Added "bindAddressFallback" setting, which defaults to "0.0.0.0".

Should fix problems on systems unable to use IPv6 wildcard (::) for listening, and avoids having to manually specify "bindAddress": "0.0.0.0" in settings.json.

src/main/java/org/qortal/api/ApiService.java

@@ -41,6 +41,7 @@ import org.glassfish.jersey.servlet.ServletContainer;
 import org.qortal.api.resource.AnnotationPostProcessor;
 import org.qortal.api.resource.ApiDefinition;
 import org.qortal.api.websocket.*;
+import org.qortal.network.Network;
 import org.qortal.settings.Settings;
 
 public class ApiService {
@@ -123,13 +124,13 @@ public class ApiService {
 				ServerConnector portUnifiedConnector = new ServerConnector(this.server,
 						new DetectorConnectionFactory(sslConnectionFactory),
 						httpConnectionFactory);
-				portUnifiedConnector.setHost(Settings.getInstance().getBindAddress());
+				portUnifiedConnector.setHost(Network.getInstance().getBindAddress());
 				portUnifiedConnector.setPort(Settings.getInstance().getApiPort());
 
 				this.server.addConnector(portUnifiedConnector);
 			} else {
 				// Non-SSL
-				InetAddress bindAddr = InetAddress.getByName(Settings.getInstance().getBindAddress());
+				InetAddress bindAddr = InetAddress.getByName(Network.getInstance().getBindAddress());
 				InetSocketAddress endpoint = new InetSocketAddress(bindAddr, Settings.getInstance().getApiPort());
 				this.server = new Server(endpoint);
 			}

src/main/java/org/qortal/api/DomainMapService.java

@@ -16,6 +16,7 @@ import org.glassfish.jersey.server.ResourceConfig;
 import org.glassfish.jersey.servlet.ServletContainer;
 import org.qortal.api.resource.AnnotationPostProcessor;
 import org.qortal.api.resource.ApiDefinition;
+import org.qortal.network.Network;
 import org.qortal.settings.Settings;
 
 import javax.net.ssl.KeyManagerFactory;
@@ -99,13 +100,13 @@ public class DomainMapService {
 				ServerConnector portUnifiedConnector = new ServerConnector(this.server,
 						new DetectorConnectionFactory(sslConnectionFactory),
 						httpConnectionFactory);
-				portUnifiedConnector.setHost(Settings.getInstance().getBindAddress());
+				portUnifiedConnector.setHost(Network.getInstance().getBindAddress());
 				portUnifiedConnector.setPort(Settings.getInstance().getDomainMapPort());
 
 				this.server.addConnector(portUnifiedConnector);
 			} else {
 				// Non-SSL
-				InetAddress bindAddr = InetAddress.getByName(Settings.getInstance().getBindAddress());
+				InetAddress bindAddr = InetAddress.getByName(Network.getInstance().getBindAddress());
 				InetSocketAddress endpoint = new InetSocketAddress(bindAddr, Settings.getInstance().getDomainMapPort());
 				this.server = new Server(endpoint);
 			}

src/main/java/org/qortal/api/GatewayService.java

@@ -15,6 +15,7 @@ import org.glassfish.jersey.server.ResourceConfig;
 import org.glassfish.jersey.servlet.ServletContainer;
 import org.qortal.api.resource.AnnotationPostProcessor;
 import org.qortal.api.resource.ApiDefinition;
+import org.qortal.network.Network;
 import org.qortal.settings.Settings;
 
 import javax.net.ssl.KeyManagerFactory;
@@ -98,13 +99,13 @@ public class GatewayService {
 				ServerConnector portUnifiedConnector = new ServerConnector(this.server,
 						new DetectorConnectionFactory(sslConnectionFactory),
 						httpConnectionFactory);
-				portUnifiedConnector.setHost(Settings.getInstance().getBindAddress());
+				portUnifiedConnector.setHost(Network.getInstance().getBindAddress());
 				portUnifiedConnector.setPort(Settings.getInstance().getGatewayPort());
 
 				this.server.addConnector(portUnifiedConnector);
 			} else {
 				// Non-SSL
-				InetAddress bindAddr = InetAddress.getByName(Settings.getInstance().getBindAddress());
+				InetAddress bindAddr = InetAddress.getByName(Network.getInstance().getBindAddress());
 				InetSocketAddress endpoint = new InetSocketAddress(bindAddr, Settings.getInstance().getGatewayPort());
 				this.server = new Server(endpoint);
 			}

src/main/java/org/qortal/network/Network.java

@@ -124,6 +124,8 @@ public class Network {
 
     private final List<PeerAddress> selfPeers = new ArrayList<>();
 
