Set HTTP hostname based on TLS server name

config/http_config.go

@@ -418,6 +418,9 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, optFuncs ...HT
 			http2t.ReadIdleTimeout = time.Minute
 		}
 
+		// hostnameRoundTripper sets the http.Request Host to the value set as the TLS server name
+		rt = newHostnameRoundTripper(tlsConfig, rt)
+
 		// If a authorization_credentials is provided, create a round tripper that will set the
 		// Authorization header correctly on each request.
 		if cfg.Authorization != nil && len(cfg.Authorization.Credentials) > 0 {
@@ -457,6 +460,28 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, optFuncs ...HT
 	return NewTLSRoundTripper(tlsConfig, cfg.TLSConfig.CAFile, newRT)
 }
 
+type hostnameRoundTripper struct {
+	tlsConfig *tls.Config
+	rt        http.RoundTripper
+}
+
+func newHostnameRoundTripper(tlsConfig *tls.Config, rt http.RoundTripper) http.RoundTripper {
+	return &hostnameRoundTripper{
+		tlsConfig: tlsConfig,
+		rt:        rt,
+	}
+}
+
+func (rt *hostnameRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	req = cloneRequest(req)
+	if rt.tlsConfig.ServerName != "" {
+		hostParts := strings.Split(req.Host, ":")
+		hostParts[0] = rt.tlsConfig.ServerName
+		req.Host = strings.Join(hostParts, ":")
+	}
+	return rt.rt.RoundTrip(req)
+}
+
 type authorizationCredentialsRoundTripper struct {
 	authType        string
 	authCredentials Secret

config/http_config_test.go

@@ -37,7 +37,7 @@ import (
 	"testing"
 	"time"
 
-	yaml "gopkg.in/yaml.v2"
+	"gopkg.in/yaml.v2"
 )
 
 const (
@@ -179,6 +179,27 @@ func TestNewClientFromConfig(t *testing.T) {
 			handler: func(w http.ResponseWriter, r *http.Request) {
 				fmt.Fprint(w, ExpectedMessage)
 			},
+		}, {
+			clientConfig: HTTPClientConfig{
+				TLSConfig: TLSConfig{
+					CAFile:             TLSCAChainPath,
+					CertFile:           ClientCertificatePath,
+					KeyFile:            ClientKeyNoPassPath,
+					ServerName:         "test-domain.com",
+					InsecureSkipVerify: true},
+			},
+			handler: func(w http.ResponseWriter, r *http.Request) {
+				srvAddr := r.Context().Value(http.LocalAddrContextKey).(net.Addr)
+				srvPort := strings.Split(srvAddr.String(), ":")[1]
+
+				expectedHostHeader := "test-domain.com:" + srvPort
+				actualHostHeader := r.Host
+				if actualHostHeader != expectedHostHeader {
+					fmt.Fprintf(w, "The expected Host header (%s) differs from the obtained Host header (%s)",
+						expectedHostHeader, actualHostHeader)
+				}
+				fmt.Fprint(w, ExpectedMessage)
+			},
 		}, {
 			clientConfig: HTTPClientConfig{
 				BearerToken: BearerToken,
@@ -512,7 +533,7 @@ func TestCustomIdleConnTimeout(t *testing.T) {
 		t.Fatalf("Can't create a round-tripper from this config: %+v", cfg)
 	}
 
-	transport, ok := rt.(*http.Transport)
+	transport, ok := rt.(*hostnameRoundTripper).rt.(*http.Transport)
 	if !ok {
 		t.Fatalf("Unexpected transport: %+v", transport)
 	}