tstest/natlab/vnet: capture network wan/lan interfaces

Updates #13038

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/natlab/vnet/pcap.go

@@ -12,6 +12,8 @@ import (
 	"github.com/google/gopacket/pcapgo"
 )
 
+// pcapWriter is a pcapgo.NgWriter that writes to a file.
+// It is safe for concurrent use. The nil value is a no-op.
 type pcapWriter struct {
 	f *os.File
 
@@ -20,6 +22,9 @@ type pcapWriter struct {
 }
 
 func (p *pcapWriter) WritePacket(ci gopacket.CaptureInfo, data []byte) error {
+	if p == nil {
+		return nil
+	}
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if p.w == nil {
@@ -28,7 +33,19 @@ func (p *pcapWriter) WritePacket(ci gopacket.CaptureInfo, data []byte) error {
 	return p.w.WritePacket(ci, data)
 }
 
+func (p *pcapWriter) AddInterface(i pcapgo.NgInterface) (int, error) {
+	if p == nil {
+		return 0, nil
+	}
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return p.w.AddInterface(i)
+}
+
 func (p *pcapWriter) Close() error {
+	if p == nil {
+		return nil
+	}
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if p.w != nil {