ipv4: introduce hardened ip_no_pmtu_disc mode

This new ip_no_pmtu_disc mode only allowes fragmentation-needed errors
to be honored by protocols which do more stringent validation on the
ICMP's packet payload. This knob is useful for people who e.g. want to
run an unmodified DNS server in a namespace where they need to use pmtu
for TCP connections (as they are used for zone transfers or fallback
for requests) but don't want to use possibly spoofed UDP pmtu information.

Currently the whitelisted protocols are TCP, SCTP and DCCP as they check
if the returned packet is in the window or if the association is valid.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David Miller <davem@davemloft.net>
Cc: John Heffner <johnwheffner@gmail.com>
Suggested-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 1 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 0d71fa962d8a..c97932c88ea3 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -26,7 +26,18 @@ ip_no_pmtu_disc - INTEGER
         discarded. Outgoing frames are handled the same as in mode 1,
         implicitly setting IP_PMTUDISC_DONT on every created socket.
 
-        Possible values: 0-2
+        Mode 3 is a hardend pmtu discover mode. The kernel will only
+        accept fragmentation-needed errors if the underlying protocol
+        can verify them besides a plain socket lookup. Current
+        protocols for which pmtu events will be honored are TCP, SCTP
+        and DCCP as they verify e.g. the sequence number or the
+        association. This mode should not be enabled globally but is
+        only intended to secure e.g. name servers in namespaces where
+        TCP path mtu must still work but path MTU information of other
+        protocols should be discarded. If enabled globally this mode
+        could break other protocols.
+
+        Possible values: 0-3
         Default: FALSE
 
 min_pmtu - INTEGER