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OPENSSH 7.5 Issue: Error occured while receiving SSH packet: Rebex.Net.SshException:

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by (120 points)

Applies to: Rebex SFTP

1 Answer

0 votes
by (150k points)
edited by 

var client = new Sftp();
client.Settings.SshParameters.KeyExchangeAlgorithms &= ~SshKeyExchangeAlgorithm.Curve25519;
client.Connect(...);

by (120 points)
Hi Lukas,
Thanks for the reply. I tried the change to the KeyExchangeAlgorithm, and removed all other configurations regarding preferences and trying to hit the magic path to get a connection. Now the log file file reads

2020-05-05 18:59:24.032 INFO FileLogWriter(1)[5] Info: Assembly: Rebex.Common 2020 R1.1 for .NET 4.0-4.8
2020-05-05 18:59:24.032 INFO FileLogWriter(1)[5] Info: Platform: Windows 6.1.7601 64-bit; CLR: 4.0.30319.36543
2020-05-05 18:59:24.032 DEBUG FileLogWriter(1)[5] Info: Culture: da; Windows-1252
2020-05-05 18:59:24.079 INFO Sftp(1)[5] Info: Connecting to 172.26.x.x:22 using Sftp.
2020-05-05 18:59:24.079 INFO Sftp(1)[5] Info: Assembly: Rebex.Sftp 2020 R1.1 for .NET 4.0-4.8
2020-05-05 18:59:24.079 INFO Sftp(1)[5] Info: Platform: Windows 6.1.7601 64-bit; CLR: 4.0.30319.36543
2020-05-05 18:59:24.079 DEBUG Sftp(1)[5] Info: Culture: da; Windows-1252
2020-05-05 18:59:24.110 DEBUG Sftp(1)[5] Proxy: Connecting to 172.26.x.x:22 (no proxy).
2020-05-05 18:59:24.126 DEBUG Sftp(1)[5] Proxy: Connection established.
2020-05-05 18:59:24.157 DEBUG Sftp(1)[5] SSH: Server is 'SSH-2.0-OpenSSH_7.5'.
2020-05-05 18:59:24.188 INFO Sftp(1)[5] SSH: Negotiation started.
2020-05-05 18:59:24.563 DEBUG Sftp(1)[5] SSH: Group exchange.
2020-05-05 18:59:24.578 DEBUG Sftp(1)[5] SSH: Negotiating key.
2020-05-05 18:59:25.031 DEBUG Sftp(1)[5] SSH: Received 4096-bit Diffie-Hellman prime (minimum allowed size is 1024 bits).
2020-05-05 18:59:25.077 ERROR Sftp(1)[7] SSH: Error occured while receiving SSH packet: Rebex.Net.SshException: The connection was closed by the server. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at Rebex.Net.SshSession.qykt(Int32 auu, Int32 auv)
   --- End of inner exception stack trace ---
   at Rebex.Net.SshSession.qykt(Int32 auu, Int32 auv)
   at Rebex.Net.SshSession.qykx(Byte[]& avc)
   at Rebex.Net.SshSession.qylf()
   at Rebex.Net.SshSession.gycv.qyme()
2020-05-05 18:59:25.077 ERROR Sftp(1)[5] SSH: Negotiation failed. The connection was closed by the server.
2020-05-05 18:59:25.077 ERROR Sftp(1)[5] Info: Rebex.Net.SshException: The connection was closed by the server. ---> Rebex.Net.SshException: The connection was closed by the server. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at Rebex.Net.SshSession.qykt(Int32 auu, Int32 auv)

The server can only be connected to from a limited number of IP adresses, but I can ask if it would be possible to open up for your IP in their firewall (I have my doubts).

