Implementing server-side file copy with Paramiko SFTP: Need guidance on custom extensions

Question

I am currently working on a project that involves server-side file copy operations using Paramiko's SFTP on the server side. I am seeking guidance on how to implement server-side file copy operations with Paramiko. Specifically, I want to add a custom extension or hook to Paramiko that supports a "copy-file" operation.

Details:

• Using Paramiko as a server
• Paramiko Version: 3.3.0
• Python Version: 3.9.11
• Operating System: Windows 11
• SSH Server: OpenSSH v9.4.0

Observations:

I have explored the Paramiko documentation and examples, but I couldn't find clear guidance on implementing server-side file copy operations. I attempted to modify the extension_pairs in paramiko/sftp.py (line 161) to ["copy-file", "md5,sha1"], but it doesn't seem to be recognized. I suspect this is definitely not be the correct approach, and I'm looking for guidance on the proper way to add a custom extension.

here's what I think the useful part of the log of client side to represent it's not working

debug2: channel_input_status_confirm: type 99 id 0
debug2: subsystem request accepted on channel 0
debug2: Remote version: 3
debug2: Unrecognised server extension "copy-data"
Connected to 127.0.0.1.
debug2: Sending SSH2_FXP_REALPATH "."
debug3: Sent message fd 6 T:16 I:1
debug3: SSH2_FXP_REALPATH . -> /
sftp> cp foo bart
Server does not support copy-data extension

Questions:

1. How can I add a custom extension or hook to Paramiko to support server-side file copy operations?
2. Are there existing hooks or methods that I can leverage for this purpose?

Thanks to anyone who can guide me on the right path or provide insights into implementing server-side file copy with Paramiko.

This question is also asked on github issue: https://github.com/paramiko/paramiko/issues/2336

Answer 1

Paramiko SFTPServer class does not seem to have any mechanism to implement custom extensions.

All you can do is to override the _process method and handle the t == CMD_EXTENDED on your own. But you will have to implement all (both) existing extensions again.

Something like:

class MySFTPServer(SFTPServer):
    def _process(self, t, request_number, msg):
        if t == CMD_EXTENDED:
            # Copy of CMD_EXTENDED implementation from SFTPServer._process
            tag = msg.get_text()
            if tag == "check-file":
                self._check_file(request_number, msg)
            elif tag == "posix-rename@openssh.com":
                oldpath = msg.get_text()
                newpath = msg.get_text()
                self._send_status(
                    request_number, self.server.posix_rename(oldpath, newpath)
                )
            elif tag == "copy-file":
                oldpath = msg.get_text()
                newpath = msg.get_text()
                # Your copy-file implementation
                # ... 
            else:
                self._send_status(request_number, SFTP_OP_UNSUPPORTED)
        else:
            super(SFTPServer, self)._process(t, request_number, msg)

(untested)

And similarly you need to reimplement _send_server_version to announce copy-file support. What you have tried already, but with wrong syntax. It's not obvious to me, what the right syntax is though. Maybe:

extension_pairs = [["check-file", "md5,sha1"], ["copy-data", "1"]]

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Though note that the OpenSSH sftp client does not support the copy-file extension. It supports the copy-data.