ipv4: new ip_no_pmtu_disc mode to always discard incoming frag needed msgs

This new mode discards all incoming fragmentation-needed notifications
as I guess was originally intended with this knob. To not break backward
compatibility too much, I only added a special case for mode 2 in the
receiving path.
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -15,12 +15,18 @@ ip_default_ttl - INTEGER
 	forwarded) IP packets. Should be between 1 and 255 inclusive.
 	Default: 64 (as recommended by RFC1700)
 
-ip_no_pmtu_disc - BOOLEAN
-	Disable Path MTU Discovery. If enabled and a
+ip_no_pmtu_disc - INTEGER
+	Disable Path MTU Discovery. If enabled in mode 1 and a
 	fragmentation-required ICMP is received, the PMTU to this
 	destination will be set to min_pmtu (see below). You will need
 	to raise min_pmtu to the smallest interface MTU on your system
 	manually if you want to avoid locally generated fragments.
+
+	In mode 2 incoming Path MTU Discovery messages will be
+	discarded. Outgoing frames are handled the same as in mode 1,
+	implicitly setting IP_PMTUDISC_DONT on every created socket.
+
+	Possible values: 0-2
 	Default: FALSE
 
 min_pmtu - INTEGER

net/ipv4/icmp.c

@@ -705,7 +705,9 @@ static void icmp_unreach(struct sk_buff *skb)
 		case ICMP_PORT_UNREACH:
 			break;
 		case ICMP_FRAG_NEEDED:
-			if (net->ipv4.sysctl_ip_no_pmtu_disc) {
+			if (net->ipv4.sysctl_ip_no_pmtu_disc == 2) {
+				goto out;
+			} else if (net->ipv4.sysctl_ip_no_pmtu_disc) {
 				LIMIT_NETDEBUG(KERN_INFO pr_fmt("%pI4: fragmentation needed and DF set\n"),
 					       &iph->daddr);
 			} else {