ares_expand_name.c

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/* Copyright 1998 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.
 */
 
 
#include <sys/types.h>
#include <stdlib.h>
 
#if !defined(WIN32) && !defined(__CYGWIN__)
#include <netinet/in.h>
#include "ares_nameser.h"
#endif
 
#include "ares.h"
 
 /* Maximum number of indirections allowed for a name */
#define MAX_INDIRS 50
 
static int name_length(const unsigned char *encoded, const unsigned char *abuf,
               int alen);
 
static int is_reservedch(int ch)
{
   switch (ch) {
   case '"':
   case '.':
   case ';':
   case '\\':
   case '(':
   case ')':
   case '@':
   case '$':
      return 1;
   default:
      break;
   }
 
   return 0;
}
 
static int ares__isprint(int ch)
{
   if (ch >= 0x20 && ch <= 0x7E)
      return 1;
   return 0;
}
 
/* 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, long* enclen)
{
   int len, remaining, 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;
 
   if (len == 0) {
      /* RFC2181 says this should be ".": the root of the DNS tree.
       * Since this function strips trailing dots though, it becomes ""
       */
      q[0] = '\0';
 
      /* indirect root label (like 0xc0 0x0c) is 2 bytes long (stupid, but
         valid) */
      if ((*encoded & INDIR_MASK) == INDIR_MASK)
         *enclen = 2L;
      else
         *enclen = 1L;  /* the caller should move one byte to get past this */
 
      return ARES_SUCCESS;
   }
 
   /* No error-checking necessary; it was all done by name_length(). */
   p = encoded;
   while (*p)
   {
      if ((*p & INDIR_MASK) == INDIR_MASK)
      {
         if (!indir)
         {
            *enclen = (long)(p + 2 - encoded);
            indir = 1;
         }
         p = abuf + ((*p & ~INDIR_MASK) << 8 | *(p + 1));
      }
      else
      {
         len = *p;
         remaining = len;
         p++;
         while (remaining--)
         {
            /* Output as \DDD for consistency with RFC1035 5.1, except
             * for the special case of a root name response  */
            if (!ares__isprint(*p) && !(len == 1 && *p == 0))
            {
               *q++ = '\\';
               *q++ = '0' + *p / 100;
               *q++ = '0' + (*p % 100) / 10;
               *q++ = '0' + (*p % 10);
            }
            else if (is_reservedch(*p))
            {
               *q++ = '\\';
               *q++ = *p;
            }
            else
            {
               *q++ = *p;
            }
            p++;
         }
         *q++ = '.';
      }
   }
   if (!indir)
      *enclen = (long)(p + 1 - encoded);
 
   /* Nuke the trailing period if we wrote one. */
   if (q > *s)
      q--;
   /* Write a trailing NUL in any case. */
   *q = 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, top;
 
   /* Allow the caller to pass us abuf + alen and have us check for it. */
   if (encoded >= abuf + alen)
      return -1;
 
   while (*encoded)
   {
      top = (*encoded & INDIR_MASK);
      if (top == INDIR_MASK)
      {
         /* Check the offset and go there. */
         if (encoded + 1 >= abuf + alen)
            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.
          */
         ++indir;
         if (indir > alen || indir > MAX_INDIRS)
            return -1;
      }
      else if (top == 0x00)
      {
         int name_len = *encoded;
         offset = name_len;
         if (encoded + offset + 1 >= abuf + alen)
            return -1;
         encoded++;
 
         while (offset--)
         {
            if (!ares__isprint(*encoded) && !(name_len == 1 && *encoded == 0))
            {
               n += 4;
            }
            else if (is_reservedch(*encoded))
            {
               n += 2;
            }
            else
            {
               n += 1;
            }
            encoded++;
         }
 
         n++;
      }
      else
      {
         /* RFC 1035 4.1.4 says other options (01, 10) for top 2
          * bits are reserved.
          */
         return -1;
      }
   }
 
   /* 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;
}