Hello everybody,
I'm reading and learning a lot about TCP/IP protocols, and i want to know how to calculate the IP header checksum manually. I've searched for it EVERYWHERE! and none of options work.
I've read a function that does it pretty good, but i can't "translate" it into a human form so i can do it manually.
here's the code:
Code:
unsigned short checksum(unsigned short *ptr, int length){
register int sum = 0;
u_short answer = 0;
register u_short *w = ptr;
register int nleft = len;
while(nleft > 1){
sum += *w++;
nleft -= 2;
}
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
answer = ~sum;
return(answer);
}
People, i've tried, i understand and know how to calculate using ones complement, hex conversion, binary conversion, AND, OR, XOR, NOT, and everything, but i can't do this .
Does anyone know the answer to this?
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1 2 3 4
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Discussion started by: verdepollo
3 Replies
LEARN ABOUT SUNOS
sum
sum(1) User Commands sum(1)NAME
sum - print checksum and block count for a file
SYNOPSIS
sum [-r] [file...]
DESCRIPTION
The sum utility calculates and prints a 16-bit checksum for the named file and the number of 512-byte blocks in the file. It is typically
used to look for bad spots, or to validate a file communicated over some transmission line.
OPTIONS
The following options are supported:
-r Use an alternate (machine-dependent) algorithm in computing the checksum.
OPERANDS
The following operands are supported:
file A path name of a file. If no files are named, the standard input is used.
USAGE
See largefile(5) for the description of the behavior of sum when encountering files greater than or equal to 2 Gbyte ( 2**31 bytes).
ENVIRONMENT VARIABLES
See environ(5) for descriptions of the following environment variables that affect the execution of sum: LC_CTYPE, LC_MESSAGES, and
NLSPATH.
EXIT STATUS
The following exit values are returned.
0 Successful completion.
>0 An error occurred.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|Availability |SUNWesu |
|CSI |enabled |
+-----------------------------+-----------------------------+
SEE ALSO cksum(1), sum(1B), wc(1), attributes(5), environ(5), largefile(5)DIAGNOSTICS
"Read error" is indistinguishable from end of file on most devices; check the block count.
NOTES
Portable applications should use cksum(1).
sum and usr/ucb/sum (see sum(1B)) return different checksums.
SunOS 5.10 7 Nov 1995 sum(1)
08-24-2009
.