Override http.Client host to ip address or separate host

override-host.go

// Override http.Client host to ip address or separate host
// Basically a programic equivalent of `/etc/hosts` in go for http clients
package main

import (
	"context"
	"fmt"
	"io/ioutil"
	"net"
	"net/http"
	"time"
)

// hostOverrides is a map of hosts or ips to replace override with the value given.
// The value of each key can be an ip address or separate hostname to lookup.
var hostOverrides = map[string]string{
	"google.com": "192.168.0.1",
}

func main() {

	myClient := &http.Client{
		Transport: NewOverrideHostTransport(hostOverrides),
	}

	resp, err := myClient.Get("https://google.com")
	if err != nil {
		panic("error making req: " + err.Error())
	}
	defer resp.Body.Close()

	data, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		panic("error reading resp: " + err.Error())
	}

	fmt.Println("Response")
	fmt.Println(string(data))
}

// NewOverrideHostTransport creates a new transport which overrides hostnames.
func NewOverrideHostTransport(overrides map[string]string) http.RoundTripper {

	// Start with default transport since it has many default proxy and timeout options you want.
	myTransport := http.DefaultTransport.(*http.Transport).Clone()

	myDialer := &net.Dialer{
		Timeout:   30 * time.Second,
		KeepAlive: 30 * time.Second,
		DualStack: true,
	}

	myTransport.DialContext = func(ctx context.Context, network, address string) (net.Conn, error) {
		fmt.Println("Resolving address:", network, address)
		// address contains `host:port` ie `google.com:443`
		host, port, err := net.SplitHostPort(address)
		if err != nil {
			return nil, err
		}

		// Lookup to see if we have a host override
		newHost, ok := overrides[host]
		if !ok {
			fmt.Println("No override found for host, using original", host)
			return myDialer.DialContext(ctx, network, address)
		}

		overrideAddress := net.JoinHostPort(newHost, port)
		fmt.Println("Overriding address:", address, overrideAddress)

		return myDialer.DialContext(ctx, network, overrideAddress)
	}

	return myTransport
}