Re: Advanced tricks I use to get rid of spam using MX 4xx

[Donald Nash <D.Nash@its.utexas.edu>]

--On November 27, 2006 8:53:58 PM +0100 Harald Hanche-Olsen 
<hanche@math.ntnu.no> wrote:
> Are there Unix variants that will let you decide in userland what to
> do about an incoming SYN?
None that I know of.  This feature was included in DEC Ultrix, but only for 
DEC's adaptation of DECnet to the socket API.  As I recall, you could use 
setsockopt() to mark an AF_DECNET socket in such a way that listen() would 
return without actually accepting the incoming connection.  You could then 
use getpeername() to determine the identity of the remote system, and then 
use setsockopt() either to accept or reject the connection.  This must have 
been at least 15 years ago, and it has been a constant source of low-level 
amazement to me ever since that it was never adapted to TCP/IP, not even by 
DEC for Ultrix.
--
Donald L. Nash, <D.Nash@its.utexas.edu>
Information Technology Services, The University of Texas at Austin