GoProxy

GoProxy is a library to create a customized HTTP/HTTPS proxy server using Go (aka Golang), with several configurable settings available. The target of this project is to offer an optimized proxy server, usable with reasonable amount of traffic, yet customizable and programmable.

The proxy itself is simply a net/http handler, so you can add multiple middlewares (panic recover, logging, compression, etc.) over it. It can be easily integrated with any other HTTP network library.

In order to use goproxy, one should set their browser (or any other client) to use goproxy as an HTTP proxy. Here is how you do that in Chrome and in Firefox. If you decide to start with the base example, the URL you should use as proxy is localhost:8080, which is the default one in our example. You also have to trust the proxy CA certificate, to avoid any certificate issue in the clients.

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Features

Perform certain actions only on specific hosts, with a single equality comparison or with regex evaluation
Manipulate requests and responses before sending them to the browser
Use a custom http.Transport to perform requests to the target server
You can specify a MITM certificates cache, to reuse them later for other requests to the same host, thus saving CPU. Not enabled by default, but you should use it in production!
Redirect normal HTTP traffic to a custom handler, when the target is a relative path (e.g. /ping)
You can choose the logger to use, by implementing the Logger interface
You can disable the HTTP request headers canonicalization, by setting PreventCanonicalization to true

Proxy modes

Regular HTTP proxy
HTTPS through CONNECT
HTTPS MITM ("Man in the Middle") proxy server, in which the server generate TLS certificates to parse request/response data and perform actions on them
"Hijacked" proxy connection, where the configured handler can access the raw net.Conn data

Maintainers

Elazar Leibovich: Creator of the project, Software Engineer
Erik Pellizzon: Maintainer, Freelancer (open to collaborations!)

If you need to integrate GoProxy into your project, or you need some custom features to maintain in your fork, you can contact Erik (the current maintainer) by email, and you can discuss together how he can help you as a paid independent consultant.

Contributions

If you have any trouble, suggestion, or if you find a bug, feel free to reach out by opening a GitHub issue. This is an open source project managed by volunteers, and we're happy to discuss anything that can improve it.

Make sure to explain everything, including the reason behind the issue and what you want to change, to make the problem easier to understand. You can also directly open a Pull Request, if it's a small code change, but you need to explain in the description everything. If you open a pull request named refactoring with 5,000 lines changed, we won't merge it... :D

The code for this project is released under the BSD 3-Clause license, making it useful for commercial uses as well.

Submit your case study

So, you have introduced & integrated GoProxy into one of your personal projects or a project inside the company you work for.

We're happy to learn about new creative solutions made with this library, so feel free to contact the maintainer listed above via e-mail, to explaining why you found this project useful for your needs.

If you have signed a Non Disclosure Agreement with the company, you can propose them to write a blog post on their official website about this topic, so this information will be public by their choice, and you can share the link of the blog post with us :)

The purpose of case studies is to share with the community why all the contributors to this project are improving the world with their help and what people are building using it.

Linter

The codebase uses an automatic lint check over your Pull Request code. Before opening it, you should check if your changes respect it, by running the linter in your local machine, so you won't have any surprise.

To install the linter:

go install github.com/golangci/golangci-lint/cmd/golangci-lint@latest

This will create an executable in your $GOPATH/bin folder ($GOPATH is an environment variable, usually its value is equivalent to ~/go, check its value in your machine if you aren't sure about it). Make sure to include the bin folder in the path of your shell, to be able to directly use the golangci-lint run command.

A taste of GoProxy

To get a taste of goproxy, here you are a basic HTTP/HTTPS proxy that just forward data to the destination:

package main

import (
    "log"
    "net/http"

    "github.com/elazarl/goproxy"
)

func main() {
    proxy := goproxy.NewProxyHttpServer()
    proxy.Verbose = true
    log.Fatal(http.ListenAndServe(":8080", proxy))
}

Request handler

This line will add X-GoProxy: yxorPoG-X header to all requests sent through the proxy, before sending them to the destination:

proxy.OnRequest().DoFunc(
    func(r *http.Request,ctx *goproxy.ProxyCtx)(*http.Request,*http.Response) {
        r.Header.Set("X-GoProxy","yxorPoG-X")
        return r,nil
    })

When the OnRequest() input is empty, the function specified in DoFunc will process all incoming requests to the proxy. In this case, it will add a header to the request and return it to the caller. The proxy will send the modified request to the destination. You can also use Do instead of DoFunc, if you implement the specified interface in your type.

⚠️ Note we returned a nil value as the response. If the returned response is not nil, goproxy will discard the request and send the specified response to the client.

Conditional Request handler

Refuse connections to www.reddit.com between 8 and 17 in the server local timezone:

proxy.OnRequest(goproxy.DstHostIs("www.reddit.com")).DoFunc(
    func(req *http.Request,ctx *goproxy.ProxyCtx)(*http.Request,*http.Response) {
        if h,_,_ := time.Now().Clock(); h >= 8 && h <= 17 {
			resp := goproxy.NewResponse(req, goproxy.ContentTypeText, http.StatusForbidden, "Don't waste your time!")
            return req, resp
        }
        return req, nil
})

DstHostIs returns a ReqCondition, which is a function receiving a *http.Request and returning a boolean that checks if the request satisfies the condition (and that will be processed). DstHostIs("www.reddit.com") will return a ReqCondition that returns true when the request is directed to "www.reddit.com". The host equality check is case-insensitive, to reflect the behaviour of DNS resolvers, so even if the user types "www.rEdDit.com", the comparison will satisfy the condition. When the hour is between 8:00am and 5:59pm, we directly return a response in DoFunc(), so the remote destination will not receive the request and the client will receive the "Don't waste your time!" response.

Let's start

import "github.com/elazarl/goproxy"

There are some proxy usage examples in the examples folder, which cover the most common cases. Take a look at them and good luck!

Request & Response manipulation

There are 3 different types of handlers to manipulate the behavior of the proxy, as follows:

// handler called after receiving HTTP CONNECT from the client, and
// before proxy establishes connection with the destination host
httpsHandlers   []HttpsHandler

// handler called before proxy sends HTTP request to destination host
reqHandlers     []ReqHandler 

// handler called after proxy receives HTTP Response from destination host,
// and before proxy forwards the Response to the client
respHandlers    []RespHandler

Depending on what you want to manipulate, the ways to add handlers to each of the previous lists are:

// Add handlers to httpsHandlers 
proxy.OnRequest(some ReqConditions).HandleConnect(YourHandlerFunc())

// Add handlers to reqHandlers
proxy.OnRequest(some ReqConditions).Do(YourReqHandlerFunc())

// Add handlers to respHandlers
proxy.OnResponse(some RespConditions).Do(YourRespHandlerFunc())

Example:

// This rejects the HTTPS request to *.reddit.com during HTTP CONNECT phase.
// Reddit URL check

Goproxy

Install / Use

README