Login page for AUTH_MODE=local instead of the Basic Auth popup

The local mode now shows a login page in the colors of the UI (light/dark theme, FR/EN)
with a signed session cookie and the log out button, like the OIDC mode. LOGIN_LOGO shows
a PNG mounted in the container on that page. SESSION_TTL applies to both modes
(OIDC_SESSION_TTL still works). HTTP Basic credentials are still accepted for scripts.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

auth_local.go

@@ -0,0 +1,167 @@
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"crypto/subtle"
+	"log"
+	"net"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"time"
+)
+
+// Local mode with AUTH_USER set: a login page (web/login.html) in the colors of the UI and a
+// session kept in a signed cookie, like the OIDC mode. HTTP Basic credentials are still
+// accepted so scripts calling the API keep working, but the browser popup is gone.
+
+const loginPage = "/login.html"
+
+var loginFailDelay = time.Second // slows down password guessing
+
+type Local struct {
+	user, pass string
+	ttl        time.Duration
+	logo       string // LOGIN_LOGO, served at /auth/logo
+	key        []byte
+	next       http.Handler
+}
+
+func newLocal(c authConfig) *Local {
+	// The key depends on the credentials: changing AUTH_USER or AUTH_PASS ends every session.
+	m := hmac.New(sha256.New, sessionKey(c.dataDir))
+	m.Write([]byte("local\x00" + c.user + "\x00" + c.pass))
+	if c.loginLogo != "" {
+		if _, err := os.Stat(c.loginLogo); err != nil {
+			log.Printf("auth: LOGIN_LOGO: %v", err)
+		}
+	}
+	log.Printf("local authentication enabled (user %s)", c.user)
+	return &Local{user: c.user, pass: c.pass, ttl: c.sessionTTL, logo: c.loginLogo, key: m.Sum(nil)}
+}
+
+func (l *Local) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	switch r.URL.Path {
+	case "/healthz", "/style.css":
+		l.next.ServeHTTP(w, r)
+		return
+	case "/auth/logo":
+		l.serveLogo(w, r)
+		return
+	case "/auth/login":
+		l.handleLogin(w, r)
+		return
+	case "/auth/logout":
+		http.SetCookie(w, &http.Cookie{Name: sessionCookie, Path: "/", MaxAge: -1, HttpOnly: true, Secure: isHTTPS(r)})
+		http.Redirect(w, r, loginPage, http.StatusFound)
+		return
+	}
+	user, ok := l.sessionUser(r)
+	if !ok {
+		if u, p, basic := r.BasicAuth(); basic && l.check(u, p) {
+			user, ok = u, true
+		}
+	}
+	switch {
+	case r.URL.Path == loginPage:
+		if ok {
+			http.Redirect(w, r, safeReturn(r.URL.Query().Get("r")), http.StatusFound)
+			return
+		}
+		w.Header().Set("Cache-Control", "no-store")
+		l.next.ServeHTTP(w, r)
+	case ok && r.URL.Path == "/auth/me":
+		writeJSON(w, http.StatusOK, map[string]string{"mode": "local", "user": user})
+	case ok:
+		l.next.ServeHTTP(w, r)
+	case r.Method == http.MethodGet && !strings.HasPrefix(r.URL.Path, "/api/") && r.URL.Path != "/auth/me":
+		target := loginPage
+		if ret := r.URL.RequestURI(); ret != "/" {
+			target += "?" + url.Values{"r": {ret}}.Encode()
+		}
+		http.Redirect(w, r, target, http.StatusFound)
+	default:
+		writeAuthRequired(w)
+	}
+}
+
+func (l *Local) handleLogin(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Redirect(w, r, loginPage, http.StatusFound)
+		return
+	}
+	user, pass := r.PostFormValue("user"), r.PostFormValue("pass")
+	ret := safeReturn(r.PostFormValue("r"))
+	if !l.check(user, pass) {
+		log.Printf("auth: failed login for %q from %s", user, clientIP(r))
+		time.Sleep(loginFailDelay)
+		q := url.Values{"e": {"1"}}
+		if ret != "/" {
+			q.Set("r", ret)
+		}
+		http.Redirect(w, r, loginPage+"?"+q.Encode(), http.StatusSeeOther)
+		return
+	}
+	log.Printf("auth: %s logged in from %s", user, clientIP(r))
+	http.SetCookie(w, &http.Cookie{
+		Name:     sessionCookie,
+		Value:    signCookie(l.key, session{User: user, Exp: time.Now().Add(l.ttl).Unix()}),
+		Path:     "/",
+		MaxAge:   int(l.ttl.Seconds()),
+		HttpOnly: true,
+		Secure:   isHTTPS(r),
+		SameSite: http.SameSiteLaxMode,
+	})
+	http.Redirect(w, r, ret, http.StatusSeeOther)
+}
+
+func (l *Local) sessionUser(r *http.Request) (string, bool) {
+	var s session
+	c, err := r.Cookie(sessionCookie)
+	if err != nil || !verifyCookie(l.key, c.Value, &s) || time.Now().Unix() >= s.Exp {
+		return "", false
+	}
+	return s.User, true
+}
+
+func (l *Local) check(user, pass string) bool {
+	u := subtle.ConstantTimeCompare([]byte(user), []byte(l.user))
+	p := subtle.ConstantTimeCompare([]byte(pass), []byte(l.pass))
+	return u&p == 1
+}
+
+// serveLogo sends LOGIN_LOGO; without it the login page hides the image.
+func (l *Local) serveLogo(w http.ResponseWriter, r *http.Request) {
+	if l.logo == "" {
+		http.NotFound(w, r)
+		return
+	}
+	w.Header().Set("Cache-Control", "no-cache")
+	http.ServeFile(w, r, l.logo)
+}
+
+// safeReturn keeps the page to open after login inside logstream.
+func safeReturn(ret string) string {
+	if !strings.HasPrefix(ret, "/") || strings.HasPrefix(ret, "//") || strings.HasPrefix(ret, "/\\") || ret == loginPage {
+		return "/"
+	}
+	return ret
+}
+
+// isHTTPS is true when the browser talks HTTPS, directly or through a reverse proxy.
+func isHTTPS(r *http.Request) bool {
+	return r.TLS != nil || strings.EqualFold(r.Header.Get("X-Forwarded-Proto"), "https")
+}
+
+func clientIP(r *http.Request) string {
+	if f := r.Header.Get("X-Forwarded-For"); f != "" {
+		return strings.TrimSpace(strings.Split(f, ",")[0])
+	}
+	host, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		return r.RemoteAddr
+	}
+	return host
+}