Reverse Tunneling Dialer

remotedialer creates a two-way connection between a server and a client, so that a net.Dial can be performed on the server and actually connects to the client running services accessible on localhost or behind a NAT or firewall.

Architecture

Abstractions

remotedialer consists of structs that organize and abstract a TCP connection between a server and a client. Both client and server use Sessions, which provides a means to make a network connection to a remote endpoint and keep track of incoming connections. A server uses the Server object which contains a sessionManager which governs one or more Sessions, while a client creates a Session directly. A connection implements the io.Reader and io.WriteCloser interfaces, so it can be read from and written to directly. The connection's internal readBuffer monitors the size of the data it is carrying and uses backPressure to pause incoming data transfer until the amount of data is below a threshold.

Data flow

A client establishes a session with a server using the server's URL. The server upgrades the connection to a websocket connection which it uses to create a Session. The client then also creates a Session with the websocket connection.

The client sits in front of some kind of HTTP server, most often a kubernetes API server, and acts as a reverse proxy for that HTTP resource. When a user requests a resource from this remote resource, request first goes to the remotedialer server. The application containing the server is responsible for routing the request to the right client connection.

The request is sent through the websocket connection to the client and read into the client's connection buffer. A pipe is created between the client and the HTTP service which continually copies data between each socket. The request is forwarded through the pipe to the remote service, draining the buffer. The service's response is copied back to the client's buffer, and then forwarded back to the server and copied into the server connection's own buffer. As the user reads the response, the server's buffer is drained.

The pause/resume mechanism checks the size of the buffer for both the client and server. If it is greater than the threshold, a PAUSE message is sent back to the remote connection, as a suggestion not to send any more data. As the buffer is drained, either by the pipe to the remote HTTP service or the user's connection, the size is checked again. When it is lower than the threshold, a RESUME message is sent, and the data transfer may continue.

remotedialer in the Rancher ecosystem

remotedialer is used to connect Rancher to the downstream clusters it manages, enabling a user agent to access the cluster through an endpoint on the Rancher server. remotedialer is used in three main ways:

Agent config and tunnel server

When the agent starts, it initially makes a client connection to the endpoint /v3/connect/register, which runs an authorizer that sets some initial data about the node. The agent continues to connect to the endpoint /v3/connect on a loop. On each connection, it runs an OnConnect handler which pulls down node configuration data from /v3/connect/config.

Steve Aggregation

The steve aggregation server on the agent establishes a remotedialer Session with Rancher, making the steve API on the downstream cluster accessible from the Rancher server and facilitating resource watches.

Health Check

The clusterconnected controller in Rancher uses the established tunnel to check that clusters are still responsive and sets alert conditions on the cluster object if they are not.

Running Locally

remotedialer provides an example client and server which can be run in standalone mode FOR TESTING ONLY. These are found in the server/ and client/ directories.`

Compile

Compile the server and client:

go build -o server/server server/main.go
go build -o client/client client/main.go

Run

Start the server first.

./server/server

The server has debug mode off by default. Enable it with --debug.

The client proxies requests from the remotedialer server to a web server, so it needs to be run somewhere where it can access the web server you want to target. The remotedialer server also needs to be reachable by the client.

For testing purposes, a basic HTTP file server is provided. Build the server with:

go build -o dummy/dummy dummy/main.go

Create a directory with files to serve, then run the web server from that directory:

mkdir www
cd www
echo 'hello' > bigfile
/path/to/dummy

Run the client with

./client/client

Both server and client can be run with even more verbose logging:

CATTLE_TUNNEL_DATA_DEBUG=true ./server/server --debug
CATTLE_TUNNEL_DATA_DEBUG=true ./client/client

Usage

If the remotedialer server is running on 192.168.0.42, and the web service that the client can access is running at address 127.0.0.1:8125, make proxied requests like this:

curl http://192.168.0.42:8123/client/foo/http/127.0.0.1:8125/bigfile

where foo is the hardcoded client ID for this test server.

This test server only supports GET requests.