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Secure copy protocol (SCP) is a means of securely transferring computer files between a local host and a remote host or between two remote hosts. It is based on the Secure Shell (SSH) protocol. "SCP" commonly refers to both the Secure Copy Protocol and the program itself. According to OpenSSH developers in April 2019 the scp protocol is outdated, inflexible and not readily fixed.
Secure Copy Protocol
The SCP is a network protocol, based on the BSD RCP protocol, which supports file transfers between hosts on a network. SCP uses Secure Shell (SSH) for data transfer and uses the same mechanisms for authentication, thereby ensuring the authenticity and confidentiality of the data in transit. A client can send (upload) files to a server, optionally including their basic attributes (permissions, timestamps). Clients can also request files or directories from a server (download). SCP runs over TCP port 22 by default. Like RCP, there is no RFC that defines the specifics of the protocol.
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). 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 opens an SSH connection to the source host and requests 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). It is important to note that SCP cannot 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 is, however, possible with key-based or GSSAPI methods that do not require user input.
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.
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 to deadlock waiting for the error message to complete.
Secure Copy (remote file copy program)
The SCP program is a software tool implementing the SCP protocol as a service daemon or client. It is a program to perform secure copying. The SCP server program is typically the same program as the SCP client.
Perhaps the most widely used SCP program is the command line scp program, which is provided in most SSH implementations. The scp program is the secure analog of the rcp command. The scp program must be part of all SSH servers that want to provide SCP service, as scp functions as SCP server too.
Some SSH implementations provide the scp2 program, which uses the SFTP protocol instead of SCP, but provides the very same command line interface as scp. scp is then typically a symbolic link to scp2.
Copying file to host:
scp SourceFile user@host:directory/TargetFile
Copying folder from host:
scp user@host:directory/SourceFile TargetFile scp -r user@host:directory/SourceFolder TargetFolder
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
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. 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). This in turn brings platform-dependency problems.
More comprehensive tools for managing files over SSH are SFTP clients.
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