以下是使用AWS SSM和.NET编写的端口转发本地RDP的解决方法：

首先，确保你已经安装了AWS Tools for PowerShell或AWS SDK for .NET。

1. 创建一个名为"PortForwarding"的Lambda函数，并将函数的运行时设置为.NET Core 3.1。

2. 在Lambda函数中添加以下代码：

using System;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;
using Amazon.Lambda.Core;
using Amazon.SimpleSystemsManagement;
using Microsoft.Extensions.Configuration;
using Renci.SshNet;

[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    private static IConfigurationRoot configuration;

    static Function()
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Environment.GetEnvironmentVariable("LAMBDA_TASK_ROOT"))
            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
            .AddEnvironmentVariables();

        configuration = builder.Build();
    }

    public async Task FunctionHandler(ILambdaContext context)
    {
        using var client = new AmazonSimpleSystemsManagementClient();

        var request = new Amazon.SimpleSystemsManagement.Model.GetParametersRequest
        {
            Names = new[] { configuration["SsmParameterName"] },
            WithDecryption = true
        };

        var response = await client.GetParametersAsync(request);

        var ssmParameter = response.Parameters.FirstOrDefault(p => p.Name == configuration["SsmParameterName"]);
        var rdpConnectionString = $"localhost:{configuration["RdpPort"]}";

        if (ssmParameter != null)
        {
            rdpConnectionString = ssmParameter.Value;
        }

        var localPort = int.Parse(configuration["LocalPort"]);

        using var listener = new TcpListener(IPAddress.Loopback, localPort);
        listener.Start();

        LambdaLogger.Log($"Listening on port {localPort}");

        while (true)
        {
            using var clientSocket = await listener.AcceptTcpClientAsync();
            LambdaLogger.Log("Client connected");

            var sshClient = new SshClient(rdpConnectionString, configuration["SshUsername"], configuration["SshPrivateKey"]);
            sshClient.Connect();

            var forwardedPort = new ForwardedPortLocal("localhost", (uint)localPort, "localhost", (uint)int.Parse(configuration["RdpPort"]));
            sshClient.AddForwardedPort(forwardedPort);

            forwardedPort.Start();

            await Task.Delay(-1);
        }
    }
}

3. 创建一个名为"appsettings.json"的配置文件，并添加以下内容：

{
    "SsmParameterName": "",
    "RdpPort": "<远程RDP端口>",
    "LocalPort": "<本地端口>",
    "SshUsername": "",
    "SshPrivateKey": ""
}

请将上述内容中的、<远程RDP端口>、<本地端口>、和替换为实际值。

4. 打包Lambda函数并将其上传到AWS Lambda。

5. 在AWS Lambda控制台中，配置适当的触发器来触发Lambda函数。

6. 现在，当有TCP连接到达本地端口时，Lambda函数将会将连接转发到远程RDP端口。

请确保你已经配置了正确的SSM参数、RDP端口、本地端口、SSH用户名和SSH私钥。此外，你还需要确保你的服务器实例具有正确的权限来访问SSM参数和RDP端口。