net/http/httputil: NewSingleHostReverseProxy returns proxy that sets default User-Agent header if client did not provided one

[artyom]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

   go version devel +babdbfb Tue May 3 12:35:09 2016 +0000 darwin/amd64
   

2. What operating system and processor architecture are you using (go env)?

   darwin/amd64, OS X 10.11.4
   

3. What did you do?

   • run program from https://play.golang.org/p/NxkSXv592J
   • make http request with User-Agent header unset: curl -v -H "User-Agent:" http://localhost:8080/

4. What did you expect to see?

   The program reports that User-Agent header is empty: User-Agent is ""

5. What did you see instead?

   The program reports that User-Agent is set to default value: User-Agent is "Go-http-client/1.1"

[artyom]

Possible fix is trivial:

diff --git i/src/net/http/httputil/reverseproxy.go w/src/net/http/httputil/reverseproxy.go
index 44d15ff..8c471a9 100644
--- i/src/net/http/httputil/reverseproxy.go
+++ w/src/net/http/httputil/reverseproxy.go
@@ -90,6 +90,11 @@ func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
        } else {
            req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
        }
+       if _, ok := req.Header["User-Agent"]; !ok {
+           // explicitly disable User-Agent so it's not set to
+           // default value
+           req.Header.Set("User-Agent", "")
+       }
    }
    return &ReverseProxy{Director: director}
 }
diff --git i/src/net/http/httputil/reverseproxy_test.go w/src/net/http/httputil/reverseproxy_test.go
index e9c0658..1065561 100644
--- i/src/net/http/httputil/reverseproxy_test.go
+++ w/src/net/http/httputil/reverseproxy_test.go
@@ -41,6 +41,9 @@ func TestReverseProxy(t *testing.T) {
        if len(r.TransferEncoding) > 0 {
            t.Errorf("backend got unexpected TransferEncoding: %v", r.TransferEncoding)
        }
+       if c := r.Header.Get("User-Agent"); c != "" {
+           t.Errorf("handler got User-Agent header value %q", c)
+       }
        if r.Header.Get("X-Forwarded-For") == "" {
            t.Errorf("didn't get X-Forwarded-For header")
        }
@@ -80,6 +83,7 @@ func TestReverseProxy(t *testing.T) {

    getReq, _ := http.NewRequest("GET", frontend.URL, nil)
    getReq.Host = "some-name"
+   getReq.Header.Set("User-Agent", "")
    getReq.Header.Set("Connection", "close")
    getReq.Header.Set("Proxy-Connection", "should be deleted")
    getReq.Header.Set("Upgrade", "foo")

Please let me know if this change makes sense, I can proceed with CL then.

[bradfitz]

@artyom, sure, send a CL. Be sure to also add a test that the original User-Agent from a user is preserved.

[artyom]

Sure, will do this later this week, likely over the weekend. Please feel free to assing this to me.

[gopherbot]

CL https://golang.org/cl/23089 mentions this issue.