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>

2 files changed, 11 insertions, 3 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index f76d177895d9..d71afa8bd828 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/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
diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 1a4ee84ca3f3..fb3c5637199d 100644
--- a/net/ipv4/icmp.c
+++ b/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 {