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When a connection is re-used, following can happen:
[ connection starts to close, fin sent in either direction ]
> syn # initator quickly reuses connection
< ack # peer sends a challenge ack
> rst # rst, sequence number == ack_seq of previous challenge ack
> syn # this syn is expected to pass
Problem is that the rst will fail window validation, so it gets
tagged as invalid.
If ruleset drops such packets, we get repeated syn-retransmits until
initator gives up or peer starts responding with syn/ack.
Before the commit indicated in the "Fixes" tag below this used to work:
The challenge-ack made conntrack re-init state based on the challenge
ack itself, so the following rst would pass window validation.
Add challenge-ack support: If we get ack for syn, record the ack_seq,
and then check if the rst sequence number matches the last ack number
seen in reverse direction.
Fixes: c7aab4f170 ("netfilter: nf_conntrack_tcp: re-init for syn packets only")
Reported-by: Michal Tesar <mtesar@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
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 Restructured Text 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.