[PATCH] UDP --badsum fix

Hi nmap-dev!

Small fix. I was reading the UDP RFC http://www.ietf.org/rfc/rfc768.txt

and noticed the following:

"If the computed checksum is zero, it is transmitted as all ones (the
 equivalent in one's complement arithmetic). An all zero transmitted
 checksum value means that the transmitter generated no checksum (for
 debugging or for higher level protocols that don't care)."

The problem is that with --badsum if a packet ends up with a checksum of
1 Nmap's --badsum will subtract 1 from it and the packet will be sent with a
checksum of 0 which actually means that UDP checksums were turned off and
this packet will not technically have a bad checksum.

This will only happen in, on average, 1 in 65535 packets, but why send
incorrect packets when you can send correct packets?

Best,

Doug

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