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/agent/client.go

@@ -319,7 +319,7 @@ func (a *Agent) pollOnce(ctx, reqCtx context.Context) error {
 		return err
 	}
 	a.applyTunnelHeader(ctx, resp.Header)
-	a.learnFallback(resp.Header.Get(proto.FallbackHeader))
+	a.learnAddrs(resp.Header.Get(proto.MainHeader), resp.Header.Get(proto.FallbackHeader))
 	a.applyTransport(resp.Header.Get(proto.TransportHeader))
 	a.dispatch(ctx, msg)
 	return nil
@@ -355,7 +355,7 @@ func (a *Agent) fetchConfig(ctx context.Context) bool {
 	}
 	defer resp.Body.Close()
 	a.applyTransport(resp.Header.Get(proto.TransportHeader))
-	a.learnFallback(resp.Header.Get(proto.FallbackHeader))
+	a.learnAddrs(resp.Header.Get(proto.MainHeader), resp.Header.Get(proto.FallbackHeader))
 	return a.applyTunnelHeader(ctx, resp.Header)
 }
 

internal/agent/hosts.go

@@ -54,10 +54,16 @@ func (a *Agent) fallback() string {
 	return a.state.Fallback
 }
 
-// learnFallback keeps the direct address the server tells.
-func (a *Agent) learnFallback(v string) {
-	v = strings.TrimRight(strings.TrimSpace(v), "/")
-	if v == "" || a.cfg.Fallback != "" {
+// learnAddrs keeps the server's other address from the two it tells: the
+// direct one, or the main one when the agent was set to the direct one.
+func (a *Agent) learnAddrs(main, direct string) {
+	main = strings.TrimRight(strings.TrimSpace(main), "/")
+	direct = strings.TrimRight(strings.TrimSpace(direct), "/")
+	v := direct
+	if direct != "" && strings.EqualFold(direct, a.cfg.Host) {
+		v = main
+	}
+	if v == "" || strings.EqualFold(v, a.cfg.Host) || a.cfg.Fallback != "" {
 		return
 	}
 	a.stateMu.Lock()
@@ -67,7 +73,7 @@ func (a *Agent) learnFallback(v string) {
 	}
 	a.stateMu.Unlock()
 	if !same {
-		log.Printf("direct address: %s", v)
+		log.Printf("fallback address: %s", v)
 		_ = a.saveState()
 	}
 }
@@ -163,7 +169,7 @@ func (a *Agent) reachableAt(ctx context.Context, base string) bool {
 	}
 	resp.Body.Close()
 	if resp.StatusCode < 300 {
-		a.learnFallback(resp.Header.Get(proto.FallbackHeader))
+		a.learnAddrs(resp.Header.Get(proto.MainHeader), resp.Header.Get(proto.FallbackHeader))
 		return true
 	}
 	return false

internal/agent/wslink.go

@@ -165,7 +165,7 @@ func (a *Agent) runWS(ctx context.Context) error {
 			}
 		case "cfg":
 			a.applyTunnelAddr(ctx, m.Tunnel)
-			a.learnFallback(m.Fallback)
+			a.learnAddrs(m.Main, m.Fallback)
 			a.applyTransport(m.Transport)
 			if a.mode() == proto.TransportLP {
 				return errors.New("set to long polling")

internal/proto/proto.go

@@ -77,6 +77,10 @@ const TransportHeader = "X-Transport"
 // to when the main one is out of reach.
 const FallbackHeader = "X-Fallback-Host"
 
+// MainHeader carries the server's main address beside FallbackHeader, for an
+// agent set to the direct one to fall back to the main one.
+const MainHeader = "X-Main-Host"
+
 // WSMsg is one message on an agent's WebSocket, either way. Server to agent:
 // "ctrl" (Ctrl, as a control poll would answer), "cfg" (Tunnel and
 // Transport, also a keep-alive), "in" (Frames for session SID), "gone"
@@ -91,6 +95,7 @@ type WSMsg struct {
 	Tunnel    string          `json:"tunnel,omitempty"`
 	Transport string          `json:"transport,omitempty"`
 	Fallback  string          `json:"fallback,omitempty"`
+	Main      string          `json:"main,omitempty"`
 }
 
 // Terminal / stream frame types on the session "in" channel (server -> agent).

