From ff2c627edec7aeeadd4a79a95f122ebe6f5e0154 Mon Sep 17 00:00:00 2001
From: Narasimha-sc <166327228+Narasimha-sc@users.noreply.github.com>
Date: Tue, 18 Aug 2026 14:35:44 +0000
Subject: [PATCH] remote control: bind to the address family enabled on the
 host

startTCPServer resolves the wildcard address with AI_PASSIVE only and then
selects IPv6 address when it is present. On the hosts where IPv6 is disabled
in the kernel (ipv6.disable=1) getaddrinfo still returns `::`, creating the
socket fails with EAFNOSUPPORT, and the desktop app cannot be connected from
the mobile app (simplex-chat/simplex-chat#5515).

Add startTCPServerConfigured that also passes AI_ADDRCONFIG, so that the
address families without any configured address are excluded - IPv6 wildcard
address is only chosen when IPv6 is enabled, and IPv4 wildcard address is used
otherwise. Use it for the remote control TLS server, as its address is
advertised as IPv4 in the invitation.

Some systems do not report configured families in all cases (e.g. Windows does
not count loopback addresses), so when resolution with AI_ADDRCONFIG fails the
addresses are resolved without it, with the previous behaviour.

startTCPServer is unchanged and is still used for SMP/XFTP/ntf servers and for
the local TCP server, so they keep failing when IPv6 is disabled.
---
 src/Simplex/Messaging/Transport/Server.hs | 28 +++++++++++++++++++----
 src/Simplex/RemoteControl/Discovery.hs    |  4 ++--
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/src/Simplex/Messaging/Transport/Server.hs b/src/Simplex/Messaging/Transport/Server.hs
index cdfc300b71..239c58d290 100644
--- a/src/Simplex/Messaging/Transport/Server.hs
+++ b/src/Simplex/Messaging/Transport/Server.hs
@@ -24,6 +24,7 @@ module Simplex.Messaging.Transport.Server
     runTransportServerSocket,
     runLocalTCPServer,
     startTCPServer,
+    startTCPServerConfigured,
     loadServerCredential,
     loadFingerprint,
     loadFileFingerprint,
@@ -52,7 +53,7 @@ import Simplex.Messaging.Transport
 import Simplex.Messaging.Transport.Shared
 import Simplex.Messaging.Util (catchAll_, labelMyThread, tshow, unlessM)
 import System.Exit (exitFailure)
-import System.IO.Error (tryIOError)
+import System.IO.Error (catchIOError, tryIOError)
 import System.Mem.Weak (Weak, deRefWeak)
 import UnliftIO (timeout)
 import UnliftIO.Concurrent
@@ -230,11 +231,30 @@ closeServer started clients sock = do
   readTVarIO clients >>= mapM_ (deRefWeak >=> mapM_ killThread)
   void . atomically $ tryPutTMVar started False
 
+-- | Start TCP server binding to IPv6 wildcard address (dual stack), when host is not passed.
+-- It fails when IPv6 is not supported by the host, e.g. disabled in the kernel.
 startTCPServer :: TMVar Bool -> Maybe HostName -> ServiceName -> IO Socket
-startTCPServer started host port = withSocketsDo $ resolve >>= open >>= setStarted
+startTCPServer = startTCPServer_ False
+
+-- | Start TCP server binding to the wildcard address of the family enabled on the host, when host is not passed.
+--
+-- It should only be used for the servers that have to accept connections when IPv6 is disabled,
+-- e.g. remote control server on the desktop, as its address is advertised as IPv4 in the invitation.
+startTCPServerConfigured :: TMVar Bool -> Maybe HostName -> ServiceName -> IO Socket
+startTCPServerConfigured = startTCPServer_ True
+
+startTCPServer_ :: Bool -> TMVar Bool -> Maybe HostName -> ServiceName -> IO Socket
+startTCPServer_ addrConfig started host port = withSocketsDo $ resolve >>= open >>= setStarted
   where
-    resolve =
-      let hints = defaultHints {addrFlags = [AI_PASSIVE], addrSocketType = Stream}
+    -- AI_ADDRCONFIG excludes address families that have no address configured on any interface,
+    -- so IPv6 wildcard address is only chosen when IPv6 is enabled, and IPv4 wildcard address is used otherwise.
+    -- Some systems do not report configured families in all cases (e.g., Windows does not count
+    -- loopback addresses), so when resolution with this flag fails the addresses are resolved without it.
+    resolve
+      | addrConfig = resolveWith [AI_PASSIVE, AI_ADDRCONFIG] `catchIOError` \_ -> resolveWith [AI_PASSIVE]
+      | otherwise = resolveWith [AI_PASSIVE]
+    resolveWith addrFlags =
+      let hints = defaultHints {addrFlags, addrSocketType = Stream}
        in select <$> getAddrInfo (Just hints) host (Just port)
     select as = fromJust $ family AF_INET6 <|> family AF_INET
       where
diff --git a/src/Simplex/RemoteControl/Discovery.hs b/src/Simplex/RemoteControl/Discovery.hs
index 4a69a57a16..697f570034 100644
--- a/src/Simplex/RemoteControl/Discovery.hs
+++ b/src/Simplex/RemoteControl/Discovery.hs
@@ -35,7 +35,7 @@ import qualified Network.UDP as UDP
 import Simplex.Messaging.Transport (TransportPeer (..), defaultSupportedParams)
 import qualified Simplex.Messaging.Transport as Transport
 import Simplex.Messaging.Transport.Client (TransportHost (..))
-import Simplex.Messaging.Transport.Server (mkTransportServerConfig, runTransportServerSocket, startTCPServer)
+import Simplex.Messaging.Transport.Server (mkTransportServerConfig, runTransportServerSocket, startTCPServerConfigured)
 import Simplex.Messaging.Util (ifM, tshow)
 import Simplex.RemoteControl.Discovery.Multicast (setMembership)
 import Simplex.RemoteControl.Types
@@ -80,7 +80,7 @@ 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 (startTCPServerConfigured started Nothing $ maybe "0" show port_) (\_e -> setPort Nothing) $ \socket ->
     ifM
       (atomically $ readTMVar started)
       (runServer started socket)