE_BAD_INPUT

Punycode invalid input.

— Return code: idn2_rc IDN2_PUNYCODE_BIG_OUTPUT

Punycode output buffer too small.

— Return code: idn2_rc IDN2_PUNYCODE_OVERFLOW

Punycode conversion would overflow.

— Return code: idn2_rc IDN2_TOO_BIG_DOMAIN

Domain name longer than 255 characters.

— Return code: idn2_rc IDN2_TOO_BIG_LABEL

Domain label longer than 63 characters.

— Return code: idn2_rc IDN2_INVALID_ALABEL

Input A-label is not valid.

— Return code: idn2_rc IDN2_UALABEL_MISMATCH

Input A-label and U-label does not match.

— Return code: idn2_rc IDN2_NOT_NFC

String is not NFC.

— Return code: idn2_rc IDN2_2HYPHEN

String has forbidden two hyphens.

— Return code: idn2_rc IDN2_HYPHEN_STARTEND

String has forbidden starting/ending hyphen.

— Return code: idn2_rc IDN2_LEADING_COMBINING

String has forbidden leading combining character.

— Return code: idn2_rc IDN2_DISALLOWED

String has disallowed character.

— Return code: idn2_rc IDN2_CONTEXTJ

String has forbidden context-j character.

— Return code: idn2_rc IDN2_CONTEXTJ_NO_RULE

String has context-j character with no rull.

— Return code: idn2_rc IDN2_CONTEXTO

String has forbidden context-o character.

— Return code: idn2_rc IDN2_CONTEXTO_NO_RULE

String has context-o character with no rull.

— Return code: idn2_rc IDN2_UNASSIGNED

String has forbidden unassigned character.

— Return code: idn2_rc IDN2_BIDI

String has forbidden bi-directional properties.

2.7 Memory Handling

idn2_free

— Function: void idn2_free (void * ptr)

ptr: pointer to deallocate

Call free(3) on the given pointer.

This function is typically only useful on systems where the library malloc heap is different from the library caller malloc heap, which happens on Windows when the library is a separate DLL.

2.8 Version Check

It is often desirable to check that the version of Libidn2 used is indeed one which fits all requirements. Even with binary compatibility new features may have been introduced but due to problem with the dynamic linker an old version is actually used. So you may want to check that the version is okay right after program startup.

idn2_check_version

— Function: const char * idn2_check_version (const char * req_version)

req_version: version string to compare with, or NULL.

Check IDN2 library version. This function can also be used to read out the version of the library code used. See IDN2_VERSION for a suitable req_version string, it corresponds to the idn2.h header file version. Normally these two version numbers match, but if you are using an application built against an older libidn2 with a newer libidn2 shared library they will be different.

Return value: Check that the version of the library is at minimum the one given as a string in req_version and return the actual version string of the library; return NULL if the condition is not met. If NULL is passed to this function no check is done and only the version string is returned.

The normal way to use the function is to put something similar to the following first in your main:

       if (!idn2_check_version (IDN2_VERSION))
         {
           printf ("idn2_check_version() failed:\n"
                   "Header file incompatible with shared library.\n");
           exit(EXIT_FAILURE);
         }


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3 Examples

This chapter contains example code which illustrate how Libidn2 is used when you write your own application.


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3.1 Lookup

This example demonstrates how a domain name is processed before it is lookup in the DNS.

#include <stdio.h> /* printf, fflush, fgets, stdin, perror, fprintf */
#include <string.h> /* strlen */
#include <locale.h> /* setlocale */
#include <stdlib.h> /* free */
#include <idn2.h> /* idn2_lookup_ul, IDN2_OK, idn2_strerror, idn2_strerror_name */

int
main (int argc, char *argv[])
{
  int rc;
  char src[BUFSIZ];
  char *lookupname;

  setlocale (LC_ALL, "");

  printf ("Enter (possibly non-ASCII) domain name to lookup: ");
  fflush (stdout);
  if (!fgets (src, sizeof (src), stdin))
    {
      perror ("fgets");
      return 1;
    }
  src[strlen (src) - 1] = '\0';

  rc = idn2_lookup_ul (src, &lookupname, 0);
  if (rc != IDN2_OK)
    {
      fprintf (stderr, "error: %s (%s, %d)\n",
	       idn2_strerror (rc), idn2_strerror_name (rc), rc);
      return 1;
    }

  printf ("IDNA2008 domain name to lookup in DNS: %s\n", lookupname);

  free (lookupname);

  return 0;
}


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3.2 Register

This example demonstrates how a domain label is processed before it is registered in the DNS.

#include <stdio.h> /* printf, fflush, fgets, stdin, perror, fprintf */
#include <string.h> /* strlen */
#include <locale.h> /* setlocale */
#include <stdlib.h> /* free */
#include <idn2.h> /* idn2_register_ul, IDN2_OK, idn2_strerror, idn2_strerror_name */

int
main (int argc, char *argv[])
{
  int rc;
  char src[BUFSIZ];
  char *insertname;

  setlocale (LC_ALL, "");

  printf ("Enter (possibly non-ASCII) label to register: ");
  fflush (stdout);
  if (!fgets (src, sizeof (src), stdin))
    {
      perror ("fgets");
      return 1;
    }
  src[strlen (src) - 1] = '\0';

  rc = idn2_register_ul (src, NULL, &insertname, 0);
  if (rc != IDN2_OK)
    {
      fprintf (stderr, "error: %s (%s, %d)\n",
	       idn2_strerror (rc), idn2_strerror_name (rc), rc);
      return 1;
    }

  printf ("IDNA2008 label to register in DNS: %s\n", insertname);

  free (insertname);

  return 0;
}


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4 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.

4.1 Options

idn2 recognizes these commands:

  -h, --help               Print help and exit

  -V, --version            Print version and exit

  -l, --lookup             Lookup domain name (default)

  -r, --register           Register label

      --debug              Print debugging information

      --quiet              Silent operation

4.2 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
     ...

4.3 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:~$

4.4 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 (http://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.


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