Allows duplicate X-Forwarded-For headers.

Fixes gh-826
Fixes gh-1760

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

@@ -207,12 +207,7 @@ public class XForwardedHeadersFilter implements HttpHeadersFilter, Ordered {
 		if (isForEnabled() && request.getRemoteAddress() != null
 				&& request.getRemoteAddress().getAddress() != null) {
 			String remoteAddr = request.getRemoteAddress().getAddress().getHostAddress();
-			List<String> xforwarded = original.get(X_FORWARDED_FOR_HEADER);
-			// prevent duplicates
-			if (remoteAddr != null
-					&& (xforwarded == null || !xforwarded.contains(remoteAddr))) {
-				write(updated, X_FORWARDED_FOR_HEADER, remoteAddr, isForAppend());
-			}
+			write(updated, X_FORWARDED_FOR_HEADER, remoteAddr, isForAppend());
 		}
 
 		String proto = request.getURI().getScheme();

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

@@ -331,4 +331,23 @@ public class XForwardedHeadersFilterTests {
 		assertThat(headers).isEmpty();
 	}
 
+	@Test
+	public void allowDuplicateEntriesInXForwardedForHeader() throws Exception {
+		MockServerHttpRequest request = MockServerHttpRequest
+				.get("http://localhost:8080/get")
+				.remoteAddress(
+						new InetSocketAddress(InetAddress.getByName("10.0.0.1"), 80))
+				.header(X_FORWARDED_FOR_HEADER, "10.0.0.1")
+				.build();
+
+		XForwardedHeadersFilter filter = new XForwardedHeadersFilter();
+
+		HttpHeaders headers = filter.filter(request.getHeaders(),
+				MockServerWebExchange.from(request));
+
+		assertThat(headers).containsKeys(X_FORWARDED_FOR_HEADER);
+
+		assertThat(headers.getFirst(X_FORWARDED_FOR_HEADER))
+				.isEqualTo("10.0.0.1,10.0.0.1");
+	}
 }