net/checksum.c

   1 /*
   2  *  IP checksumming functions.
   3  *  (c) 2008 Gerd Hoffmann <kraxel@redhat.com>
   4  *
   5  *  This program is free software; you can redistribute it and/or modify
   6  *  it under the terms of the GNU General Public License as published by
   7  *  the Free Software Foundation; under version 2 or later of the License.
   8  *
   9  *  This program is distributed in the hope that it will be useful,
  10  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  *  GNU General Public License for more details.
  13  *
  14  *  You should have received a copy of the GNU General Public License
  15  *  along with this program; if not, see <http://www.gnu.org/licenses/>.
  16  */
  17
  18 #include "net/checksum.h"
  19
  20 #define PROTO_TCP  6
  21 #define PROTO_UDP 17
  22
  23 uint32_t net_checksum_add(int len, uint8_t *buf)
  24 {
  25     uint32_t sum = 0;
  26     int i;
  27
  28     for (i = 0; i < len; i++) {
  29         if (i & 1)
  30             sum += (uint32_t)buf[i];
  31         else
  32             sum += (uint32_t)buf[i] << 8;
  33     }
  34     return sum;
  35 }
  36
  37 uint16_t net_checksum_finish(uint32_t sum)
  38 {
  39     while (sum>>16)
  40         sum = (sum & 0xFFFF)+(sum >> 16);
  41     return ~sum;
  42 }
  43
  44 uint16_t net_checksum_tcpudp(uint16_t length, uint16_t proto,
  45                              uint8_t *addrs, uint8_t *buf)
  46 {
  47     uint32_t sum = 0;
  48
  49     sum += net_checksum_add(length, buf);         // payload
  50     sum += net_checksum_add(8, addrs);            // src + dst address
  51     sum += proto + length;                        // protocol & length
  52     return net_checksum_finish(sum);
  53 }
  54
  55 void net_checksum_calculate(uint8_t *data, int length)
  56 {
  57     int hlen, plen, proto, csum_offset;
  58     uint16_t csum;
  59
  60     if ((data[14] & 0xf0) != 0x40)
  61         return; /* not IPv4 */
  62     hlen  = (data[14] & 0x0f) * 4;
  63     plen  = (data[16] << 8 | data[17]) - hlen;
  64     proto = data[23];
  65
  66     switch (proto) {
  67     case PROTO_TCP:
  68         csum_offset = 16;
  69         break;
  70     case PROTO_UDP:
  71         csum_offset = 6;
  72         break;
  73     default:
  74         return;
  75     }
  76
  77     if (plen < csum_offset+2)
  78         return;
  79
  80     data[14+hlen+csum_offset]   = 0;
  81     data[14+hlen+csum_offset+1] = 0;
  82     csum = net_checksum_tcpudp(plen, proto, data+14+12, data+14+hlen);
  83     data[14+hlen+csum_offset]   = csum >> 8;
  84     data[14+hlen+csum_offset+1] = csum & 0xff;
  85 }