Apparently bad checksums in tcpdump/wireshark captures for outgoing
traffic are normal, when hardware offload is enabled. The NIC only
fills in the correct value after data has been copied to the NIC, which
is after capture takes a snapshot of the buffer with whatever garbage
was there. See the last point in http://docs.gz.ro/node/282.
Since disabling chksum offload actually fixed your problem, maybe your
NIC really was filling in checksums incorrectly (or it wasn't really
enabled, so neither the kernel nor the NIC were generating correct
Anyway, I think to properly debug this, you should have tried capturing
packets from a different computer, so you can be sure you're seeing what
went out over the wire with/without HW chksum enabled.
TL:DR: you probably had a real issue (since disabling offload got your
ssh working), but your debugging / verification technique was flawed,
and would show a problem even on working hardware.
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Bad TCP/UDP checksum on e1000e with tx-checksumming on
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