HTTP Request with Custom DNS Resolver & Transport

main.go

package main

import (
	"context"
	"crypto/tls"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"net/url"
	"os"
	"time"
)

func main() {
	customDialContext := CustomDialContext("1.1.1.1:53") // Enter your DNS IP here with port, default 53
	customTransport := SetProxyTransport(customDialContext)
	client := &http.Client{Transport: customTransport}
	URL := "http://aseemshrey.in"

	resp, err := client.Get(URL)
	if err != nil {
		log.Fatalln(err)
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		log.Fatalln(err)
	}
	log.Println(string(body))
}

func SetProxyTransport(dialContext func(ctx context.Context, network string, addr string) (net.Conn, error)) *http.Transport {
	// create proxy transport
	proxyURL := os.Getenv("PROXY_URL")
	// Allow insecure, expired certificates
	proxyTransport := &http.Transport{TLSClientConfig: &tls.Config{InsecureSkipVerify: true}, DialContext: dialContext}

	if proxyURL != "" {
		proxy, err := url.Parse(proxyURL)
		if err != nil {
			log.Printf("Error in parsing PROXY_URL from env vars, going with no proxy : %s", err.Error())
		}
		proxyTransport.Proxy = http.ProxyURL(proxy)
		log.Printf("Proxy set in http.transport : %s", proxyURL)
	} else {
		log.Printf("PROXY_URL empty. Hence, no proxy set in http.transport.")
	}

	return proxyTransport
}

func CustomDialContext(dns string) func(ctx context.Context, network string, addr string) (net.Conn, error) {
	var (
		dnsResolverIP        = dns   // Google DNS resolver.
		dnsResolverProto     = "udp" // Protocol to use for the DNS resolver
		dnsResolverTimeoutMs = 5000  // Timeout (ms) for the DNS resolver (optional)
	)

	dialer := &net.Dialer{
		Resolver: &net.Resolver{
			PreferGo: true,
			Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
				d := net.Dialer{
					Timeout: time.Duration(dnsResolverTimeoutMs) * time.Millisecond,
				}
				return d.DialContext(ctx, dnsResolverProto, dnsResolverIP)
			},
		},
	}

	dialContext := func(ctx context.Context, network, addr string) (net.Conn, error) {
		return dialer.DialContext(ctx, network, addr)
	}

	return dialContext
}