Falls back to unresolved hostname in ForwardedHeadersFilter to prevent NPE.

fixes gh-1601

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/headers/ForwardedHeadersFilter.java

@@ -116,7 +116,10 @@ public class ForwardedHeadersFilter implements HttpHeadersFilter, Ordered {
 
 		InetSocketAddress remoteAddress = request.getRemoteAddress();
 		if (remoteAddress != null) {
-			String forValue = remoteAddress.getAddress().getHostAddress();
+			// If remoteAddress is unresolved, calling getHostAddress() would cause a
+			// NullPointerException.
+			String forValue = remoteAddress.isUnresolved() ? remoteAddress.getHostName()
+					: remoteAddress.getAddress().getHostAddress();
 			int port = remoteAddress.getPort();
 			if (port >= 0) {
 				forValue = forValue + ":" + port;

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/headers/ForwardedHeadersFilterTests.java

@@ -131,6 +131,30 @@ public class ForwardedHeadersFilterTests {
 				.containsEntry("for", "\"10.0.0.1:80\"");
 	}
 
+	@Test
+	public void unresolvedRemoteAddressFallsBackToHostName() throws UnknownHostException {
+		MockServerHttpRequest request = MockServerHttpRequest.get("http://localhost/get")
+				.remoteAddress(
+						InetSocketAddress.createUnresolved("unresolvable-hostname", 80))
+				.build();
+
+		ForwardedHeadersFilter filter = new ForwardedHeadersFilter();
+
+		HttpHeaders headers = filter.filter(request.getHeaders(),
+				MockServerWebExchange.from(request));
+
+		assertThat(headers.get(FORWARDED_HEADER)).hasSize(1);
+
+		List<Forwarded> forwardeds = ForwardedHeadersFilter
+				.parse(headers.get(FORWARDED_HEADER));
+
+		assertThat(forwardeds).hasSize(1);
+		Forwarded forwarded = forwardeds.get(0);
+
+		assertThat(forwarded.getValues()).containsEntry("proto", "http")
+				.containsEntry("for", "\"unresolvable-hostname:80\"");
+	}
+
 	@Test
 	public void forwardedParsedCorrectly() {
 		String[] valid = new String[] { "for=\"_gazonk\"",