wgengine: support FreeBSD with IPv6.

Fixes https://github.com/tailscale/tailscale/issues/1307 for keepsies.

We cannot set the tun interface address as a /128 on FreeBSD,
due to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218508
Instead we set the interface address as a /48, which is enabled
by commit 82edf94df7.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

wgengine/router/router_userspace_bsd.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"log"
 	"os/exec"
+	"runtime"
 
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
@@ -123,7 +124,17 @@ func (r *userspaceBSDRouter) Set(cfg *Config) (reterr error) {
 		}
 	}
 	for _, addr := range r.addrsToAdd(cfg.LocalAddrs) {
-		arg := []string{"ifconfig", r.tunname, inet(addr), addr.String(), addr.IP.String()}
+		var arg []string
+		if runtime.GOOS == "freebsd" && addr.IP.Is6() && addr.Bits == 128 {
+			// FreeBSD rejects tun addresses of the form fc00::1/128 -> fc00::1,
+			// https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218508
+			// Instead add our whole /48, which works because we use a /48 route.
+			// Full history: https://github.com/tailscale/tailscale/issues/1307
+			tmp := netaddr.IPPrefix{addr.IP, 48}
+			arg = []string{"ifconfig", r.tunname, inet(tmp), tmp.String()}
+		} else {
+			arg = []string{"ifconfig", r.tunname, inet(addr), addr.String(), addr.IP.String()}
+		}
 		out, err := cmd(arg...).CombinedOutput()
 		if err != nil {
 			r.logf("addr add failed: %v => %v\n%s", arg, err, out)