initial: ZeroTier-like P2P mesh VPN server with multi-tenant Web UI

internal/tap/tap.go

@@ -0,0 +1,24 @@
+package tap
+
+type Interface struct {
+	Name string
+	MTU  int
+	fd   int
+}
+
+// Open creates a TAP interface on Linux. On other platforms it returns an error.
+func Open(name string, mtu int) (*Interface, error) {
+	return openTap(name, mtu)
+}
+
+func (t *Interface) Read(buf []byte) (int, error) {
+	return readBuf(t.fd, buf)
+}
+
+func (t *Interface) Write(buf []byte) (int, error) {
+	return writeBuf(t.fd, buf)
+}
+
+func (t *Interface) Close() error {
+	return closeFD(t.fd)
+}

internal/tap/tap_linux.go

@@ -0,0 +1,71 @@
+//go:build linux
+
+package tap
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	cIFF_TAP   = 0x0002
+	cIFF_NO_PI = 0x1000
+	cTUNSETIFF = 0x400454ca
+	cSIOCSIFMTU = 0x8922
+)
+
+func openTap(name string, mtu int) (*Interface, error) {
+	fd, err := syscall.Open("/dev/net/tun", syscall.O_RDWR, 0)
+	if err != nil {
+		return nil, fmt.Errorf("open /dev/net/tun: %w (is TUN/TAP supported?)", err)
+	}
+
+	ifr := make([]byte, 40)
+	copy(ifr, []byte(name))
+	*(*uint16)(unsafe.Pointer(&ifr[16])) = uint16(cIFF_TAP | cIFF_NO_PI)
+
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), cTUNSETIFF, uintptr(unsafe.Pointer(&ifr[0])))
+	if errno != 0 {
+		syscall.Close(fd)
+		return nil, fmt.Errorf("ioctl TUNSETIFF: %w", errno)
+	}
+
+	if mtu > 0 {
+		mtuIfr := make([]byte, 40)
+		copy(mtuIfr, ifr[:16])
+		*(*int32)(unsafe.Pointer(&mtuIfr[16])) = int32(mtu)
+		_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), cSIOCSIFMTU, uintptr(unsafe.Pointer(&mtuIfr[0])))
+		if errno != 0 {
+			syscall.Close(fd)
+			return nil, fmt.Errorf("ioctl SIOCSIFMTU: %w", errno)
+		}
+	}
+
+	return &Interface{
+		Name: name,
+		MTU:  mtu,
+		fd:   fd,
+	}, nil
+}
+
+func readBuf(fd int, buf []byte) (int, error) {
+	n, err := syscall.Read(fd, buf)
+	if err != nil {
+		return 0, os.NewSyscallError("read", err)
+	}
+	return n, nil
+}
+
+func writeBuf(fd int, buf []byte) (int, error) {
+	n, err := syscall.Write(fd, buf)
+	if err != nil {
+		return 0, os.NewSyscallError("write", err)
+	}
+	return n, nil
+}
+
+func closeFD(fd int) error {
+	return syscall.Close(fd)
+}

internal/tap/tap_stub.go

@@ -0,0 +1,21 @@
+//go:build !linux
+
+package tap
+
+import "fmt"
+
+func openTap(_ string, _ int) (*Interface, error) {
+	return nil, fmt.Errorf("TAP devices are only supported on Linux")
+}
+
+func readBuf(_ int, _ []byte) (int, error) {
+	return 0, fmt.Errorf("not supported")
+}
+
+func writeBuf(_ int, _ []byte) (int, error) {
+	return 0, fmt.Errorf("not supported")
+}
+
+func closeFD(_ int) error {
+	return fmt.Errorf("not supported")
+}