net/http/httputil: don't insert default User-Agent header in proxied requests

When ReverseProxy forwards a request with no User-Agent header, leave
the header in the forwarded request blank rather than inserting the
default Go HTTP clent User-Agent.

We already did this for NewSingleHostReverseProxy; generalize it to
every ReverseProxy.

Change-Id: Id81a230cb8d384acdfae190b78a4265d80720388
Reviewed-on: https://go-review.googlesource.com/c/go/+/407375
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index a5a3900fb3..0c52497177 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -156,10 +156,6 @@ 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}
 }
@@ -321,6 +317,12 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}
 	}
 
+	if _, ok := outreq.Header["User-Agent"]; !ok {
+		// If the outbound request doesn't have a User-Agent header set,
+		// don't send the default Go HTTP client User-Agent.
+		outreq.Header.Set("User-Agent", "")
+	}
+
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		p.getErrorHandler()(rw, outreq, err)