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/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