SSH, The Secure Shell: The Definitive Guide, 2nd Edition

SSH and File Transfers (scp and sftp)

The first thing to understand about SSH and file transfers is this: SSH doesn’t really do file transfers. That is, the core SSH protocol as implemented by a program such as ssh (SSH-TRANS, SSH-AUTH, and SSH-CONN) has no file-transfer capability at all. Following good modular design, file transfer is simply one of many services that might be run over an SSH connection channel. In fact, the file-transfer programs bundled with most Unix-based SSH products, scp and sftp, typically don’t even implement SSH in themselves; they simply run ssh in a subprocess to connect to the remote host, start the remote file-transfer agent, and talk to it.

Historically, the first file-transfer mechanism implemented with SSH was the program scp, included with the original SSH1 product. scp is simply an “ssh-ification” of the venerable Unix rcp program; just as rcp runs the rsh program to contact the remote host, scp runs ssh instead. If existing rsh software had supported a switch to select a different program than the default rsh (like scp -S), scp might never have been written; there would have been no need.

The rcp protocol used by scp is very limited. In a single session it can only transfer a set of whole files in one direction; there’s no directory browsing, partial transfer, resumption of interrupted transfers, multiple transfer directions—in other words, it’s nothing like FTP. When SSH Communications Security (SCS) defined the first version of the SSH-2 protocol ...