steward/internal/server/server.go

// Package server is Steward's HTTP API and serves the web UI.
package server

import (
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"io/fs"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"os"
	"path"
	"strconv"
	"strings"
	"sync"
	"syscall"
	"time"

	"steward/internal/auth"
	"steward/internal/certs"
	"steward/internal/dhcpd"
	"steward/internal/engine"
	"steward/internal/store"
	"steward/internal/version"
)

// Config configures the HTTP server.
type Config struct {
	Listen          []string
	TLSCertFile     string
	TLSKeyFile      string
	TrustedProxies  []netip.Prefix
	SessionLifetime time.Duration
	SessionIdle     time.Duration
	// CookieSecure is auto (secure over HTTPS), always or never.
	CookieSecure string
	MaxBodyBytes int64
	// Socket is the local unix socket ("" for none).
	Socket string
	// PingProblem says why the ping check can't run (nil: always fine).
	PingProblem func() string
	// ACME keeps the certificate stewardd obtains itself (tls.acme; nil
	// when off).
	ACME *certs.Manager
	// ExcludeInterfaces are interface names or path.Match patterns left
	// out of GET /interfaces (interfaces.exclude).
	ExcludeInterfaces []string
}

// excludedInterface reports whether interfaces.exclude hides the
// interface name.
func (s *Server) excludedInterface(name string) bool {
	for _, p := range s.cfg.ExcludeInterfaces {
		if ok, _ := path.Match(p, name); ok {
			return true
		}
	}
	return false
}

// Interfaces lists the host's interfaces.
type Interfaces interface {
	All() (map[string][]netip.Prefix, error)
}

// Server is the HTTP server.
type Server struct {
	cfg     Config
	eng     *engine.Engine
	dhcp    *dhcpd.Server
	ifaces  Interfaces
	store   *store.Store
	log     *slog.Logger
	mux     *http.ServeMux
	limiter *rateLimiter
	web     fs.FS
	started time.Time
	certs   *certReloader

	mu   sync.Mutex
	srvs []*http.Server
}

// New makes a server. web holds the web UI (nil: API only).
func New(cfg Config, eng *engine.Engine, dhcp *dhcpd.Server, ifaces Interfaces, log *slog.Logger, web fs.FS) *Server {
	if cfg.SessionLifetime == 0 {
		cfg.SessionLifetime = 12 * time.Hour
	}
	if cfg.SessionIdle == 0 {
		cfg.SessionIdle = 2 * time.Hour
	}
	if cfg.MaxBodyBytes == 0 {
		cfg.MaxBodyBytes = 8 << 20
	}
	if cfg.CookieSecure == "" {
		cfg.CookieSecure = "auto"
	}
	if log == nil {
		log = slog.Default()
	}
	s := &Server{cfg: cfg, eng: eng, dhcp: dhcp, ifaces: ifaces, store: eng.Store(), log: log, mux: http.NewServeMux(), limiter: newRateLimiter(), web: web, started: time.Now()}
	s.routes()
	return s
}

// Handler returns the HTTP handler with all middleware.
func (s *Server) Handler() http.Handler {
	return s.recoverer(s.securityHeaders(s.logRequests(s.mux)))
}

func lowPort(addr string) bool {
	_, port, err := net.SplitHostPort(addr)
	if err != nil {
		return false
	}
	n, err := strconv.Atoi(port)
	return err == nil && n > 0 && n < 1024
}

