[SSHD-998] respect SftpVersionSelector when establishing a new connection

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.4.0
• Fix Version/s: 2.5.0
• Labels: features patch performance
• Flags: Patch, Important
• Assignee: Lyor Goldstein
• Reporter: Holger Rehn
• Created: 17/May/20 19:09
• Updated: 19/May/20 15:54
• Resolved: 19/May/20 15:54

Description

Even if using an SftpVersionSelector (e.g. created by SftpVersionSelector.fixedVersionSelector), SSHD sends the maximum protocol version that is supported (namely 6) in the initial SSH_FXP_INIT datagram. This behaviour is hard-coded and may cause problems if the server doesn't correctly handle the request/support protocol version 6. One of our customers is using a server that initially accepts protocol version 6, but then fails every single request by sending invalid data. Actually, the server only correctly supports SFTP 3, so this clearly is a bug in the server implementation - but could be circumvented quite easily by starting with protocol version 3. Furthermore an additional (and actually unnecessary) version renegotiation may be required later.

If using an SftpVersionSelector that prefers a certain protocol version other than 6 (custom or created by SftpVersionSelector.preferredVersionSelector), an additional (and actually unnecessary) network round trip is required for protocol version renegotiation. 

 

Please find attached a minimally invasive patch against the official 2.4.0 source to respect the version(s) accepted by an SftpVersionSelector for choosing the initial protocol version.  

Issue Links

• links to: https://github.com/apache/mina-sshd/pull/133