derp/xdp,cmd/xdpderper: initial skeleton (#12390)

This commit introduces a userspace program for managing an experimental
eBPF XDP STUN server program. derp/xdp contains the eBPF pseudo-C along
with a Go pkg for loading it and exporting its metrics.
cmd/xdpderper is a package main user of derp/xdp.

Updates tailscale/corp#20689

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/xdpderper/xdpderper.go

@@ -0,0 +1,76 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"flag"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"tailscale.com/derp/xdp"
+	"tailscale.com/tsweb"
+)
+
+var (
+	flagDevice  = flag.String("device", "", "target device name")
+	flagPort    = flag.Int("dst-port", 0, "destination UDP port to serve")
+	flagVerbose = flag.Bool("verbose", false, "verbose output including verifier errors")
+	flagMode    = flag.String("mode", "xdp", "XDP mode; valid modes: [xdp, xdpgeneric, xdpdrv, xdpoffload]")
+	flagHTTP    = flag.String("http", ":8230", "HTTP listen address")
+)
+
+func main() {
+	flag.Parse()
+	var attachFlags xdp.XDPAttachFlags
+	switch strings.ToLower(*flagMode) {
+	case "xdp":
+		attachFlags = 0
+	case "xdpgeneric":
+		attachFlags = xdp.XDPGenericMode
+	case "xdpdrv":
+		attachFlags = xdp.XDPDriverMode
+	case "xdpoffload":
+		attachFlags = xdp.XDPOffloadMode
+	default:
+		log.Fatal("invalid mode")
+	}
+	server, err := xdp.NewSTUNServer(&xdp.STUNServerConfig{
+		DeviceName:      *flagDevice,
+		DstPort:         *flagPort,
+		AttachFlags:     attachFlags,
+		FullVerifierErr: *flagVerbose,
+	})
+	if err != nil {
+		log.Fatalf("failed to init XDP STUN server: %v", err)
+	}
+	defer server.Close()
+	err = prometheus.Register(server)
+	if err != nil {
+		log.Fatalf("failed to register XDP STUN server as a prometheus collector: %v", err)
+	}
+	log.Println("XDP STUN server started")
+
+	mux := http.NewServeMux()
+	tsweb.Debugger(mux)
+	errCh := make(chan error, 1)
+	go func() {
+		err := http.ListenAndServe(*flagHTTP, mux)
+		errCh <- err
+	}()
+
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
+	select {
+	case err := <-errCh:
+		log.Printf("HTTP serve err: %v", err)
+	case sig := <-sigCh:
+		log.Printf("received signal: %s", sig)
+	}
+
+}