Adds a filter to replace the host header

fixes gh-530
fixes gh-1344

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -1596,6 +1596,33 @@ errorMessage` : `Request size is larger than permissible limit. Request size is
 
 NOTE: The default request size is set to five MB if not provided as a filter argument in the route definition.
 
+=== The `SetRequestHost` `GatewayFilter` Factory
+
+There are certain situation when the host header may need to be overridden. In this situation, the `SetRequestHost` `GatewayFilter` factory can replace the existing host header with a specified vaue.
+The filter takes a `host` parameter.
+The following listing configures a `SetRequestHost` `GatewayFilter`:
+
+.application.yml
+====
+[source,yaml]
+----
+spring:
+  cloud:
+    gateway:
+      routes:
+      - id: set_request_host_header_route
+        uri: http://localhost:8080/headers
+        predicates:
+        - Path=/headers
+        filters:
+        - name: SetRequestHost
+          args:
+            host: example.org
+----
+====
+
+The `SetRequestHost` `GatewayFilter` factory replaces the value of the host header with `example.org`.
+
 === Modify a Request Body `GatewayFilter` Factory
 
 You can use the `ModifyRequestBody` filter  filter to modify the request body before it is sent downstream by the gateway.

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/config/GatewayAutoConfiguration.java

@@ -90,6 +90,7 @@ import org.springframework.cloud.gateway.filter.factory.SecureHeadersGatewayFilt
 import org.springframework.cloud.gateway.filter.factory.SecureHeadersProperties;
 import org.springframework.cloud.gateway.filter.factory.SetPathGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetRequestHeaderGatewayFilterFactory;
+import org.springframework.cloud.gateway.filter.factory.SetRequestHostHeaderGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetResponseHeaderGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetStatusGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.StripPrefixGatewayFilterFactory;
@@ -525,6 +526,11 @@ public class GatewayAutoConfiguration {
 		return new SetRequestHeaderGatewayFilterFactory();
 	}
 
+	@Bean
+	public SetRequestHostHeaderGatewayFilterFactory setRequestHostHeaderGatewayFilterFactory() {
+		return new SetRequestHostHeaderGatewayFilterFactory();
+	}
+
 	@Bean
 	public SetResponseHeaderGatewayFilterFactory setResponseHeaderGatewayFilterFactory() {
 		return new SetResponseHeaderGatewayFilterFactory();

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/factory/SetRequestHostHeaderGatewayFilterFactory.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2013-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.gateway.filter.factory;
+
+import java.util.Collections;
+import java.util.List;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.cloud.gateway.filter.GatewayFilter;
+import org.springframework.cloud.gateway.filter.GatewayFilterChain;
+import org.springframework.cloud.gateway.support.ServerWebExchangeUtils;
+import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.web.server.ServerWebExchange;
+
+import static org.springframework.cloud.gateway.support.GatewayToStringStyler.filterToStringCreator;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.PRESERVE_HOST_HEADER_ATTRIBUTE;
+
+/**
+ * @author Andrew Fitzgerald
+ */
+public class SetRequestHostHeaderGatewayFilterFactory extends
+		AbstractGatewayFilterFactory<SetRequestHostHeaderGatewayFilterFactory.Config> {
+
+	public SetRequestHostHeaderGatewayFilterFactory() {
+		super(Config.class);
+	}
+
+	@Override
+	public List<String> shortcutFieldOrder() {
+		return Collections.singletonList("host");
+	}
+
+	@Override
+	public GatewayFilter apply(Config config) {
+		return new GatewayFilter() {
+			@Override
+			public Mono<Void> filter(ServerWebExchange exchange,
+					GatewayFilterChain chain) {
+				String value = ServerWebExchangeUtils.expand(exchange, config.getHost());
+
+				ServerHttpRequest request = exchange.getRequest().mutate()
+						.headers(httpHeaders -> {
+							httpHeaders.remove("Host");
+							httpHeaders.add("Host", value);
+						}).build();
+
+				// Make sure the header we just set is preserved
+				exchange.getAttributes().put(PRESERVE_HOST_HEADER_ATTRIBUTE, true);
+
+				return chain.filter(exchange.mutate().request(request).build());
+			}
+
+			@Override
+			public String toString() {
+				return filterToStringCreator(
+						SetRequestHostHeaderGatewayFilterFactory.this)
+								.append(config.getHost()).toString();
+			}
+		};
+	}
+
+	public static class Config {
+
+		private String host;
+
+		public String getHost() {
+			return host;
+		}
+
+		public void setHost(String host) {
+			this.host = host;
+		}
+
+	}
+
+}

