The Tcpdump Group git mirrors - tcpdump/blob - checksum.c

   1 /*
   2  * Copyright (c) 1998-2006 The TCPDUMP project
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that: (1) source code
   6  * distributions retain the above copyright notice and this paragraph
   7  * in its entirety, and (2) distributions including binary code include
   8  * the above copyright notice and this paragraph in its entirety in
   9  * the documentation or other materials provided with the distribution.
  10  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND
  11  * WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT
  12  * LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  13  * FOR A PARTICULAR PURPOSE.
  14  *
  15  * miscellaneous checksumming routines
  16  *
  17  * Original code by Hannes Gredler (hannes@juniper.net)
  18  */
  19
  20 #ifndef lint
  21 static const char rcsid[] _U_ =
  22     "@(#) $Header: /tcpdump/master/tcpdump/checksum.c,v 1.4 2006-09-25 09:23:32 hannes Exp $";
  23 #endif
  24
  25 #ifdef HAVE_CONFIG_H
  26 #include "config.h"
  27 #endif
  28
  29 #include <tcpdump-stdinc.h>
  30
  31 #include <stdio.h>
  32 #include <stdlib.h>
  33 #include <string.h>
  34
  35 #include "interface.h"
  36
  37 #define CRC10_POLYNOMIAL 0x633
  38 static u_int16_t crc10_table[256];
  39
  40 static void
  41 init_crc10_table(void)
  42 {
  43     register int i, j;
  44     register u_int16_t accum;
  45
  46     for ( i = 0;  i < 256;  i++ )
  47     {
  48         accum = ((unsigned short) i << 2);
  49         for ( j = 0;  j < 8;  j++ )
  50         {
  51             if ((accum <<= 1) & 0x400) accum ^= CRC10_POLYNOMIAL;
  52         }
  53         crc10_table[i] = accum;
  54     }
  55     return;
  56 }
  57
  58 u_int16_t
  59 verify_crc10_cksum(u_int16_t accum, const u_char *p, int length)
  60 {
  61     register int i;
  62
  63     for ( i = 0;  i < length;  i++ )
  64     {
  65         accum = ((accum << 8) & 0x3ff)
  66             ^ crc10_table[( accum >> 2) & 0xff]
  67             ^ *p++;
  68     }
  69     return accum;
  70 }
  71
  72 /* precompute checksum tables */
  73 void
  74 init_checksum(void) {
  75
  76     init_crc10_table();
  77
  78 }
  79
  80 /*
  81  * Creates the OSI Fletcher checksum. See 8473-1, Appendix C, section C.3.
  82  * The checksum field of the passed PDU does not need to be reset to zero.
  83  */
  84 u_int16_t
  85 create_osi_cksum (const u_int8_t *pptr, int checksum_offset, int length)
  86 {
  87
  88     int x;
  89     int y;
  90     u_int32_t mul;
  91     u_int32_t c0;
  92     u_int32_t c1;
  93     u_int16_t checksum;
  94     int index;
  95
  96     checksum = 0;
  97
  98     c0 = 0;
  99     c1 = 0;
 100
 101     for (index = 0; index < length; index++) {
 102         /*
 103          * Ignore the contents of the checksum field.
 104          */
 105         if (index == checksum_offset ||
 106             index == checksum_offset+1) {
 107             c1 += c0;
 108             pptr++;
 109         } else {
 110             c0 = c0 + *(pptr++);
 111             c1 += c0;
 112         }
 113     }
 114
 115     c0 = c0 % 255;
 116     c1 = c1 % 255;
 117
 118     mul = (length - checksum_offset)*(c0);
 119
 120     x = mul - c0 - c1;
 121     y = c1 - mul - 1;
 122
 123     if ( y >= 0 ) y++;
 124     if ( x < 0 ) x--;
 125
 126     x %= 255;
 127     y %= 255;
 128
 129
 130     if (x == 0) x = 255;
 131     if (y == 0) y = 255;
 132
 133     y &= 0x00FF;
 134     checksum = ((x << 8) | y);
 135
 136     return checksum;
 137 }