linux-kernel - RE: [PATCH net 6/7] tcp: increase source port perturb table to 2^16

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Message-ID: <7372f788762140d496c157813b0173e5@AcuMS.aculab.com>
Date:   Wed, 27 Apr 2022 08:07:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "Eric Dumazet" <edumazet@...gle.com>,
        Moshe Kol <moshe.kol@...l.huji.ac.il>,
        "Yossi Gilad" <yossi.gilad@...l.huji.ac.il>,
        Amit Klein <aksecurity@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net 6/7] tcp: increase source port perturb table to 2^16

From: Willy Tarreau
> Sent: 27 April 2022 07:53
> 
> Moshe Kol, Amit Klein, and Yossi Gilad reported being able to accurately
> identify a client by forcing it to emit only 40 times more connections
> than there are entries in the table_perturb[] table. The previous two
> improvements consisting in resalting the secret every 10s and adding
> randomness to each port selection only slightly improved the situation,
> and the current value of 2^8 was too small as it's not very difficult
> to make a client emit 10k connections in less than 10 seconds.
> 
> Thus we're increasing the perturb table from 2^8 to 2^16 so that the the
> same precision now requires 2.6M connections, which is more difficult in
> this time frame and harder to hide as a background activity. The impact
> is that the table now uses 256 kB instead of 1 kB, which could mostly
> affect devices making frequent outgoing connections. However such
> components usually target a small set of destinations (load balancers,
> database clients, perf assessment tools), and in practice only a few
> entries will be visited, like before.

Increasing the table size has a bigger impact on anyone trying
to get the kernel to run in a limited memory footprint.

All these large tables (often hash tables) soon add up.

	David

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