diff --git a/src/environment.cc b/src/environment.cc
index bc9ac79b..c3d2e843 100644
--- a/src/environment.cc
+++ b/src/environment.cc
@@ -78,12 +78,14 @@ constexpr const char kGceInstanceResourceType[] = "gce_instance";
 }
 
 Environment::Environment(const Configuration& config)
-    : config_(config), application_default_credentials_read_(false) {}
+    : config_(config),
+      metadata_server_url_(kGceMetadataServerAddress),
+      application_default_credentials_read_(false) {}
 
 std::string Environment::GetMetadataString(const std::string& path) const {
   http::client::options options;
   http::client client(options.timeout(2));
-  http::client::request request(kGceMetadataServerAddress + path);
+  http::client::request request(metadata_server_url_ + path);
   request << boost::network::header("Metadata-Flavor", "Google");
   try {
     http::client::response response = client.get(request);
@@ -98,7 +100,7 @@ std::string Environment::GetMetadataString(const std::string& path) const {
     }
   } catch (const boost::system::system_error& e) {
     LOG(ERROR) << "Exception: " << e.what()
-               << ": '" << kGceMetadataServerAddress << path << "'";
+               << ": '" << metadata_server_url_ << path << "'";
     return "";
   }
 }
diff --git a/src/environment.h b/src/environment.h
index aa356b61..2ca4c079 100644
--- a/src/environment.h
+++ b/src/environment.h
@@ -48,6 +48,11 @@ class Environment {
 
   void ReadApplicationDefaultCredentials() const;
 
+  // The url must end in a '/'.
+  void SetMetadataServerUrlForTest(const std::string& url) {
+    metadata_server_url_ = url;
+  }
+
   const Configuration& config_;
 
   // Cached data.
@@ -60,6 +65,7 @@ class Environment {
   mutable std::string kubernetes_cluster_location_;
   mutable std::string client_email_;
   mutable std::string private_key_;
+  mutable std::string metadata_server_url_;
   mutable bool application_default_credentials_read_;
 };
 
diff --git a/test/Makefile b/test/Makefile
index 5129a492..3ad7a3bc 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -121,7 +121,7 @@ base64_unittest: base64_unittest.o $(SRC_DIR)/base64.o
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
 configuration_unittest: configuration_unittest.o $(SRC_DIR)/configuration.o
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
-environment_unittest: environment_unittest.o $(SRC_DIR)/environment.o $(SRC_DIR)/configuration.o $(SRC_DIR)/format.o $(SRC_DIR)/json.o $(SRC_DIR)/logging.o $(SRC_DIR)/time.o
+environment_unittest: environment_unittest.o fake_http_server.o $(SRC_DIR)/environment.o $(SRC_DIR)/configuration.o $(SRC_DIR)/format.o $(SRC_DIR)/json.o $(SRC_DIR)/logging.o $(SRC_DIR)/time.o
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
 format_unittest: format_unittest.o $(SRC_DIR)/format.o
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
diff --git a/test/environment_unittest.cc b/test/environment_unittest.cc
index 09ab5655..a76e530b 100644
--- a/test/environment_unittest.cc
+++ b/test/environment_unittest.cc
@@ -1,4 +1,5 @@
 #include "../src/environment.h"
+#include "fake_http_server.h"
 #include "gtest/gtest.h"
 
 #include <fstream>
@@ -12,9 +13,15 @@ class EnvironmentTest : public ::testing::Test {
   static void ReadApplicationDefaultCredentials(const Environment& environment) {
     environment.ReadApplicationDefaultCredentials();
   }
+
+  static void SetMetadataServerUrlForTest(Environment* environment,
+                                          const std::string& url) {
+    environment->SetMetadataServerUrlForTest(url);
+  }
 };
 
 namespace {
+
 // A file with a given name in a temporary (unique) directory.
 boost::filesystem::path TempPath(const std::string& filename) {
   boost::filesystem::path path = boost::filesystem::temp_directory_path();
@@ -42,6 +49,7 @@ class TemporaryFile {
  private:
   boost::filesystem::path path_;
 };
+
 }  // namespace
 
