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netfilter: nf_nat: skip nat clash resolution for same-origin entries

It is possible that two concurrent packets originating from the same
socket of a connection-less protocol (e.g. UDP) can end up having
different IP_CT_DIR_REPLY tuples which results in one of the packets
being dropped.

To illustrate this, consider the following simplified scenario:

1. Packet A and B are sent at the same time from two different threads
   by same UDP socket.  No matching conntrack entry exists yet.
   Both packets cause allocation of a new conntrack entry.
2. get_unique_tuple gets called for A.  No clashing entry found.
   conntrack entry for A is added to main conntrack table.
3. get_unique_tuple is called for B and will find that the reply
   tuple of B is already taken by A.
   It will allocate a new UDP source port for B to resolve the clash.
4. conntrack entry for B cannot be added to main conntrack table
   because its ORIGINAL direction is clashing with A and the REPLY
   directions of A and B are not the same anymore due to UDP source
   port reallocation done in step 3.

This patch modifies nf_conntrack_tuple_taken so it doesn't consider
colliding reply tuples if the IP_CT_DIR_ORIGINAL tuples are equal.

[ Florian: simplify patch to not use .allow_clash setting
  and always ignore identical flows ]

Signed-off-by: Martynas Pumputis <>
Signed-off-by: Florian Westphal <>
Signed-off-by: Pablo Neira Ayuso <>
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Martynas Pumputis authored and ummakynes committed Jan 29, 2019
1 parent 98bfc34 commit 4e35c1cb9460240e983a01745b5f29fe3a4d8e39
Showing with 16 additions and 0 deletions.
  1. +16 −0 net/netfilter/nf_conntrack_core.c
@@ -1007,6 +1007,22 @@ nf_conntrack_tuple_taken(const struct nf_conntrack_tuple *tuple,

if (nf_ct_key_equal(h, tuple, zone, net)) {
/* Tuple is taken already, so caller will need to find
* a new source port to use.
* Only exception:
* If the *original tuples* are identical, then both
* conntracks refer to the same flow.
* This is a rare situation, it can occur e.g. when
* more than one UDP packet is sent from same socket
* in different threads.
* Let nf_ct_resolve_clash() deal with this later.
if (nf_ct_tuple_equal(&ignored_conntrack->tuplehash[IP_CT_DIR_ORIGINAL].tuple,

NF_CT_STAT_INC_ATOMIC(net, found);
return 1;

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