// Serve listens until ctx ends. ready is called once listening.
func (s *Server) Serve(ctx context.Context, ready func()) error {
	if len(s.cfg.Listen) == 0 {
		return errors.New("no listen addresses configured")
	}
	// With a certificate (tls.cert_file, or one stewardd obtains, which
	// may turn up later), each listener serves HTTPS and plain HTTP side
	// by side (sniff.go); plain HTTP is redirected once there is one.
	var tlsCfg *tls.Config
	if s.cfg.TLSCertFile != "" {
		cr, err := newCertReloader(s.cfg.TLSCertFile, s.cfg.TLSKeyFile, s.log)
		if err != nil {
			return err
		}
		s.certs = cr
	}
	if s.certs != nil || s.cfg.ACME != nil {
		tlsCfg = &tls.Config{MinVersion: tls.VersionTLS12, GetCertificate: s.webCertificate, NextProtos: []string{"h2", "http/1.1"}}
	}
	errc := make(chan error, len(s.cfg.Listen)+1)
	var listeners []net.Listener
	for _, addr := range s.cfg.Listen {
		ln, err := net.Listen("tcp", addr)
		if err != nil {
			for _, l := range listeners {
				l.Close()
			}
			if errors.Is(err, syscall.EACCES) && lowPort(addr) {
				return fmt.Errorf("listening on %s: %w (ports below 1024 need CAP_NET_BIND_SERVICE, which steward.service grants)", addr, err)
			}
			return fmt.Errorf("listening on %s: %w", addr, err)
		}
		listeners = append(listeners, ln)
	}
	for i, ln := range listeners {
		handler := s.Handler()
		if tlsCfg != nil {
			handler = s.requireHTTPS(handler)
		}
		srv := &http.Server{
			Handler:           handler,
			ReadHeaderTimeout: 10 * time.Second,
			ReadTimeout:       2 * time.Minute,
			WriteTimeout:      5 * time.Minute,
			IdleTimeout:       2 * time.Minute,
			TLSConfig:         tlsCfg,
			ErrorLog:          slog.NewLogLogger(s.log.Handler(), slog.LevelWarn),
		}
		s.mu.Lock()
		s.srvs = append(s.srvs, srv)
		s.mu.Unlock()
		scheme := "http"
		if tlsCfg != nil {
			if s.httpsAvailable() {
				scheme = "https"
			}
			ln = newSniffListener(ln, tlsCfg)
		}
		if !s.httpsAvailable() && s.cfg.ACME == nil {
			if host, _, err := net.SplitHostPort(s.cfg.Listen[i]); err == nil {
				if ip, err := netip.ParseAddr(host); host == "" || err == nil && !ip.IsLoopback() {
					s.log.Warn("serving plain HTTP on a non-loopback address: passwords and API tokens travel unencrypted; configure tls or a TLS reverse proxy", "addr", s.cfg.Listen[i])
				}
			}
		}
		s.log.Info("listening", "addr", s.cfg.Listen[i], "url", scheme+"://"+ln.Addr().String())
		go func(srv *http.Server, ln net.Listener) {
			if err := srv.Serve(ln); !errors.Is(err, http.ErrServerClosed) {
				errc <- err
			}
		}(srv, ln)
	}
	s.serveSocket(errc)
	if ready != nil {
		ready()
	}
	go s.housekeeping(ctx)
	select {
	case <-ctx.Done():
	case err := <-errc:
		s.shutdown()
		return err
	}
	s.shutdown()
	return nil
}

func (s *Server) serveSocket(errc chan error) {
	if s.cfg.Socket == "" {
		return
	}
	ln, err := listenSocket(s.cfg.Socket)
	if err != nil {
		s.log.Warn("not listening on the local socket", "path", s.cfg.Socket, "err", err)
		return
	}
	srv := &http.Server{
		Handler:           s.Handler(),
		ConnContext:       socketConnContext,
		ReadHeaderTimeout: 10 * time.Second,
		WriteTimeout:      5 * time.Minute,
		IdleTimeout:       2 * time.Minute,
		ErrorLog:          slog.NewLogLogger(s.log.Handler(), slog.LevelWarn),
	}
	s.mu.Lock()
	s.srvs = append(s.srvs, srv)
	s.mu.Unlock()
	s.log.Info("listening", "socket", s.cfg.Socket)
	go func() {
		if err := srv.Serve(ln); !errors.Is(err, http.ErrServerClosed) {
			errc <- err
		}
	}()
}

func (s *Server) shutdown() {
	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	defer cancel()
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, srv := range s.srvs {
		srv.Shutdown(ctx)
	}
	if s.cfg.Socket != "" {
		os.Remove(s.cfg.Socket)
	}
}

func (s *Server) housekeeping(ctx context.Context) {
	t := time.NewTicker(10 * time.Minute)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			if err := s.store.PurgeSessions(ctx, time.Now()); err != nil {
				s.log.Warn("purging sessions failed", "err", err)
			}
			s.limiter.cleanup()
			if s.certs != nil {
				s.certs.reloadIfChanged()
			}
		}
	}
}

// ReloadTLS rereads the certificate (on SIGHUP).
func (s *Server) ReloadTLS() {
	if s.certs != nil {
		s.certs.reload()
	}
}

// certReloader serves a certificate that is reread when its file changes.
type certReloader struct {
	certFile, keyFile string
	log               *slog.Logger
	mu                sync.RWMutex
	cert              *tls.Certificate
	modTime           time.Time
}

func newCertReloader(certFile, keyFile string, log *slog.Logger) (*certReloader, error) {
	cr := &certReloader{certFile: certFile, keyFile: keyFile, log: log}
	if err := cr.load(); err != nil {
		return nil, err
	}
	return cr, nil
}

func (cr *certReloader) load() error {
	cert, err := tls.LoadX509KeyPair(cr.certFile, cr.keyFile)
	if err != nil {
		return fmt.Errorf("loading the TLS certificate: %w", err)
	}
	fi, _ := os.Stat(cr.certFile)
	cr.mu.Lock()
	cr.cert = &cert
	if fi != nil {
		cr.modTime = fi.ModTime()
	}
	cr.mu.Unlock()
	return nil
}

