Implementing a Reverse Proxy in Go with Custom Enhancements

Background: a reverse proxy acts as an intermediary between a client and a target server; Go's net/http package provides a built‑in ReverseProxy but its capabilities are limited for production needs.

Simple usage shows how to parse the target URL, instantiate httputil.NewSingleHostReverseProxy, and start an HTTP server.

target, err := url.Parse("http://www.domain.com")</code>
<code>if err != nil { panic(err) }</code>
<code>proxy := httputil.NewSingleHostReverseProxy(target)</code>
<code>log.Fatal(http.ListenAndServe(":8082", proxy))

The official ReverseProxy struct contains fields such as Director, Transport, FlushInterval, ErrorLog, BufferPool, ModifyResponse, and ErrorHandler, each responsible for different aspects of request forwarding and response handling.

type ReverseProxy struct { Director func(*http.Request); Transport http.RoundTripper; FlushInterval time.Duration; ErrorLog *log.Logger; BufferPool BufferPool; ModifyResponse func(*http.Response) error; ErrorHandler func(http.ResponseWriter, *http.Request, error) }

Initialization via NewSingleHostReverseProxy sets the request scheme, host, path, query string, and clears the default User-Agent header to avoid unwanted defaults.

func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy { /* sets target scheme, host, path, query, and User-Agent */ }

The core ServeHTTP method clones the incoming request, manages context cancellation, adjusts headers (including removal of hop headers and addition of X-Forwarded-For), handles protocol upgrades, forwards the request using the configured transport, processes the response, copies headers, writes the status code, streams the body, and flushes trailers when needed.

func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) { /* request cloning, header manipulation, transport.RoundTrip, response handling */ }

A custom ServeHTTP wrapper is introduced to provide tracing, timeout extraction, structured logging, and custom error/response handling while still leveraging the underlying ReverseProxy for actual forwarding.

type ServeHTTP struct { targetUrl string; reverseProxy *httputil.ReverseProxy; proxyErrorHandler ProxyErrorHandler; logger log.Logger }

Transport tuning demonstrates increasing MaxIdleConns and MaxIdleConnsPerHost to 200 to improve performance under HTTP/1.1 workloads.

netHttp.Transport{ MaxIdleConns: 200, MaxIdleConnsPerHost: 200, IdleConnTimeout: 90*time.Second, /* other fields */ }

Utility functions generate a trace ID, determine request timeout, log detailed request/response information, and set appropriate response headers, illustrating a complete, production‑ready reverse‑proxy solution in Go.