fix(shadowsocks): check IPv4-mapped-IPv6 compatiblity with UDP socket

- ref #1543

crates/shadowsocks/src/net/sys/mod.rs

@@ -1,7 +1,6 @@
 use std::{
     io::{self, ErrorKind},
     net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr},
-    time::Duration,
 };
 
 use cfg_if::cfg_if;
@@ -177,39 +176,34 @@ static IP_STACK_CAPABILITIES: Lazy<IpStackCapabilities> = Lazy::new(|| {
 });
 
 fn check_ipv4_mapped_ipv6_capability() -> io::Result<()> {
+    // https://stackoverflow.com/questions/30184377/how-to-detect-if-dual-stack-socket-is-supported
+
     let local_host = SockAddr::from(SocketAddr::new(Ipv4Addr::LOCALHOST.to_ipv6_mapped().into(), 0));
+    let local_host_53 = SockAddr::from(SocketAddr::new(Ipv4Addr::LOCALHOST.to_ipv6_mapped().into(), 53));
 
-    let socket1 = Socket::new(Domain::IPV6, Type::STREAM, Some(Protocol::TCP))?;
-    socket1.set_only_v6(false)?;
-    socket1.bind(&local_host)?;
-    socket1.listen(128)?;
+    let socket = Socket::new(Domain::IPV6, Type::DGRAM, Some(Protocol::UDP))?;
+    socket.set_only_v6(false)?;
+    socket.bind(&local_host)?;
+    socket.connect(&local_host_53)?;
 
-    let socket1_address = socket1.local_addr()?;
-    match socket1_address.as_socket() {
+    // getsockname()
+    let local_addr = socket.local_addr()?;
+    match local_addr.as_socket() {
         None => return Err(io::Error::new(ErrorKind::Other, "invalid local_addr")),
-        Some(socket_addr) => match socket_addr {
-            SocketAddr::V4(..) => {
-                return Err(io::Error::new(
-                    ErrorKind::Other,
-                    "local_addr shouldn't be an IPv4 address",
-                ));
-            }
-            SocketAddr::V6(ref v6) => {
-                if let None = v6.ip().to_ipv4_mapped() {
+        Some(addr) => match addr {
+            SocketAddr::V4(..) => return Err(io::Error::new(ErrorKind::Other, "local_addr returned an IPv4 address")),
+            SocketAddr::V6(v6) => match v6.ip().to_ipv4_mapped() {
+                Some(_) => {}
+                None => {
                     return Err(io::Error::new(
                         ErrorKind::Other,
-                        "local_addr is not an IPv4-mapped-IPv6 address",
+                        "local_addr returned an non IPv4-mapped-IPv6 address",
                     ));
                 }
-            }
+            },
         },
     }
 
-    let socket2 = Socket::new(Domain::IPV6, Type::STREAM, Some(Protocol::TCP))?;
-    socket2.set_only_v6(false)?;
-    socket2.bind(&local_host)?;
-    socket2.connect_timeout(&socket1_address, Duration::from_secs(1))?;
-
     Ok(())
 }