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, 1 insertions, 0 deletions

diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index 88299c29101d..22b5d818b200 100644
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -989,6 +989,7 @@ static const struct net_protocol dccp_v4_protocol = {
         .err_handler    = dccp_v4_err,
         .no_policy      = 1,
         .netns_ok       = 1,
+        .icmp_strict_tag_validation = 1,
 };
 
 static const struct proto_ops inet_dccp_ops = {