SSHSession.KeepAlive method

Question

Hi

Since

A - its a method

B - the help says it "Sends an SSH_MSG_IGNORE packet to the server"

C - The release notes said : Added SshSession.KeepAlive method to make it possible to periodically "ping" the SSH connection to keep it working and detect failures

I need confirmation that I simply call the method after the connection is established, and that I have nothing else to do. That help page isn't for end users.

I looked up the protocol specification at http://www.ietf.org/rfc/rfc4251.txt, but all mentions of that message are security or attack related.

Thanks

Answer 1

Calling the method once after the connection is established is not useful - to prevent the connection from being closed by SSH-unfriendly routers along the way during no activity, you have to call it periodically (once every minute or two, for example).

For example, to add keep-alive functionality to Sftp object, the following class could be used instead of the Sftp itself:

/// <summary>
/// Enhanced Sftp class which implements the keep-alive functionality.
/// Use this instead of Sftp in your code.
/// </summary>
public class SftpKeepAlive : Sftp
{
    // Timer object (needs to be referenced to prevent it from being
    // claimed by the garbage collector).
    private readonly System.Threading.Timer _keepAliveTimer;

    public SftpKeepAlive()
    {
        // initialize the timer
        var oneMinute = new TimeSpan(0, 1, 0);
        _keepAliveTimer = new System.Threading.Timer(KeepAliveCallback, null, oneMinute, oneMinute);
    }

    private void KeepAliveCallback(object state)
    {
        try
        {
            if (State == SftpState.Ready)
            {
                // send keep-alive packet to the server
                Session.KeepAlive();
            }
        }
        catch (Exception ex)
        {
            // log the exception here
        }
    }
}

(RFC 4253, which defines the SSH_MSG_IGNORE message, states that it can be used as an additional protection measure against advanced traffic analysis techniques, but that's by no means the only usage. It has in fact been utilized as a simple keep-alive measure for years - check out http://www.sc.isc.tohoku.ac.jp/~hgot/sources/openssh-watchdog.html#others or http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/two-way-keepalives.html).