You have to correctly set your checksum options according to your
hardware checksum capabilities.
Setting all 1 or all 0 is not correct per se.
Ethernet hardware is also capable of generating IP checksums, TCP
checksums, UDP checksums, all of them, some of them, none of them.
Check your hardware docs, decide whether you want your hardware or lwIP
to be the one in charge of checksumming (usually hw is a better option),
and set both to do its part.
If you set lwIP to not do checksumming, it will leave that field at
zero, so the hardware can fill in. But if you don't tell the hardware to
do it... it will go out as zeroes.
If you don't know how to set your hardware, or it is not able to
generate one (or all) checksums ("checksum offloading"), then set lwIP
to do it for that one (or all of them).
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