domain_codec.c 6.6 KB

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  1. /* vi: set sw=4 ts=4: */
  2. /* RFC1035 domain compression routines (C) 2007 Gabriel Somlo <somlo at cmu.edu>
  3. *
  4. * Loosely based on the isc-dhcpd implementation by dhankins@isc.org
  5. *
  6. * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  7. */
  8. #ifdef DNS_COMPR_TESTING
  9. # define FAST_FUNC /* nothing */
  10. # define xmalloc malloc
  11. # include <stdlib.h>
  12. # include <stdint.h>
  13. # include <string.h>
  14. # include <stdio.h>
  15. #else
  16. # include "common.h"
  17. #endif
  18. #define NS_MAXDNAME 1025 /* max domain name length */
  19. #define NS_MAXCDNAME 255 /* max compressed domain name length */
  20. #define NS_MAXLABEL 63 /* max label length */
  21. #define NS_MAXDNSRCH 6 /* max domains in search path */
  22. #define NS_CMPRSFLGS 0xc0 /* name compression pointer flag */
  23. /* Expand a RFC1035-compressed list of domain names "cstr", of length "clen";
  24. * returns a newly allocated string containing the space-separated domains,
  25. * prefixed with the contents of string pre, or NULL if an error occurs.
  26. */
  27. char* FAST_FUNC dname_dec(const uint8_t *cstr, int clen, const char *pre)
  28. {
  29. char *ret = ret; /* for compiler */
  30. char *dst = NULL;
  31. /* We make two passes over the cstr string. First, we compute
  32. * how long the resulting string would be. Then we allocate a
  33. * new buffer of the required length, and fill it in with the
  34. * expanded content. The advantage of this approach is not
  35. * having to deal with requiring callers to supply their own
  36. * buffer, then having to check if it's sufficiently large, etc.
  37. */
  38. while (1) {
  39. /* note: "return NULL" below are leak-safe since
  40. * dst isn't yet allocated */
  41. const uint8_t *c;
  42. unsigned crtpos, retpos, depth, len;
  43. crtpos = retpos = depth = len = 0;
  44. while (crtpos < clen) {
  45. c = cstr + crtpos;
  46. if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) {
  47. /* pointer */
  48. if (crtpos + 2 > clen) /* no offset to jump to? abort */
  49. return NULL;
  50. if (retpos == 0) /* toplevel? save return spot */
  51. retpos = crtpos + 2;
  52. depth++;
  53. crtpos = ((c[0] & 0x3f) << 8) | c[1]; /* jump */
  54. } else if (*c) {
  55. /* label */
  56. if (crtpos + *c + 1 > clen) /* label too long? abort */
  57. return NULL;
  58. if (dst)
  59. memcpy(dst + len, c + 1, *c);
  60. len += *c + 1;
  61. crtpos += *c + 1;
  62. if (dst)
  63. dst[len - 1] = '.';
  64. } else {
  65. /* NUL: end of current domain name */
  66. if (retpos == 0) {
  67. /* toplevel? keep going */
  68. crtpos++;
  69. } else {
  70. /* return to toplevel saved spot */
  71. crtpos = retpos;
  72. retpos = depth = 0;
  73. }
  74. if (dst)
  75. dst[len - 1] = ' ';
  76. }
  77. if (depth > NS_MAXDNSRCH /* too many jumps? abort, it's a loop */
  78. || len > NS_MAXDNAME * NS_MAXDNSRCH /* result too long? abort */
  79. ) {
  80. return NULL;
  81. }
  82. }
  83. if (!len) /* expanded string has 0 length? abort */
  84. return NULL;
  85. if (!dst) { /* first pass? */
  86. /* allocate dst buffer and copy pre */
  87. unsigned plen = strlen(pre);
  88. ret = dst = xmalloc(plen + len);
  89. memcpy(dst, pre, plen);
  90. dst += plen;
  91. } else {
  92. dst[len - 1] = '\0';
  93. break;
  94. }
  95. }
  96. return ret;
  97. }
  98. /* Convert a domain name (src) from human-readable "foo.blah.com" format into
  99. * RFC1035 encoding "\003foo\004blah\003com\000". Return allocated string, or
  100. * NULL if an error occurs.
