SFTP The server has closed the connection.

Question

Exception "The server has closed the connection." is thrown with
_sftp.Login(username, pwd);
Below settings are applied.
_sftp.Settings.DisableTransferQueue = true;
_sftp.Settings.UploadBufferSize = 48 * 1024;

The log is below.

2019-09-24 19:49:02.780 INFO FileLogWriter(1)[13] Info: Assembly: Rebex.Common 2018 R2 for .NET 4.0-4.7
2019-09-24 19:49:02.784 INFO FileLogWriter(1)[13] Info: Platform: Windows 10.0.17134 32-bit; CLR: 4.0.30319.42000
2019-09-24 19:49:02.785 DEBUG FileLogWriter(1)[13] Info: Culture: en; Windows-1252
2019-09-24 19:49:02.815 DEBUG Sftp(1)[13] SSH: Allowed authentication methods for 'root': publickey, password.
2019-09-24 19:49:02.817 DEBUG Sftp(1)[13] SSH: Trying password authentication for 'root'.
2019-09-24 19:49:02.858 DEBUG Sftp(1)[13] SSH: Authentication successful.
2019-09-24 19:49:02.881 DEBUG Sftp(1)[13] SSH: Requesting subsystem 'sftp'.
2019-09-24 19:49:02.897 INFO Sftp(1)[13] Command: SSH_FXP_INIT (4)
2019-09-24 19:49:03.126 ERROR Sftp(1)[13] Info: Rebex.Net.SftpException: The server has closed the connection.
   at ctcy.mpez(Int32 arz)
   at ctcy.mpfb(hffs& asc)
   at ctcy.mpdu(SftpServerType aoe, ctch aof)
   at Rebex.Net.Sftp.ypkg(ctch qu)
   at Rebex.Net.Sftp.ypke(String qo, String qp, SshPrivateKey qq, ctch qr)

Comments

This looks like the server is disconnecting the client when it sends the initial SFTP protocol's SSH_FXP_INIT packet. If you add the following setting, does it have any effect?
  _sftp.Settings.DisableSftp4 = true;

