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Top Forums Programming calculate IP Header checksum manually Post 302346695 by semash! on Sunday 23rd of August 2009 11:09:29 PM
Old 08-24-2009
calculate IP Header checksum manually

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 Smilie.

Please help,
Thanks.
 

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sum(1)							      General Commands Manual							    sum(1)

NAME
sum - Displays the checksum and byte count in block-size (1024) units of a file SYNOPSIS
sum [-o | -r] [file...] The sum command reads file and calculates a 16-bit checksum and the byte count in block-size (1024) units in the file. If the file argu- ment is omitted, sum reads standard input. STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: sum: XCU5.0 Refer to the standards(5) reference page for more information about industry standards and associated tags. OPTIONS
[Tru64 UNIX] Uses an algorithm to compute the checksum using word-by-word computation. Uses an alternate algorithm to compute the check- sum (rigorous byte-by-byte computation rather than the word-by-word computation). [Tru64 UNIX] This is the default. OPERANDS
The file for which a checksum is to be computed. If this operand is omitted, standard input is used. DESCRIPTION
The checksum and number of blocks are written to standard output. The sum command is generally used to determine if a file that was copied or communicated over transmission lines is an exact copy of the original. The sum command writes the space used in 1024-byte units. [Tru64 UNIX] The checksum is calculated using a rigorous byte-byte computation by default. System V Compatibility [Tru64 UNIX] The checksum algorithms for the default sum command and the SVID 2 compliant sum command are reversed. The SVID 2 compliant sum command uses the word-by-word algorithm by default and uses the byte-by-byte algorithm if you specify the -r option on the command line. NOTES
The sum utility is marked LEGACY in XCU Issue 5. [Tru64 UNIX] The default algorithm is no longer the word-by-word computation algorithm. It was changed to the 4.3BSD default algorithm. The algorithms used may not be portable, that is, the same checksum may not be produced for the same input on different systems. Portable applications should use cksum. EXIT STATUS
The following exit values are returned: Successful completion. An error occurred. EXAMPLES
To display the checksum of datafile and the number of blocks in this file, enter: sum datafile If the checksum of datafile is 1605 and if the file contains 3 blocks, sum displays: 1605 3 datafile ENVIRONMENT VARIABLES
The following environment variables affect the execution of sum: Provides a default value for the internationalization variables that are unset or null. If LANG is unset or null, the corresponding value from the default locale is used. If any of the internationalization vari- ables contain an invalid setting, the utility behaves as if none of the variables had been defined. If set to a non-empty string value, overrides the values of all the other internationalization variables. Determines the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multibyte characters in arguments). Determines the locale for the for- mat and contents of diagnostic messages written to standard error. Determines the location of message catalogues for the processing of LC_MESSAGES. SEE ALSO
Commands: cksum(1), wc(1) Standards: standards(5) sum(1)
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