Index: sys/net/bpf.c
===================================================================
--- sys/net/bpf.c
+++ sys/net/bpf.c
@@ -213,6 +213,7 @@
 static int	bpf_setdlt(struct bpf_d *, u_int);
 static void	filt_bpfdetach(struct knote *);
 static int	filt_bpfread(struct knote *, long);
+static int	filt_bpfwrite(struct knote *, long);
 static void	bpf_drvinit(void *);
 static int	bpf_stats_sysctl(SYSCTL_HANDLER_ARGS);
 
@@ -257,6 +258,12 @@
 	.f_event = filt_bpfread,
 };
 
+static struct filterops bpfwrite_filtops = {
+	.f_isfd = 1,
+	.f_detach = filt_bpfdetach,
+	.f_event = filt_bpfwrite,
+};
+
 /*
  * LOCKING MODEL USED BY BPF
  *
@@ -2158,19 +2165,37 @@
 {
 	struct bpf_d *d;
 
-	if (devfs_get_cdevpriv((void **)&d) != 0 ||
-	    kn->kn_filter != EVFILT_READ)
+	if (devfs_get_cdevpriv((void **)&d) != 0)
 		return (1);
 
-	/*
-	 * Refresh PID associated with this descriptor.
-	 */
-	BPFD_LOCK(d);
-	BPF_PID_REFRESH_CUR(d);
-	kn->kn_fop = &bpfread_filtops;
-	kn->kn_hook = d;
-	knlist_add(&d->bd_sel.si_note, kn, 1);
-	BPFD_UNLOCK(d);
+	switch (kn->kn_filter) {
+	case EVFILT_READ:
+		/*
+		 * Refresh PID associated with this descriptor.
+		 */
+		BPFD_LOCK(d);
+		BPF_PID_REFRESH_CUR(d);
+		kn->kn_fop = &bpfread_filtops;
+		kn->kn_hook = d;
+		knlist_add(&d->bd_sel.si_note, kn, 1);
+		BPFD_UNLOCK(d);
+		break;
+
+	case EVFILT_WRITE:
+		/*
+		 * Refresh PID associated with this descriptor.
+		 */
+		BPFD_LOCK(d);
+		BPF_PID_REFRESH_CUR(d);
+		kn->kn_fop = &bpfwrite_filtops;
+		kn->kn_hook = d;
+		knlist_add(&d->bd_sel.si_note, kn, 1);
+		BPFD_UNLOCK(d);
+		break;
+
+	default:
+		return (1);
+	}
 
 	return (0);
 }
@@ -2207,6 +2232,17 @@
 	return (ready);
 }
 
+static int
+filt_bpfwrite(struct knote *kn, long hint)
+{
+	struct bpf_d *d = (struct bpf_d *)kn->kn_hook;
+	BPFD_LOCK_ASSERT(d);
+
+	kn->kn_data = d->bd_bif->bif_ifp->if_mtu;
+
+	return (1);
+}
+
 #define	BPF_TSTAMP_NONE		0
 #define	BPF_TSTAMP_FAST		1
 #define	BPF_TSTAMP_NORMAL	2