---

With DisableSftp4 = true, I get the below log

2019-09-24 20:15:32.223 Opening log file.
2019-09-24 20:15:32.225 INFO FileLogWriter(1)[12] Info: Assembly: Rebex.Common 2018 R2 for .NET 4.0-4.7
2019-09-24 20:15:32.227 INFO FileLogWriter(1)[12] Info: Platform: Windows 10.0.17134 32-bit; CLR: 4.0.30319.42000
2019-09-24 20:15:32.228 DEBUG FileLogWriter(1)[12] Info: Culture: en; Windows-1252
2019-09-24 20:15:33.387 VERBOSE Sftp(1)[12] SSH: Sending packet SSH_MSG_SERVICE_REQUEST (17 bytes).
 0000 |05-00-00-00-0C-73-73-68 2D-75-73-65-72-61-75-74| .....ssh-useraut
 0010 |68                                             | h
2019-09-24 20:15:33.398 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_SERVICE_ACCEPT (17 bytes).
 0000 |06-00-00-00-0C-73-73-68 2D-75-73-65-72-61-75-74| .....ssh-useraut
 0010 |68                                             | h
2019-09-24 20:15:33.399 VERBOSE Sftp(1)[12] SSH: Sending packet SSH_MSG_USERAUTH_REQUEST (35 bytes).
 0000 |32-00-00-00-04-72-6F-6F 74-00-00-00-0E-73-73-68| 2....root....ssh
 0010 |2D-63-6F-6E-6E-65-63-74 69-6F-6E-00-00-00-04-6E| -connection....n
 0020 |6F-6E-65                                       | one
2019-09-24 20:15:33.404 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_USERAUTH_FAILURE (24 bytes).
 0000 |33-00-00-00-12-70-75-62 6C-69-63-6B-65-79-2C-70| 3....publickey,p
 0010 |61-73-73-77-6F-72-64-00                        | assword.
2019-09-24 20:15:33.406 DEBUG Sftp(1)[12] SSH: Allowed authentication methods for 'root': publickey, password.
2019-09-24 20:15:33.408 DEBUG Sftp(1)[12] SSH: Trying password authentication for 'root'.
2019-09-24 20:15:33.409 VERBOSE Sftp(1)[12] SSH: Sending packet SSH_MSG_USERAUTH_REQUEST (48 bytes).
 0000 |32-00-00-00-04-72-6F-6F 74-00-00-00-0E-73-73-68| 2....root....ssh
 0010 |2D-63-6F-6E-6E-65-63-74 69-6F-6E-00-00-00-08-70| -connection....p
 0020 |61-73-73-77-6F-72-64-00 00-00-00-04-72-6F-6F-74| assword.....root
2019-09-24 20:15:33.523 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_USERAUTH_SUCCESS (1 bytes).
 0000 |34                                             | 4
2019-09-24 20:15:33.523 DEBUG Sftp(1)[12] SSH: Authentication successful.
2019-09-24 20:15:33.529 VERBOSE Sftp(1)[12] SSH: Sending packet SSH_MSG_CHANNEL_OPEN (24 bytes).
 0000 |5A-00-00-00-07-73-65-73 73-69-6F-6E-00-00-00-00| Z....session....
 0010 |00-02-00-00-00-01-F8-00                        | ........
2019-09-24 20:15:33.531 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_CHANNEL_OPEN_CONFIRMATION (17 bytes).
 0000 |5B-00-00-00-00-00-00-00 00-00-00-60-00-00-00-7F| [..........`....
 0010 |F7                                             | .
2019-09-24 20:15:33.538 DEBUG Sftp(1)[12] SSH: Requesting subsystem 'sftp'.
2019-09-24 20:15:33.540 VERBOSE Sftp(1)[12] SSH: Sending packet SSH_MSG_CHANNEL_REQUEST (27 bytes).
 0000 |62-00-00-00-00-00-00-00 09-73-75-62-73-79-73-74| b........subsyst
 0010 |65-6D-01-00-00-00-04-73 66-74-70               | em.....sftp
2019-09-24 20:15:33.542 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_CHANNEL_SUCCESS (5 bytes).
 0000 |63-00-00-00-00                                 | c....
2019-09-24 20:15:33.557 VERBOSE Sftp(1)[12] SSH: Sending packet SSH_MSG_CHANNEL_DATA (18 bytes).
 0000 |5E-00-00-00-00-00-00-00 09-00-00-00-05-01-00-00| ^...............
 0010 |00-03                                          | ..
2019-09-24 20:15:33.559 INFO Sftp(1)[12] Command: SSH_FXP_INIT (3)
2019-09-24 20:15:33.591 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_CHANNEL_EXTENDED_DATA (69 bytes).
 0000 |5F-00-00-00-00-00-00-00 01-00-00-00-38-73-68-3A| _...........8sh:
 0010 |20-2F-75-73-72-2F-6C-69 62-65-78-65-63-2F-73-66|  /usr/libexec/sf
 0020 |74-70-2D-73-65-72-76-65 72-3A-20-4E-6F-20-73-75| tp-server: No su
 0030 |63-68-20-66-69-6C-65-20 6F-72-20-64-69-72-65-63| ch file or direc
 0040 |74-6F-72-79-0A                                 | tory.
2019-09-24 20:15:33.634 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_CHANNEL_EOF (5 bytes).
 0000 |60-00-00-00-00                                 | `....
2019-09-24 20:15:33.634 VERBOSE Sftp(1)[14] SSH: Sending packet SSH_MSG_CHANNEL_EOF (5 bytes).
 0000 |60-00-00-00-00                                 | `....
2019-09-24 20:15:33.635 VERBOSE Sftp(1)[14] SSH: Sending packet SSH_MSG_CHANNEL_CLOSE (5 bytes).
 0000 |61-00-00-00-00                                 | a....
2019-09-24 20:15:33.635 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_CHANNEL_REQUEST (25 bytes).
 0000 |62-00-00-00-00-00-00-00 0B-65-78-69-74-2D-73-74| b........exit-st
 0010 |61-74-75-73-00-00-00-00 7F                     | atus.....
2019-09-24 20:15:33.636 VERBOSE Sftp(1)[14] SSH: Received packet SSH_MSG_CHANNEL_CLOSE (5 bytes).
 0000 |61-00-00-00-00                                 | a....
2019-09-24 20:15:33.731 ERROR Sftp(1)[12] Info: Rebex.Net.SftpException: The server has closed the connection.
   at ctcy.mpez(Int32 arz)
   at ctcy.mpfb(hffs& asc)
   at ctcy.mpdu(SftpServerType aoe, ctch aof)
   at Rebex.Net.Sftp.ypkg(ctch qu)
   at Rebex.Net.Sftp.ypke(String qo, String qp, SshPrivateKey qq, ctch qr)

Answer 1

It seems the server wasn't implemented properly. So we can close this ticket.