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About calculating packet checksum

 
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souleaper
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Joined: 08 Jul 2012
Posts: 24

PostPosted: Thu Aug 23, 2018 11:15 pm    Post subject: About calculating packet checksum Reply with quote

i am following a guide in
securitynik calculating-udp-checksum-with-taste-of_3 (since i can't post link)

50 C7 BF 65 B5 CA 70 F3 95 00 A0 AF 08 00 45 00 00 28 3F 1C 40 00 80 06 00 00 C0 A8 01 A3 D2 F2 F3 65 C2 00 1A 4C 9F 5E 98 B7 70 8A A9 B8 50 10 40 18 88 BE 00 00

This is my packet..

source Ip 192.168.1.163 1100 0000 1010 1000 C0 A8
0000 0001 1010 0011 01 A3
dest IP 210.242.243.101 1101 0010 1111 0010 D2 F2
1111 0011 0110 0101 F3 65

TCP pro 0/6 0000 0000 0000 0110 00 06

Padding
Length 0000 0000 0001 1110 00 14

S-Port 49664 1100 0010 0000 0000 C2 00

D-Port 6732 0001 1010 0100 1100 1A 4C

Seq # 2673776823 1001 1111 0101 1110 9F 5E
1001 1000 1011 0111 98 B7

Ack # 1888135608 0111 0000 1000 1010 70 8A
1010 1001 1011 1000 A9 B8

Flag # 0101 0000 0001 0000 50 10

Window # 16408 0100 0000 0001 1000 40 18
Sum: 5 86 49
Adding first byte: 86 4E
in Binary: 1000 0110 0100 1110
Flip bit: 0111 1001 1011 0001
My checksum is 79B1
but the packet checksum is 88 BE

the only info i was not sure of is the padding length.. which it was not explained well.. My understanding is.. it is either

by adding header length which in this case is 20 bytes + any payload bytes.. which there is none.. so 14 in hex is 20.. or maybe adding the flag number to it.. which 20 + 10 is 30.. i also tried that and the checksum doesn't match?

Is my math wrong? or logic wrong..

Thanks for your help..

Chelvan.
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