diff --git a/main.go b/main.go
index ebb1620..2c07154 100644
--- a/main.go
+++ b/main.go
@@ -158,10 +158,13 @@ type responseLogger struct {
 	http.ResponseWriter
 }
 
+// WriteHeader writes header code into responser writer.
 func (r *responseLogger) WriteHeader(code int) {
 	r.code = code
 	r.ResponseWriter.WriteHeader(code)
 }
+
+// ServeHTTP implements http handler.
 func (l *logger) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	start := time.Now()
 	rl := &responseLogger{code: 200, ResponseWriter: w}
@@ -172,9 +175,12 @@ func (l *logger) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // An ops is a proxy.ServerOps implementation.
 type ops struct{}
 
+// NewContext crates a context.
 func (*ops) NewContext(r *http.Request) (context.Context, error) {
 	return context.Background(), nil
 }
+
+// List lists proxy files.
 func (*ops) List(ctx context.Context, mpath string) (proxy.File, error) {
 	escMod, err := module.EscapePath(mpath)
 	if err != nil {
@@ -209,6 +215,8 @@ func (*ops) List(ctx context.Context, mpath string) (proxy.File, error) {
 
 	return os.Open(file)
 }
+
+// Latest fetch latest file.
 func (*ops) Latest(ctx context.Context, path string) (proxy.File, error) {
 	d, err := download(module.Version{Path: path, Version: "latest"})
 	if err != nil {
@@ -216,6 +224,8 @@ func (*ops) Latest(ctx context.Context, path string) (proxy.File, error) {
 	}
 	return os.Open(d.Info)
 }
+
+// Info fetch info file.
 func (*ops) Info(ctx context.Context, m module.Version) (proxy.File, error) {
 	d, err := download(m)
 	if err != nil {
@@ -223,6 +233,8 @@ func (*ops) Info(ctx context.Context, m module.Version) (proxy.File, error) {
 	}
 	return os.Open(d.Info)
 }
+
+// GoMod fetch go mod file.
 func (*ops) GoMod(ctx context.Context, m module.Version) (proxy.File, error) {
 	d, err := download(m)
 	if err != nil {
@@ -230,6 +242,8 @@ func (*ops) GoMod(ctx context.Context, m module.Version) (proxy.File, error) {
 	}
 	return os.Open(d.GoMod)
 }
+
+// Zip fetch zip file.
 func (*ops) Zip(ctx context.Context, m module.Version) (proxy.File, error) {
 	d, err := download(m)
 	if err != nil {
diff --git a/proxy/router.go b/proxy/router.go
index ca13662..8780fe4 100644
--- a/proxy/router.go
+++ b/proxy/router.go
@@ -34,16 +34,98 @@ type RouterOptions struct {
 // which implements Route Filter to
 // routing private module or public module .
 type Router struct {
+	opts         *RouterOptions
 	srv          *Server
 	proxy        *httputil.ReverseProxy
 	pattern      string
 	downloadRoot string
 }
 
