ares_expand_name.c

/* Copyright 1998, 2002 by the Massachusetts Institute of Technology.
 *
 * Permission to use, copy, modify, and distribute this
 * software and its documentation for any purpose and without
 * fee is hereby granted, provided that the above copyright
 * notice appear in all copies and that both that copyright
 * notice and this permission notice appear in supporting
 * documentation, and that the name of M.I.T. not be used in
 * advertising or publicity pertaining to distribution of the
 * software without specific, written prior permission.
 * M.I.T. makes no representations about the suitability of
 * this software for any purpose.  It is provided "as is"
 * without express or implied warranty.
 */

static const char rcsid[] = "$Id: ares_expand_name.c,v 1.4 2002-08-26 06:48:53 ghudson Exp $";

#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <stdlib.h>
#include "ares.h"

static int name_length(const unsigned char *encoded, const unsigned char *abuf,
		       int alen);

/* Expand an RFC1035-encoded domain name given by encoded.  The
 * containing message is given by abuf and alen.  The result given by
 * *s, which is set to a NUL-terminated allocated buffer.  *enclen is
 * set to the length of the encoded name (not the length of the
 * expanded name; the goal is to tell the caller how many bytes to
 * move forward to get past the encoded name).
 *
 * In the simple case, an encoded name is a series of labels, each
 * composed of a one-byte length (limited to values between 0 and 63
 * inclusive) followed by the label contents.  The name is terminated
 * by a zero-length label.
 *
 * In the more complicated case, a label may be terminated by an
 * indirection pointer, specified by two bytes with the high bits of
 * the first byte (corresponding to INDIR_MASK) set to 11.  With the
 * two high bits of the first byte stripped off, the indirection
 * pointer gives an offset from the beginning of the containing
 * message with more labels to decode.  Indirection can happen an
 * arbitrary number of times, so we have to detect loops.
 *
 * Since the expanded name uses '.' as a label separator, we use
 * backslashes to escape periods or backslashes in the expanded name.
 */

int ares_expand_name(const unsigned char *encoded, const unsigned char *abuf,
		     int alen, char **s, int *enclen)
{
  int len, indir = 0;
  char *q;
  const unsigned char *p;

  len = name_length(encoded, abuf, alen);
  if (len == -1)
    return ARES_EBADNAME;

  *s = malloc(len + 1);
  if (!*s)
    return ARES_ENOMEM;
  q = *s;

  /* No error-checking necessary; it was all done by name_length(). */
  p = encoded;
  while (*p)
    {
      if ((*p & INDIR_MASK) == INDIR_MASK)
	{
	  if (!indir)
	    *enclen = p + 2 - encoded;
	  indir = 1;
	  p = abuf + ((*p & ~INDIR_MASK) << 8 | *(p + 1));
	}
      else
	{
	  len = *p;
	  p++;
	  while (len--)
	    {
	      if (*p == '.' || *p == '\\')
		*q++ = '\\';
	      *q++ = *p;
	      p++;
	    }
	  *q++ = '.';
	}
    }
  if (!indir)
    *enclen = p + 1 - encoded;

  /* Nuke the trailing period if we wrote one. */
  if (q > *s)
    *(q - 1) = 0;

  return ARES_SUCCESS;
}

/* Return the length of the expansion of an encoded domain name, or
 * -1 if the encoding is invalid.
 */
static int name_length(const unsigned char *encoded, const unsigned char *abuf,
		       int alen)
{
  int n = 0, offset, indir = 0;

  /* An encoded domain name must contain at least one byte. */
  if (encoded == abuf + alen)
    return -1;

  while (*encoded)
    {
      if ((*encoded & INDIR_MASK) == INDIR_MASK)
	{
	  /* Check the offset and go there. */
	  if (abuf + alen - encoded < 2)
	    return -1;
	  offset = (*encoded & ~INDIR_MASK) << 8 | *(encoded + 1);
	  if (offset >= alen)
	    return -1;
	  encoded = abuf + offset;

	  /* If we've seen more indirects than the message length,
	   * then there's a loop.
	   */
	  if (++indir > alen)
	    return -1;
	}
      else
	{
	  offset = *encoded;
	  /* There must be at least one byte for the next label. */
	  if (abuf + alen - encoded < offset + 2)
	    return -1;
	  encoded++;
	  while (offset--)
	    {
	      n += (*encoded == '.' || *encoded == '\\') ? 2 : 1;
	      encoded++;
	    }
	  n++;
	}
    }

  /* If there were any labels at all, then the number of dots is one
   * less than the number of labels, so subtract one.
   */
  return (n) ? n - 1 : n;
}