Comment 5 for bug 1251464

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 checksums).

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.