Folks,
Looking at FreeBSD's TCP implementation, I see that by default,
ephemeral ports are selected from the range 49152-65535. This means
that only 15K ports out of the available 65K port range are used for
ephemeral port selection.
This has at least two implications:
* Ephemeral ports are easier to predict (as you are picking them from
a smaller range)
* There is a higher chance of facing the interoperability problems
described in Mike Silbersack's presentation at EuroBSDCon 2005
(http://www.silby.com/eurobsdcon05/eurobsdcon_silbersack.pdf).
A first and small proposal would be to change the range of ephemeral
port numbers to use the range 1024-65535. An array of bits could be
maintained in memory to avoid the selection of ports that are used
for services (e.g., X).
We have also been working on an alternative port randomization
scheme, that would help to avoid the problems described in Mike's
presentation. Our work on the subject is available at:
http://www.gont.com.ar/drafts/port-randomization/draft-larsen-tsvwg-port-randomization-01.txt
We would be willing to provide patches for these things if there is
interest in implementing the proposed changes (extending the port
range and possibly implementing the RFC1948-like scheme for ephemeral
port selection).
Any comments will be more than welcome.
Thanks,
--
Fernando Gont
e-mail: [EMAIL PROTECTED] || [EMAIL PROTECTED]
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1
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