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A conntrack entry can be inserted to the connection tracking table if there
is no existing entry with an identical tuple in either direction.
Example:
INITIATOR -> NAT/PAT -> RESPONDER
Initiator passes through NAT/PAT ("us") and SNAT is done (saddr rewrite).
Then, later, NAT/PAT machine itself also wants to connect to RESPONDER.
This will not work if the SNAT done earlier has same IP:PORT source pair.
Conntrack table has:
ORIGINAL: $IP_INITATOR:$SPORT -> $IP_RESPONDER:$DPORT
REPLY: $IP_RESPONDER:$DPORT -> $IP_NAT:$SPORT
and new locally originating connection wants:
ORIGINAL: $IP_NAT:$SPORT -> $IP_RESPONDER:$DPORT
REPLY: $IP_RESPONDER:$DPORT -> $IP_NAT:$SPORT
This is handled by the NAT engine which will do a source port reallocation
for the locally originating connection that is colliding with an existing
tuple by attempting a source port rewrite.
This is done even if this new connection attempt did not go through a
masquerade/snat rule.
There is a rare race condition with connection-less protocols like UDP,
where we do the port reallocation even though its not needed.
This happens when new packets from the same, pre-existing flow are received
in both directions at the exact same time on different CPUs after the
conntrack table was flushed (or conntrack becomes active for first time).
With strict ordering/single cpu, the first packet creates new ct entry and
second packet is resolved as established reply packet.
With parallel processing, both packets are picked up as new and both get
their own ct entry.
In this case, the 'reply' packet (picked up as ORIGINAL) can be mangled by
NAT engine because a port collision is detected.
This change isn't enough to prevent a packet drop later during
nf_conntrack_confirm(), the existing clash resolution strategy will not
detect such reverse clash case. This is resolved by a followup patch.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.