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Secure copy protocol
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<h2 id="secure-copy-protocol">Secure Copy Protocol</h2> <p>The SCP is a <a href="/facts/Network_protocol/5uady7cB">network protocol</a>, based on the <a href="/facts/BSD/XQKcURrh">BSD</a> <a href="/facts/Rcp_(Unix)/fB8rLwNJ">RCP</a> protocol,<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> which supports <a href="/facts/File_transfer/9qP8HDW8">file transfers</a> between hosts on a network. SCP uses <a href="/facts/Secure_Shell/wjVRw5zv">Secure Shell</a> (SSH) for data transfer and uses the same mechanisms for authentication, thereby ensuring the <a href="/facts/Information_security/horlrCln">authenticity</a> and <a href="/facts/Confidentiality/JFMubsQm">confidentiality</a> of the <a href="/facts/Data_in_transit/8f1ztg6C">data in transit</a>. 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 <a href="/facts/TCP_port/ygoHVKNC">TCP port</a> 22 by default.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> Like RCP, there is no <a href="/facts/Request_for_Comments/esftTvE7">RFC</a> that defines the specifics of the protocol. </p> <h3>Function</h3> <p>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: </p> <ul><li>source mode, which reads files (usually from disk) and sends them back to the client, or</li> <li>sink mode, which accepts the files sent by the client and writes them (usually to disk) on the remote host.</li></ul> <p>For most SCP clients, source mode is generally triggered with the -f flag (from), while sink mode is triggered with -t (to).<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> These flags are used internally and are not documented outside the SCP source code. </p> <h3>Remote to remote mode</h3> <p>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 <a href="/facts/GSSAPI/7rTyX3x5">GSSAPI</a> methods that do not require user input.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p><p>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.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h3>Issues using talkative shell profiles</h3> <p>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.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p> <h2 id="scp-program">scp program</h2> <p>The SCP program<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> is a software tool implementing the SCP protocol as a service daemon or client. It is a program to perform secure copying. </p><p>Perhaps the most widely used SCP program is the OpenSSH <a href="/facts/Command_line/R9JpGmXQ">command line</a> 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. Since OpenSSH 9.0, the program has been updated to use the newer, more secure <a href="/facts/SSH_file_transfer_protocol/88YptbER">SFTP</a> protocol; an -O option is added for using SCP with old SCP-only servers.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h3>Syntax</h3> <p>Typically, a syntax of scp program<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> is like the syntax of <a href="/facts/Cp_(Unix)/T7n2B1Fg">cp (copy)</a>: </p><p>Copying local file to a remote host: </p> scp LocalSourceFile user@remotehost:directory/TargetFile <p>Copying file from remote host and recursively copying folder (with -r switch) from remote host: </p> scp user@remotehost:directory/SourceFile LocalTargetFile scp -r user@host:directory/SourceFolder LocalTargetFolder <p>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: </p> scp -P 2222 user@host:directory/SourceFile TargetFile <h3>Other clients</h3> <p>As the Secure Copy Protocol implements file transfers only, <a href="/facts/GUI/wBlQFWDP">GUI</a> SCP clients are rare, as implementing it requires additional functionality (<a href="/facts/Directory_(file_systems)/VLtLcEXY">directory</a> listing at least). For example, <a href="/facts/WinSCP/7tagd8zu">WinSCP</a> defaults to the SFTP protocol.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> 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 <a href="/facts/Ls/5GsH3v2a">ls command</a>).<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> This in turn brings platform-dependency problems. </p><p>More comprehensive tools for managing files over SSH are <a href="/facts/SSH_File_Transfer_Protocol/88YptbER">SFTP</a> clients. </p> <h2 id="security">Security</h2> <p>In 2019 vulnerability <a href="/facts/CVE_(identifier)/r6uEABNB">CVE</a>-<a href="https://nvd.nist.gov/vuln/detail/CVE-2019-6111">2019-6111</a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> was announced related to the <a href="/facts/Openssh/2qxJ3Fe7">openssh</a> SCP tool and protocol allowing users to overwrite arbitrary files in the SCP client target directory. </p> <h2 id="see-also">See also</h2> Wikibooks has a book on the topic of: <i>Guide to Windows Commands</i> <ul><li><a href="/facts/SSH_File_Transfer_Protocol/88YptbER">SSH