+    private String bindAddress = null;
+
     private final ExecuteProduceConsume networkEPC;
     private Selector channelSelector;
     private ServerSocketChannel serverChannel;
@@ -159,9 +161,19 @@ public class Network {
         // Grab P2P port from settings
         int listenPort = Settings.getInstance().getListenPort();
 
-        // Grab P2P bind address from settings
+        // Grab P2P bind addresses from settings
+        List<String> bindAddresses = new ArrayList<>();
+        if (Settings.getInstance().getBindAddress() != null) {
+            bindAddresses.add(Settings.getInstance().getBindAddress());
+        }
+        if (Settings.getInstance().getBindAddressFallback() != null) {
+            bindAddresses.add(Settings.getInstance().getBindAddressFallback());
+        }
+
+        for (int i=0; i<bindAddresses.size(); i++) {
             try {
-            InetAddress bindAddr = InetAddress.getByName(Settings.getInstance().getBindAddress());
+                String bindAddress = bindAddresses.get(i);
+                InetAddress bindAddr = InetAddress.getByName(bindAddress);
                 InetSocketAddress endpoint = new InetSocketAddress(bindAddr, listenPort);
 
                 channelSelector = Selector.open();
@@ -172,13 +184,21 @@ public class Network {
                 serverChannel.setOption(StandardSocketOptions.SO_REUSEADDR, true);
                 serverChannel.bind(endpoint, LISTEN_BACKLOG);
                 serverSelectionKey = serverChannel.register(channelSelector, SelectionKey.OP_ACCEPT);
+
+                this.bindAddress = bindAddress; // Store the selected address, so that it can be used by other parts of the app
+                break; // We don't want to bind to more than one address
             } catch (UnknownHostException e) {
                 LOGGER.error("Can't bind listen socket to address {}", Settings.getInstance().getBindAddress());
+                if (i == bindAddresses.size()-1) { // Only throw an exception if all addresses have been tried
                     throw new IOException("Can't bind listen socket to address", e);
+                }
             } catch (IOException e) {
                 LOGGER.error("Can't create listen socket: {}", e.getMessage());
+                if (i == bindAddresses.size()-1) { // Only throw an exception if all addresses have been tried
                     throw new IOException("Can't create listen socket", e);
                 }
+            }
+        }
 
         // Load all known peers from repository
         synchronized (this.allKnownPeers) {
@@ -228,6 +248,10 @@ public class Network {
         return this.maxPeers;
     }
 
+    public String getBindAddress() {
+        return this.bindAddress;
+    }
+
     public byte[] getMessageMagic() {
         return Settings.getInstance().isTestNet() ? TESTNET_MESSAGE_MAGIC : MAINNET_MESSAGE_MAGIC;
     }
@@ -1556,7 +1580,7 @@ public class Network {
         this.isShuttingDown = true;
 
         // Close listen socket to prevent more incoming connections
-        if (this.serverChannel.isOpen()) {
+        if (this.serverChannel != null && this.serverChannel.isOpen()) {
             try {
                 this.serverChannel.close();
             } catch (IOException e) {

src/main/java/org/qortal/settings/Settings.java

@@ -61,6 +61,7 @@ public class Settings {
 
 	// Common to all networking (API/P2P)
 	private String bindAddress = "::"; // Use IPv6 wildcard to listen on all local addresses
+	private String bindAddressFallback = "0.0.0.0"; // Some systems are unable to bind using IPv6
 
 	// UI servers
 	private int uiPort = 12388;
@@ -684,6 +685,10 @@ public class Settings {
 		return this.bindAddress;
 	}
 
+	public String getBindAddressFallback() {
+		return this.bindAddressFallback;
+	}
+
 	public boolean isUPnPEnabled() {
 		return this.uPnPEnabled;
 	}