coder
diff --git a/‎coderd/coderd.go
+17 b/‎coderd/coderd.go
+17
diff --git a/‎coderd/debug.go
+19-1 b/‎coderd/debug.go
+19-1
@@ -5,6 +5,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"database/sql"
+	"expvar"
 	"flag"
 	"fmt"
 	"io"
@@ -85,6 +86,8 @@ func init() {
 	globalHTTPSwaggerHandler = httpSwagger.Handler(httpSwagger.URL("/swagger/doc.json"))
 }
 
+var expDERPOnce = sync.Once{}
+
 // Options are requires parameters for Coder to start.
 type Options struct {
 	AccessURL *url.URL
@@ -561,6 +564,16 @@ func New(options *Options) *API {
 
 	derpHandler := derphttp.Handler(api.DERPServer)
 	derpHandler, api.derpCloseFunc = tailnet.WithWebsocketSupport(api.DERPServer, derpHandler)
+	// Register DERP on expvar HTTP handler, which we serve below in the router, c.f. expvar.Handler()
+	// These are the metrics the DERP server exposes.
+	// TODO: export via prometheus
+	expDERPOnce.Do(func() {
+		// We need to do this via a global Once because expvar registry is global and panics if we
+		// register multiple times.  In production there is only one Coderd and one DERP server per
+		// process, but in testing, we create multiple of both, so the Once protects us from
+		// panicking.
+		expvar.Publish("derp", api.DERPServer.ExpVar())
+	})
 	cors := httpmw.Cors(options.DeploymentValues.Dangerous.AllowAllCors.Value())
 	prometheusMW := httpmw.Prometheus(options.PrometheusRegistry)
 
@@ -1038,6 +1051,10 @@ func New(options *Options) *API {
 				r.Use(httpmw.ExtractUserParam(options.Database))
 				r.Get("/debug-link", api.userDebugOIDC)
 			})
+			r.Route("/derp", func(r chi.Router) {
+				r.Get("/traffic", options.DERPServer.ServeDebugTraffic)
+			})
+			r.Mount("/expvar", expvar.Handler()) // contains DERP metrics as well as cmdline and memstats
 		})
 	})
 
 
@@ -276,7 +276,7 @@ func validateHealthSettings(settings codersdk.HealthSettings) error {
 }
 
 // For some reason the swagger docs need to be attached to a function.
-//
+
 // @Summary Debug Info Websocket Test
 // @ID debug-info-websocket-test
 // @Security CoderSessionToken
@@ -287,6 +287,24 @@ func validateHealthSettings(settings codersdk.HealthSettings) error {
 // @x-apidocgen {"skip": true}
 func _debugws(http.ResponseWriter, *http.Request) {} //nolint:unused
 
+// @Summary Debug DERP traffic
+// @ID debug-derp-traffic
+// @Security CoderSessionToken
+// @Produce json
+// @Tags Debug
+// @Router /debug/derp/traffic [get]
+// @x-apidocgen {"skip": true}
+func _debugDERPTraffic(http.ResponseWriter, *http.Request) {} //nolint:unused
+
+// @Summary Debug expvar
+// @ID debug-expvar
+// @Security CoderSessionToken
+// @Produce json
+// @Tags Debug
+// @Router /debug/expvar [get]
+// @x-apidocgen {"skip": true}
+func _debugExpVar(http.ResponseWriter, *http.Request) {} //nolint:unused
+
 func loadDismissedHealthchecks(ctx context.Context, db database.Store, logger slog.Logger) []codersdk.HealthSection {
 	dismissedHealthchecks := []codersdk.HealthSection{}
 	settingsJSON, err := db.GetHealthSettings(ctx)