feat: installer fetched from the direct address sets it as the agent's own with the main one as fallback (and downloads from it); agents fall back to whichever address is not theirs; (direct) beside the connection status

internal/server/handlers_panel.go

@@ -11,6 +11,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/x-grk/longpolling-remote-access/internal/proto"
@@ -227,6 +228,7 @@ func (s *Server) handleAgentEdit(w http.ResponseWriter, r *http.Request) {
 		"UserErr":       r.URL.Query().Get("uerr"),
 		"Transport":     firstNonEmpty(a.Transport, proto.TransportAuto),
 		"OnWS":          s.wsAgents.On(a.ID),
+		"ViaDirect":     viaDirectOf(&s.viaDirectA, a.ID),
 		"Online":        s.online(a),
 	}
 	// The user list lives on the machine, so it is only known while it is online.
@@ -437,7 +439,7 @@ func (s *Server) handleSettings(w http.ResponseWriter, r *http.Request) {
 		"Host":          host,
 		"Direct":        direct,
 		"DirectShort":   fmt.Sprintf("wget -qO- %s/i/%s | sudo sh", direct, code),
-		"DirectCurl":    fmt.Sprintf("curl -fsSL %s/install.sh | sudo HOST=%s FALLBACK_HOST=%s ENROLL_SECRET=%s sh", direct, host, direct, secret),
+		"DirectCurl":    fmt.Sprintf("curl -fsSL %s/install.sh | sudo HOST=%s FALLBACK_HOST=%s ENROLL_SECRET=%s sh", direct, direct, host, secret),
 		"Secret":        secret,
 		"CodeLen":       s.db.InstallCodeLen(),
 		"Admins":        admins,
@@ -664,7 +666,7 @@ func (s *Server) handleUfwRevoke(w http.ResponseWriter, r *http.Request) {
 // handleInstallScript serves the generic installer; the secret is NOT baked in,
 // so it must be supplied via env or the interactive prompt.
 func (s *Server) handleInstallScript(w http.ResponseWriter, r *http.Request) {
-	s.renderInstall(w, "")
+	s.renderInstall(w, r, "")
 }
 
 // handleShortInstall serves the installer behind the short /i/<code> link with
@@ -686,16 +688,35 @@ func (s *Server) handleShortInstall(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	s.renderInstall(w, s.db.EnrollSecret())
+	s.renderInstall(w, r, s.db.EnrollSecret())
 }
 
-// renderInstall writes install.sh with the current host and an optional secret.
-func (s *Server) renderInstall(w http.ResponseWriter, secret string) {
+// viaDirect reports whether the request came to the direct address.
+func (s *Server) viaDirect(r *http.Request) bool {
+	d := directHost(s.db.DirectAddress())
+	if d == "" {
+		return false
+	}
+	host := r.Host
+	if h, _, err := net.SplitHostPort(host); err == nil {
+		host = h
+	}
+	return strings.EqualFold(host, d)
+}
+
+// renderInstall writes install.sh with the server's addresses and an
+// optional secret. Fetched from the direct address, it makes that the
+// agent's address and the main one its fallback; otherwise the other way.
+func (s *Server) renderInstall(w http.ResponseWriter, r *http.Request, secret string) {
+	host, fallback := s.db.HostAddress(), s.db.DirectAddress()
+	if s.viaDirect(r) {
+		host, fallback = fallback, host
+	}
 	w.Header().Set("Content-Type", "text/x-shellscript; charset=utf-8")
 	if err := s.installTmpl.Execute(w, map[string]any{
-		"Host":   s.db.HostAddress(),
-		"Direct": s.db.DirectAddress(),
-		"Secret": secret,
+		"Host":     host,
+		"Fallback": fallback,
+		"Secret":   secret,
 	}); err != nil {
 		log.Printf("install.sh render: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -707,3 +728,11 @@ func (s *Server) downloadHandler() http.Handler {
 	// install one-liner works before the machine is enrolled.
 	return http.FileServer(http.Dir(s.distDir))
 }
+
+// viaDirectOf tells whether the agent last reached the server at the direct
+// address.
+func viaDirectOf(m *sync.Map, id string) bool {
+	v, _ := m.Load(id)
+	b, _ := v.(bool)
+	return b
+}