Ephemeral port range (patch)

Mike Silbersack silby at silby.com
Mon Mar 3 06:43:10 UTC 2008

On Sat, 1 Mar 2008, Fernando Gont wrote:

> I will also start working on the double-hash ephemeral port selection 
> algorithm described in the draft (this is, IMHO, the right approach to 
> ephemeral port randomization)
> Kind regards,
> --
> Fernando Gont

Earlier in the week, I had commented (via private e-mail?) that I thought 
that Amit Klein's algorithm which I recently implemented in ip_id.c might 
be adapted to serve as an ephemeral port allocator.  Now that I've thought 
more about it, I'm not as certain that it would fit well.  I'll try to 
sketch out my ideas and see if I can figure out how it could fit.

The double-hash concept sounds pretty good, but there's a major problem 
with it.  If an application does a bind() to get a local port before doing 
a connect(), you don't know the remote IP or the remote port.

There's a related "feature" in the BSD TCP stack that all local ports are 
considered equal; even for applications that do a connect() call and 
specify a remote IP/port, we do not let them use the same local port to 
two different remote IPs at the same time.  This puts a limit on the total 
number of outgoing connections that one machine can have.


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