[NET-747] Preserve IPv4-mapped IPv6 addresses in SubnetUtils6

src/changes/changes.xml

@@ -63,6 +63,7 @@ The <action> type attribute can be add,update,fix,remove.
   <body>
     <release version="3.13.1" date="YYYY-MM-DD" description="This is a feature and maintenance release. Java 8 or later is required.">
       <!-- FIX -->
+      <action type="fix" issue="NET-747" due-to="Aryan Ganjoo">Preserve IPv4-mapped IPv6 addresses in SubnetUtils6 parsing and formatting.</action>
       <action type="fix" dev="ggregory" due-to="Javid Khan, Gary Gregory">Reject CR/LF in SimpleSMTPHeader and SimpleNNTPHeader fields #397.</action>
       <action type="fix" dev="sebb" due-to="Javid Khan">Reject CR/LF in IMAP quoteMailboxName (#399).</action>
       <action type="fix" dev="ggregory" due-to="Javid Khan, Gary Gregory">Reject CR/LF in FTP, POP3, NNTP and SMTP command senders (#400).</action>

src/main/java/org/apache/commons/net/util/SubnetUtils6.java

@@ -17,6 +17,7 @@
 package org.apache.commons.net.util;
 
 import java.math.BigInteger;
+import java.net.Inet4Address;
 import java.net.Inet6Address;
 import java.net.InetAddress;
 import java.net.UnknownHostException;
@@ -191,7 +192,8 @@ public String toString() {
     private static String format(final BigInteger addr) {
         final byte[] bytes = toByteArray16(addr);
         try {
-            return InetAddress.getByAddress(bytes).getHostAddress();
+            // Inet6Address.getByAddress keeps IPv4-mapped addresses (::ffff:0:0/96) as IPv6; InetAddress.getByAddress would convert them to IPv4.
+            return Inet6Address.getByAddress(null, bytes, -1).getHostAddress();
         } catch (final UnknownHostException e) {
             // Should never happen with a valid 16-byte array
             throw new IllegalStateException("Unexpected error formatting IPv6 address", e);
@@ -233,6 +235,14 @@ private static byte[] toBytes(final String address) {
             if (inetAddr instanceof Inet6Address) {
                 return inetAddr.getAddress();
             }
+            if (inetAddr instanceof Inet4Address && address.indexOf(':') >= 0) {
+                // The JDK parses IPv4-mapped IPv6 literals such as "::ffff:192.0.2.1" to an Inet4Address.
+                final byte[] mapped = new byte[16];
+                mapped[10] = (byte) 0xff;
+                mapped[11] = (byte) 0xff;
+                System.arraycopy(inetAddr.getAddress(), 0, mapped, 12, 4);
+                return mapped;
+            }
             throw new IllegalArgumentException(String.format(PARSE_FAIL, address) + " - not an IPv6 address");
         } catch (final UnknownHostException e) {
             throw new IllegalArgumentException(String.format(PARSE_FAIL, address), e);

src/test/java/org/apache/commons/net/util/SubnetUtils6Test.java

@@ -171,6 +171,23 @@ void testInvalidIPv4Address() {
         assertThrows(IllegalArgumentException.class, () -> new SubnetUtils6("192.168.1.1/24"));
     }
 
+    @Test
+    void testIPv4MappedAddresses() {
+        final SubnetInfo info = new SubnetUtils6("::ffff:0:0/96").getInfo();
+        assertEquals("0:0:0:0:0:ffff:0:0", info.getNetworkAddress());
+        assertEquals("0:0:0:0:0:ffff:ffff:ffff", info.getHighAddress());
+        assertTrue(info.isInRange("::ffff:192.0.2.1"));
+        assertTrue(info.isInRange("::ffff:c000:201"));
+        assertFalse(info.isInRange("::192.0.2.1"));
+        assertEquals("0:0:0:0:0:ffff:c000:201/128", new SubnetUtils6("::ffff:192.0.2.1/128").getInfo().getCidrSignature());
+        // A high address that falls in the IPv4-mapped range must be formatted as IPv6 and round-trip.
+        final SubnetInfo low = new SubnetUtils6("::/80").getInfo();
+        assertEquals("0:0:0:0:0:ffff:ffff:ffff", low.getHighAddress());
+        assertTrue(low.isInRange(low.getHighAddress()));
+        // Plain IPv4 text is still rejected.
+        assertThrows(IllegalArgumentException.class, () -> info.isInRange("192.0.2.1"));
+    }
+
     @Test
     void testInvalidTwoArgConstructor() {
         assertThrows(IllegalArgumentException.class, () -> new SubnetUtils6("2001:db8::1", 129));