+func (router *Router) customModResponse(r *http.Response) error {
+	var err error
+	if r.StatusCode == http.StatusOK {
+		var buf []byte
+		if strings.Contains(r.Header.Get("Content-Encoding"), "gzip") {
+			gr, err := gzip.NewReader(r.Body)
+			if err != nil {
+				return err
+			}
+			defer gr.Close()
+			buf, err = ioutil.ReadAll(gr)
+			if err != nil {
+				return err
+			}
+			r.Header.Del("Content-Encoding")
+		} else {
+			buf, err = ioutil.ReadAll(r.Body)
+			if err != nil {
+				return err
+			}
+		}
+		r.Body = ioutil.NopCloser(bytes.NewReader(buf))
+		if buf != nil {
+			file := filepath.Join(router.opts.DownloadRoot, r.Request.URL.Path)
+			os.MkdirAll(path.Dir(file), os.ModePerm)
+			err = renameio.WriteFile(file, buf, 0666)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	// support 302 status code.
+	if r.StatusCode == http.StatusFound {
+		loc := r.Header.Get("Location")
+		if loc == "" {
+			return fmt.Errorf("%d response missing Location header", r.StatusCode)
+		}
+
+		// TODO: location is relative.
+		_, err := url.Parse(loc)
+		if err != nil {
+			return fmt.Errorf("failed to parse Location header %q: %v", loc, err)
+		}
+		resp, err := http.Get(loc)
+		if err != nil {
+			return err
+		}
+		defer resp.Body.Close()
+
+		var buf []byte
+		if strings.Contains(resp.Header.Get("Content-Encoding"), "gzip") {
+			gr, err := gzip.NewReader(resp.Body)
+			if err != nil {
+				return err
+			}
+			defer gr.Close()
+			buf, err = ioutil.ReadAll(gr)
+			if err != nil {
+				return err
+			}
+			resp.Header.Del("Content-Encoding")
+		} else {
+			buf, err = ioutil.ReadAll(resp.Body)
+			if err != nil {
+				return err
+			}
+		}
+		resp.Body = ioutil.NopCloser(bytes.NewReader(buf))
+		if buf != nil {
+			file := filepath.Join(router.opts.DownloadRoot, r.Request.URL.Path)
+			os.MkdirAll(path.Dir(file), os.ModePerm)
+			err = renameio.WriteFile(file, buf, 0666)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
 // NewRouter returns a new Router using the given operations.
 func NewRouter(srv *Server, opts *RouterOptions) *Router {
 	rt := &Router{
-		srv: srv,
+		opts: opts,
+		srv:  srv,
 	}
 	if opts != nil {
 		if opts.Proxy == "" {
@@ -61,91 +143,14 @@ func NewRouter(srv *Server, opts *RouterOptions) *Router {
 			director(r)
 			r.Host = remote.Host
 		}
+
 		rt.proxy = proxy
 
 		rt.proxy.Transport = &http.Transport{
 			Proxy:           http.ProxyFromEnvironment,
 			TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
 		}
-		rt.proxy.ModifyResponse = func(r *http.Response) error {
-			if r.StatusCode == http.StatusOK {
-				var buf []byte
-				if strings.Contains(r.Header.Get("Content-Encoding"), "gzip") {
-					gr, err := gzip.NewReader(r.Body)
-					if err != nil {
-						return err
-					}
-					defer gr.Close()
-					buf, err = ioutil.ReadAll(gr)
-					if err != nil {
-						return err
-					}
-					r.Header.Del("Content-Encoding")
-				} else {
-					buf, err = ioutil.ReadAll(r.Body)
-					if err != nil {
-						return err
-					}
-				}
-				r.Body = ioutil.NopCloser(bytes.NewReader(buf))
-				if buf != nil {
-					file := filepath.Join(opts.DownloadRoot, r.Request.URL.Path)
-					os.MkdirAll(path.Dir(file), os.ModePerm)
-					err = renameio.WriteFile(file, buf, 0666)
-					if err != nil {
-						return err
-					}
-				}
-			}
-
-			// support 302 status code.
-			if r.StatusCode == http.StatusFound {
-				loc := r.Header.Get("Location")
-				if loc == "" {
-					return fmt.Errorf("%d response missing Location header", r.StatusCode)
-				}
-
-				// TODO: location is relative.
-				_, err := url.Parse(loc)
-				if err != nil {
-					return fmt.Errorf("failed to parse Location header %q: %v", loc, err)
-				}
-				resp, err := http.Get(loc)
-				if err != nil {
-					return err
-				}
-				defer resp.Body.Close()
-
-				var buf []byte
-				if strings.Contains(resp.Header.Get("Content-Encoding"), "gzip") {
-					gr, err := gzip.NewReader(resp.Body)
-					if err != nil {
-						return err
-					}
-					defer gr.Close()
-					buf, err = ioutil.ReadAll(gr)
-					if err != nil {
-						return err
-					}
-					resp.Header.Del("Content-Encoding")
-				} else {
-					buf, err = ioutil.ReadAll(resp.Body)
-					if err != nil {
-						return err
-					}
-				}
-				resp.Body = ioutil.NopCloser(bytes.NewReader(buf))
-				if buf != nil {
-					file := filepath.Join(opts.DownloadRoot, r.Request.URL.Path)
-					os.MkdirAll(path.Dir(file), os.ModePerm)
-					err = renameio.WriteFile(file, buf, 0666)
-					if err != nil {
-						return err
-					}
-				}
-			}
-			return nil
-		}
+		rt.proxy.ModifyResponse = rt.customModResponse
 		rt.pattern = opts.Pattern
 		rt.downloadRoot = opts.DownloadRoot
 	}
@@ -160,6 +165,7 @@ func (rt *Router) Direct(path string) bool {
 	return GlobsMatchPath(rt.pattern, path)
 }
 
