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 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  *  Contributions after 2012-01-13 are licensed under the terms of the
  18  *  GNU GPL, version 2 or (at your option) any later version.
  19  */
  20
  21 #include "net/checksum.h"
  22
  23 #define PROTO_TCP  6
  24 #define PROTO_UDP 17
  25
  26 uint32_t net_checksum_add(int len, uint8_t *buf)
  27 {
  28     uint32_t sum = 0;
  29     int i;
  30
  31     for (i = 0; i < len; i++) {
  32         if (i & 1)
  33             sum += (uint32_t)buf[i];
  34         else
  35             sum += (uint32_t)buf[i] << 8;
  36     }
  37     return sum;
  38 }
  39
  40 uint16_t net_checksum_finish(uint32_t sum)
  41 {
  42     while (sum>>16)
  43         sum = (sum & 0xFFFF)+(sum >> 16);
  44     return ~sum;
  45 }
  46
  47 uint16_t net_checksum_tcpudp(uint16_t length, uint16_t proto,
  48                              uint8_t *addrs, uint8_t *buf)
  49 {
  50     uint32_t sum = 0;
  51
  52     sum += net_checksum_add(length, buf);         // payload
  53     sum += net_checksum_add(8, addrs);            // src + dst address
  54     sum += proto + length;                        // protocol & length
  55     return net_checksum_finish(sum);
  56 }
  57
  58 void net_checksum_calculate(uint8_t *data, int length)
  59 {
  60     int hlen, plen, proto, csum_offset;
  61     uint16_t csum;
  62
  63     if ((data[14] & 0xf0) != 0x40)
  64         return; /* not IPv4 */
  65     hlen  = (data[14] & 0x0f) * 4;
  66     plen  = (data[16] << 8 | data[17]) - hlen;
  67     proto = data[23];
  68
  69     switch (proto) {
  70     case PROTO_TCP:
  71         csum_offset = 16;
  72         break;
  73     case PROTO_UDP:
  74         csum_offset = 6;
  75         break;
  76     default:
  77         return;
  78     }
  79
  80     if (plen < csum_offset+2)
  81         return;
  82
  83     data[14+hlen+csum_offset]   = 0;
  84     data[14+hlen+csum_offset+1] = 0;
  85     csum = net_checksum_tcpudp(plen, proto, data+14+12, data+14+hlen);
  86     data[14+hlen+csum_offset]   = csum >> 8;
  87     data[14+hlen+csum_offset+1] = csum & 0xff;
  88 }