tcp: change source port randomizarion at connect() time
RFC 6056 (Recommendations for Transport-Protocol Port Randomization) provides good summary of why source selection needs extra care. David Dworken reminded us that linux implements Algorithm 3 as described in RFC 6056 3.3.3 Quoting David : In the context of the web, this creates an interesting info leak where websites can count how many TCP connections a user's computer is establishing over time. For example, this allows a website to count exactly how many subresources a third party website loaded. This also allows: - Distinguishing between different users behind a VPN based on distinct source port ranges. - Tracking users over time across multiple networks. - Covert communication channels between different browsers/browser profiles running on the same computer - Tracking what applications are running on a computer based on the pattern of how fast source ports are getting incremented. Section 3.3.4 describes an enhancement, that reduces attackers ability to use the basic information currently stored into the shared 'u32 hint'. This change also decreases collision rate when multiple applications need to connect() to different destinations. Signed-off-by: Eric Dumazet <edumazet@google.com> Reported-by: David Dworken <ddworken@google.com> Cc: Willem de Bruijn <willemb@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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