File Transfer Protocol</a> (SFTP)</li> <li><a href="/facts/Rcp_(Unix)/fB8rLwNJ">Remote copy</a> (rcp)</li> <li><a href="/facts/Comparison_of_file_transfer_protocols/XGBF8I9C">Comparison of file transfer protocols</a></li> <li><a href="/facts/Files_transferred_over_shell_protocol/M7FeRApC">Files transferred over shell protocol</a> (FISH)</li> <li><a href="/facts/FTP_over_SSH/THuWgzfw">FTP over SSH</a></li> <li><a href="/facts/FTPS/6DMVBUCX">FTPS</a></li> <li><a href="/facts/Rsync/H4UlsQ3U">rsync</a></li> <li><a href="/facts/Secure_shell/wjVRw5zv">Secure shell</a></li> <li><a href="/facts/WinSCP/7tagd8zu">WinSCP</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Linux and Unix scp command". Computer Hope. Retrieved 4 August 2015. <a href="https://www.computerhope.com/unix/scp.htm" target="_blank">https://www.computerhope.com/unix/scp.htm</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Pechanec, Jan. "How the SCP protocol works". Jan Pechanec's weblog. Oracle. Archived from the original on 2017-02-15. Retrieved 4 August 2015. <a href="/w/index.php?title=Jan_Pechanec&action=edit&redlink=1" target="_blank">/w/index.php?title=Jan_Pechanec&action=edit&redlink=1</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"OpenSSH 8.0". OpenSSH Release Notes. 17 April 2019. <a href="https://www.openssh.com/txt/release-8.0" target="_blank">https://www.openssh.com/txt/release-8.0</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"OpenSSH 9.0". OpenSSH Release Notes. 8 April 2022. <a href="https://www.openssh.com/txt/release-9.0" target="_blank">https://www.openssh.com/txt/release-9.0</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"scp(1) - OpenBSD manual pages (history section)". Retrieved 25 June 2012. <a href="https://man.openbsd.org/scp#HISTORY" target="_blank">https://man.openbsd.org/scp#HISTORY</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"SCP - Secure Copy Protocol - What is it & Full Definition & Example Cmds!". PC & Network Downloads - PCWDLD.com. 2019-06-29. Retrieved 2020-05-22. <a href="https://www.pcwdld.com/what-is-scp" target="_blank">https://www.pcwdld.com/what-is-scp</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Pechanec, Jan. "How the SCP protocol works". Jan Pechanec's weblog. Oracle. Archived from the original on 2017-02-15. Retrieved 4 August 2015. <a href="/w/index.php?title=Jan_Pechanec&action=edit&redlink=1" target="_blank">/w/index.php?title=Jan_Pechanec&action=edit&redlink=1</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Pechanec, Jan. "How the SCP protocol works". Jan Pechanec's weblog. Oracle. Archived from the original on 2017-02-15. Retrieved 4 August 2015. <a href="/w/index.php?title=Jan_Pechanec&action=edit&redlink=1" target="_blank">/w/index.php?title=Jan_Pechanec&action=edit&redlink=1</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Carroll, Brandon (16 August 2017). "How to use Secure Copy for file transfer". TechRepublic. Retrieved 2020-05-22. <a href="https://www.techrepublic.com/article/how-to-use-secure-copy-for-file-transfer/" target="_blank">https://www.techrepublic.com/article/how-to-use-secure-copy-for-file-transfer/</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Pechanec, Jan. "How the SCP protocol works". Jan Pechanec's weblog. Oracle. Archived from the original on 2017-02-15. Retrieved 4 August 2015. <a href="/w/index.php?title=Jan_Pechanec&action=edit&redlink=1" target="_blank">/w/index.php?title=Jan_Pechanec&action=edit&redlink=1</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"Portable OpenSSH". GitHub. 13 May 2022. <a href="https://github.com/openssh/openssh-portable/blob/master/scp.c" target="_blank">https://github.com/openssh/openssh-portable/blob/master/scp.c</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>"OpenSSH 9.0". OpenSSH Release Notes. 8 April 2022. <a href="https://www.openssh.com/txt/release-9.0" target="_blank">https://www.openssh.com/txt/release-9.0</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>"scp(1) - OpenBSD manual pages" https://man.openbsd.org/scp <a href="https://man.openbsd.org/scp" target="_blank">https://man.openbsd.org/scp</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>"Supported File Transfer Protocols :: WinSCP". winscp.net. Retrieved 2020-05-22. <a href="https://winscp.net/eng/docs/protocols" target="_blank">https://winscp.net/eng/docs/protocols</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>"The SCP/Shell Page (Advanced Site Settings dialog) :: WinSCP". winscp.net. Retrieved 2020-05-22. <a href="https://winscp.net/eng/docs/ui_login_scp" target="_blank">https://winscp.net/eng/docs/ui_login_scp</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>"NVD - Cve-2019-6111". <a href="https://nvd.nist.gov/vuln/detail/CVE-2019-6111" target="_blank">https://nvd.nist.gov/vuln/detail/CVE-2019-6111</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> </ol>