Index: sbin/ifconfig/ifconfig.c
===================================================================
RCS file: /cvs/src/sbin/ifconfig/ifconfig.c,v
retrieving revision 1.432
diff -u -p -r1.432 ifconfig.c
--- sbin/ifconfig/ifconfig.c	16 Jan 2021 17:44:29 -0000	1.432
+++ sbin/ifconfig/ifconfig.c	27 Jan 2021 06:57:37 -0000
@@ -469,6 +469,8 @@ const struct	cmd {
 	{ "soii",	-IFXF_INET6_NOSOII,	0,	setifxflags },
 	{ "-soii",	IFXF_INET6_NOSOII,	0,	setifxflags },
 #ifndef SMALL
+	{ "monitor",	IFXF_MONITOR,	0,		setifxflags },
+	{ "-monitor",	-IFXF_MONITOR,	0,		setifxflags },
 	{ "hwfeatures", NEXTARG0,	0,		printifhwfeatures },
 	{ "metric",	NEXTARG,	0,		setifmetric },
 	{ "powersave",	NEXTARG0,	0,		setifpowersave },
@@ -675,7 +677,7 @@ const struct	cmd {
 	"\7RUNNING\10NOARP\11PROMISC\12ALLMULTI\13OACTIVE\14SIMPLEX"	\
 	"\15LINK0\16LINK1\17LINK2\20MULTICAST"				\
 	"\23INET6_NOPRIVACY\24MPLS\25WOL\26AUTOCONF6\27INET6_NOSOII"	\
-	"\30AUTOCONF4"
+	"\30AUTOCONF4" "\32MONITOR"
 
 int	getinfo(struct ifreq *, int);
 void	getsock(int);
Index: sys/net/if.c
===================================================================
RCS file: /cvs/src/sys/net/if.c,v
retrieving revision 1.625
diff -u -p -r1.625 if.c
--- sys/net/if.c	18 Jan 2021 09:55:43 -0000	1.625
+++ sys/net/if.c	27 Jan 2021 06:57:37 -0000
@@ -860,7 +860,8 @@ if_vinput(struct ifnet *ifp, struct mbuf
 	}
 #endif
 
-	(*ifp->if_input)(ifp, m);
+	if (__predict_true(!ISSET(ifp->if_xflags, IFXF_MONITOR)))
+		(*ifp->if_input)(ifp, m);
 }
 
 void
Index: sys/net/if.h
===================================================================
RCS file: /cvs/src/sys/net/if.h,v
retrieving revision 1.205
diff -u -p -r1.205 if.h
--- sys/net/if.h	18 Jan 2021 09:55:43 -0000	1.205
+++ sys/net/if.h	27 Jan 2021 06:57:37 -0000
@@ -230,6 +230,7 @@ struct if_status_description {
 #define	IFXF_AUTOCONF6		0x20	/* [N] v6 autoconf enabled */
 #define IFXF_INET6_NOSOII	0x40	/* [N] don't do RFC 7217 */
 #define	IFXF_AUTOCONF4		0x80	/* [N] v4 autoconf (aka dhcp) enabled */
+#define	IFXF_MONITOR		0x200	/* [N] only used for bpf */
 
 #define	IFXF_CANTCHANGE \
 	(IFXF_MPSAFE|IFXF_CLONED)
Index: sys/net/ifq.c
===================================================================
RCS file: /cvs/src/sys/net/ifq.c,v
retrieving revision 1.41
diff -u -p -r1.41 ifq.c
--- sys/net/ifq.c	7 Jul 2020 00:00:03 -0000	1.41
+++ sys/net/ifq.c	27 Jan 2021 06:57:37 -0000
@@ -715,10 +715,12 @@ ifiq_input(struct ifiqueue *ifiq, struct
 	ifiq->ifiq_bytes += bytes;
 
 	len = ml_len(&ifiq->ifiq_ml);
-	if (len > ifiq_maxlen_drop)
-		ifiq->ifiq_qdrops += ml_len(ml);
-	else
-		ml_enlist(&ifiq->ifiq_ml, ml);
+	if (__predict_true(!ISSET(ifp->if_xflags, IFXF_MONITOR))) {
+		if (len > ifiq_maxlen_drop)
+			ifiq->ifiq_qdrops += ml_len(ml);
+		else
+			ml_enlist(&ifiq->ifiq_ml, ml);
+	}
 	mtx_leave(&ifiq->ifiq_mtx);
 
 	if (ml_empty(ml))