Running remote commands 

Remote command execution is a very useful feature provided by Sftp object's SSH core. To run a command at the server, use Sftp object's Session property:

// open an SSH session channel over a connected SFTP client
SshChannel channel = sftp.Session.OpenSession();

// execute the 'uname' command to get OS info
channel.RequestExec("uname -a");

// process the command response
var response = new StringBuilder();
var buffer = new byte[4096];
while (true)
{
    int n = channel.Receive(buffer, 0, buffer.Length);
    if (n == 0)
    {
        channel.Close();
        break;
    }

    response.Append(Encoding.Default.GetString(buffer, 0, n));
}
Console.WriteLine("OS info: {0}", response);

' open an SSH session channel over a connected SFTP client
Dim channel As SshChannel = sftp.Session.OpenSession()

' execute the 'uname' command to get OS info
channel.RequestExec("uname -a")

' process the command response
Dim response = New StringBuilder()
Dim buffer(0 To 4095) As Byte
While True
    Dim n = channel.Receive(buffer, 0, buffer.Length)
    If n = 0 Then
        channel.Close()
        Exit While
    End If

    response.Append(Encoding.Default.GetString(buffer, 0, n))
End While
Console.WriteLine("OS info: {0}", response)

The code above is quite complicated. Fortunately, Rebex SSH Shell library not only makes it much easier, but adds terminal emulation support as well.

SSH session sharing 

Session sharing is another useful feature of the underlying SSH protocol. It's possible to share a single SSH connection between several instances of Sftp or Scp objects. When using Rebex SFTP with Rebex SSH Shell library, you can even transfer files and run SSH shell sessions over a single connection.

To create a shared session, connect and authenticate an SshSession object. Then, instead of calling Connect and Login for each Sftp or Scp object, use Bind to attach it to the shared SshSession.

The same SshSession object can be used to execute commands at the server as well

// establish the shared SSH connection
var session = new Rebex.Net.SshSession();
session.Connect(hostname);
// check session.Fingerprint here

// authenticate
session.Authenticate(username, password);

// bind an SFTP client to the SSH session
Sftp sftp = new Sftp();
sftp.Bind(session);

// bind an SCP client to the SSH session
Scp scp = new Scp();
scp.Bind(session);

' establish the shared SSH session
Dim session = New Rebex.Net.SshSession()
session.Connect(hostname)
' check session.Fingerprint here

' authenticate
session.Authenticate(username, password)

' bind an SFTP client to the SSH session
Dim sftp = New Sftp()
sftp.Bind(session)

' bind an SCP client to the SSH session
Dim scp = New Scp()
scp.Bind(session)

Changing password 

Some SSH servers make it possible to change user's password programmatically. This feature is accessible through SshSession object's ChangePassword method.

Please note that password can only be changed before the authentication has begun.

// connect to an SSH server but don't log in
var session = new SshSession();
session.Connect(hostname, port);

// try changing the password
SshPasswordChangeResult result
    = session.ChangePassword(username, password, newPassword);

// check the result
switch (result)
{
    case SshPasswordChangeResult.Success:
        // password changed;
        // now you are logged on using the new password
        break;
    case SshPasswordChangeResult.ChangedButNotAuthenticated:
        // password changed;
        // you should authenticate explicitly
        session.Authenticate(username, newPassword);
        break;
    case SshPasswordChangeResult.Failure:
        // password not changed
        throw new Exception("Password change failed.");
}

' connect to an SSH server but don't log in
Dim session = New SshSession()
session.Connect(hostname, port)

' try changing the password
Dim result As SshPasswordChangeResult = session.ChangePassword(username, password, newPassword)

' check the result
Select Case result
    Case SshPasswordChangeResult.Success
        ' password changed;
        ' now you are logged on using the new password
        Exit Select
    Case SshPasswordChangeResult.ChangedButNotAuthenticated
        ' password changed;
        ' you should authenticate explicitly
        session.Authenticate(username, newPassword)
        Exit Select
    Case SshPasswordChangeResult.Failure
        ' password not changed
        Throw New Exception("Password change failed.")
End Select

Keep-alive packet (pinging SSH server) 

Some firewall/router devices have a habit of dropping connections that have been inactive for a certain amount of time (such as 30 minutes). To prevent this, the client can send a special ignore packet periodically to keep the connection alive.

To send such packet, use SshSession.KeepAlive method. For a sample code to send the packet periodically, check out this forum post.

Tunneling TCP protocols through SSH 

SSH can be used as a transport layer for various TCP communication protocols such as FTP, HTTP, SCP, SMTP, IMAP, POP3 or Telnet. Most Rebex communication libraries support tunneling through SSH. In other words, it's possible to use an SSH server as a proxy server.

// establish the shared SSH connection
var session = new SshSession();
session.Connect(hostname);
session.Authenticate(username, password);

// use SSH session as proxy to establish FTP connection
var ftp = new Ftp();
ftp.SetSocketFactory(session.ToSocketFactory());
ftp.Connect(sftpHostname);
ftp.Login(sftpUsername, sftpPassword);
// use Ftp object to upload or download files, etc.

// use SSH session as proxy to establish IMAP connection
var imap = new Imap();
imap.SetSocketFactory(session.ToSocketFactory());
imap.Connect(imapHostname);
imap.Login(imapUsername, imapPassword);
// use Imap object to download mail messages, etc.

// use it with any Rebex library in the same way
// (Ssh, Sftp, Ftp, Scp, Smtp, Imap, Pop3, Telnet, WebClient objects)

' establish the shared SSH connection
Dim session = New Rebex.Net.SshSession()
session.Connect(hostname)
session.Authenticate(username, password)

' use SSH session as proxy to establish FTP connection
Dim ftp = New Ftp()
ftp.SetSocketFactory(session.ToSocketFactory())
ftp.Connect(sftpHostname)
ftp.Login(sftpUsername, sftpPassword)
' use Ftp object to upload or download files ...