internal/server/handlers_agent.go

@@ -45,6 +45,7 @@ func (s *Server) handlePoll(w http.ResponseWriter, r *http.Request) {
 	}
 	// The agent sends its version and OS user occasionally; record them when
 	// present. Agents older than 0.0.2 send no user, so keep what we have.
+	s.viaDirectA.Store(a.ID, s.viaDirect(r))
 	v := r.Header.Get("X-Agent-Version")
 	u := firstNonEmpty(r.Header.Get("X-Agent-User"), a.RunUser)
 	if v != "" && (v != a.Version || u != a.RunUser) {
@@ -69,6 +70,7 @@ func (s *Server) setTunnelHeader(w http.ResponseWriter) {
 	}
 	if d := s.db.DirectAddress(); d != "" {
 		w.Header().Set(proto.FallbackHeader, d)
+		w.Header().Set(proto.MainHeader, s.db.HostAddress())
 	}
 }
 

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
+}

internal/server/server.go

@@ -60,6 +60,7 @@ type Server struct {
 	meters     speedMeters       // byte counts of the speed tests under way
 	xfers      xfers             // files on their way between a browser and an agent
 	ship       shipping          // files on their way from one machine to another
+	viaDirectA sync.Map          // agent id -> true while it reaches the server at the direct address
 	challenges challenges        // passkey challenges handed out
 	wsAgents   wsAgents          // agents on their WebSocket now
 	pinMisses  pinMisses         // wrong PINs per session

internal/server/web/install.sh.tmpl

@@ -7,9 +7,10 @@
 set -eu
 
 HOST="${HOST:-{{.Host}}}"
-# The direct address (not behind a proxy like Cloudflare): the agent turns to
-# it when the main one is out of reach.
-FALLBACK_HOST="${FALLBACK_HOST:-{{.Direct}}}"
+# The server's other address (the direct one, not behind a proxy like
+# Cloudflare, or the main one when this script came from the direct one): the
+# agent turns to it when HOST is out of reach.
+FALLBACK_HOST="${FALLBACK_HOST:-{{.Fallback}}}"
 {{if .Secret}}ENROLL_SECRET="${ENROLL_SECRET:-{{.Secret}}}"
 {{end}}
 BIN_PATH="/usr/local/bin/lpagent"
@@ -123,7 +124,7 @@ bold "Скачивание агента ($ARCH) с $HOST…"
 TMP="$(mktemp)"
 if ! download "$HOST/download/agent-linux-$ARCH" "$TMP"; then
   if [ -n "$FALLBACK_HOST" ] && [ "$FALLBACK_HOST" != "$HOST" ]; then
-    bold "Основной адрес недоступен, скачивание с $FALLBACK_HOST…"
+    bold "$HOST недоступен, скачивание с $FALLBACK_HOST…"
     download "$FALLBACK_HOST/download/agent-linux-$ARCH" "$TMP"
   else
     red "Не удалось скачать агента."; exit 1

internal/server/web/templates/edit.html

@@ -76,7 +76,7 @@
   <div class="card">
     <h2>Связь с сервером</h2>
     <p class="muted">
-      Сейчас: {{if not .Online}}<span>офлайн</span>{{else if .OnWS}}<span style="color:var(--ok)">WebSocket</span>{{else}}<span>long polling</span>{{end}}
+      Сейчас: {{if not .Online}}<span>офлайн</span>{{else if .OnWS}}<span style="color:var(--ok)">WebSocket</span>{{else}}<span>long polling</span>{{end}}{{if and .Online .ViaDirect}} <span class="muted">(direct)</span>{{end}}
     </p>
     <form method="post" action="/agents/{{.Agent.ID}}/transport" class="row">
       <input type="hidden" name="csrf" value="{{.CSRF}}">

internal/server/wsagent.go

@@ -84,6 +84,7 @@ func (s *Server) handleAgentWS(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		return
 	}
+	s.viaDirectA.Store(a.ID, s.viaDirect(r))
 	conn.SetReadLimit(4 << 20)
 	ctx, cancel := context.WithCancel(r.Context())
 	defer cancel()
@@ -97,7 +98,7 @@ func (s *Server) handleAgentWS(w http.ResponseWriter, r *http.Request) {
 		return wsjson.Write(wctx, conn, m)
 	}
 	cfg := func() proto.WSMsg {
-		return proto.WSMsg{T: "cfg", Tunnel: s.tunnelAdvert(), Transport: s.transportOf(a.ID), Fallback: s.db.DirectAddress()}
+		return proto.WSMsg{T: "cfg", Tunnel: s.tunnelAdvert(), Transport: s.transportOf(a.ID), Fallback: s.db.DirectAddress(), Main: s.db.HostAddress()}
 	}
 	if send(cfg()) != nil {
 		return