allow customizing response headers passed to client

A nil slice of headers will use the previous set of default response
headers to maintain existing behavior. The same list of headers is
repeated as the default flag value in `cmd/imageproxy` as documentation
for users to know what values they are overriding (and might want to
still include).

Fixes #387

Author: willnorris

cmd/imageproxy/main.go

@@ -39,6 +39,7 @@ var includeReferer = flag.Bool("includeReferer", false, "include referer header
 var followRedirects = flag.Bool("followRedirects", true, "follow redirects")
 var baseURL = flag.String("baseURL", "", "default base URL for relative remote URLs")
 var passRequestHeaders = flag.String("passRequestHeaders", "", "comma separatetd list of request headers to pass to remote server")
+var passResponseHeaders = flag.String("passResponseHeaders", "Cache-Control,Last-Modified,Expires,Etag,Link", "comma separated list of response headers to pass from remote server")
 var cache tieredCache
 var signatureKeys signatureKeyList
 var scaleUp = flag.Bool("scaleUp", false, "allow images to scale beyond their original dimensions")
@@ -75,6 +76,12 @@ func main() {
 	if *passRequestHeaders != "" {
 		p.PassRequestHeaders = strings.Split(*passRequestHeaders, ",")
 	}
+	if *passResponseHeaders != "" {
+		p.PassResponseHeaders = strings.Split(*passResponseHeaders, ",")
+	} else {
+		// set to a non-nil empty slice to pass no headers.
+		p.PassResponseHeaders = []string{}
+	}
 	p.SignatureKeys = signatureKeys
 	if *baseURL != "" {
 		var err error

imageproxy.go

@@ -94,6 +94,10 @@ type Proxy struct {
 	// requests to the proxied server.
 	PassRequestHeaders []string
 
+	// PassResponseHeaders identifies HTTP headers to pass from server responses to the proxy client.
+	// If nil, a default set of headers is passed: Cache-Control, Last-Modified, Expires, Etag, Link.
+	PassResponseHeaders []string
+
 	// MinimumCacheDuration is the minimum duration to cache remote images.
 	// This will override cache duration from the remote server.
 	MinimumCacheDuration time.Duration
@@ -312,7 +316,12 @@ func (p *Proxy) serveImage(w http.ResponseWriter, r *http.Request) {
 		metricServedFromCache.Inc()
 	}
 
-	copyHeader(w.Header(), resp.Header, "Cache-Control", "Last-Modified", "Expires", "Etag", "Link")
+	if p.PassResponseHeaders == nil {
+		// pass default set of response headers
+		copyHeader(w.Header(), resp.Header, "Cache-Control", "Last-Modified", "Expires", "Etag", "Link")
+	} else {
+		copyHeader(w.Header(), resp.Header, p.PassResponseHeaders...)
+	}
 
 	if should304(r, resp) {
 		w.WriteHeader(http.StatusNotModified)