Try make IPv6 work on travis

.travis.yml

@@ -4,6 +4,13 @@ branches:
   only:
   - master
 
+before_install:
+  # Add an IPv6 config - see the corresponding Travis issue
+  # https://github.com/travis-ci/travis-ci/issues/8361
+  - if [ "${TRAVIS_OS_NAME}" == "linux" ]; then 
+      sudo sh -c 'echo 0 > /proc/sys/net/ipv6/conf/all/disable_ipv6';
+    fi
+
 cache: cargo
 
 matrix:

libp2p-tcp-transport/src/lib.rs

@@ -258,7 +258,7 @@ mod tests {
     }
 
     #[test]
-    fn replace_port_0_in_returned_multiaddr() {
+    fn replace_port_0_in_returned_multiaddr_ipv4() {
         let core = Core::new().unwrap();
         let tcp = TcpConfig::new(core.handle());
 
@@ -269,6 +269,18 @@ mod tests {
         assert!(!new_addr.to_string().contains("tcp/0"));
     }
 
+    #[test]
+    fn replace_port_0_in_returned_multiaddr_ipv6() {
+        let core = Core::new().unwrap();
+        let tcp = TcpConfig::new(core.handle());
+
+        let addr = Multiaddr::new("/ip6/::1/tcp/0").unwrap();
+        assert!(addr.to_string().contains("tcp/0"));
+
+        let (_, new_addr) = tcp.listen_on(addr).unwrap();
+        assert!(!new_addr.to_string().contains("tcp/0"));
+    }
+
     #[test]
     fn larger_addr_denied() {
         let core = Core::new().unwrap();