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<td width="68%" background="DDl_back2.jpg" height="44"><big><big><font face="黑体"><p align="center">               ● UNIX网络编程                       (BM: clown)                </font></big></big></td></tr>
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<tr><td width="100%" colspan="2" height="100" align="center" valign="top"><br><p align="center">[<a href="index.htm">回到开始</a>][<a href="473.htm">上一层</a>][<a href="498.htm">下一篇</a>]
<hr><p align="left"><small>发信人: guru ( Darkness), 信区: UNP <br>

标  题: 两个TCP校验和例子 <br>

发信站: UNIX编程 (2001年07月29日13:59:49 星期天), 站内信件 <br>

  <br>

>第一个是精华区中源例分析中blat.c里的,加了点注释 <br>

u_short checksum(u_short * data,u_short length) { <br>

         register long value;     //32bit <br>

         u_short i;               //16bit <br>

         for(i=0;i<(length>>1);i++)   //length/2 ---- 16bit <br>

                 value+=data[i]; <br>

         if((length&1)==1)           //length last bit = 1 -- 奇数 <br>

                 value+=(data[i]<<8);   //填充8bit 0; <br>

  <br>

         value=(value&65535)+(value>>16); //add low 16biy to high 16 bit <br>

         return(~value); <br>

 } <br>

>第二个是phrack valume 7 issue 51中 the art of port scanning例子 <br>

>昨天看这期文章的时候看到的.稍微加了点注释 <br>

/* <br>

  <br>

 *      I'll bet you've never seen this function before (yeah right)! <br>

  <br>

 *      standard swiped checksum routine. <br>



  <br>

 */ <br>

  <br>

unsigned short in_cksum(unsigned short *ptr,int nbytes) { <br>

  <br>

  <br>

  <br>

register long           sum;            /* assumes long == 32 bits */ <br>

  <br>

u_short                 oddbyte; <br>

  <br>

register u_short        answer;         /* assumes u_short == 16 bits <br>

  <br>

*/ <br>

  <br>

  <br>

  <br>

/* <br>

  <br>

 * Our algorithm is simple, using a 32-bit accumulator (sum), <br>

  <br>

 * we add sequential 16-bit words to it, and at the end, fold back <br>



  <br>

 * all the carry bits from the top 16 bits into the lower 16 bits. <br>

  <br>

 */ <br>

  <br>

  <br>

  <br>

sum = 0; <br>

  <br>

while (nbytes > 1)  { <br>

  <br>

sum += *ptr++; <br>

  <br>

nbytes -= 2; <br>

  <br>

} <br>

  <br>

  <br>

/* mop up an odd byte, if necessary */ <br>

if (nbytes == 1) { <br>

oddbyte = 0;            /* make sure top half is zero */ <br>

*((u_char *) &oddbyte) = *(u_char *)ptr;   /* one byte only */ <br>



sum += oddbyte; <br>

} <br>

  <br>

/* <br>

 * Add back carry outs from top 16 bits to low 16 bits. <br>

 */ <br>

  <br>

sum  = (sum >> 16) + (sum & 0xffff);    /* add high-16 to low-16 */ <br>

sum += (sum >> 16);                     /* add carry */ <br>

answer = ~sum;          /* ones-complement, then truncate to 16 bits <br>

*/ <br>

return(answer); <br>

} <br>

>>其他校验和相关的东西放入精华区TCP/IP相关目录的校验和目录下. <br>

-- <br>

Target Locked:Guru In Darkness. <br>

我只是一只静静卧着的狮子。。。 <br>

※ 来源:·UNIX编程 www.tiaozhan.com/unixbbs/·[FROM: 202.114.36.176] <br>

</small><hr>
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