fleet-manager: drop self-TLS admin mode — the manager never terminates TLS

Traefik (and the k8s Ingress later) owns UI TLS, so remove the vestigial
self-TLS path: adminTLS(), the certReloader type, the adminCert/adminKey config
+ FLEET_MGR_ADMIN_CERT/_KEY env, and the now-redundant adminBehindProxy flag.
The admin plane always serves plaintext now — two modes: INSECURE (dev, no auth)
or default (passkey behind a trusted TLS proxy). Drops the crypto/tls, fmt, sync
imports and the errString helper.

serving/fleet-manager/main.go

@@ -12,25 +12,21 @@
 package main
 
 import (
-	"crypto/tls"
-	"fmt"
 	"log"
 	"net/http"
 	"os"
 	"strconv"
 	"strings"
-	"sync"
 	"time"
 
 	"github.com/go-webauthn/webauthn/webauthn"
 )
 
 type config struct {
+	// The admin plane always serves plaintext: passkey auth behind a trusted
+	// TLS-terminating proxy (Traefik / k8s Ingress), or no auth in insecure dev.
 	adminAddr     string
 	adminInsecure bool
-	adminBehindProxy bool // passkey auth ON, but TLS terminated by a trusted upstream proxy
-	adminCert        string
-	adminKey         string
 
 	agentAddr     string
 	agentCert     string
@@ -63,9 +59,6 @@ func loadConfig() config {
 	return config{
 		adminAddr:     env("FLEET_MGR_ADMIN_ADDR", "127.0.0.1:9500"),
 		adminInsecure: env("FLEET_MGR_ADMIN_INSECURE", "") == "true",
-		adminBehindProxy: env("FLEET_MGR_ADMIN_BEHIND_PROXY", "") == "true",
-		adminCert:        env("FLEET_MGR_ADMIN_CERT", ""),
-		adminKey:         env("FLEET_MGR_ADMIN_KEY", ""),
 
 		agentAddr:     env("FLEET_MGR_AGENT_ADDR", "127.0.0.1:9600"),
 		agentCert:     env("FLEET_MGR_AGENT_CERT", ""),
@@ -235,84 +228,18 @@ func main() {
 		IdleTimeout:       60 * time.Second,
 	}
 
-	switch {
-	case cfg.adminInsecure:
+	if cfg.adminInsecure {
 		log.Printf("WARNING: admin plane INSECURE (plaintext, NO passkey auth) on %s — dev/lab only", cfg.adminAddr)
 		log.Fatal(srv.ListenAndServe())
+	}
 
-	case cfg.adminBehindProxy:
-		// Passkey auth ON, but a trusted upstream proxy (Traefik / k8s Ingress)
-		// terminates TLS and forwards plaintext. WebAuthn only needs RP_ORIGIN to
-		// match the browser's HTTPS origin, not our own transport; session cookies
-		// stay Secure (the browser sees HTTPS). The listener MUST be reachable ONLY
-		// via the proxy — bind it private (loopback / a private net), never a
-		// routable interface.
+	// Secure mode: passkey auth over PLAINTEXT behind a trusted TLS-terminating
+	// proxy (Traefik / k8s Ingress) that owns the cert — the manager never
+	// terminates TLS. WebAuthn only needs RP_ORIGIN to match the browser's HTTPS
+	// origin, not our transport; session cookies stay Secure. The listener MUST be
+	// bound private (loopback / a private net) so only the proxy reaches it.
 	log.Printf("fleet-manager admin (passkey, behind TLS proxy) plaintext listening on %s (rp=%s) — ensure only the proxy can reach this address", cfg.adminAddr, cfg.rpID)
 	log.Fatal(srv.ListenAndServe())
-
-	default:
-		tlsCfg, err := adminTLS(cfg)
-		if err != nil {
-			log.Fatalf("admin TLS: %v", err)
-		}
-		srv.TLSConfig = tlsCfg
-		log.Printf("fleet-manager admin (HTTPS + passkey) listening on %s (rp=%s)", cfg.adminAddr, cfg.rpID)
-		log.Fatal(srv.ListenAndServeTLS("", "")) // certs are in TLSConfig
-	}
-}
-
-// adminTLS builds the admin plane's SERVER-AUTH TLS. Browser identity is a passkey
-// (WebAuthn), not a client cert, so there is no ClientCAs/ClientAuth here — just
-// our server cert. (manager→broker mTLS is separate; see broker.go.)
-//
-// The cert is served via a hot-reloading GetCertificate so an out-of-band renewal
-// is picked up WITHOUT a restart. The intended deployment is a publicly-trusted
-// cert obtained by an external ACME client via Let's Encrypt DNS-01 (the UI is on
-// the mgmt network, not internet-reachable, so HTTP-01/TLS-ALPN-01 can't validate;
-// DNS-01 proves control via a TXT record and supports wildcards). The ACME client
-// (lego/certbot + a DNS provider) writes cert+key to FLEET_MGR_ADMIN_CERT/_KEY on
-// a shared volume; this reloader notices the mtime bump and swaps the cert live.
-func adminTLS(cfg config) (*tls.Config, error) {
-	if cfg.adminCert == "" || cfg.adminKey == "" {
-		return nil, errString("FLEET_MGR_ADMIN_CERT and FLEET_MGR_ADMIN_KEY are required (or set FLEET_MGR_ADMIN_INSECURE=true for dev)")
-	}
-	rl := &certReloader{certFile: cfg.adminCert, keyFile: cfg.adminKey}
-	if _, err := rl.GetCertificate(nil); err != nil { // fail fast on a bad/missing cert
-		return nil, err
-	}
-	return &tls.Config{
-		MinVersion:     tls.VersionTLS12,
-		GetCertificate: rl.GetCertificate,
-	}, nil
-}
-
-// certReloader serves the admin server cert, reloading from disk when the file's
-// mtime advances (an ACME renewal), so Let's Encrypt DNS-01 rotations apply live.
-type certReloader struct {
-	certFile, keyFile string
-	mu                sync.Mutex
-	cached            *tls.Certificate
-	mtime             time.Time
-}
-
-func (c *certReloader) GetCertificate(*tls.ClientHelloInfo) (*tls.Certificate, error) {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-	fi, statErr := os.Stat(c.certFile)
-	if statErr == nil && (c.cached == nil || fi.ModTime().After(c.mtime)) {
-		cert, err := tls.LoadX509KeyPair(c.certFile, c.keyFile)
-		if err != nil {
-			if c.cached != nil {
-				return c.cached, nil // serve the last good cert through a transient mid-rotation error
-			}
-			return nil, err
-		}
-		c.cached, c.mtime = &cert, fi.ModTime()
-	}
-	if c.cached == nil {
-		return nil, fmt.Errorf("admin cert unavailable: %v", statErr)
-	}
-	return c.cached, nil
 }
 
 // ---- tiny helpers ---------------------------------------------------------
@@ -348,7 +275,3 @@ func envInt(k string, def int) int {
 	}
 	return def
 }
-
-type errString string
-
-func (e errString) Error() string { return string(e) }