func (cr *certReloader) reload() {
	if err := cr.load(); err != nil {
		cr.log.Error("reloading the TLS certificate failed", "err", err)
		return
	}
	cr.log.Info("TLS certificate reloaded", "file", cr.certFile)
}

func (cr *certReloader) reloadIfChanged() {
	fi, err := os.Stat(cr.certFile)
	if err != nil {
		return
	}
	cr.mu.RLock()
	changed := fi.ModTime().After(cr.modTime)
	cr.mu.RUnlock()
	if changed {
		cr.reload()
	}
}

func (cr *certReloader) get(*tls.ClientHelloInfo) (*tls.Certificate, error) {
	cr.mu.RLock()
	defer cr.mu.RUnlock()
	return cr.cert, nil
}

// ---- middleware

type ctxKey int

const (
	keyPrincipal ctxKey = iota
	keySession
)

func (s *Server) recoverer(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		defer func() {
			if v := recover(); v != nil {
				if v == http.ErrAbortHandler {
					panic(v)
				}
				s.log.Error("panic serving a request", "path", r.URL.Path, "panic", v)
				writeError(w, http.StatusInternalServerError, "internal", "internal server error")
			}
		}()
		next.ServeHTTP(w, r)
	})
}

func (s *Server) securityHeaders(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		h := w.Header()
		h.Set("Server", "Steward/"+version.Short())
		h.Set("X-Content-Type-Options", "nosniff")
		h.Set("X-Frame-Options", "DENY")
		h.Set("Referrer-Policy", "no-referrer")
		h.Set("Content-Security-Policy", "default-src 'self'; img-src 'self' data:; style-src 'self'; script-src 'self'; connect-src 'self'; frame-ancestors 'none'; base-uri 'none'; form-action 'self'")
		h.Set("Cross-Origin-Opener-Policy", "same-origin")
		if s.isHTTPS(r) {
			h.Set("Strict-Transport-Security", "max-age=31536000")
		}
		next.ServeHTTP(w, r)
	})
}

type statusWriter struct {
	http.ResponseWriter
	status int
}

func (w *statusWriter) WriteHeader(code int) {
	w.status = code
	w.ResponseWriter.WriteHeader(code)
}

func (w *statusWriter) Unwrap() http.ResponseWriter { return w.ResponseWriter }

func (s *Server) logRequests(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		start := time.Now()
		sw := &statusWriter{ResponseWriter: w, status: 200}
		next.ServeHTTP(sw, r)
		if r.URL.Path == "/healthz" || strings.HasPrefix(r.URL.Path, "/assets/") {
			return
		}
		user := ""
		if p, ok := r.Context().Value(keyPrincipal).(*auth.Principal); ok {
			user = p.Name()
		}
		s.log.Debug("http", "method", r.Method, "path", r.URL.Path, "status", sw.status, "duration", time.Since(start).Round(time.Millisecond), "ip", s.clientIP(r), "user", user)
	})
}

// clientIP is the client's address, from X-Forwarded-For when the request
// came through a trusted proxy.
func (s *Server) clientIP(r *http.Request) string {
	if peerOf(r.Context()) != nil {
		return "local"
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	peer, err := netip.ParseAddr(host)
	if err != nil || !s.trusted(peer) {
		return host
	}
	parts := strings.Split(r.Header.Get("X-Forwarded-For"), ",")
	for i := len(parts) - 1; i >= 0; i-- {
		a, err := netip.ParseAddr(strings.TrimSpace(parts[i]))
		if err != nil {
			return host
		}
		if !s.trusted(a) {
			return a.String()
		}
	}
	return host
}

func (s *Server) trusted(a netip.Addr) bool {
	a = a.Unmap()
	for _, p := range s.cfg.TrustedProxies {
		if p.Contains(a) {
			return true
		}
	}
	return false
}

// isHTTPS reports whether the request reached Steward over HTTPS: TLS to
// stewardd, the local socket, or a trusted proxy that says so (the last
// X-Forwarded-Proto value).
func (s *Server) isHTTPS(r *http.Request) bool {
	if r.TLS != nil || peerOf(r.Context()) != nil {
		return true
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return false
	}
	peer, err := netip.ParseAddr(host)
	if err != nil || !s.trusted(peer) {
		return false
	}
	vals := r.Header.Values("X-Forwarded-Proto")
	if len(vals) == 0 {
		return false
	}
	parts := strings.Split(strings.Join(vals, ","), ",")
	return strings.EqualFold(strings.TrimSpace(parts[len(parts)-1]), "https")
}