From d340e19e72ffe277d47940d4cc54a3042a93dd71 Mon Sep 17 00:00:00 2001
From: Narasimha-sc <166327228+Narasimha-sc@users.noreply.github.com>
Date: Mon, 20 Jul 2026 19:36:58 +0000
Subject: [PATCH] remote control: fall back to IPv4 when IPv6 bind fails

The remote-host TLS listener binds via startTCPServer with host=Nothing,
which prefers the IPv6 wildcard (::). The invitation sent to the mobile
device only ever advertises an IPv4 address, so when IPv6 is disabled the
bind to :: throws and the desktop app cannot be reached (issue #5515).

Keep the existing IPv6-preferring bind and only fall back to binding
0.0.0.0 when it fails, so behaviour is unchanged whenever IPv6 is
available and the SMP server transport path is not touched.
---
 src/Simplex/RemoteControl/Discovery.hs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/Simplex/RemoteControl/Discovery.hs b/src/Simplex/RemoteControl/Discovery.hs
index 4a69a57a16..7f47959326 100644
--- a/src/Simplex/RemoteControl/Discovery.hs
+++ b/src/Simplex/RemoteControl/Discovery.hs
@@ -80,12 +80,17 @@ preferAddress RCCtrlAddress {address, interface} addrs =
 startTLSServer :: Maybe Word16 -> TMVar (Maybe N.PortNumber) -> TLS.Credential -> TLS.ServerHooks -> (Transport.TLS 'TServer -> IO ()) -> IO (Async ())
 startTLSServer port_ startedOnPort credentials hooks server = async . liftIO $ do
   started <- newEmptyTMVarIO
-  bracketOnError (startTCPServer started Nothing $ maybe "0" show port_) (\_e -> setPort Nothing) $ \socket ->
+  bracketOnError (startTCPServerCtrl started $ maybe "0" show port_) (\_e -> setPort Nothing) $ \socket ->
     ifM
       (atomically $ readTMVar started)
       (runServer started socket)
       (setPort Nothing)
   where
+    -- Bind preferring IPv6 (dual-stack) as before; if that fails (e.g. IPv6 is
+    -- disabled), fall back to IPv4 so the (IPv4-only) invitation stays reachable.
+    startTCPServerCtrl started port =
+      startTCPServer started Nothing port
+        `catchAny` \_ -> startTCPServer started (Just "0.0.0.0") port
     runServer started socket = do
       port <- N.socketPort socket
       logInfo $ "System-assigned port: " <> tshow port