As the Tunnel device has no real inherrent MRU limit,

so don't enforce the MTU as an MRU. Allow bidirectional ppp MTU
negotiation, by checking against a differnt figure for MRU.
Make it large enough for ATM frames at least.

Submitted by:   archie@whistle.com (archie cobbs)

sys/net/if_tun.c

@@ -511,7 +511,7 @@ tunwrite(dev_t dev, struct uio *uio, int flag)
 
 	TUNDEBUG("%s%d: tunwrite\n", ifp->if_name, ifp->if_unit);
 
-	if (uio->uio_resid < 0 || uio->uio_resid > ifp->if_mtu) {
+	if (uio->uio_resid < 0 || uio->uio_resid > TUNMRU) {
 		TUNDEBUG("%s%d: len=%d!\n", ifp->if_name, ifp->if_unit,
 		    uio->uio_resid);
 		return EIO;

sys/net/if_tun.h

@@ -38,9 +38,12 @@ struct tun_softc {
 	struct	selinfo	tun_wsel;	/* write select (not used) */
 };
 
-/* Default maximum packet size */
+/* Maximum transmit packet size (default) */
 #define	TUNMTU		1500
 
+/* Maximum receive packet size (hard limit) */
+#define	TUNMRU		16384
+
 struct tuninfo {
 	int	baudrate;		/* linespeed */
 	short	mtu;			/* maximum transmission unit */