  101. */
  102. static uint8_t *convert_dname(const char *src)
  103. {
  104. uint8_t c, *res, *lenptr, *dst;
  105. int len;
  106. res = xmalloc(strlen(src) + 2);
  107. dst = lenptr = res;
  108. dst++;
  109. for (;;) {
  110. c = (uint8_t)*src++;
  111. if (c == '.' || c == '\0') { /* end of label */
  112. len = dst - lenptr - 1;
  113. /* label too long, too short, or two '.'s in a row? abort */
  114. if (len > NS_MAXLABEL || len == 0 || (c == '.' && *src == '.')) {
  115. free(res);
  116. return NULL;
  117. }
  118. *lenptr = len;
  119. if (c == '\0' || *src == '\0') /* "" or ".": end of src */
  120. break;
  121. lenptr = dst++;
  122. continue;
  123. }
  124. if (c >= 'A' && c <= 'Z') /* uppercase? convert to lower */
  125. c += ('a' - 'A');
  126. *dst++ = c;
  127. }
  128. if (dst - res >= NS_MAXCDNAME) { /* dname too long? abort */
  129. free(res);
  130. return NULL;
  131. }
  132. *dst = 0;
  133. return res;
  134. }
  135. /* Returns the offset within cstr at which dname can be found, or -1 */
  136. static int find_offset(const uint8_t *cstr, int clen, const uint8_t *dname)
  137. {
  138. const uint8_t *c, *d;
  139. int off;
  140. /* find all labels in cstr */
  141. off = 0;
  142. while (off < clen) {
  143. c = cstr + off;
  144. if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) { /* pointer, skip */
  145. off += 2;
  146. continue;
  147. }
  148. if (*c) { /* label, try matching dname */
  149. d = dname;
  150. while (1) {
  151. unsigned len1 = *c + 1;
  152. if (memcmp(c, d, len1) != 0)
  153. break;
  154. if (len1 == 1) /* at terminating NUL - match, return offset */
  155. return off;
  156. d += len1;
  157. c += len1;
  158. if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) /* pointer, jump */
  159. c = cstr + (((c[0] & 0x3f) << 8) | c[1]);
  160. }
  161. off += cstr[off] + 1;
  162. continue;
  163. }
  164. /* NUL, skip */
  165. off++;
  166. }
  167. return -1;
  168. }
  169. /* Computes string to be appended to cstr so that src would be added to
  170. * the compression (best case, it's a 2-byte pointer to some offset within
  171. * cstr; worst case, it's all of src, converted to <4>host<3>com<0> format).
  172. * The computed string is returned directly; its length is returned via retlen;
  173. * NULL and 0, respectively, are returned if an error occurs.
  174. */
  175. uint8_t* FAST_FUNC dname_enc(const uint8_t *cstr, int clen, const char *src, int *retlen)
  176. {
  177. uint8_t *d, *dname;
  178. int off;
  179. dname = convert_dname(src);
  180. if (dname == NULL) {
  181. *retlen = 0;
  182. return NULL;
  183. }
  184. d = dname;
  185. while (*d) {
  186. if (cstr) {
  187. off = find_offset(cstr, clen, d);
  188. if (off >= 0) { /* found a match, add pointer and return */
  189. *d++ = NS_CMPRSFLGS | (off >> 8);
  190. *d = off;
  191. break;
  192. }
  193. }
  194. d += *d + 1;
  195. }
  196. *retlen = d - dname + 1;
  197. return dname;
  198. }
  199. #ifdef DNS_COMPR_TESTING
  200. /* gcc -Wall -DDNS_COMPR_TESTING domain_codec.c -o domain_codec && ./domain_codec */
  201. int main(int argc, char **argv)
  202. {
  203. int len;
  204. uint8_t *encoded;
  205. #define DNAME_DEC(encoded,pre) dname_dec((uint8_t*)(encoded), sizeof(encoded), (pre))
  206. printf("'%s'\n", DNAME_DEC("\4host\3com\0", "test1:"));
  207. printf("test2:'%s'\n", DNAME_DEC("\4host\3com\0\4host\3com\0", ""));
  208. printf("test3:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\0", ""));
  209. printf("test4:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\5", ""));
  210. printf("test5:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\5\1z\xC0\xA", ""));
  211. #define DNAME_ENC(cache,source,lenp) dname_enc((uint8_t*)(cache), sizeof(cache), (source), (lenp))
  212. encoded = dname_enc(NULL, 0, "test.net", &len);
  213. printf("test6:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
  214. encoded = DNAME_ENC("\3net\0", "test.net", &len);
  215. printf("test7:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
  216. encoded = DNAME_ENC("\4test\3net\0", "test.net", &len);
  217. printf("test8:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
  218. return 0;
  219. }
  220. #endif