 TEST(TemporaryFile, Basic) {
@@ -70,7 +78,6 @@ TEST_F(EnvironmentTest, ReadApplicationDefaultCredentialsSucceeds) {
   TemporaryFile credentials_file(
     std::string(test_info_->name()) + "_creds.json",
     "{\"client_email\":\"user@example.com\",\"private_key\":\"some_key\"}");
-  std::string cfg;
   Configuration config(std::istringstream(
       "CredentialsFile: '" + credentials_file.FullPath().native() + "'\n"
   ));
@@ -98,4 +105,17 @@ TEST_F(EnvironmentTest, ReadApplicationDefaultCredentialsCaches) {
   );
   EXPECT_EQ("some_key", environment.CredentialsPrivateKey());
 }
+
+TEST_F(EnvironmentTest, GetMetadataString) {
+  testing::FakeServer server;
+  server.SetResponse("/a/b/c", "hello");
+
+  Configuration config;
+  Environment environment(config);
+  SetMetadataServerUrlForTest(&environment, server.GetUrl());
+
+  EXPECT_EQ("hello", environment.GetMetadataString("a/b/c"));
+  EXPECT_EQ("", environment.GetMetadataString("unknown/path"));
+}
+
 }  // namespace google
diff --git a/test/fake_http_server.cc b/test/fake_http_server.cc
new file mode 100644
index 00000000..d5980dec
--- /dev/null
+++ b/test/fake_http_server.cc
@@ -0,0 +1,47 @@
+#include "fake_http_server.h"
+
+namespace google {
+namespace testing {
+
+FakeServer::FakeServer()
+    // Note: An empty port selects a random available port (this behavior
+    // is not documented).
+    : server_(Server::options(handler_).address("127.0.0.1").port("")) {
+  server_.listen();
+  server_thread_ = std::thread([this] { server_.run(); });
+}
+
+FakeServer::~FakeServer() {
+  server_.stop();
+  server_thread_.join();
+}
+
+std::string FakeServer::GetUrl() {
+  network::uri_builder builder;
+  builder.scheme("http").host(server_.address()).port(server_.port()).path("/");
+  return builder.uri().string();
+}
+
+void FakeServer::SetResponse(const std::string& path,
+                             const std::string& response) {
+  handler_.path_responses[path] = response;
+}
+
+void FakeServer::Handler::operator()(Server::request const &request,
+                                     Server::connection_ptr connection) {
+  auto it = path_responses.find(request.destination);
+  if (it != path_responses.end()) {
+    connection->set_status(Server::connection::ok);
+    connection->set_headers(std::map<std::string, std::string>({
+        {"Content-Type", "text/plain"},
+    }));
+    connection->write(it->second);
+  } else {
+    // Note: We have to set headers; otherwise, an exception is thrown.
+    connection->set_status(Server::connection::not_found);
+    connection->set_headers(std::map<std::string, std::string>());
+  }
+}
+
+}  // testing
+}  // google
diff --git a/test/fake_http_server.h b/test/fake_http_server.h
new file mode 100644
index 00000000..7eb5881a
--- /dev/null
+++ b/test/fake_http_server.h
@@ -0,0 +1,38 @@
+#ifndef FAKE_HTTP_SERVER_H_
+#define FAKE_HTTP_SERVER_H_
+
+#include <boost/network/protocol/http/server.hpp>
+
+namespace google {
+namespace testing {
+
+// Starts a server in a separate thread, allowing it to choose an
+// available port.
+class FakeServer {
+ public:
+  FakeServer();
+  ~FakeServer();
+
+  std::string GetUrl();
+  void SetResponse(const std::string& path, const std::string& response);
+
+ private:
+  struct Handler;
+  typedef boost::network::http::server<Handler> Server;
+
+  // Handler that maps paths to response strings.
+  struct Handler {
+    void operator()(Server::request const &request,
+                    Server::connection_ptr connection);
+    std::map<std::string, std::string> path_responses;
+  };
+
+  Handler handler_;
+  Server server_;
+  std::thread server_thread_;
+};
+
+}  // testing
+}  // google
+
+#endif  // FAKE_HTTP_SERVER_H_