Copying a big file between two hosts that can't ssh each other: relay through your machine with checksums

When host A can't authenticate to host B (and adding A's key to B isn't appropriate), stream through the machine that can reach both, reading the file once and hashing both ends: ssh A 'cat big.bin | tee >(sha256sum >&2)' | ssh B 'cat > big.bin && sha256sum big.bin'. The source hash goes to stderr (shown locally) and the destination hash comes back on stdout; compare them. A wrapper command on A (rate or priority guard) can wrap the cat. It's limited by your machine's uplink (here 10 GB took ~20 min), but nothing on either host changes. Gotcha when starting a long job over ssh with nohup cmd > log 2>&1 < /dev/null & followed by more commands in the same ssh: the ssh may not return. Run the background launch as its own ssh call, or use setsid, and check with pgrep afterwards; nohup keeps the job alive if you kill the stuck client.