Index: head/share/man/man4/tcp.4
===================================================================
--- head/share/man/man4/tcp.4
+++ head/share/man/man4/tcp.4
@@ -34,7 +34,7 @@
 .\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd March 29, 2020
+.Dd March 31, 2020
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -628,21 +628,31 @@
 This is needed to help with connection establishment
 when a broken firewall is in the network path.
 .It Va pmtud_blackhole_detection
-Turn on automatic path MTU blackhole detection.
-In case of retransmits OS will
-lower the MSS to check if it's MTU problem.
-If current MSS is greater than
-configured value to try
+Enable automatic path MTU blackhole detection.
+In case of retransmits of MSS sized segments,
+the OS will lower the MSS to check if it's an MTU problem.
+If the current MSS is greater than the configured value to try
 .Po Va net.inet.tcp.pmtud_blackhole_mss
 and
 .Va net.inet.tcp.v6pmtud_blackhole_mss
 .Pc ,
 it will be set to this value, otherwise,
-MSS will be set to default values
+the MSS will be set to the default values
 .Po Va net.inet.tcp.mssdflt
 and
 .Va net.inet.tcp.v6mssdflt
 .Pc .
+Settings:
+.Bl -tag -compact
+.It 0
+Disable path MTU blackhole detection.
+.It 1
+Enable path MTU blackhole detection for IPv4 and IPv6.
+.It 2
+Enable path MTU blackhole detection only for IPv4.
+.It 3
+Enable path MTU blackhole detection only for IPv6.
+.El
 .It Va pmtud_blackhole_mss
 MSS to try for IPv4 if PMTU blackhole detection is turned on.
 .It Va v6pmtud_blackhole_mss
Index: head/sys/netinet/tcp_stacks/bbr.c
===================================================================
--- head/sys/netinet/tcp_stacks/bbr.c
+++ head/sys/netinet/tcp_stacks/bbr.c
@@ -5041,6 +5041,7 @@
 {
 	int32_t rexmt;
 	int32_t retval = 0;
+	bool isipv6;
 
 	bbr->r_ctl.rc_hpts_flags &= ~PACE_TMR_RXT;
 	if (bbr->rc_all_timers_stopped) {
@@ -5127,11 +5128,16 @@
 	 * of packets and process straight to FIN. In that case we won't
 	 * catch ESTABLISHED state.
 	 */
-	if (V_tcp_pmtud_blackhole_detect && (((tp->t_state == TCPS_ESTABLISHED))
-	    || (tp->t_state == TCPS_FIN_WAIT_1))) {
 #ifdef INET6
-		int32_t isipv6;
+	isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) ? true : false;
+#else
+	isipv6 = false;
 #endif
+	if (((V_tcp_pmtud_blackhole_detect == 1) ||
+	    (V_tcp_pmtud_blackhole_detect == 2 && !isipv6) ||
+	    (V_tcp_pmtud_blackhole_detect == 3 && isipv6)) &&
+	    ((tp->t_state == TCPS_ESTABLISHED) ||
+	    (tp->t_state == TCPS_FIN_WAIT_1))) {
 
 		/*
 		 * Idea here is that at each stage of mtu probe (usually,
Index: head/sys/netinet/tcp_stacks/rack.c
===================================================================
--- head/sys/netinet/tcp_stacks/rack.c
+++ head/sys/netinet/tcp_stacks/rack.c
@@ -3123,6 +3123,7 @@
 	int32_t rexmt;
 	struct inpcb *inp;
 	int32_t retval = 0;
+	bool isipv6;
 
 	inp = tp->t_inpcb;
 	if (tp->t_timers->tt_flags & TT_STOPPED) {
@@ -3209,11 +3210,16 @@
 	 * of packets and process straight to FIN. In that case we won't
 	 * catch ESTABLISHED state.
 	 */
-	if (V_tcp_pmtud_blackhole_detect && (((tp->t_state == TCPS_ESTABLISHED))
-	    || (tp->t_state == TCPS_FIN_WAIT_1))) {
 #ifdef INET6
-		int32_t isipv6;
+	isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) ? true : false;
+#else
+	isipv6 = false;
 #endif
+	if (((V_tcp_pmtud_blackhole_detect == 1) ||
+	    (V_tcp_pmtud_blackhole_detect == 2 && !isipv6) ||
+	    (V_tcp_pmtud_blackhole_detect == 3 && isipv6)) &&
+	    ((tp->t_state == TCPS_ESTABLISHED) ||
+	    (tp->t_state == TCPS_FIN_WAIT_1))) {
 
 		/*
 		 * Idea here is that at each stage of mtu probe (usually,
@@ -3243,7 +3249,6 @@
 			 * default in an attempt to retransmit.
 			 */
 #ifdef INET6
-			isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) ? 1 : 0;
 			if (isipv6 &&
 			    tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss) {
 				/* Use the sysctl tuneable blackhole MSS. */
Index: head/sys/netinet/tcp_timer.c
===================================================================
--- head/sys/netinet/tcp_timer.c
+++ head/sys/netinet/tcp_timer.c
@@ -614,6 +614,7 @@
 	int rexmt;
 	struct inpcb *inp;
 	struct epoch_tracker et;
+	bool isipv6;
 #ifdef TCPDEBUG
 	int ostate;
 
@@ -712,12 +713,16 @@
 	 * packets and process straight to FIN. In that case we won't catch
 	 * ESTABLISHED state.
 	 */
-	if (V_tcp_pmtud_blackhole_detect && (((tp->t_state == TCPS_ESTABLISHED))
-	    || (tp->t_state == TCPS_FIN_WAIT_1))) {
 #ifdef INET6
-		int isipv6;
+	isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) ? true : false;
+#else
+	isipv6 = false;
 #endif
-
+	if (((V_tcp_pmtud_blackhole_detect == 1) ||
+	    (V_tcp_pmtud_blackhole_detect == 2 && !isipv6) ||
+	    (V_tcp_pmtud_blackhole_detect == 3 && isipv6)) &&
+	    ((tp->t_state == TCPS_ESTABLISHED) ||
+	    (tp->t_state == TCPS_FIN_WAIT_1))) {
 		/*
 		 * Idea here is that at each stage of mtu probe (usually, 1448
 		 * -> 1188 -> 524) should be given 2 chances to recover before
@@ -746,7 +751,6 @@
 			 * in an attempt to retransmit.
 			 */
 #ifdef INET6
-			isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) ? 1 : 0;
 			if (isipv6 &&
 			    tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss) {
 				/* Use the sysctl tuneable blackhole MSS. */