src/java.base/share/classes/sun/net/util/IPAddressUtil.java

*** 23,32 ****
--- 23,41 ----
   * questions.
   */
  
  package sun.net.util;
  
+ import java.net.Inet6Address;
+ import java.net.InetAddress;
+ import java.net.NetworkInterface;
+ import java.net.SocketException;
+ import java.security.AccessController;
+ import java.security.PrivilegedAction;
+ import java.util.Map;
+ import java.util.concurrent.ConcurrentHashMap;
+ 
  public class IPAddressUtil {
      private static final int INADDR4SZ = 4;
      private static final int INADDR16SZ = 16;
      private static final int INT16SZ = 2;
  
*** 285,290 ****
--- 294,352 ----
              (addr[11] == (byte)0xff))  {
              return true;
          }
          return false;
      }
+ 
+     /** Returns a scopeId suitable for binding, or -1 if none */
+     public static int bindingScopeId(InetAddress address) {
+         if (!(address instanceof Inet6Address))
+             return -1;
+ 
+         Inet6Address ia6 = (Inet6Address)address;
+         if (ia6.getScopeId() != 0) {  // explicit scopeId
+             return ia6.getScopeId();
+         }
+ 
+         int scopeId = -1;
+         if (ia6.isLinkLocalAddress()) {
+             scopeId = IPAddressUtil.scopeFromLocalAddress(ia6);
+         }
+ 
+         //TODO: if macOS default interface stuff
+ 
+         return scopeId;
+     }
+ 
+     private static Map<Inet6Address,Integer> localAddressScopeIdMap = new ConcurrentHashMap<>();
+ 
+     private static int scopeFromLocalAddress(Inet6Address addr) {
+         assert addr.isLinkLocalAddress() :  addr;
+ 
+         int scopeId = privilegedLookupScopeId(addr);
+         if (scopeId == -1)
+             return -1;
+         int cur = localAddressScopeIdMap.putIfAbsent(addr, scopeId);
+         assert cur == scopeId;
+         return scopeId;
+     }
+ 
+     private static int privilegedLookupScopeId(Inet6Address ia6) {
+         PrivilegedAction<Integer> pa = () -> lookupScopeID(ia6);
+         return AccessController.doPrivileged(pa);
+     }
+ 
+     private static int lookupScopeID(Inet6Address ia6) {
+         try {
+             return NetworkInterface.networkInterfaces()
+                     .filter(nif -> nif.inetAddresses()
+                                       .filter(addr -> addr.equals(ia6))
+                                       .findFirst().isPresent())
+                     .findFirst()
+                     .map(NetworkInterface::getIndex)
+                     .orElse(-1);
+         } catch (SocketException ignore) {
+             // ignore, handle as not found
+         }
+         return -1;
+     }
  }