Index: sys/net/if_tun.c
===================================================================
RCS file: /cvs/src/sys/net/if_tun.c,v
retrieving revision 1.181
diff -u -p -r1.181 if_tun.c
--- sys/net/if_tun.c	24 Feb 2018 07:20:04 -0000	1.181
+++ sys/net/if_tun.c	29 Oct 2018 02:03:44 -0000
@@ -667,8 +667,7 @@ tun_dev_ioctl(struct tun_softc *tp, u_lo
 			tp->tun_flags &= ~TUN_ASYNC;
 		break;
 	case FIONREAD:
-		*(int *)data = ifq_empty(&tp->tun_if.if_snd) ?
-		    0 : tp->tun_if.if_mtu;
+		*(int *)data = ifq_hdatalen(&tp->tun_if.if_snd);
 		break;
 	case TIOCSPGRP:
 		tp->tun_pgid = *(int *)data;
Index: sys/net/ifq.c
===================================================================
RCS file: /cvs/src/sys/net/ifq.c,v
retrieving revision 1.22
diff -u -p -r1.22 ifq.c
--- sys/net/ifq.c	25 Jan 2018 14:04:36 -0000	1.22
+++ sys/net/ifq.c	29 Oct 2018 02:03:44 -0000
@@ -357,6 +357,21 @@ ifq_dequeue(struct ifqueue *ifq)
 	return (m);
 }
 
+int
+ifq_hdatalen(struct ifqueue *ifq)
+{
+	struct mbuf *m;
+	int len = 0;
+
+	m = ifq_deq_begin(ifq);
+	if (m != NULL) {
+		len = m->m_pkthdr.len;
+		ifq_deq_commit(ifq, m);
+	}
+
+	return (len);
+}
+
 unsigned int
 ifq_purge(struct ifqueue *ifq)
 {
Index: sys/net/ifq.h
===================================================================
RCS file: /cvs/src/sys/net/ifq.h,v
retrieving revision 1.20
diff -u -p -r1.20 ifq.h
--- sys/net/ifq.h	4 Jan 2018 11:02:57 -0000	1.20
+++ sys/net/ifq.h	29 Oct 2018 02:03:44 -0000
@@ -409,6 +409,7 @@ struct mbuf	*ifq_deq_begin(struct ifqueu
 void		 ifq_deq_commit(struct ifqueue *, struct mbuf *);
 void		 ifq_deq_rollback(struct ifqueue *, struct mbuf *);
 struct mbuf	*ifq_dequeue(struct ifqueue *);
+int		 ifq_hdatalen(struct ifqueue *);
 void		 ifq_mfreem(struct ifqueue *, struct mbuf *);
 void		 ifq_mfreeml(struct ifqueue *, struct mbuf_list *);
 unsigned int	 ifq_purge(struct ifqueue *);
Index: share/man/man9/ifq_enqueue.9
===================================================================
RCS file: /cvs/src/share/man/man9/ifq_enqueue.9,v
retrieving revision 1.9
diff -u -p -r1.9 ifq_enqueue.9
--- share/man/man9/ifq_enqueue.9	10 Dec 2015 06:58:56 -0000	1.9
+++ share/man/man9/ifq_enqueue.9	29 Oct 2018 02:03:44 -0000
@@ -23,6 +23,7 @@
 .Nm ifq_purge ,
 .Nm ifq_len ,
 .Nm ifq_empty ,
+.Nm ifq_hdatalen ,
 .Nm ifq_set_oactive ,
 .Nm ifq_clr_oactive ,
 .Nm ifq_is_oactive ,
@@ -41,6 +42,8 @@
 .Fn ifq_len "struct ifqueue *ifq"
 .Ft unsigned int
 .Fn ifq_empty "struct ifqueue *ifq"
+.Ft int
+.Fn ifq_hdatalen "struct ifqueue *ifq"
 .Ft void
 .Fn ifq_set_oactive "struct ifqueue *ifq"
 .Ft void
@@ -87,6 +90,10 @@ or
 Return if the interface send queue
 .Fa ifq
 is empty.
+.It Fn ifq_hdatalen "struct ifqueue *ifq"
+Return the number of bytes in the mbuf at the head of the interface
+send queue
+.Fa ifq .
 .It Fn ifq_set_oactive "struct ifqueue *ifq"
 .Fn ifq_set_oactive
 is called by the relevant driver to mark the hardware associated
@@ -125,6 +132,7 @@ returns.
 .Fn ifq_purge ,
 .Fn ifq_len ,
 .Fn ifq_empty ,
+.Fn ifq_hdatalen ,
 .Fn ifq_set_oactive ,
 .Fn ifq_clr_oactive ,
 .Fn ifq_is_oactive ,
@@ -152,6 +160,9 @@ returns the number of mbufs on the queue
 .Pp
 .Fn ifq_empty
 returns a non-zero value if the queue is empty, otherwise 0.
+.Pp
+.Fn ifq_hdatalen
+returns the size of a packet on the queue, or 0 if the queue is empty;
 .Pp
 .Fn ifq_is_oactive
 returns a non-zero value if the hardware associated with the interface