reverse proxy within the same host, as a workaround to access ports inside WSL2 from local-network devices.

main.go

package main

import (
	"bytes"
	"flag"
	"io"
	"log"
	"net/http"
	"net/http/httputil"
	"regexp"
)

func main() {
	listenPort := flag.String("listenPort", "80", "Port for the proxy to listen on")
	targetPort := flag.String("targetPort", "8081", "Target port on localhost to forward requests to")
	flag.Parse()

	proxy := &httputil.ReverseProxy{
		Director: func(r *http.Request) {
			r.URL.Host = "localhost:" + *targetPort
			r.URL.Scheme = "http"
			r.Header.Del("Accept-Encoding")
			log.Printf("%s %s\n", r.Method, r.URL)
		},
	}

	proxy.ModifyResponse = func(resp *http.Response) error {
		if resp.StatusCode == http.StatusSwitchingProtocols || resp.Body == nil || resp.Body == http.NoBody {
			return nil
		}

		for _, values := range resp.Header {
			for i := range values {
				values[i] = string(replacePort([]byte(values[i]), *targetPort, *listenPort))
			}
		}

		bodyBytes, err := io.ReadAll(resp.Body)
		defer resp.Body.Close()
		if err != nil {
			return err
		}

		modifiedBody := replacePort(bodyBytes, *targetPort, *listenPort)
		resp.Body = io.NopCloser(bytes.NewReader(modifiedBody))
		return nil
	}

	log.Println("starting reverse proxy server, listening on port", *listenPort)
	log.Fatal(http.ListenAndServe(":"+*listenPort, proxy))
}

func replacePort(input []byte, port1, port2 string) []byte {
	escapedPort := regexp.QuoteMeta(port1)
	pattern := `((https?://)?[^:]+):` + escapedPort + `\b`
	re := regexp.MustCompile(pattern)
	replacement := []byte("${1}:" + port2)
	return re.ReplaceAll(input, replacement)
}