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/* vi: set sw=4 ts=4: */
/* RFC1035 domain compression routines (C) 2007 Gabriel Somlo <somlo at cmu.edu>
*
* Loosely based on the isc-dhcpd implementation by dhankins@isc.org
*
* Licensed under GPLv2 or later, see file LICENSE in this source tree.
*/
#ifdef DNS_COMPR_TESTING
# define FAST_FUNC /* nothing */
# define xmalloc malloc
# include <stdlib.h>
# include <stdint.h>
# include <string.h>
# include <stdio.h>
#else
# include "common.h"
#endif
#define NS_MAXDNAME 1025 /* max domain name length */
#define NS_MAXCDNAME 255 /* max compressed domain name length */
#define NS_MAXLABEL 63 /* max label length */
#define NS_MAXDNSRCH 6 /* max domains in search path */
#define NS_CMPRSFLGS 0xc0 /* name compression pointer flag */
/* Expand a RFC1035-compressed list of domain names "cstr", of length "clen";
* returns a newly allocated string containing the space-separated domains,
* prefixed with the contents of string pre, or NULL if an error occurs.
*/
char* FAST_FUNC dname_dec(const uint8_t *cstr, int clen, const char *pre)
{
char *ret = ret; /* for compiler */
char *dst = NULL;
/* We make two passes over the cstr string. First, we compute
* how long the resulting string would be. Then we allocate a
* new buffer of the required length, and fill it in with the
* expanded content. The advantage of this approach is not
* having to deal with requiring callers to supply their own
* buffer, then having to check if it's sufficiently large, etc.
*/
while (1) {
/* note: "return NULL" below are leak-safe since
* dst isn't allocated yet */
const uint8_t *c;
unsigned crtpos, retpos, depth, len;
crtpos = retpos = depth = len = 0;
while (crtpos < clen) {
c = cstr + crtpos;
if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) {
/* pointer */
if (crtpos + 2 > clen) /* no offset to jump to? abort */
return NULL;
if (retpos == 0) /* toplevel? save return spot */
retpos = crtpos + 2;
depth++;
crtpos = ((c[0] & 0x3f) << 8) | c[1]; /* jump */
} else if (*c) {
/* label */
if (crtpos + *c + 1 > clen) /* label too long? abort */
return NULL;
if (dst)
memcpy(dst + len, c + 1, *c);
len += *c + 1;
crtpos += *c + 1;
if (dst)
dst[len - 1] = '.';
} else {
/* NUL: end of current domain name */
if (retpos == 0) {
/* toplevel? keep going */
crtpos++;
} else {
/* return to toplevel saved spot */
crtpos = retpos;
retpos = depth = 0;
}
if (dst)
dst[len - 1] = ' ';
}
if (depth > NS_MAXDNSRCH /* too many jumps? abort, it's a loop */
|| len > NS_MAXDNAME * NS_MAXDNSRCH /* result too long? abort */
) {
return NULL;
}
}
if (!len) /* expanded string has 0 length? abort */
return NULL;
if (!dst) { /* first pass? */
/* allocate dst buffer and copy pre */
unsigned plen = strlen(pre);
ret = xmalloc(plen + len);
dst = stpcpy(ret, pre);
} else {
dst[len - 1] = '\0';
break;
}
}
return ret;
}
/* Convert a domain name (src) from human-readable "foo.blah.com" format into
* RFC1035 encoding "\003foo\004blah\003com\000". Return allocated string, or
* NULL if an error occurs.
*/
static uint8_t *convert_dname(const char *src)
{
uint8_t c, *res, *lenptr, *dst;
int len;
res = xmalloc(strlen(src) + 2);
dst = lenptr = res;
dst++;
for (;;) {
c = (uint8_t)*src++;
if (c == '.' || c == '\0') { /* end of label */
len = dst - lenptr - 1;
/* label too long, too short, or two '.'s in a row? abort */
if (len > NS_MAXLABEL || len == 0 || (c == '.' && *src == '.')) {
free(res);
return NULL;
}
*lenptr = len;
if (c == '\0' || *src == '\0') /* "" or ".": end of src */
break;
lenptr = dst++;
continue;
}
if (c >= 'A' && c <= 'Z') /* uppercase? convert to lower */
c += ('a' - 'A');
*dst++ = c;
}
if (dst - res >= NS_MAXCDNAME) { /* dname too long? abort */
free(res);
return NULL;
}
*dst = 0;
return res;
}
/* Returns the offset within cstr at which dname can be found, or -1 */
static int find_offset(const uint8_t *cstr, int clen, const uint8_t *dname)
{
const uint8_t *c, *d;
int off;
/* find all labels in cstr */
off = 0;
while (off < clen) {
c = cstr + off;
if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) { /* pointer, skip */
off += 2;
continue;
}
if (*c) { /* label, try matching dname */
d = dname;
while (1) {
unsigned len1 = *c + 1;
if (memcmp(c, d, len1) != 0)
break;
if (len1 == 1) /* at terminating NUL - match, return offset */
return off;
d += len1;
c += len1;
if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) /* pointer, jump */
c = cstr + (((c[0] & 0x3f) << 8) | c[1]);
}
off += cstr[off] + 1;
continue;
}
/* NUL, skip */
off++;
}
return -1;
}
/* Computes string to be appended to cstr so that src would be added to
* the compression (best case, it's a 2-byte pointer to some offset within
* cstr; worst case, it's all of src, converted to <4>host<3>com<0> format).
* The computed string is returned directly; its length is returned via retlen;
* NULL and 0, respectively, are returned if an error occurs.
*/
uint8_t* FAST_FUNC dname_enc(const uint8_t *cstr, int clen, const char *src, int *retlen)
{
uint8_t *d, *dname;
int off;
dname = convert_dname(src);
if (dname == NULL) {
*retlen = 0;
return NULL;
}
d = dname;
while (*d) {
if (cstr) {
off = find_offset(cstr, clen, d);
if (off >= 0) { /* found a match, add pointer and return */
*d++ = NS_CMPRSFLGS | (off >> 8);
*d = off;
break;
}
}
d += *d + 1;
}
*retlen = d - dname + 1;
return dname;
}
#ifdef DNS_COMPR_TESTING
/* gcc -Wall -DDNS_COMPR_TESTING domain_codec.c -o domain_codec && ./domain_codec */
int main(int argc, char **argv)
{
int len;
uint8_t *encoded;
#define DNAME_DEC(encoded,pre) dname_dec((uint8_t*)(encoded), sizeof(encoded), (pre))
printf("'%s'\n", DNAME_DEC("\4host\3com\0", "test1:"));
printf("test2:'%s'\n", DNAME_DEC("\4host\3com\0\4host\3com\0", ""));
printf("test3:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\0", ""));
printf("test4:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\5", ""));
printf("test5:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\5\1z\xC0\xA", ""));
#define DNAME_ENC(cache,source,lenp) dname_enc((uint8_t*)(cache), sizeof(cache), (source), (lenp))
encoded = dname_enc(NULL, 0, "test.net", &len);
printf("test6:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
encoded = DNAME_ENC("\3net\0", "test.net", &len);
printf("test7:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
encoded = DNAME_ENC("\4test\3net\0", "test.net", &len);
printf("test8:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
return 0;
}
#endif
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