/* 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 tarball for details.
 */

#if ENABLE_FEATURE_UDHCP_RFC3397

#include "common.h"
#include "options.h"

#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)
{
        const uint8_t *c;
        int crtpos, retpos, depth, plen = 0, len = 0;
        char *dst = NULL;

        if (!cstr)
                return NULL;

        if (pre)
                plen = strlen(pre);

        /* 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 (!dst) {

                if (len > 0) {  /* second pass? allocate dst buffer and copy pre */
                        dst = xmalloc(len + plen);
                        memcpy(dst, pre, plen);
                }

                crtpos = retpos = depth = len = 0;

                while (crtpos < clen) {
                        c = cstr + crtpos;

                        if ((*c & NS_CMPRSFLGS) != 0) { /* 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 & 0x3f) << 8) | (*(c + 1) & 0xff); /* jump */
                        } else if (*c) {                        /* label */
                                if (crtpos + *c + 1 > clen)             /* label too long? abort */
                                        return NULL;
                                if (dst)
                                        memcpy(dst + plen + len, c + 1, *c);
                                len += *c + 1;
                                crtpos += *c + 1;
                                if (dst)
                                        *(dst + plen + len - 1) = '.';
                        } else {                                        /* null: 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 + plen + 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)
                        *(dst + plen + len - 1) = '\0';
        }

        return dst;
}

/* 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, *lp, *rp;
        int len;

        res = xmalloc(strlen(src) + 2);
        rp = lp = res;
        rp++;

        for (;;) {
                c = (uint8_t)*src++;
                if (c == '.' || c == '\0') {    /* end of label */
                        len = rp - lp - 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;
                        }
                        *lp = len;
                        lp = rp++;
                        if (c == '\0' || *src == '\0')  /* end of dname */
                                break;
                } else {
                        if (c >= 0x41 && c <= 0x5A)             /* uppercase? convert to lower */
                                c += 0x20;
                        *rp++ = c;
                }
        }

        *lp = 0;
        if (rp - res > NS_MAXCDNAME) {  /* dname too long? abort */
                free(res);
                return NULL;
        }
        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, inc;

        /* find all labels in cstr */
        off = 0;
        while (off < clen) {
                c = cstr + off;

                if ((*c & NS_CMPRSFLGS) != 0) { /* pointer, skip */
                        off += 2;
                } else if (*c) {        /* label, try matching dname */
                        inc = *c + 1;
                        d = dname;
                        while (*c == *d && memcmp(c + 1, d + 1, *c) == 0) {
                                if (*c == 0)    /* match, return offset */
                                        return off;
                                d += *c + 1;
                                c += *c + 1;
                                if ((*c & NS_CMPRSFLGS) != 0)   /* pointer, jump */
                                        c = cstr + (((*c & 0x3f) << 8) | (*(c + 1) & 0xff));
                        }
                        off += inc;
                } else {        /* null, 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 rfc3011 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;
        }

        for (d = dname; *d != 0; d += *d + 1) {
                off = find_offset(cstr, clen, d);
                if (off >= 0) { /* found a match, add pointer and terminate string */
                        *d++ = NS_CMPRSFLGS;
                        *d = off;
                        break;
                }
        }

        *retlen = d - dname + 1;
        return dname;
}

#endif /* ENABLE_FEATURE_UDHCP_RFC3397 */