How to implement a network proxy server in Go for routing and filtering traffic?

To implement a network proxy server in Go for routing and filtering traffic, you can follow these steps:

Set up a basic HTTP server: Create a Go program that will act as an HTTP server. Use the net/http package to handle incoming requests.
Listen and handle requests: Use the ListenAndServe function to listen for incoming requests and handle them using the http package's Handler interface. Create a custom handler function that will receive all incoming requests.
Parse the incoming request: Inside the custom handler function, parse the incoming request to extract important information like the request URL, headers, and body. Modify the request as needed to add any necessary routing or filtering logic.
Set up an outbound request: Create an outbound HTTP request based on the modified incoming request. Use the http.NewRequest function to create a new HTTP request object.
Modify outbound request: Modify the outbound request as needed, such as adding or modifying headers, or filtering out certain methods or URLs. Perform any routing logic based on the request.
Send outbound request: Use the http.DefaultClient.Do function to send the modified outbound request to the desired destination server. Capture the response returned by the server.
Modify and send response: Modify the response received from the destination server as needed, such as modifying headers or filtering out certain content. Write the response to the client using the http.ResponseWriter interface.
Run the proxy server: Start the proxy server by calling ListenAndServe with your custom handler.

Here is a basic example to get started:

package main

import (
	"fmt"
	"log"
	"net/http"
)

func proxyHandler(w http.ResponseWriter, r *http.Request) {
	fmt.Println("Proxying request:", r.URL.Path)
	
	// Modify incoming request or perform routing logic
	
	// Create outbound request
	outReq, err := http.NewRequest(r.Method, "https://destination-server.com"+r.URL.Path, r.Body)
	if err != nil {
		log.Println("Error creating outbound request:", err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	outReq.Header = r.Header
	
	// Send outbound request
	client := &http.Client{}
	resp, err := client.Do(outReq)
	if err != nil {
		log.Println("Error sending outbound request:", err)
		http.Error(w, "Bad Gateway", http.StatusBadGateway)
		return
	}
	defer resp.Body.Close()
	
	// Modify and send response
	for key, value := range resp.Header {
		w.Header().Set(key, value[0])
	}
	w.WriteHeader(resp.StatusCode)
	
	// Copy response body
	_, err = io.Copy(w, resp.Body)
	if err != nil {
		log.Println("Error copying response body:", err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
}

func main() {
	http.HandleFunc("/", proxyHandler)
	err := http.ListenAndServe(":8080", nil)
	if err != nil {
		log.Fatal("Error starting proxy server:", err)
	}
}

This is a basic example to get you started with a network proxy server in Go. You can modify and expand upon it as per your specific requirements, such as adding more routing and filtering logic, handling different types of requests, or implementing additional functionalities.