source: MondoRescue/branches/stable/mindi-busybox/networking/udhcp/domain_codec.c@ 1770

/* 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_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 *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 *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_RFC3397 */