Sender and receiver to transfer files over ssh on request?

I found a solution from another angle. Since version 3.9, OpenSSH supports session multiplexing: a single connection can carry multiple login or file transfer sessions. This avoids the set-up cost per connection.

For the case of the question, I can first open a connection with sets up a control master (-M) with a socket (-S) in a specific location. I don't need a session (-N).

ssh user@host -M -S /tmp/%r@%h:%p -N

Next, I can invoke scp for each file and instruct it to use the same socket:

scp -o 'ControlPath /tmp/%r@%h:%p' <file> user@host:<remotefile>

This command starts copying almost instantaneously!

You can also use the control socket for normal ssh connections, which will then open immediately:

ssh user@host -S /tmp/%r@%h:%p

If the control socket is no longer available (e.g. because you killed the master), this falls back to a normal connection. More information is available in this article.

It might work to use sftp instead of scp, and to place it into batch mode. Make the batch command file a pipe or UNIX domain socket and feed commands to it as you want them executed.

Security on this might be a little tricky at the client end.

Have you tried sshfs?You could:

sshfs remote_user@remote_host:/remote_dir /mnt/local_dir

Where

• /remote_dir was the directory you want to send files to on the system you are sshing into
• /mnt/local_dir was the local mount location

With this setup you can just cp a file into the local_dir and it would be sent over sftp to remote_host in its remote_dir

Note that there is a single connection, so there is little in the way of overhead

You may need to use the flag -o ServerAliveInterval=15 to maintain an indefinite connection

You will need to have fuse installed locally and an SSH server supporting (and configured for) sftp