text: /* 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 _GNU_SOURCE
  # 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)
  					/* \3com ---> "com." */
  					((char*)mempcpy(dst + len, c + 1, *c))[0] = '.';
  				len += *c + 1;
  				crtpos += *c + 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 && len != 0)
  					/* \4host\3com\0\4host and we are at \0:
  					 * \3com was converted to "com.", change dot to space.
  					 */
  					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;
  
          uint8_t str[6] = { 0x00, 0x00, 0x02, 0x65, 0x65, 0x00 };
          printf("NUL:'%s'\n",   dname_dec(str, 6, ""));
  
  #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
  