ip-echo/internal/httpapi/server.go

package httpapi

import (
	"encoding/json"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"sort"
	"strings"
	"time"

	"ipecho/internal/clientip"
)

type Server struct {
	resolver clientip.Resolver
	logger   *slog.Logger
}

type ClientInfo struct {
	IP         string              `json:"ip"`
	RemoteAddr string              `json:"remote_addr"`
	Method     string              `json:"method"`
	Path       string              `json:"path"`
	UserAgent  string              `json:"user_agent"`
	Headers    map[string][]string `json:"headers"`
}

func New(resolver clientip.Resolver) http.Handler {
	return NewWithLogger(resolver, discardLogger())
}

func NewWithLogger(resolver clientip.Resolver, logger *slog.Logger) http.Handler {
	if logger == nil {
		logger = discardLogger()
	}

	server := &Server{
		resolver: resolver,
		logger:   logger,
	}

	mux := http.NewServeMux()

	mux.HandleFunc("GET /{$}", server.handleRoot)
	mux.HandleFunc("GET /ip", server.handleIP)
	mux.HandleFunc("GET /json", server.handleJSON)
	mux.HandleFunc("GET /headers", server.handleHeaders)
	mux.HandleFunc("GET /user-agent", server.handleUserAgent)
	mux.HandleFunc("GET /healthz", server.handleHealth)
	mux.HandleFunc("GET /favicon.ico", server.handleFavicon)

	return accessLog(secureHeaders(mux), logger, resolver)
}

func (s *Server) handleRoot(w http.ResponseWriter, req *http.Request) {
	if wantsHTML(req) {
		s.handleHome(w, req)
		return
	}

	s.handleIP(w, req)
}

func (s *Server) handleIP(w http.ResponseWriter, req *http.Request) {
	writePlain(w, http.StatusOK, s.resolver.FromRequest(req))
}

func (s *Server) handleJSON(w http.ResponseWriter, req *http.Request) {
	info := s.clientInfo(req)

	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.WriteHeader(http.StatusOK)

	encoder := json.NewEncoder(w)
	encoder.SetIndent("", "  ")
	_ = encoder.Encode(info)
}

func (s *Server) handleHeaders(w http.ResponseWriter, req *http.Request) {
	writePlain(w, http.StatusOK, formatHeaders(req))
}

func (s *Server) handleUserAgent(w http.ResponseWriter, req *http.Request) {
	writePlain(w, http.StatusOK, req.UserAgent())
}

func (s *Server) handleHealth(w http.ResponseWriter, _ *http.Request) {
	writePlain(w, http.StatusOK, "ok")
}

func (s *Server) clientInfo(req *http.Request) ClientInfo {
	return ClientInfo{
		IP:         s.resolver.FromRequest(req),
		RemoteAddr: req.RemoteAddr,
		Method:     req.Method,
		Path:       req.URL.Path,
		UserAgent:  req.UserAgent(),
		Headers:    cloneHeaders(req),
	}
}

func secureHeaders(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
		w.Header().Set("X-Content-Type-Options", "nosniff")
		w.Header().Set("X-Frame-Options", "DENY")
		w.Header().Set("Referrer-Policy", "no-referrer")
		w.Header().Set("Cache-Control", "no-store")
		w.Header().Set("Content-Security-Policy", "default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; script-src 'self' 'unsafe-inline'; base-uri 'none'; frame-ancestors 'none'; form-action 'none'")
		next.ServeHTTP(w, req)
	})
}

func accessLog(next http.Handler, logger *slog.Logger, resolver clientip.Resolver) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
		startedAt := time.Now()
		recorder := &responseRecorder{ResponseWriter: w}

		next.ServeHTTP(recorder, req)

		status := recorder.status
		if status == 0 {
			status = http.StatusOK
		}

		logger.Info("http_request",
			"method", req.Method,
			"path", req.URL.Path,
			"status", status,
			"bytes", recorder.bytes,
			"duration", time.Since(startedAt).String(),
			"client_ip", resolver.FromRequest(req),
			"remote_addr", req.RemoteAddr,
			"user_agent", req.UserAgent(),
		)
	})
}

type responseRecorder struct {
	http.ResponseWriter
	status int
	bytes  int
}

func (r *responseRecorder) WriteHeader(status int) {
	if r.status != 0 {
		return
	}

	r.status = status
	r.ResponseWriter.WriteHeader(status)
}

func (r *responseRecorder) Write(body []byte) (int, error) {
	if r.status == 0 {
		r.WriteHeader(http.StatusOK)
	}

	written, err := r.ResponseWriter.Write(body)
	r.bytes += written

	return written, err
}

func (r *responseRecorder) Unwrap() http.ResponseWriter {
	return r.ResponseWriter
}

func wantsHTML(req *http.Request) bool {
	for _, value := range req.Header.Values("Accept") {
		for _, part := range strings.Split(value, ",") {
			mediaType := strings.TrimSpace(strings.SplitN(part, ";", 2)[0])
			if mediaType == "text/html" || mediaType == "application/xhtml+xml" {
				return true
			}
		}
	}

	return false
}

func writePlain(w http.ResponseWriter, status int, body string) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	w.WriteHeader(status)
	_, _ = fmt.Fprintln(w, body)
}

func cloneHeaders(req *http.Request) map[string][]string {
	headers := make(map[string][]string, len(req.Header)+1)
	headers["Host"] = []string{req.Host}

	for name, values := range req.Header {
		headers[name] = append([]string(nil), values...)
	}

	return headers
}

func formatHeaders(req *http.Request) string {
	headers := cloneHeaders(req)
	names := make([]string, 0, len(headers))

	for name := range headers {
		names = append(names, name)
	}

	sort.Strings(names)

	var builder strings.Builder
	for _, name := range names {
		for _, value := range headers[name] {
			builder.WriteString(name)
			builder.WriteString(": ")
			builder.WriteString(value)
			builder.WriteByte('\n')
		}
	}

	return strings.TrimRight(builder.String(), "\n")
}

func discardLogger() *slog.Logger {
	return slog.New(slog.NewTextHandler(io.Discard, nil))
}