diff --git a/Shorewall/Perl/Shorewall/Rules.pm b/Shorewall/Perl/Shorewall/Rules.pm
index 17f6d4279..6c5495ad9 100644
--- a/Shorewall/Perl/Shorewall/Rules.pm
+++ b/Shorewall/Perl/Shorewall/Rules.pm
@@ -4589,6 +4589,52 @@ sub process_mangle_rule1( $$$$$$$$$$$$$$$$$$$ ) {
 	    },
 	},
 
+	TCPMSS     => {
+	    defaultchain   => FORWARD,
+	    allowedchains  => FORWARD | POSTROUTING,
+	    minparams      => 0,
+	    maxparams      => 2,
+	    function       => sub () {
+		if ( $proto eq '-' ) {
+		    $proto = TCP;
+		} else {
+		    fatal_error 'TCPMSS only valid with TCP' unless $proto eq '6' || $proto eq 'tcp';
+		}
+
+		$target   = 'TCPMSS ';
+		$matches .= '--tcp-flags SYN,RST SYN ';
+
+		if ( supplied $params ) {
+		    my ( $mss, $ipsec ) = split /,/, $params;
+
+		    if ( supplied $mss ) {
+			if ( $mss eq 'pmtu' ) {
+			    $target .= '--clamp-mss-to-pmtu';
+			} else {
+			    my $num = numeric_value $mss;
+			    fatal_error "Invalid MSS ($mss)" unless defined $num && $num >= 500 && $num < 65534;
+			    $target .= "--set-mss $num";
+			}
+		    } else {
+			$target .= '--clamp-mss-to-pmtu';
+		    }
+		    if ( supplied $ipsec && $ipsec ne 'all' ) {
+			if ( $ipsec eq '-' || $ipsec eq 'none' ) {
+			    $matches .= '-m policy --pol none --dir out ';
+			} elsif ( $ipsec eq 'ipsec' ) {
+			    $matches .= '-m policy --pol ipsec --dir out ';
+			} else {
+			    fatal_error "Invalid ipsec parameter ($ipsec)";
+			}
+
+			require_capability 'POLICY_MATCH', "The $ipsec ipsec option", 's';
+		    }
+		} else {
+		    $target .= '--clamp-mss-to-pmtu';
+		}
+	    },
+	},
+
 	TOS        => {
 	    defaultchain   => 0,
 	    allowedchains  => PREROUTING | FORWARD | OUTPUT | POSTROUTING,
diff --git a/Shorewall/manpages/shorewall-mangle.xml b/Shorewall/manpages/shorewall-mangle.xml
index fd847ec3f..b9e99579e 100644
--- a/Shorewall/manpages/shorewall-mangle.xml
+++ b/Shorewall/manpages/shorewall-mangle.xml
@@ -674,6 +674,43 @@ SAME              $FW            0.0.0.0/0    tcp        80,443</programlisting>
               </listitem>
             </varlistentry>
 
+            <varlistentry>
+              <term><emphasis
+              role="bold">TCPMSS</emphasis>([<replaceable>mss</replaceable>][,<replaceable>ipsec</replaceable>])</term>
+
+              <listitem>
+                <para>Added in Shorewall 5.1.9. This target only applies to
+                TCP traffic and alters the MSS value in SYN packets. It may be
+                used in the FOWARD and POSTROUTING chains; the default is
+                FORWARD.</para>
+
+                <para>The <replaceable>mss</replaceable> parameter may be
+                either <option>pmtu</option> or an integer in the range
+                500:65533. The value <option>pmtu</option> automatically
+                clamps the MSS value to (path_MTU - 40 for IPv4; -60 for
+                IPv6). This may not function as desired where asymmetric
+                routes with differing path MTU exist — the kernel uses the
+                path MTU which it would use to send packets from itself to the
+                source and destination IP addresses. Prior to Linux 2.6.25,
+                only the path MTU to the destination IP address was considered
+                by this option; subsequent kernels also consider the path MTU
+                to the source IP address. If an integer is given, the MSS
+                option is set to the specified value. If the MSS of the packet
+                is already lower than <replaceable>mss</replaceable>, it will
+                not be increased (from Linux 2.6.25 onwards) to avoid more
+                problems with hosts relying on a proper MSS. If
+                <replaceable>mss</replaceable> is omitted,
+                <option>pmtu</option> is assumed.</para>
+
+                <para>The <replaceable>ipsec</replaceable> parameter
+                determines whether the rule applies to IPSEC traffic
+                (<option>ipsec</option> is passed), non-IPSEC traffic
+                (<option>none</option> is passed) or both
+                (<option>all</option> is passed). If omitted,
+                <option>all</option> is assumed.</para>
+              </listitem>
+            </varlistentry>
+
             <varlistentry>
               <term><emphasis
               role="bold">TOS</emphasis>(<replaceable>tos</replaceable>[/<replaceable>mask</replaceable>])</term>