pz8-relay/main.go

package main

import (
	"crypto/subtle"
	"log/slog"
	"net/http"
	"net/http/httputil"
	"net/url"
	"os"
	"strings"
)

func main() {
	logger := slog.New(slog.NewTextHandler(os.Stdout, nil))
	slog.SetDefault(logger)

	target := mustEnv("PZ8_RELAY_TARGET_URL")
	username := mustEnv("PZ8_RELAY_USERNAME")
	password := mustEnv("PZ8_RELAY_PASSWORD")

	addr := os.Getenv("PZ8_RELAY_LISTEN_ADDR")
	if addr == "" {
		addr = ":8080"
	}

	targetURL, err := url.Parse(target)
	if err != nil {
		slog.Error("invalid PZ8_RELAY_TARGET_URL", "err", err)
		os.Exit(1)
	}

	proxy := &httputil.ReverseProxy{
		Rewrite: func(preq *httputil.ProxyRequest) {
			preq.SetURL(targetURL)
		},
	}

	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		u, p, ok := r.BasicAuth()
		if !ok || subtle.ConstantTimeCompare([]byte(u), []byte(username)) != 1 ||
			subtle.ConstantTimeCompare([]byte(p), []byte(password)) != 1 {
			slog.Warn("unauthorized request", "client", clientIP(r))
			w.Header().Set("WWW-Authenticate", `Basic realm="pz8-relay"`)
			http.Error(w, "unauthorized", http.StatusUnauthorized)
			return
		}
		proxy.ServeHTTP(w, r)
	})

	slog.Info("pz8-relay listening", "addr", addr, "target", targetURL.Redacted())
	if err := http.ListenAndServe(addr, nil); err != nil {
		slog.Error("server stopped", "err", err)
		os.Exit(1)
	}
}

// clientIP returns the original client address, trusting X-Forwarded-For
// because this service runs behind Traefik. Do not expose directly.
func clientIP(r *http.Request) string {
	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
		if i := strings.IndexByte(xff, ','); i >= 0 {
			return strings.TrimSpace(xff[:i])
		}
		return strings.TrimSpace(xff)
	}
	return r.RemoteAddr
}

func mustEnv(key string) string {
	v := os.Getenv(key)
	if v == "" {
		slog.Error("missing required env var", "name", key)
		os.Exit(1)
	}
	return v
}