Re: [RFC] tcp: randomize TCP source ports

Hannes Frederic Sowa
2013-11-08T13:02:46Z

On Thu, Nov 07, 2013 at 04:54:09PM -0800, Eric Dumazet wrote:
> TCP does proper randomization of ports on active connections only if
> bind() is used between socket() and connect()
>
> If bind() is not specifically used, kernel performs autobind, and TCP
> autobind typically uses a sequential allocation for a given (dst
> address, dst port, src address) tuple.
>
> UDP autobind does a randomization, as part of the effort to make DNS
> more secure.

If I understand the code correctly the UDP ports are fully randomized? This
is good as per-peer randomization and then incrementation seems to be
theoretically broken:

<https://sites.google.com/site/hayashulman/files/NIC-derandomisation.pdf>

Looking at the code I somehow would like to check the use of net_random there.
The prandom function is reseeded as late_initcall and then only seeded by some
network addresses.

At the time the late_initcall reseeds the PRNG my tests have shown that
the nonblockingpool was still not fully initialized where the PRNG gets
reseeded from.

Hm, I propose a patch which does reseed the pool as soon as the nonblocking
pool got credited enough entropy in credit_entropy_bits. This should help
later binds().

> TCP autobind uses a global sequential number (called @hint in source
> code) with a perturbation done by secure_ipv4_port_ephemeral(),
> so that the 'hint' of the next port is per (saddr, daddr, dport) tuple
>
> This was probably done to maximize port use and avoid hitting timewait
> sockets, but I think it should be OK to replace this stuff by a random
> selection to have more entropy in the various flow hashing functions,
> and in general higher security levels. TCP timestamps are now well
> deployed.

We recently had a thread that Windows (since Vista?) disabled tcp
timestamps by default. But I don't see how this should make a great
difference (and still wonder why they give up PAWS.)

> Patch would be trivial, but I'd like to get some comments if
> you think this idea is wrong.

I would like to see this happening.

Thanks,

Hannes