Secure copy protocol

Function

Normally, a client initiates an SSH connection to the remote host, and requests an SCP process to be started on the remote server. The remote SCP process can operate in one of two modes:

• source mode, which reads files (usually from disk) and sends them back to the client, or
• sink mode, which accepts the files sent by the client and writes them (usually to disk) on the remote host.

For most SCP clients, source mode is generally triggered with the -f flag (from), while sink mode is triggered with -t (to).^[2] These flags are used internally and are not documented outside the SCP source code.

Remote to remote mode

In the past, in remote-to-remote secure copy, the SCP client would open an SSH connection to the source host and request that it, in turn, open an SCP connection to the destination. (Remote-to-remote mode did not support opening two SCP connections and using the originating client as an intermediary). SCP thus could not be used to remotely copy from the source to the destination when operating in password or keyboard-interactive authentication mode, as this would reveal the destination server's authentication credentials to the source. It was, however, possible with key-based or GSSAPI methods that do not require user input.^[2]

More recently, remote-to-remote mode supports routing traffic through the client which originated the transfer, even though it is a 3rd party to the transfer. This way, authorization credentials must reside only on the originating client, the 3rd party.^[7]

Issues using talkative shell profiles

SCP does not expect text communicating with the SSH login shell. Text transmitted due to the SSH profile (e.g. echo "Welcome" in the .bashrc file) is interpreted as an error message, and a null line (echo "") causes SCP client to deadlock waiting for the error message to complete.^[2]

Syntax

Typically, a syntax of scp program^[9] is like the syntax of cp (copy):

Copying local file to a remote host:

scp LocalSourceFile user@remotehost:directory/TargetFile

Copying file from remote host and recursively copying folder (with -r switch) from remote host:

scp user@remotehost:directory/SourceFile LocalTargetFile
scp -r user@host:directory/SourceFolder LocalTargetFolder

Note that if the remote host uses a port other than the default of 22, it can be specified in the command. For example, copying a file from host:

scp -P 2222 user@host:directory/SourceFile TargetFile

Other clients

As the Secure Copy Protocol implements file transfers only, GUI SCP clients are rare, as implementing it requires additional functionality (directory listing at least). For example, WinSCP defaults to the SFTP protocol.^[10] Even when operating in SCP mode, clients like WinSCP are typically not pure SCP clients, as they must use other means to implement the additional functionality (like the ls command).^[11] This in turn brings platform-dependency problems.

More comprehensive tools for managing files over SSH are SFTP clients.