+// ServveHTTP implements http handler.
 func (rt *Router) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// sumdb handler
 	if strings.HasPrefix(r.URL.Path, "/sumdb/") {
diff --git a/proxy/server.go b/proxy/server.go
index c3bbbd3..95f4180 100644
--- a/proxy/server.go
+++ b/proxy/server.go
@@ -221,8 +221,13 @@ type memFile struct {
 	stat memStat
 }
 
-func (f *memFile) Close() error                             { return nil }
-func (f *memFile) Stat() (os.FileInfo, error)               { return &f.stat, nil }
+// Close closes file.
+func (f *memFile) Close() error { return nil }
+
+// Stat stats file.
+func (f *memFile) Stat() (os.FileInfo, error) { return &f.stat, nil }
+
+// Readdir read dir.
 func (f *memFile) Readdir(count int) ([]os.FileInfo, error) { return nil, os.ErrInvalid }
 
 type memStat struct {
@@ -230,12 +235,23 @@ type memStat struct {
 	size int64
 }
 
-func (s *memStat) Name() string       { return "memfile" }
-func (s *memStat) Size() int64        { return s.size }
-func (s *memStat) Mode() os.FileMode  { return 0444 }
+// Name returns file name.
+func (s *memStat) Name() string { return "memfile" }
+
+// Size returns file size.
+func (s *memStat) Size() int64 { return s.size }
+
+// Mode returns file mode.
+func (s *memStat) Mode() os.FileMode { return 0444 }
+
+// ModTime returns file modtime.
 func (s *memStat) ModTime() time.Time { return s.t }
-func (s *memStat) IsDir() bool        { return false }
-func (s *memStat) Sys() interface{}   { return nil }
+
+// IsDir returns if file is a dir.
+func (s *memStat) IsDir() bool { return false }
+
+// Sys return nil.
+func (s *memStat) Sys() interface{} { return nil }
 
 // NewInfo returns a formatted info file for the given version, time pair.
 // The version should be a canonical semantic version.
diff --git a/sumdb/handler.go b/sumdb/handler.go
index 2787c8a..fbb9ad0 100644
--- a/sumdb/handler.go
+++ b/sumdb/handler.go
@@ -50,30 +50,15 @@ func Handler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	p := "https://" + strings.TrimPrefix(r.URL.Path, "/sumdb/")
-	_, err := url.Parse(p)
-	if err != nil {
-		w.WriteHeader(http.StatusGone)
-		fmt.Fprintf(w, err.Error())
-		return
-	}
-
-	resp, err := http.Get(p)
-	if err != nil {
-		w.WriteHeader(http.StatusGone)
-		fmt.Fprintf(w, err.Error())
-		return
-	}
-	defer resp.Body.Close()
-	w.WriteHeader(resp.StatusCode)
-	if _, err := io.Copy(w, resp.Body); err != nil {
-		fmt.Fprintf(w, err.Error())
-		return
-	}
-	return
+	proxySumdb(p, w, r)
 }
 
 func sumViaGoproxy(w http.ResponseWriter, r *http.Request) {
 	p := "https://goproxy.io" + r.URL.Path
+	proxySumdb(p, w, r)
+}
+
+func proxySumdb(p string, w http.ResponseWriter, r *http.Request) {
 	_, err := url.Parse(p)
 	if err != nil {
 		w.WriteHeader(http.StatusGone)
@@ -94,4 +79,5 @@ func sumViaGoproxy(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	return
+
 }