feat: add verify client config for embedded DERP (#2260)

* feat: add verify client config for embedded DERP

* refactor: embedded DERP no longer verify clients via HTTP

- register the `headscale://` protocol in `http.DefaultTransport` to intercept network requests
- update configuration to use a single boolean option `verify_clients`

* refactor: use `http.HandlerFunc` for type definition

* refactor: some renaming and restructuring

* chore: some renaming and fix lint

* test: fix TestDERPVerifyEndpoint

- `tailscale debug derp` use random node private key

* test: add verify clients integration test for embedded DERP server

* fix: apply code review suggestions

* chore: merge upstream changes

* fix: apply code review suggestions

---------

Co-authored-by: Kristoffer Dalby <kristoffer@dalby.cc>

hscontrol/derp/server/derp_server.go

@@ -2,9 +2,11 @@ package server
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"net/netip"
@@ -28,7 +30,10 @@ import (
 // server that the DERP HTTP client does not want the HTTP 101 response
 // headers and it will begin writing & reading the DERP protocol immediately
 // following its HTTP request.
-const fastStartHeader = "Derp-Fast-Start"
+const (
+	fastStartHeader  = "Derp-Fast-Start"
+	DerpVerifyScheme = "headscale-derp-verify"
+)
 
 type DERPServer struct {
 	serverURL     string
@@ -45,6 +50,11 @@ func NewDERPServer(
 	log.Trace().Caller().Msg("Creating new embedded DERP server")
 	server := derp.NewServer(derpKey, util.TSLogfWrapper()) // nolint // zerolinter complains
 
+	if cfg.ServerVerifyClients {
+		server.SetVerifyClientURL(DerpVerifyScheme + "://verify")
+		server.SetVerifyClientURLFailOpen(false)
+	}
+
 	return &DERPServer{
 		serverURL:     serverURL,
 		key:           derpKey,
@@ -360,3 +370,29 @@ func serverSTUNListener(ctx context.Context, packetConn *net.UDPConn) {
 		}
 	}
 }
+
+func NewDERPVerifyTransport(handleVerifyRequest func(*http.Request, io.Writer) error) *DERPVerifyTransport {
+	return &DERPVerifyTransport{
+		handleVerifyRequest: handleVerifyRequest,
+	}
+}
+
+type DERPVerifyTransport struct {
+	handleVerifyRequest func(*http.Request, io.Writer) error
+}
+
+func (t *DERPVerifyTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	buf := new(bytes.Buffer)
+	if err := t.handleVerifyRequest(req, buf); err != nil {
+		log.Error().Caller().Err(err).Msg("Failed to handle client verify request: ")
+
+		return nil, err
+	}
+
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Body:       io.NopCloser(buf),
+	}
+
+	return resp, nil
+}