Resolve target hostnames to IPv4 before connecting the OSC sender

CMakeLists.txt

@@ -42,6 +42,7 @@ function(configure_osccontrol_target TARGET)
         Source/ControlElementToggle.h
         Source/ControlElementUI.cpp
         Source/ControlElementUI.h
+        Source/HostResolver.h
         Source/LayoutHints.cpp
         Source/LayoutHints.h
         Source/PluginEditor.cpp

Source/HostResolver.h

@@ -0,0 +1,94 @@
+/*
+
+  osccontrol-light - An audio plugin that speaks OSC.
+  Copyright (C) 2020 Patric Schmitz
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+#pragma once
+
+#include "JuceHeader.h"
+
+#if JUCE_WINDOWS
+  #include <ws2tcpip.h>
+#else
+  #include <netdb.h>
+  #include <netinet/in.h>
+  #include <sys/socket.h>
+#endif
+
+/** Resolve a hostname to its first IPv4 address, as a dotted-quad string.
+
+    JUCE creates its datagram socket as IPv4-only:
+
+        handle = (int) socket (AF_INET, SOCK_DGRAM, 0);   // juce_Socket.cpp
+
+    but resolves target names with hints.ai_family = AF_UNSPEC, and
+    DatagramSocket::write() then calls sendto() on info->ai_addr - the first
+    entry of the returned list, without walking it. When a name resolves to an
+    IPv6 address first, that address is handed to an AF_INET socket and every
+    send fails.
+
+    This is easy to hit with mDNS ".local" names, which commonly resolve to an
+    IPv6 link-local address ahead of the IPv4 one. The failure is silent from
+    the user's point of view: OSCSender::connect() only binds a local port and
+    never resolves the target, so it still reports success, and the plugin
+    appears to work while no packets leave the machine.
+
+    Resolving to an IPv4 literal before handing the host to JUCE avoids the
+    problem entirely, since a dotted-quad can only produce an AF_INET result.
+
+    Returns the input unchanged if the lookup fails or yields no IPv4 address,
+    so behaviour is never worse than before this call existed. Addresses that
+    are already literals pass through untouched.
+*/
+inline juce::String resolveHostToIPv4 (const juce::String & host)
+{
+    if (host.isEmpty())
+        return host;
+
+    struct addrinfo hints;
+    juce::zerostruct (hints);
+
+    hints.ai_family   = AF_INET;       // IPv4 only, to match JUCE's socket
+    hints.ai_socktype = SOCK_DGRAM;
+
+    struct addrinfo * info = nullptr;
+
+    if (getaddrinfo (host.toRawUTF8(), nullptr, &hints, &info) != 0
+        || info == nullptr)
+        return host;
+
+    juce::String result (host);
+
+    for (auto * i = info; i != nullptr; i = i->ai_next)
+    {
+        if (i->ai_family != AF_INET || i->ai_addr == nullptr)
+            continue;
+
+        auto * addr = reinterpret_cast<struct sockaddr_in *> (i->ai_addr);
+        auto raw = (juce::uint32) ntohl (addr->sin_addr.s_addr);
+
+        result = juce::String ((raw >> 24) & 0xff) + "."
+               + juce::String ((raw >> 16) & 0xff) + "."
+               + juce::String ((raw >>  8) & 0xff) + "."
+               + juce::String ( raw        & 0xff);
+        break;
+    }
+
+    freeaddrinfo (info);
+    return result;
+}

Source/PluginProcessor.cpp

@@ -26,6 +26,8 @@
 
 #include "PresetParser.h"
 
+#include "HostResolver.h"
+
 #include "ControlElementHost.h"
 #include "ControlElementFactory.h"
 
@@ -130,7 +132,8 @@ initializeHeadless
     PresetParser preset (inputStream);
 
     oscSender = std::make_unique<OSCSender> ();
-    oscSender->connect (preset.getHost (), preset.getPort ());
+    oscSender->connect (resolveHostToIPv4 (preset.getHost ()),
+                        preset.getPort ());
 
     ControlElementFactory factory (*oscSender);
     for(auto control : preset.getControlElements ()) {

Source/PresetPage.cpp

@@ -20,6 +20,8 @@
 
 #include "PresetParser.h"
 
+#include "HostResolver.h"
+
 #include "LayoutHints.h"
 
 #include "ControlContainer.h"
@@ -128,7 +130,7 @@ connectOsc ()
         + host.toString () + ":" + port.toString();
     Logger::writeToLog(message);
 
-    oscSender.connect(host.getValue (), port.getValue ());
+    oscSender.connect(resolveHostToIPv4 (host.toString ()), port.getValue ());
 
     for (auto & control : container->getControlElements ()) {
         control->setEnabled(true);