How to use a Forwarding Proxy with golang

21 Mar 2018 · Seven minute read · on Gianluca's blog

A forwarding proxy is a proxy configuration that handle requests from a set of internal clients that are trying to create a connection to the outside.

In practice is a man in the middle between your application and the server that you are trying to connect. It works over the HTTP(S) protocol and it is implemented at the edge of your infrastructure.

Usually, you can find it in large organizations or universities and it is used as additional control mechanism for authorization and security.

I find it useful when you work with containers or in a dynamic cloud environment because you will have a set of servers for all the outbound network communication.

If you work in a dynamic environment as AWS, Azure and so on you will end up having a variable number of servers and also a dynamic number of public IPs. Same if your application runs on a Kubernetes cluster. Your container can be everywhere.

Now let’s suppose that a customer asks you to provide a range of public IPs because he needs to set up a firewall… How can you provide this feature? In some environments can be very simple, in others very complicated.

1st December 2015 a users asked this question on the CircleCI forum this request is still open. This is just an example, CircleCi is great. I am not complaining about them.

One of the possible ways to fix this problem is via the forwarding proxy. You can spin up a set of nodes with a static ip and you can offer the list to the customer.

Almost all cloud providers have a way to do that, floating ip on DigitalOcean or Elastic IP on AWS.

You can configure your applications to forward the requests to that pool and the end services will get the ip from the forward proxy nodes and not from the internal one.

This can be another security layer for your infrastructure because you will be able to control and scan packages that are going outside from your network in a really simple way and in a centralized place.

This is not a single point of failure because you can spin up more than one forward proxies and they scale really well.

Under the hood, a forward proxy is the HTTP method CONNECT.

The CONNECT method converts the request connection to a transparent TCP/IP tunnel, usually to facilitate SSL-encrypted communication (HTTPS) through an unencrypted HTTP proxy.

A lot of HTTP Client across languages already support this in a very transparent way. I built a very small example using golang and privoxy to show you how simple it is.

First of all, let’s build an application called whoyare. It is an HTTP server that returns your remote address:

package main

import (

func main() {
	http.HandleFunc("/whoyare", func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "application/json")
		body, _ := json.Marshal(map[string]string{
			"addr": r.RemoteAddr,
	http.ListenAndServe(":8080", nil)

You can call the GET the route /whoyare and you will receive a JSON like {"addr": ""} where is your public address. Running whoyare from your laptop if you make a request on your terminal you should get localhost as remote address. You can use curl to try it:

18:36 $ curl -v http://localhost:8080/whoyare
> GET /whoyare HTTP/1.1
> User-Agent: curl/7.58.0
> Accept: */*
< HTTP/1.1 200 OK
< Content-Type: application/json
< Date: Sun, 18 Mar 2018 17:36:40 GMT
< Content-Length: 31
* Connection #0 to host localhost left intact

I wrote another application, it uses http.Client to print the response on stdout. If you have the server running you can run it:

package main

import (

type whoiam struct {
	Addr string

func main() {
	url := "http://localhost:8080"
	if "" != os.Getenv("URL") {
		url = os.Getenv("URL")
	log.Printf("Target %s.", url)
	resp, err := http.Get(url + "/whoyare")
	if err != nil {
	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
	println("You are " + string(body))

So, this is a very simple example, but you can apply this example to more complex environments.

To make this example a bit more clear I created two virtual machines on DigitalOcean, one will run privoxy the other one will run whoyare.

Privoxy is a very simple to setup forward proxy, nginx, haproxy doesn’t fit very well for this use case because they do not support the CONNECT method.

I built a docker image gianarb/privoxy, it’s on Docker Hub. You can run and by default, it runs on port 8118.

core@coreos-s-1vcpu-1gb-ams3-01 ~ $ docker run -it --rm -p 8118:8118
2018-03-18 17:28:05.589 7fbbf41dab88 Info: Privoxy version 3.0.26
2018-03-18 17:28:05.589 7fbbf41dab88 Info: Program name: privoxy
2018-03-18 17:28:05.591 7fbbf41dab88 Info: Loading filter file:
2018-03-18 17:28:05.599 7fbbf41dab88 Info: Loading filter file:
2018-03-18 17:28:05.599 7fbbf41dab88 Info: Loading actions file:
2018-03-18 17:28:05.600 7fbbf41dab88 Info: Loading actions file:
2018-03-18 17:28:05.607 7fbbf41dab88 Info: Loading actions file:
2018-03-18 17:28:05.611 7fbbf41dab88 Info: Listening on port 8118 on IP address

The second step is to build and scp whoyare in your server. You can build it using the command:

$ CGO_ENABLED=0 GOOS=linux go build -o bin/server_linux -a ./whoyare

Now that we have the application up and running we can try via cURL to query it directly and via privoxy.

Let’s try directly as we did previously:

$ curl -v http://your-ip:8080/whoyare

cURL uses an environment variable http_proxy to forward the requests through the proxy:

$ http_proxy= curl -v
*   Trying
* Connected to ( port 8118 (#0)
> GET HTTP/1.1
> Host:
> User-Agent: curl/7.58.0
> Accept: */*
> Proxy-Connection: Keep-Alive
< HTTP/1.1 200 OK
< Content-Type: application/json
< Date: Sun, 18 Mar 2018 17:37:02 GMT
< Content-Length: 29
< Proxy-Connection: keep-alive
* Connection #0 to host left intact

As you can see I have set http_proxy= and the response doesn’t contain my public ip but the proxy one.

These are the logs that you should expect from privoxy for the requests crossing it:

2018-03-18 17:28:22.886 7fbbf41d5ae8 Request:
2018-03-18 17:32:29.495 7fbbf41d5ae8 Request:

The client that you run it previously by default it connects to localhost:8080 but you can override the target URL via env var URL= Running the following command I reached directly whoyare.

$ URL= ./bin/client_linux
2018/03/18 18:37:59 Target
You are {"addr":""}

The golang HTTP.Client supports a set of environment variables to configure the proxy, it makes everything very flexible because passing these variables to any service already running it will just work.

export HTTP_PROXY=http://http_proxy:port/
export HTTPS_PROXY=http://https_proxy:port/
export NO_PROXY=, localhost

The first two are very simple, one is the proxy for the HTTP requests, the second for HTTPS. NO_PROXY excludes a set of hostname, the hostname listed there won’t cross the proxy. In my case localhost and

     +--------------+           +----------------+         +----------------+
     |              |           |                |         |                |
     |   client     +----------^+ forward proxy  +--------^+    whoyare     |
     |              |           |                |         |                |
     +--------------+           +----------------+         +----^-----------+
    +---------------+                                           |
    |               |                                           |
    |   client      +-------------------------------------------+
    |               |
   HTTP_PROXY not configured

The client with the environment variables configured will cross the forward proxy. Other client will reach it directly.

This granularity is very important. It’s very flexible because other than a “per-process” you can also select what request to forward and what to exclude.

2018/03/18 18:39:18 Target
You are {"addr":""}

As you can see we just reached whoyare via proxy and the addr in response is now ours but the proxy one.

The last command is a bit weird but it is just to show how the NO_PROXY works. We are calling the proxy excluding the whoyare URL, and as expected it doesn’t cross the proxy:

$ HTTP_PROXY= URL= NO_PROXY= ./bin/client_linux
2018/03/18 18:42:03 Target
You are {"addr":""}

Let’s read this article as a practical introduction to golang, forward proxy. You can subscribe to my rss feed or you can follow me on @twitter. Probably I will write about how to replace privoxy with golang and about how to setup and deploy this solution on Kubernetes. So let me know what to write first!

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