Paramiko server responds to stat() with FAILURE rather than NO_SUCH_FILE

I have a Kotlin/Java application that interacts with several SFTP and FTP servers. SFTP is provided by SSH-2.0-SSHJ_0.34.0. One of these servers is based on: SSH-2.0-paramiko_1.16.0. This server offers only: diffie-hellman-group1-sha1,diffie-hellman-group14-sha1

Searching for an existing resolution, I found issue https://github.com/hierynomus/sshj/issues/670, which gave me a plausible reason for this problem… After some searching around, and failing to get the application’s logger to output SSHJ’s SLF4J output, I reached the point at which the original BufferException is thrown:

ensureAvailable:137, Buffer (net.schmizz.sshj.common)
readUInt32:302, Buffer (net.schmizz.sshj.common)
readUInt32AsInt:297, Buffer (net.schmizz.sshj.common)
readStatusCode:107, Response (net.schmizz.sshj.sftp)
ensurePacketTypeIs:117, Response (net.schmizz.sshj.sftp)
stat:283, SFTPEngine (net.schmizz.sshj.sftp)
stat:227, SFTPEngine (net.schmizz.sshj.sftp)
prepareFile:340, SFTPFileTransfer$Uploader (net.schmizz.sshj.sftp)
uploadFile:256, SFTPFileTransfer$Uploader (net.schmizz.sshj.sftp)
upload:209, SFTPFileTransfer$Uploader (net.schmizz.sshj.sftp)
access$100:192, SFTPFileTransfer$Uploader (net.schmizz.sshj.sftp)
upload:81, SFTPFileTransfer (net.schmizz.sshj.sftp)
upload:76, SFTPFileTransfer (net.schmizz.sshj.sftp)
put:263, SFTPClient (net.schmizz.sshj.sftp)

The value of the target is:

this = Buffer [rpos=21, wpos=24, size=24]
this.data = [101, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 7, 70, 97, 105, 108, 117, 114, 101, 0, 0, 0, 0]
this.type = PacketType.STATUS
this.protocolVersion = 3
this.available() = **3 (yes, not 4!)**

When targeting a test server (SSH-2.0-OpenSSH_9.0, ), everything works perfectly. When using the (debian) sftp CLI application, the upload succeeds. As a result, like @jmatias-assia and yourself, I suspect that there is a problem introduced by the use of diffie-hellman-group1-sha1.

The fault is persistent, and happens on every attempt to put().

Regrettably, the server is not open to the public, so I can’t give you access to it, but I can either do your bidding, or, alternatively, can clone the project and try to find a resolution myself.

Which would you prefer? Be advised that I have no background in cryptography.

[ghherlin]

I agree. But I cannot duplicate the “problematic” server, since it is a “homebrew” application made by the vendor of the services it renders (hence the use of the paramiko library).

So I’ll check-out the repo and fix it myself, then make a pull request. The SFTP part of it is (relatively) easy to read. I’ll keep this issue posted with my observations on the way.

Thanks for your help.

[ghherlin]

One night’s musings later, I think that we are facing a “trident” of issues:

1. how to deal best with the vagaries and errors in the plethora of servers faced by SSH4J
2. how to fix the error-reporting issue identified above
3. How to reduce the time wasted in the diagnostic of these issues.

I have created 2 new issues, in order that the wanderings at the beginning do not deter the reader from continuing.

Thanks to @vladimirlagunov for his aid in reaching this point.