IPv4 Checksum Routine in Perl

by Eric on Feb.09, 2008, under Code

Here’s a subroutine that I wrote a few years ago to calculate IPv4 checksums.

sub ipchecksum {
# Calculate the Internet Protocol checksum of the given hex string
# pskl.us January 2006

my $sum=0;
my $i=0;
my $word;

for ($i=0; $i<length($_[0]); $i=$i+4) {
$word=substr($_[0],$i,4);
if ( length($word) eq 2 ) { $word=$word.’00’; };
$sum=$sum + hex $word;
}

return sprintf(“%04X”, 65535 – (( hex substr(sprintf(“%X”, $sum), -4,4) ) +
( hex substr(sprintf(“%X”, $sum), -8,4) )) );

};

jeremy
February 9th, 2008 on 9:48 am
Will that make my internets faster?

Reply
John
February 19th, 2008 on 12:53 pm
No, you need bigger tubes for the faster internets.

Reply
Yoram Stern
March 1st, 2011 on 9:17 am
Hi.
Thanks for the code BUT you have a bug.
There is the scenario that the sum of the carry is greater than 65535.
In this case your function gives wrong result.

for example assume the header:

my $dw0 = “45ff0034” ;
my $dw1 = “a6199362” ;
my $dw2 = “ff110000” ;
my $dw3 = “0b5097c5” ;
my $dw4 = “7fad5e7c” ;
my $pkt = $dw0 . $dw1. $dw2 . $dw3 . $dw4 ;

my $checksum = ipchecksum( $pkt) ;

printf “ipv4 checksum= $checksum\n” ;

The $sum will be 0x3fffd
Your function will give checksum of 0xffff
while the correct checksum is 0xfffe .

Regards.

Reply

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