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/* vi: set sw=4 ts=4: */
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* RFC1035 domain compression routines (C) 2007 Gabriel Somlo <somlo at cmu.edu>
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* Loosely based on the isc-dhcpd implementation by dhankins@isc.org
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* Licensed under GPLv2 or later, see file LICENSE in this source tree.
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#ifdef DNS_COMPR_TESTING
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# define FAST_FUNC /* nothing */
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# define xmalloc malloc
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#define NS_MAXDNAME 1025 /* max domain name length */
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#define NS_MAXCDNAME 255 /* max compressed domain name length */
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#define NS_MAXLABEL 63 /* max label length */
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#define NS_MAXDNSRCH 6 /* max domains in search path */
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#define NS_CMPRSFLGS 0xc0 /* name compression pointer flag */
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/* Expand a RFC1035-compressed list of domain names "cstr", of length "clen";
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* returns a newly allocated string containing the space-separated domains,
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* prefixed with the contents of string pre, or NULL if an error occurs.
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char* FAST_FUNC dname_dec(const uint8_t *cstr, int clen, const char *pre)
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char *ret = ret; /* for compiler */
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/* We make two passes over the cstr string. First, we compute
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* how long the resulting string would be. Then we allocate a
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* new buffer of the required length, and fill it in with the
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* expanded content. The advantage of this approach is not
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* having to deal with requiring callers to supply their own
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* buffer, then having to check if it's sufficiently large, etc.
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/* note: "return NULL" below are leak-safe since
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* dst isn't allocated yet */
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unsigned crtpos, retpos, depth, len;
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crtpos = retpos = depth = len = 0;
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while (crtpos < clen) {
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if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) {
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if (crtpos + 2 > clen) /* no offset to jump to? abort */
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if (retpos == 0) /* toplevel? save return spot */
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crtpos = ((c[0] & 0x3f) << 8) | c[1]; /* jump */
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if (crtpos + *c + 1 > clen) /* label too long? abort */
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/* \3com ---> "com." */
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((char*)mempcpy(dst + len, c + 1, *c))[0] = '.';
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/* NUL: end of current domain name */
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/* toplevel? keep going */
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/* return to toplevel saved spot */
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/* \4host\3com\0\4host and we are at \0:
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* \3com was converted to "com.", change dot to space.
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if (depth > NS_MAXDNSRCH /* too many jumps? abort, it's a loop */
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|| len > NS_MAXDNAME * NS_MAXDNSRCH /* result too long? abort */
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if (!len) /* expanded string has 0 length? abort */
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if (!dst) { /* first pass? */
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/* allocate dst buffer and copy pre */
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unsigned plen = strlen(pre);
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ret = xmalloc(plen + len);
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dst = stpcpy(ret, pre);
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/* Convert a domain name (src) from human-readable "foo.blah.com" format into
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* RFC1035 encoding "\003foo\004blah\003com\000". Return allocated string, or
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* NULL if an error occurs.
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static uint8_t *convert_dname(const char *src)
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uint8_t c, *res, *lenptr, *dst;
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res = xmalloc(strlen(src) + 2);
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if (c == '.' || c == '\0') { /* end of label */
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len = dst - lenptr - 1;
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/* label too long, too short, or two '.'s in a row? abort */
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if (len > NS_MAXLABEL || len == 0 || (c == '.' && *src == '.')) {
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if (c == '\0' || *src == '\0') /* "" or ".": end of src */
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if (c >= 'A' && c <= 'Z') /* uppercase? convert to lower */
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if (dst - res >= NS_MAXCDNAME) { /* dname too long? abort */
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/* Returns the offset within cstr at which dname can be found, or -1 */
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static int find_offset(const uint8_t *cstr, int clen, const uint8_t *dname)
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const uint8_t *c, *d;
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/* find all labels in cstr */
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if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) { /* pointer, skip */
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if (*c) { /* label, try matching dname */
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unsigned len1 = *c + 1;
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if (memcmp(c, d, len1) != 0)
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if (len1 == 1) /* at terminating NUL - match, return offset */
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if ((*c & NS_CMPRSFLGS) == NS_CMPRSFLGS) /* pointer, jump */
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c = cstr + (((c[0] & 0x3f) << 8) | c[1]);
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off += cstr[off] + 1;
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/* Computes string to be appended to cstr so that src would be added to
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* the compression (best case, it's a 2-byte pointer to some offset within
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* cstr; worst case, it's all of src, converted to <4>host<3>com<0> format).
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* The computed string is returned directly; its length is returned via retlen;
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* NULL and 0, respectively, are returned if an error occurs.
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uint8_t* FAST_FUNC dname_enc(const uint8_t *cstr, int clen, const char *src, int *retlen)
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dname = convert_dname(src);
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off = find_offset(cstr, clen, d);
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if (off >= 0) { /* found a match, add pointer and return */
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*d++ = NS_CMPRSFLGS | (off >> 8);
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*retlen = d - dname + 1;
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#ifdef DNS_COMPR_TESTING
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/* gcc -Wall -DDNS_COMPR_TESTING domain_codec.c -o domain_codec && ./domain_codec */
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int main(int argc, char **argv)
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uint8_t str[6] = { 0x00, 0x00, 0x02, 0x65, 0x65, 0x00 };
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printf("NUL:'%s'\n", dname_dec(str, 6, ""));
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#define DNAME_DEC(encoded,pre) dname_dec((uint8_t*)(encoded), sizeof(encoded), (pre))
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printf("'%s'\n", DNAME_DEC("\4host\3com\0", "test1:"));
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printf("test2:'%s'\n", DNAME_DEC("\4host\3com\0\4host\3com\0", ""));
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printf("test3:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\0", ""));
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printf("test4:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\5", ""));
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printf("test5:'%s'\n", DNAME_DEC("\4host\3com\0\xC0\5\1z\xC0\xA", ""));
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#define DNAME_ENC(cache,source,lenp) dname_enc((uint8_t*)(cache), sizeof(cache), (source), (lenp))
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encoded = dname_enc(NULL, 0, "test.net", &len);
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printf("test6:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
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encoded = DNAME_ENC("\3net\0", "test.net", &len);
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printf("test7:'%s' len:%d\n", dname_dec(encoded, len, ""), len);
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encoded = DNAME_ENC("\4test\3net\0", "test.net", &len);
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printf("test8:'%s' len:%d\n", dname_dec(encoded, len, ""), len);