Rsync + SSH ServerAliveInterval

Did you ever leave scp running overnight to copy a file from a remote server over some slow WAN link? And only to find the dreaded message on the console (I was using port 2222):

scp -c blowfish -P 2222 user@remote_host:huge_remote_file .
huge_remote_file            43%  182MB   0.0KB/s - stalled -

Yeah - "stalled". At 43%. So you pull out rsync to, at least, re-start from where it stopped:

rsync --progress -e 'ssh  -c blowfish -p 2222' user@remote_host:huge_remote_file .

But what if it stalls again? You will not be around to re-start the transfer.

You can use ssh's ServerAliveInterval option and rsync's exit value to solve this problem. The remote host will need to accept public key authentication but this is easy to set-up, even if only temporarily.

This is the command that does the trick:

while rsync --progress -e 'ssh -o ServerAliveInterval=300 -c blowfish -p 2222' \
user@remote_host:huge_remote_file . ; do sleep 1; done

Explaining: if there is no traffic through ssh in 300 seconds, the client will request a ping reply from the ssh server and drop the connection in case none is received. Rsync will then exit with a non-zero status (probably 20), which will allow the loop condition to run again.

Thus, when the transfer is successfully finished, rsync will exit with status 0, ending the loop. And the file will be waiting for you in the morning. Nice.

By the way, I use blowfish as the cipher algorithm because it is faster than the default, 3des. In my case it doesn't matter too much since the connection was being tunnelled through a VPN but you should use a stronger cipher if that's not your case.