Set default user agent in proxy client

Signed-off-by: Han Verstraete (OpenFaaS Ltd) <han@openfaas.com>

gateway/types/proxy_client.go

@@ -4,10 +4,13 @@
 package types
 
 import (
+	"fmt"
 	"net"
 	"net/http"
 	"net/url"
 	"time"
+
+	"github.com/openfaas/faas/gateway/version"
 )
 
 // NewHTTPClientReverseProxy proxies to an upstream host through the use of a http.Client
@@ -31,18 +34,20 @@ func NewHTTPClientReverseProxy(baseURL *url.URL, timeout time.Duration, maxIdleC
 	// https://github.com/minio/minio/pull/5860
 
 	// Taken from http.DefaultTransport in Go 1.11
-	h.Client.Transport = &http.Transport{
-		Proxy: http.ProxyFromEnvironment,
-		DialContext: (&net.Dialer{
-			Timeout:   timeout,
-			KeepAlive: timeout,
-			DualStack: true,
-		}).DialContext,
-		MaxIdleConns:          maxIdleConns,
-		MaxIdleConnsPerHost:   maxIdleConnsPerHost,
-		IdleConnTimeout:       90 * time.Second,
-		TLSHandshakeTimeout:   10 * time.Second,
-		ExpectContinueTimeout: 1 * time.Second,
+	h.Client.Transport = &proxyTransport{
+		Transport: &http.Transport{
+			Proxy: http.ProxyFromEnvironment,
+			DialContext: (&net.Dialer{
+				Timeout:   timeout,
+				KeepAlive: timeout,
+				DualStack: true,
+			}).DialContext,
+			MaxIdleConns:          maxIdleConns,
+			MaxIdleConnsPerHost:   maxIdleConnsPerHost,
+			IdleConnTimeout:       90 * time.Second,
+			TLSHandshakeTimeout:   10 * time.Second,
+			ExpectContinueTimeout: 1 * time.Second,
+		},
 	}
 
 	return &h
@@ -54,3 +59,19 @@ type HTTPClientReverseProxy struct {
 	Client  *http.Client
 	Timeout time.Duration
 }
+
+// proxyTransport is an http.RoundTripper for the reverse proxy client.
+// It ensures default headers like the `User-Agent` are set on requests.
+type proxyTransport struct {
+	// Transport is the underlying HTTP transport to use when making requests.
+	Transport http.RoundTripper
+}
+
+// RoundTrip implements the RoundTripper interface.
+func (t *proxyTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	if _, ok := req.Header["User-Agent"]; !ok {
+		req.Header.Set("User-Agent", fmt.Sprintf("openfaas-ce-gateway/%s", version.BuildVersion()))
+	}
+
+	return t.Transport.RoundTrip(req)
+}