The Relationship Between File Transfer, SSH, SCP2 (SCP), and SFTP.

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A Brief Introduction to SSH

The SSH protocol provides strong encrypted authentication and a secure encrypted tunnel through which you can move data and execute remote commands securely.

There are two different and incompatible implementations of the SSH protocol; SSH-2 and SSH-1. While the Reflection Windows clients support both protocols, it is highly recommended that you use the newer protocol, SSH-2, rather than SSH-1, which is deprecated.


The Secure File Transfer Utilities

The file transfer capabilities of SSH are provided by utilities included with most SSH products, such as Reflection and the Reflection for Secure IT Windows and UNIX Clients. Typically, these utilities are called scp and sftp.

The scp and sftp utilities use the SCP and SFTP protocols (respectively) to provide file transfer capabilities and use the encrypted SSH tunnel to provide security.

SCP (Secure Copy Protocol)  is part of the OpenSSH suite of tools. Basically it's an encrypted/strongly authenticated version of rcp.

Problem using scp:
I issued an command on a local machine
scp -r download/ tim@foo.xxx.xxx.xxx
Then I checked at the remote foo.xxx.xxx.xxx and found under my home directory.

Sol/ Colon is missing at the end, it should be scp -r download/ tim@foo.xxx.xxx.xxx:. The colon shows you're transferring to a remote host and not just a local file called tim@foo.xxx.xxx.xxx.



Important distinction: The file transfer protocol SFTP is not a 'secure version' of the standard FTP protocol. It is a completely different protocol. You cannot connect to an FTP server using SFTP or to an SFTP server using FTP.

See this Link in detail.

There are two ways that scp utilities can implement file transfers; one implementation is based on OpenSSH and uses SCP over SSH, the other uses SFTP over SSH.

The following sections detail what happens in the background when scp based on OpenSSH (using SCP over SSH) or scp/sftp (using SFTP) are used.

Transferring Files Using scp / sftp (using SFTP)

When an sftp or scp (using SFTP) command is issued, the following occurs:
The command opens the sftp command line interface.
It runs the ssh command with the –s option to start an sftp subsystem.
It sets up a secure tunnel and securely authenticates.
It starts the host SSH daemon's sftp-server subsystem (the file transfer server).
And it waits for interactive sftp commands.

The SSH tunnel remains open until the bye command is issued, allowing multiple sftp commands to be issued before it is closed.

Transferring Files Using scp Based on OpenSSH (using SCP over SSH)

When an scp command is issued, the following occurs:
scp starts ssh with the specified options and the remote command to execute scp on the remote host. This initiates an SSH connection, sets up an encrypted tunnel, and securely authenticates to the SSH server.
scp executes on the server, using the -t (uploads) or -f (gets) options.
scp transfers the specified file(s) or folder(s), using the SCP protocol.
Once the transfer is complete, the SSH tunnel is closed.

A new SSH tunnel is created and shut down for each scp command issued.



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