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FTP FTPS SFTP Concept, install, config and usage with certificate file

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Proftpd
 32 # To cause every FTP user to be "jailed" (chrooted) into their home
 33 # directory, uncomment this line.
 34 #DefaultRoot ~

Compared to the SCP protocol, which only allows file transfers, the SFTP protocol allows for a range of operations on remote files which make it more like a remote file system protocol. An SFTP client's extra capabilities include resuming interrupted transfers, directory listings, and remote file removal.

SFTP is not FTP run over SSH, but rather a new protocol designed from the ground up by the IETF SECSH working group.

  • 2yz – Success reply
  • 4yz or 5yz – Failure reply
  • 1yz or 3yz – Error or Incomplete reply
The second digit defines the kind of error:
  • x0z – Syntax. These replies refer to syntax errors.
  • x1z – Information. Replies to requests for information.






Differences from HTTP

HTTP essentially fixes the bugs in FTP that made it inconvenient to use for many small ephemeral transfers as are typical in web pages.

FTP has a stateful control connection which maintains a current working directory and other flags, and each transfer requires a secondary connection through which the data are transferred. In "passive" mode this secondary connection is from client to server, whereas in the default "active" mode this connection is from server to client. This apparent role reversal when in active mode, and random port numbers for all transfers, is why firewalls and NAT gateways have such a hard time with FTP. HTTP is stateless and multiplexes control and data over a single connection from client to server on well-known port numbers, which trivially passes through NAT gateways and is simple for firewalls to manage.

Setting up an FTP control connection is quite slow due to the round-trip delays of sending all of the required commands and awaiting responses, so it is customary to bring up a control connection and hold it open for multiple file transfers rather than drop and re-establish the session afresh each time. In contrast, HTTP originally dropped the connection after each transfer because doing so was so cheap. While HTTP has subsequently gained the ability to reuse the TCP connection for multiple transfers, the conceptual model is still of independent requests rather than a session.

When FTP is transferring over the data connection, the control connection is idle. If the transfer takes too long, the firewall or NAT may decide that the control connection is dead and stop tracking it, effectively breaking the connection and confusing the download. The single HTTP connection is only idle between requests and it is normal and expected for such connections to be dropped after a time-out.



Explicit
In explicit mode, a FTPS client must “explicitly request” security from a FTPS server and then step-up to a mutually agreed encryption method. If a client does not request security, the FTPS server can either allow the client to continue insecure or refuse/limit the connection.

In Explicit Mode, the client has full control over what areas of the connection are to be encrypted. Enabling and disabling of encryption for the FTPS control channel and FTPS data channel can occur at any time. WinSCP though requests encryption for both control and data channel unconditionally during whole session.

Implicit
Negotiation is not allowed with implicit FTPS configurations. A client is immediately expected to challenge the FTPS server with a connection encrypted using TLS/SSL. If it does not, the server should drop the connection.

In order to maintain compatibility with existing non-TLS/SSL aware FTP clients, implicit FTPS was expected to listen on the IANA Well Known Port 990/TCP for the FTPS control channel and 989/TCP for the FTPS data channel. This allowed administrators to retain legacy compatible services on the original 21/TCP FTP control channel.

In Implicit Mode, the entire FTPS session (both control and data channels) is unconditionally encrypted.


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