You've forgotten to include the UDP payload in your calculation. For the calculation the pseudo header must be in front of UDP header and UDP payload.
Helge
-------- Original-Nachricht --------
> Datum: Fri, 5 Aug 2011 15:45:38 +0200
> Von: "news.gmane.com" <AndreasSander1@xxxxxxx>
> An: wireshark-dev@xxxxxxxxxxxxx
> Betreff: [Wireshark-dev] Question about UDP checksum
> Hello,
>
> I try to implement a UDP checksum routine. Unfortunately it calculates a
> completely different value than Wireshark does. I don't known what I am
> doing wrong. Can you help me?
>
> The calculation is done using a UDP pseudo header with the structure
>
> struct pseudo
> {
> uint32 source;
> uint32 destination;
> uint8 zero;
> uint8 protocol;
> uint16 udp_length;
> };
>
> When I have a UDP frame with IP source = "192.168.100.132" and IP
> destination = "192.168.144.255", and UDP Length = 66 (IP Total_Length =
> 86)
> I get this data in the pseudo header:
>
> c0 a8 64 84
> c0 a8 90 ff
> 00 11 00 42
>
> The sum is 0x32874 which is must be folded to 0x2874+0x0003 = 0x8277. The
> complement is 0xd788.
>
> But Wireshark detects an error and says the checksum must be 0x5528. Can
> anybody give me a hint what's wrong?
>
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