add security issue.

kame/kame/man/man4/gif.4

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"     $Id: gif.4,v 1.4 2000/03/12 13:12:27 itojun Exp $
+.\"     $Id: gif.4,v 1.4.2.1 2000/04/14 09:07:33 itojun Exp $
 .\"
 .Dd April 10, 1999
 .Dt GIF 4
@@ -209,6 +209,13 @@ For compatibility, IPv4-in-IPv4 traffic will be matched to
 interfaces first, and then sent to
 .Xr vif 4
 if no match is found.
+.Pp
+.Ss Security
+Malicious party may try to circumvent security filters by using
+tunnelled packets.
+As mentioned above, multi-destination mode
+.Pq Dv IFF_LINK0
+is far less secure than bidirectional mode.
 .\"
 .Sh SEE ALSO
 .Xr inet 4 ,