Key exchange negotiation failed

[okarpov]

Renci.SshNet.Common.SshConnectionException: Key exchange negotiation failed.
at Renci.SshNet.Session.WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout) in \SSH.NET-rsa-sha2-256\src\Renci.SshNet\Session.cs:line 977
at Renci.SshNet.Session.WaitOnHandle(WaitHandle waitHandle) in \SSH.NET-rsa-sha2-256\src\Renci.SshNet\Session.cs:line 874
at Renci.SshNet.Session.ConnectAsync(CancellationToken cancellationToken) in \SSH.NET-rsa-sha2-256\src\Renci.SshNet\Session.cs:line 739
at Renci.SshNet.BaseClient.CreateAndConnectSessionAsync(CancellationToken cancellationToken) in \SSH.NET-rsa-sha2-256\src\Renci.SshNet\BaseClient.cs:line 545
at Renci.SshNet.BaseClient.ConnectAsync(CancellationToken cancellationToken) in \SSH.NET-rsa-sha2-256\src\Renci.SshNet\BaseClient.cs:line 284
at

[scott-xu]

Is the server OpenSSH 6.5 or 6.6? @okarpov

[okarpov]

ssh -V
OpenSSH_8.2p1 Ubuntu-4ubuntu0.8, OpenSSL 1.1.1f 31 Mar 2020

[scott-xu]

Then the case is different with mine. I got the same error message when connect to OpenSSH 6.6 and I believe it is caused by the wrong Bignum encoding in OpenSSH 6.5/6.6. OpenSSH fixed the issue in 6.7 (and 6.6.1). See https://www.openssh.com/txt/release-6.7
BTW, I think ssh -V is to display SSH client version but not server version.

[scott-xu]

Then the case is different with mine. I got the same error message when connect to OpenSSH 6.6 and I believe it is caused by the wrong Bignum encoding in OpenSSH 6.5/6.6. OpenSSH fixed the issue in 6.7 (and 6.6.1). See https://www.openssh.com/txt/release-6.7 BTW, I think ssh -V is to display SSH client version but not server version.

Hi @WojciechNagorski, could we consider removing curve25519-sha256 and curve25519-sha256@libssh.org key exchange algorithms when speaking to OpenSSH 6.5 and 6.6? See openssh/openssh-portable@9395b28

Alternatively, we could expose some event during negotiation so that the caller can attach a handler to the event and filter the key exchange algorithems.

Here's the line where we could add the filter logic: https://github.com/sshnet/SSH.NET/blob/develop/src/Renci.SshNet/Session.cs#L608

I can create PR if that's okay.

[scott-xu]

FYI, this is the root fix in OpenSSH: openssh/openssh-portable@adbfdbb

[WojciechNagorski]

@scott-xu can you create a new issue for this. I didn't know about this problem.

[scott-xu]

#1191 is created.

[WojciechNagorski]

Version 2023.0.0 has been published https://www.nuget.org/packages/SSH.NET/2023.0.0