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.

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

Proxy modes

1. Regular HTTP proxy
2. HTTPS through CONNECT
3. 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
4. "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!)

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.

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(r, 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 is case-insensitive because of (?i), so the block will work also if the user types something like rEdDit.com.
proxy.OnRequest(goproxy.ReqHostMatches(regexp.MustCompile("(?i)reddit.*:443$"))).HandleConnect(goproxy.AlwaysReject)

// Be careful about this example! It shows you a common error that you
// need to avoid.
// This will NOT reject the HTTPS request with URL ending with .gif because,
// if the scheme is HTTPS, the proxy will receive only URL.Hostname
// and URL.Port during the HTTP CONNECT phase.
proxy.OnRequest(goproxy.UrlMatches(regexp.MustCompile(`.*gif$`))).HandleConnect(goproxy.AlwaysReject)

// To fix the previous example, here there is the correct way to manipulate
// an HTTP request using URL.Path (target path) as a condition.
proxy.OnRequest(goproxy.UrlMatches(regexp.MustCompile(`.*gif$`))).Do(YourReqHandlerFunc())

Error handling

Generic error

If an error occurs while handling a request through the proxy, by default the proxy returns HTTP error 500 (Internal Server Error) with the error message as the body content.

If you want to override this behaviour, you can define your own RespHandler that changes the error response. Among the context parameters, ctx.Error contains the error occurred, if any, or the nil value, if no error happened.

You can handle it as you wish, including returning a custom JSON as the body. Example of an error handler:

proxy.OnResponse().DoFunc(func(resp *http.Response, ctx *goproxy.ProxyCtx) *http.Response {
	var dnsError *net.DNSError
	if errors.As(ctx.Error, &dnsError) {
		// Do not leak our DNS server's address
		dnsError.Server = "<server-redacted>"
		return goproxy.NewResponse(ctx.Req, goproxy.ContentTypeText, http.StatusBadGateway, dnsError.Error())
	}
	return resp
})

Connection error

If an error occurs while sending data to the target remote server (or to the proxy client), the proxy.ConnectionErrHandler is called to handle the error, if present, else a default handler will be used. The error is passed as function parameter and not inside the proxy context, so you don't have to check the ctx.Error field in this handler.

In this handler you have access to the raw connection with the proxy client (as an io.Writer), so you could send any HTTP data over it, if needed, containing the error data. There is no guarantee that the connection hasn't already been closed, so the Write() could return an error.

The connection will be automatically closed by the proxy library after the error handler call, so you don't have to worry about it.

Project Status

This project has been created 10 years ago, and has reached a stage of maturity. It can be safely used in production, and many projects already do that.

If there will be any breaking change in the future, a new version of the Go module will be released (e.g. v2).

Trusted, as a direct dependency, by: