/* 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