TcpTunnel

TcpTunnel is a program implemented in C# (.NET 8.0) that allows to tunnel TCP connections through a server (gateway) to a remote machine, for example to access services that are running behind a firewall or NAT (like a reverse-connect mechanism).

A working configuration consists of three instances:

Gateway: Runs on a (server) machine that is accessible for both proxy endpoints (e.g. on a public server). It listens for incoming TCP connections from the proxy endpoints (proxy-client and proxy-server) and forwards data from one proxy endpoint to the corresponding partner proxy endpoint.
Proxy-Server: Connects to the Gateway and listens for incoming TCP connections on previously configured ports. When a connection arrives, it forwards it to the Gateway, which in turn forwards the connection to the Proxy-Client.
Proxy-Client: Connects to the Gateway and waits for forwarded connections received through the Gateway from the Proxy-Server. When receiving such a forwarded connection, it opens a TCP connection to the specified target endpoint and forwards the data to it.

For example, imagine you have some TCP services (like a VNC and a SSH server) running on a machine within a LAN that has internet access (maybe only through NAT so it's not possible to use port forwarding or a VPN), and you want to securely connect to this service from a machine on another network.
Additionally, you have a server (e.g. virtual private server, VPS) with a public domain and you have a SSL (TLS) certificate for it.

In this case, you could use the TcpTunnel with a configuration as shown in the following image:

That is:

Run the Gateway on the VPS and configure it to listen at a specific TCP port using SSL (TLS), and to allow a session with an ID and password.
Run the Proxy-Client on the machine that has access to the TCP services (VNC/SSH server), and configure it to connect to the host and port of the Gateway.
Run the Proxy-Server on your machine where you want to access the TCP services (with a VNC/SSH client), and configure it to connect to the host and port of the Gateway, and to listen on a specific TCP port (like 5920) that should get forwarded to the Proxy-Client to a specific target host and TCP port (like 192.168.40.80:5900).

Configuration

TcpTunnel is configured via an XML file with the name settings.xml in the application's directory. When building the application, sample setting files will get copied to the output directory which you can use as a template. You can also find them here for Gateway, Proxy-Server and Proxy-Client.

You can define multiple instances (e.g. a Gateway and a Proxy-Server instance) in the settings file, which will then be run by a single application process.

Features

Uses async I/O for high scalability.
Supports SSL (TLS) and password authentication for secure connections between the Gateway and the Proxies.
Multiplexes multiple (tunneled) TCP connections over a single connection, similar to the stream concept in HTTP/2.
Uses flow control for the tunneled TCP connections (using a initial window size of 384 KiB), similar to the flow control mechanism in HTTP/2.
Multiple Proxy-Server instances can connect to a session, so it's possible to connect to the target endpoints from different machines at the same time.
Automatically recovers after one of the instances (Gateway, Proxy-Server, Proxy-Client) was temporarily unavailable.
On Windows, it can be installed as service.

Security Considerations

The Gateway can define one or more password-protected sessions (which associate a Proxy-Client with one or more Proxy-Servers). In combination with enabling SSL (TLS) in the Gateway and the Proxies, this ensures the connections between the Proxies and the Gateway are secure, and only the intended Proxies can connect to each other.
You can specify different passwords for the Proxy-Client and the Proxy-Server(s) in the Gateway configuration. Additionally, it's possible to restrict the possible target endpoints (host and port) accepted by the Proxy-Client when receiving a forwarded connection from the Proxy-Server.
This allows the Proxy-Server and the Proxy-Client to operate at different trust levels, e.g. if you want to share the Proxy-Server to other people but want to allow them to only be able to connect to specific target endpoints.
There are currently no special DoS protection mechanisms implemented (to handle the case when you don't control the partner proxy or the gateway), e.g. to limit the number of received forwarded connections, but there are basic limitations implemented, like limiting the max. received message size and ensuring that the partner proxy doesn't send more data for a connection than allowed by the flow control window.

Building

Install the .NET 8.0 SDK or higher.
On Windows, you can use one of the PUBLISH-xyz.cmd files to publish the app, either as self-contained app (with native AOT compilation), or as framework-dependent app (so it needs the .NET Runtime to be installed).

Otherwise, you can publish for the current platform with the following command (as self-contained app):

dotnet publish "TcpTunnel/TcpTunnel.csproj" -f net8.0 -c Release -p:PublishSingleFile=true --self-contained

Possible Development TODOs