如何通过固定出站端口连接SFTP服务器？

我用C#编写了一个命令行程序，通过Renci.SSHNet库连接不同SFTP服务器下载文件。其中某台SFTP服务器的管理员要求，连接该服务器时不仅要对接指定端口，还必须从客户端的特定本地端口发起连接。我希望通过代码实现这一要求，不想求助网管修改路由规则。

我尝试用本地端口转发做测试，但发现只有SSH ACK报文通过指定的localPort，其余通信仍然使用随机空闲端口。我的测试代码如下：

string sshSrv = "127.0.0.1"; // Local SSH server
string sshUser = "user";
string sshPass = "pass";
int sshPort = 22;

string sftpSrv = "192.168.1.120"; // SFTP test server
string sftpUser = "tester";
string sftpPass = "password";
uint sftpPort = 2222;

string localSrv = "127.0.0.1";
int localPort = 51000;  // The outgoing traffic must go through this port.

ConnectionInfo sshCI = new ConnectionInfo(sshSrv, sshPort, sshUser, 
                        new PasswordAuthenticationMethod(sshUser, sshPass));

using (SshClient sshClient = new SshClient(sshCI))
{
    ForwardedPort fwdPortL = null;
    try
    {
        sshClient.KeepAliveInterval = new TimeSpan(0, 0, 20);
        sshClient.Connect();

        fwdPortL = new ForwardedPortLocal(localSrv, (uint)localPort, sftpSrv, sftpPort);
        fwdPortL.Exception += delegate (object sender, ExceptionEventArgs e)
        {
            Console.WriteLine("ForwardedPortLocal Exception: " + e.Exception.ToString());
        };
        fwdPortL.RequestReceived += delegate (object sender, PortForwardEventArgs e)
        {
            Console.WriteLine($"Request Received: {e.OriginatorHost}:{e.OriginatorPort}");
        };
        sshClient.AddForwardedPort(fwdPortL);
        fwdPortL.Start();
                    
        using (SftpClient sftpClient = new SftpClient(localSrv, localPort, sftpUser, sftpPass))
        {
            sftpClient.Connect();
            // Stuff to test SFTP connection...
            sftpClient.Disconnect();
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine("Error: " + ex.ToString());
    }
    finally
    {
        fwdPortL?.Stop();
        sshClient.Disconnect();
    }
}

内容的提问来源于stack exchange，提问作者Óscar Alberto Fernández Muñoz