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/route/builder/GatewayFilterSpec.java

@@ -63,6 +63,7 @@ import org.springframework.cloud.gateway.filter.factory.SaveSessionGatewayFilter
 import org.springframework.cloud.gateway.filter.factory.SecureHeadersGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetPathGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetRequestHeaderGatewayFilterFactory;
+import org.springframework.cloud.gateway.filter.factory.SetRequestHostHeaderGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetResponseHeaderGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SetStatusGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.factory.SpringCloudCircuitBreakerFilterFactory;
@@ -383,6 +384,17 @@ public class GatewayFilterSpec extends UriSpec {
 		return filter(getBean(PreserveHostHeaderGatewayFilterFactory.class).apply());
 	}
 
+	/**
+	 * A filter that will set the Host header to
+	 * {@param hostName} on the outgoing request
+	 * @param hostName the updated Host header value
+	 * @return a {@link GatewayFilterSpec} that can be used to apply additional filters
+	 */
+	public GatewayFilterSpec setHostHeader(String hostName) {
+		return filter(getBean(SetRequestHostHeaderGatewayFilterFactory.class)
+				.apply(c -> c.setHost(hostName)));
+	}
+
 	/**
 	 * A filter that will return a redirect response back to the client.
 	 * @param status an HTTP status code, should be a {@code 300} series redirect

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/factory/SetRequestHostHeaderGatewayFilterFactoryTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2013-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.gateway.filter.factory;
+
+import java.util.Map;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.cloud.gateway.filter.GatewayFilter;
+import org.springframework.cloud.gateway.test.BaseWebClientTests;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.boot.test.context.SpringBootTest.WebEnvironment.RANDOM_PORT;
+import static org.springframework.cloud.gateway.test.TestUtils.getMap;
+
+@RunWith(SpringRunner.class)
+@SpringBootTest(webEnvironment = RANDOM_PORT)
+@DirtiesContext
+public class SetRequestHostHeaderGatewayFilterFactoryTests extends BaseWebClientTests {
+
+	@Test
+	public void setRequestHostHeaderFilterWorks() {
+		testClient.get().uri("/headers").header("Host", "www.setrequesthostheader.org")
+				.exchange().expectStatus().isOk().expectBody(Map.class)
+				.consumeWith(result -> {
+					Map<String, Object> headers = getMap(result.getResponseBody(),
+							"headers");
+					assertThat(headers).hasEntrySatisfying("Host",
+							val -> assertThat(val).isEqualTo("otherhost.io"));
+				});
+	}
+
+	@Test
+	public void toStringFormat() {
+		SetRequestHostHeaderGatewayFilterFactory.Config config = new SetRequestHostHeaderGatewayFilterFactory.Config();
+		config.setHost("myhost");
+		GatewayFilter filter = new SetRequestHostHeaderGatewayFilterFactory()
+				.apply(config);
+		assertThat(filter.toString()).contains("myhost");
+	}
+
+	@EnableAutoConfiguration
+	@SpringBootConfiguration
+	@Import(DefaultTestConfig.class)
+	public static class TestConfig {
+
+	}
+
+}

spring-cloud-gateway-core/src/test/resources/application.yml

@@ -364,6 +364,15 @@ spring:
         filters:
         - SecureHeaders
 
+        # =====================================
+      - id: set_request_host_header_test
+        uri: ${test.uri}
+        predicates:
+          - Host=**.setrequesthostheader.org
+          - Path=/headers
+        filters:
+          - SetRequestHostHeader=otherhost.io
+
       # =====================================
       - id: set_path_test
         uri: ${test.uri}