cmd/derper: move 204 handler from package main to derphttp

Updates #13038

Change-Id: I28a8284dbe49371cae0e9098205c7c5f17225b40
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/derper.go

@@ -237,7 +237,7 @@ func main() {
 		tsweb.AddBrowserHeaders(w)
 		io.WriteString(w, "User-agent: *\nDisallow: /\n")
 	}))
-	mux.Handle("/generate_204", http.HandlerFunc(serveNoContent))
+	mux.Handle("/generate_204", http.HandlerFunc(derphttp.ServeNoContent))
 	debug := tsweb.Debugger(mux)
 	debug.KV("TLS hostname", *hostname)
 	debug.KV("Mesh key", s.HasMeshKey())
@@ -337,7 +337,7 @@ func main() {
 		if *httpPort > -1 {
 			go func() {
 				port80mux := http.NewServeMux()
-				port80mux.HandleFunc("/generate_204", serveNoContent)
+				port80mux.HandleFunc("/generate_204", derphttp.ServeNoContent)
 				port80mux.Handle("/", certManager.HTTPHandler(tsweb.Port80Handler{Main: mux}))
 				port80srv := &http.Server{
 					Addr:        net.JoinHostPort(listenHost, fmt.Sprintf("%d", *httpPort)),
@@ -378,31 +378,6 @@ func main() {
 	}
 }
 
-const (
-	noContentChallengeHeader = "X-Tailscale-Challenge"
-	noContentResponseHeader  = "X-Tailscale-Response"
-)
-
-// For captive portal detection
-func serveNoContent(w http.ResponseWriter, r *http.Request) {
-	if challenge := r.Header.Get(noContentChallengeHeader); challenge != "" {
-		badChar := strings.IndexFunc(challenge, func(r rune) bool {
-			return !isChallengeChar(r)
-		}) != -1
-		if len(challenge) <= 64 && !badChar {
-			w.Header().Set(noContentResponseHeader, "response "+challenge)
-		}
-	}
-	w.WriteHeader(http.StatusNoContent)
-}
-
-func isChallengeChar(c rune) bool {
-	// Semi-randomly chosen as a limited set of valid characters
-	return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') ||
-		('0' <= c && c <= '9') ||
-		c == '.' || c == '-' || c == '_'
-}
-
 var validProdHostname = regexp.MustCompile(`^derp([^.]*)\.tailscale\.com\.?$`)
 
 func prodAutocertHostPolicy(_ context.Context, host string) error {

cmd/derper/derper_test.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"tailscale.com/derp/derphttp"
 	"tailscale.com/tstest/deptest"
 )
 
@@ -76,20 +77,20 @@ func TestNoContent(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			req, _ := http.NewRequest("GET", "https://localhost/generate_204", nil)
 			if tt.input != "" {
-				req.Header.Set(noContentChallengeHeader, tt.input)
+				req.Header.Set(derphttp.NoContentChallengeHeader, tt.input)
 			}
 			w := httptest.NewRecorder()
-			serveNoContent(w, req)
+			derphttp.ServeNoContent(w, req)
 			resp := w.Result()
 
 			if tt.want == "" {
-				if h, found := resp.Header[noContentResponseHeader]; found {
+				if h, found := resp.Header[derphttp.NoContentResponseHeader]; found {
 					t.Errorf("got %+v; expected no response header", h)
 				}
 				return
 			}
 
-			if got := resp.Header.Get(noContentResponseHeader); got != tt.want {
+			if got := resp.Header.Get(derphttp.NoContentResponseHeader); got != tt.want {
 				t.Errorf("got %q; want %q", got, tt.want)
 			}
 		})

derp/derphttp/derphttp_server.go

@@ -18,6 +18,7 @@ import (
 // following its HTTP request.
 const fastStartHeader = "Derp-Fast-Start"
 
+// Handler returns an http.Handler to be mounted at /derp, serving s.
 func Handler(s *derp.Server) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		// These are installed both here and in cmd/derper. The check here
@@ -79,3 +80,29 @@ func ProbeHandler(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "bogus probe method", http.StatusMethodNotAllowed)
 	}
 }
+
+// ServeNoContent generates the /generate_204 response used by Tailscale's
+// captive portal detection.
+func ServeNoContent(w http.ResponseWriter, r *http.Request) {
+	if challenge := r.Header.Get(NoContentChallengeHeader); challenge != "" {
+		badChar := strings.IndexFunc(challenge, func(r rune) bool {
+			return !isChallengeChar(r)
+		}) != -1
+		if len(challenge) <= 64 && !badChar {
+			w.Header().Set(NoContentResponseHeader, "response "+challenge)
+		}
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func isChallengeChar(c rune) bool {
+	// Semi-randomly chosen as a limited set of valid characters
+	return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') ||
+		('0' <= c && c <= '9') ||
+		c == '.' || c == '-' || c == '_'
+}
+
+const (
+	NoContentChallengeHeader = "X-Tailscale-Challenge"
+	NoContentResponseHeader  = "X-Tailscale-Response"
+)