cmd/derper, net/netcheck: add challenge/response to generate_204 endpoint

The Lufthansa in-flight wifi generates a synthetic 204 response to the
DERP server's /generate_204 endpoint. This PR adds a basic
challenge/response to the endpoint; something sufficiently complicated
that it's unlikely to be implemented by a captive portal. We can then
check for the expected response to verify whether we're being MITM'd.

Follow-up to #5601

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I94a68c9a16a7be7290200eea6a549b64f02ff48f

cmd/derper/derper.go

@@ -325,11 +325,31 @@ 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 == '_'
+}
+
 // probeHandler is the endpoint that js/wasm clients hit to measure
 // DERP latency, since they can't do UDP STUN queries.
 func probeHandler(w http.ResponseWriter, r *http.Request) {

cmd/derper/derper_test.go

@@ -7,6 +7,9 @@ package main
 import (
 	"context"
 	"net"
+	"net/http"
+	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"tailscale.com/net/stun"
@@ -67,3 +70,57 @@ func BenchmarkServerSTUN(b *testing.B) {
 	}
 
 }
+
+func TestNoContent(t *testing.T) {
+	testCases := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name: "no challenge",
+		},
+		{
+			name:  "valid challenge",
+			input: "input",
+			want:  "response input",
+		},
+		{
+			name:  "invalid challenge",
+			input: "foo\x00bar",
+			want:  "",
+		},
+		{
+			name:  "whitespace invalid challenge",
+			input: "foo bar",
+			want:  "",
+		},
+		{
+			name:  "long challenge",
+			input: strings.Repeat("x", 65),
+			want:  "",
+		},
+	}
+	for _, tt := range testCases {
+		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)
+			}
+			w := httptest.NewRecorder()
+			serveNoContent(w, req)
+			resp := w.Result()
+
+			if tt.want == "" {
+				if h, found := resp.Header[noContentResponseHeader]; found {
+					t.Errorf("got %+v; expected no response header", h)
+				}
+				return
+			}
+
+			if got := resp.Header.Get(noContentResponseHeader); got != tt.want {
+				t.Errorf("got %q; want %q", got, tt.want)
+			}
+		})
+	}
+}

net/netcheck/netcheck.go

@@ -1116,13 +1116,20 @@ func (c *Client) checkCaptivePortal(ctx context.Context, dm *tailcfg.DERPMap, pr
 	if err != nil {
 		return false, err
 	}
+
+	chal := "tailscale " + node.HostName
+	req.Header.Set("X-Tailscale-Challenge", chal)
 	r, err := noRedirectClient.Do(req)
 	if err != nil {
 		return false, err
 	}
-	c.logf("[v2] checkCaptivePortal url=%q status_code=%d", req.URL.String(), r.StatusCode)
+	defer r.Body.Close()
+
+	expectedResponse := "response " + chal
+	validResponse := r.Header.Get("X-Tailscale-Response") == expectedResponse
 
-	return r.StatusCode != 204, nil
+	c.logf("[v2] checkCaptivePortal url=%q status_code=%d valid_response=%v", req.URL.String(), r.StatusCode, validResponse)
+	return r.StatusCode != 204 || !validResponse, nil
 }
 
 // runHTTPOnlyChecks is the netcheck done by environments that can