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Index: share/man/man4/tcp.4
===================================================================
--- share/man/man4/tcp.4
+++ 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 30, 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: sys/netinet/tcp_timer.c
===================================================================
--- sys/netinet/tcp_timer.c
+++ 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. */

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