Other suggestions?
Br
Kim
by (150k points)
Oops, this looks like my guess about this being related to Curve25519 key exchange might be wrong. Could you please try setting the KeyExchangeAlgorithms property to this instead?

client.Settings.SshParameters.KeyExchangeAlgorithms =
    SshKeyExchangeAlgorithm.ECDiffieHellmanNistP256 |
    SshKeyExchangeAlgorithm.ECDiffieHellmanNistP384 |
    SshKeyExchangeAlgorithm.ECDiffieHellmanNistP521;
by (120 points)
Hi Lukas,
No change in outcome setting the KeyExchangeAlgorithms to the above three.

2020-05-05 19:55:56.890 INFO Sftp(1)[5] SSH: Negotiation started.
2020-05-05 19:55:57.265 DEBUG Sftp(1)[5] SSH: Negotiating key.
2020-05-05 19:55:57.592 ERROR Sftp(1)[7] SSH: Error occured while receiving SSH packet: Rebex.Net.SshException: The connection was closed by the server. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at Rebex.Net.SshSession.qykt(Int32 auu, Int32 auv)
   --- End of inner exception stack trace ---
   at Rebex.Net.SshSession.qykt(Int32 auu, Int32 auv)
   at Rebex.Net.SshSession.qykx(Byte[]& avc)
   at Rebex.Net.SshSession.qylf()
   at Rebex.Net.SshSession.gycv.qyme()
2020-05-05 19:55:57.592 ERROR Sftp(1)[5] SSH: Negotiation failed. The connection was closed by the server.
2020-05-05 19:55:57.608 ERROR Sftp(1)[5] Info: Rebex.Net.SshException: The connection was closed by the server. ---> Rebex.Net.SshException: The connection was closed by the server. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
 
Br
Kim
by (120 points)
I tried to connect with putty sftp - psftp and it too connects without further configuration.

We claim version: SSH-2.0-PuTTY_Release_0.73
Remote version: SSH-2.0-OpenSSH_7.5
Using SSH protocol version 2
No GSSAPI security context available
Doing ECDH key exchange with curve Curve25519 and hash SHA-256 (unaccelerated)
Server also has ecdsa-sha2-nistp256/ssh-rsa host keys, but we don't know any of them
Host key fingerprint is:
ssh-ed25519 255 c3:8e:8f:1b:40:e3:dd:a9:9d:7b:27:63:1c:41:85:04
Initialised AES-256 SDCTR (AES-NI accelerated) outbound encryption
Initialised HMAC-SHA-256 (unaccelerated) outbound MAC algorithm
Initialised AES-256 SDCTR (AES-NI accelerated) inbound encryption
Initialised HMAC-SHA-256 (unaccelerated) inbound MAC algorithm
Using username "xxx".
Attempting keyboard-interactive authentication
Server refused keyboard-interactive authentication

With psftp it is possible to make a logfile with outgoing and incoming packets. Could this help in figuring out what is going on?

Br
Kim
by (150k points)
Thanks for giving this a try! A PuTTY log with incoming/outgoing packets would be very helpful, especially if you could also provide a Rebex log (with packets). Comparing the two logs should reveal the differences that's triggering the disconnection. To create a Rebex log, use Sftp's LogWriter property (see https://www.rebex.net/kb/logging/ for details, but use LogLevel.Verbose instead of LogLevel.Debug). Thanks!

The content of packets exchanged before authentication should not contain any sensitive data (except IP addresses and such - feel free to remove these), but if you prefer, mail it to support@rebex.net instead of posting the logs here.
by (120 points)
Hi Lukas,
Sent you the logs per email.
Br
Kim
by (150k points)
Thanks for all the logs!

Based on what we have learned so far, it looks like there is something wrong with this particular server’s RSA key.
A PuTTY log with RSA as preferred host key algorithm would make it possible to determine whether this is really the case

To work around the problem, add and register Ed25519 plugin in your application (see https://www.rebex.net/kb/elliptic-curve-plugins/ for details) and force Ed25519 host key algorithm:
    sftp.Settings.SshParameters.HostKeyAlgorithms = SshHostKeyAlgorithm.ED25519;
...