Unbreak building for FreeBSD (#103)

Add support for freebsd. You have to set `tun.dev` in your config. The second pass of this would be to remove the exec calls and use ioctl(2) and route(4) instead, but we can do that in a second PR.

Co-authored-by: Wade Simmons <wade@wades.im>

tun_freebsd.go

@@ -0,0 +1,77 @@
+package nebula
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+var deviceNameRE = regexp.MustCompile(`^tun[0-9]+$`)
+
+type Tun struct {
+	Device       string
+	Cidr         *net.IPNet
+	MTU          int
+	UnsafeRoutes []route
+
+	io.ReadWriteCloser
+}
+
+func newTun(deviceName string, cidr *net.IPNet, defaultMTU int, routes []route, unsafeRoutes []route, txQueueLen int) (ifce *Tun, err error) {
+	if len(routes) > 0 {
+		return nil, fmt.Errorf("Route MTU not supported in FreeBSD")
+	}
+	if strings.HasPrefix(deviceName, "/dev/") {
+		deviceName = strings.TrimPrefix(deviceName, "/dev/")
+	}
+	if !deviceNameRE.MatchString(deviceName) {
+		return nil, fmt.Errorf("tun.dev must match `tun[0-9]+`")
+	}
+	return &Tun{
+		Device:       deviceName,
+		Cidr:         cidr,
+		MTU:          defaultMTU,
+		UnsafeRoutes: unsafeRoutes,
+	}, nil
+}
+
+func (c *Tun) Activate() error {
+	var err error
+	c.ReadWriteCloser, err = os.OpenFile("/dev/"+c.Device, os.O_RDWR, 0)
+	if err != nil {
+		return fmt.Errorf("Activate failed: %v", err)
+	}
+
+	// TODO use syscalls instead of exec.Command
+	l.Debug("command: ifconfig", c.Device, c.Cidr.String(), c.Cidr.IP.String())
+	if err = exec.Command("/sbin/ifconfig", c.Device, c.Cidr.String(), c.Cidr.IP.String()).Run(); err != nil {
+		return fmt.Errorf("failed to run 'ifconfig': %s", err)
+	}
+	l.Debug("command: route", "-n", "add", "-net", c.Cidr.String(), "-interface", c.Device)
+	if err = exec.Command("/sbin/route", "-n", "add", "-net", c.Cidr.String(), "-interface", c.Device).Run(); err != nil {
+		return fmt.Errorf("failed to run 'route add': %s", err)
+	}
+	l.Debug("command: ifconfig", c.Device, "mtu", strconv.Itoa(c.MTU))
+	if err = exec.Command("/sbin/ifconfig", c.Device, "mtu", strconv.Itoa(c.MTU)).Run(); err != nil {
+		return fmt.Errorf("failed to run 'ifconfig': %s", err)
+	}
+	// Unsafe path routes
+	for _, r := range c.UnsafeRoutes {
+		l.Debug("command: route", "-n", "add", "-net", r.route.String(), "-interface", c.Device)
+		if err = exec.Command("/sbin/route", "-n", "add", "-net", r.route.String(), "-interface", c.Device).Run(); err != nil {
+			return fmt.Errorf("failed to run 'route add' for unsafe_route %s: %s", r.route.String(), err)
+		}
+	}
+
+	return nil
+}
+
+func (c *Tun) WriteRaw(b []byte) error {
+	_, err := c.Write(b)
+	return err
+}