pxe: move dhcp-host allowlist into a SIGHUP-reloadable file

dnsmasq's SIGHUP re-reads /etc/ethers and any --dhcp-hostsfile= paths,
but NOT dhcp-host= lines from the main conf. Reload() was faithfully
rewriting dnsmasq.conf with the new MAC, sending SIGHUP, and then
dnsmasq kept serving its startup view — so a freshly-registered host
still showed up as "proxy-ignored, tags: eth0" with no "known" tag.

Split the allowlist into ${RuntimeDir}/dhcp-hosts, referenced from the
main conf via dhcp-hostsfile=. writeConf() is static-ish now; Reload
just rewrites the hosts file and SIGHUPs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/pxe/dnsmasq.go

@@ -120,7 +120,10 @@ func (s *Supervisor) Start(ctx context.Context, hosts []model.Host) error {
 	if err := os.MkdirAll(s.cfg.RuntimeDir, 0o755); err != nil {
 		return fmt.Errorf("mkdir runtime: %w", err)
 	}
-	if err := s.writeConf(hosts); err != nil {
+	if err := s.writeHosts(hosts); err != nil {
+		return err
+	}
+	if err := s.writeConf(); err != nil {
 		return err
 	}
 	subCtx, cancel := context.WithCancel(ctx)
@@ -152,14 +155,14 @@ func (s *Supervisor) Start(ctx context.Context, hosts []model.Host) error {
 	return nil
 }
 
-// Reload rewrites the conf with the latest host registry and sends
-// SIGHUP. It will restart the subprocess if SIGHUP is unsupported
-// (e.g. when running behind an OS that doesn't support it).
+// Reload rewrites the dhcp-hosts allowlist with the latest host
+// registry and SIGHUPs dnsmasq to pick it up. The main dnsmasq.conf
+// is unchanged — it only references the hosts file by path.
 func (s *Supervisor) Reload(hosts []model.Host) error {
 	if !s.cfg.Enabled {
 		return nil
 	}
-	if err := s.writeConf(hosts); err != nil {
+	if err := s.writeHosts(hosts); err != nil {
 		return err
 	}
 	s.mu.Lock()
@@ -201,7 +204,7 @@ func (s *Supervisor) Shutdown(timeout time.Duration) error {
 	return nil
 }
 
-func (s *Supervisor) writeConf(hosts []model.Host) error {
+func (s *Supervisor) writeConf() error {
 	tmpl, err := template.New("dnsmasq").Parse(dnsmasqTemplate)
 	if err != nil {
 		return err
@@ -219,10 +222,9 @@ func (s *Supervisor) writeConf(hosts []model.Host) error {
 	}
 	data := struct {
 		Cfg     SupervisorConfig
-		Hosts   []model.Host
 		Network string
 		Netmask string
-	}{s.cfg, hosts, ipnet.IP.String(), net.IP(ipnet.Mask).String()}
+	}{s.cfg, ipnet.IP.String(), net.IP(ipnet.Mask).String()}
 	if err := tmpl.Execute(f, data); err != nil {
 		_ = f.Close()
 		return fmt.Errorf("render conf: %w", err)
@@ -240,6 +242,32 @@ func (s *Supervisor) writeConf(hosts []model.Host) error {
 	return nil
 }
 
+// writeHosts renders the dhcp-hostsfile referenced by dnsmasq.conf.
+// Each registered host contributes one line:
+//
+//	<mac>,set:known
+//
+// dnsmasq re-reads this file on SIGHUP — that's the whole point of
+// keeping it separate from the main conf.
+func (s *Supervisor) writeHosts(hosts []model.Host) error {
+	if err := os.MkdirAll(s.cfg.RuntimeDir, 0o755); err != nil {
+		return fmt.Errorf("mkdir runtime: %w", err)
+	}
+	path := filepath.Join(s.cfg.RuntimeDir, "dhcp-hosts")
+	tmp := path + ".new"
+	var b strings.Builder
+	for _, h := range hosts {
+		fmt.Fprintf(&b, "%s,set:known\n", h.MAC)
+	}
+	if err := os.WriteFile(tmp, []byte(b.String()), 0o644); err != nil {
+		return fmt.Errorf("write dhcp-hosts: %w", err)
+	}
+	if err := os.Rename(tmp, path); err != nil {
+		return fmt.Errorf("rename dhcp-hosts: %w", err)
+	}
+	return nil
+}
+
 // Exposed for the UI handlers to show operators what config is live.
 func (s *Supervisor) ConfPath() string {
 	return filepath.Join(s.cfg.RuntimeDir, "dnsmasq.conf")
@@ -275,11 +303,12 @@ no-resolv
 # CIDR — we split Subnet upstream in writeConf().
 dhcp-range={{ .Network }},proxy,{{ .Netmask }}
 
-# MAC allowlist: dnsmasq only answers DHCP for MACs with a dhcp-host= below.
+# MAC allowlist: dnsmasq only answers DHCP for MACs tagged "known".
+# The per-MAC dhcp-host= entries live in a separate file so SIGHUP
+# can reload them — dnsmasq does NOT re-read dhcp-host= from the
+# main conf on SIGHUP, only from dhcp-hostsfile=.
 dhcp-ignore=tag:!known
-{{- range .Hosts }}
-dhcp-host={{ .MAC }},set:known
-{{- end }}
+dhcp-hostsfile={{ .Cfg.RuntimeDir }}/dhcp-hosts
 
 # Keep runtime state inside RuntimeDir so the systemd sandbox
 # (ReadWritePaths=/var/lib/vetting ...) doesn't block writes to the