' use SSH session as proxy to establish IMAP connection
Dim imap = New Imap()
imap.SetSocketFactory(session.ToSocketFactory())
imap.Connect(imapHostname)
imap.Login(imapUsername, imapPassword)
' use Imap object to download mails ...

' use it with any Rebex library in the same way
' (Ssh, Sftp, Ftp, Scp, Smtp, Imap, Pop3, Telnet objects)

Key re-exchange 

SSH recommends changing encryption/decryption keys after each gigabyte of transmitted data or after each hour of connection time (whichever comes sooner). To achieve this, the client or server can initiate a key re-exchange process. This involves a public key operation that requires a fair amount of processing power; it is not initiated by Rebex SFTP automatically. To start a key re-exchange, simply call SshSession.Negotiate method.

Usually, however, it's the server that initiates the re-exchange. Rebex SFTP handles it transparently and there is no need to explicitly enable this feature.

SSH ciphers 

Rebex SFTP's SSH module supports a number of security algorithms:

• Encryption algorithms (AES, AES/GCM, ChaCha20/Poly1305, Triple DES, Twofish, Blowfish, RC4).
• Encryption modes (CBC, CTR, AEAD).
• Host key algorithms (RSA, DSS, ECDSA, EdDSA).
• Key exchange algorithms (Diffie-Hellman or Elliptic Curve Diffie-Hellman).
• MAC algorithms (SHA-2, SHA-1, MD5).

Use Sftp.Settings.SshParameters property to specify all kinds of SSH ciphers:

Key exchange ciphers

Use SshParameters.KeyExchangeAlgorithms property to enable/disable whole categories of key exchange ciphers. If you need more control over key exchange ciphers, use SshParameters.SetKeyExchangeAlgorithms(...) method to specify supported ciphers in order of preference. The following table lists supported key exchange ciphers:

Host key algorithms

Use SshParameters.HostKeyAlgorithms property to enable/disable whole categories of host key algorithms. If you need more control over host key algorithms, use SshParameters.SetHostKeyAlgorithms(...) method to specify supported algorithms in order of preference. The following table lists supported host key algorithms:

Encryption ciphers

Use SshParameters.EncryptionAlgorithms and SshParameters.EncryptionModes properties to enable/disable whole categories of encryption ciphers. If you need more control over encryption ciphers, use SshParameters.SetEncryptionAlgorithms(...) method to specify supported ciphers in order of preference. The following table lists supported encryption ciphers:

MAC ciphers

Use SshParameters.MacAlgorithms property to enable/disable whole categories of message authentication code (MAC) ciphers. If you need more control over MAC ciphers, use SshParameters.SetMacAlgorithms(...) method to specify supported ciphers in order of preference. The following table lists supported MAC ciphers:

* Might be very slow on legacy Windows CE platforms

** Windows 7 SP1 and higher, or Windows Embedded Compact 2013

Server ciphers information 

To retrieve lists of SSH ciphers supported by your SFTP/SSH server, connect to it and inspect the Sftp.Session.ServerInfo property:

var sftp = new Sftp();
sftp.Connect("test.rebex.net");

SshServerInfo serverInfo = sftp.Session.ServerInfo;

Console.WriteLine("Key exchange algorithms: " + string.Join(", ", serverInfo.KeyExchangeAlgorithms));
Console.WriteLine("Host key algorithms: " + string.Join(", ", serverInfo.ServerHostKeyAlgorithms));

Console.WriteLine("MAC algorithms (server -> client): " + string.Join(", ", serverInfo.MacAlgorithmsServerToClient));
Console.WriteLine("MAC algorithms (client -> server): " + string.Join(", ", serverInfo.MacAlgorithmsClientToServer));

Console.WriteLine("Encryption algorithms (server -> client): " + string.Join(", ", serverInfo.EncryptionAlgorithmsServerToClient));
Console.WriteLine("Encryption algorithms (client -> server): " + string.Join(", ", serverInfo.EncryptionAlgorithmsClientToServer));

Console.WriteLine("Compression algorithms (server -> client): " + string.Join(", ", serverInfo.CompressionAlgorithmsServerToClient));
Console.WriteLine("Compression algorithms (client -> server): " + string.Join(", ", serverInfo.CompressionAlgorithmsClientToServer));

Dim sftp As New Sftp()
sftp.Connect("test.rebex.net")

Dim serverInfo As SshServerInfo = sftp.Session.ServerInfo

Console.WriteLine("Key exchange algorithms: " + String.Join(", ", serverInfo.KeyExchangeAlgorithms))
Console.WriteLine("Host key algorithms: " + String.Join(", ", serverInfo.ServerHostKeyAlgorithms))

Console.WriteLine("MAC algorithms (server -> client): " + String.Join(", ", serverInfo.MacAlgorithmsServerToClient))
Console.WriteLine("MAC algorithms (client -> server): " + String.Join(", ", serverInfo.MacAlgorithmsClientToServer))

Console.WriteLine("Encryption algorithms (server -> client): " + String.Join(", ", serverInfo.EncryptionAlgorithmsServerToClient))
Console.WriteLine("Encryption algorithms (client -> server): " + String.Join(", ", serverInfo.EncryptionAlgorithmsClientToServer))

Console.WriteLine("Compression algorithms (server -> client): " + String.Join(", ", serverInfo.CompressionAlgorithmsServerToClient))
Console.WriteLine("Compression algorithms (client -> server): " + String.Join(", ", serverInfo.CompressionAlgorithmsClientToServer))