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Posted by Philip on 08/03/05 22:05
I've wanted to make a ping 'program' in PHP, so after reading up on
this, it occured to me that the only difficult thing was to calculate
the ICMP checksum. So after some heavy reading I could calculate this by
pen and paper. And after some hours trying to make my calculations into
PHP code, I discovered this piece of code in the online doc:
(by Khaless [at] bigpond [dot] com)
http://dk2.php.net/manual/en/ref.sockets.php
<?PHP
// Computes Internet Checksum for $data
// will return a 16-bit internet checksum for $data
function inetChecksum($data)
{
// 32-bit accumilator, 16 bits at a time, adds odd bit on at end
for($i=0;$i<strlen($data);$i += 2)
{
if($data[$i+1]) $bits = unpack('n*',$data[$i].$data[$i+1]);
else $bits = unpack('C*',$data[$i]);
$sum += $bits[1];
}
// Fold 32-bit sum to 16 bits
while ($sum>>16) $sum = ($sum & 0xffff) + ($sum >> 16);
$checksum = pack('n1',~$sum);
return $checksum;
}
?>
Now, after discovering the unpack function it all became a lot easier,
so I've made my own checksum calculator, which works approximatly 5
times as fast (and it dosn't have the for loop limitation of data
length), and all the tests I've done it gives the same result as above:
<?php
function myChecksum($data)
{
$bit = unpack('n*', $data);
$sum = array_sum($bit);
if (strlen($data)%2) {
$temp = unpack('C*',$data[strlen($data)-1]);
$sum += $temp[1];
}
// The next 2 lines of code, is stolen from the
// original ping program written in C
// Stolen code start:
$sum = ($sum >> 16) + ($sum & 0xffff);
$sum += ($sum >> 16);
// End of stolen code
return pack('n*', ~$sum);
}
?>
Can anyone tell me, if there will be any example, where the 2 functions
wont give the same output? The piece of code I think could be the most
trouble is the piece I've stolen, in the other function there's a while
loop, so if I a very long data input, it wouldn't be the same output,
but I've tried with large strings and they still give the same.
So to sum up, will the 2 functions always calculate the same.
--
Philip Birk-Jensen
URL: http://birk-jensen.dk
ICQ: 14789099
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