Which IP implementations can send traffic to a process?

[John Chambers]

Just recently, it came to my attention that at least one commercial 
version of Unix has a driver that implements a very useful feature:
It creates a pseudo-interface that has an IP address, and is a file
that can be opened by a process.  Thus one can write a program that
tells the kernel "If you get any traffic for a.b.c.d, give it to me
and I'll handle it."   I can think of lots & lots of uses for this.

This driver is on Ultrix; it is unsupported, but it is available.

Do any other extant Unix releases implement this capability?  If
so, can you say what they are, what sort of special hooks (if any)
are needed, and whether programming examples are available?

It's not obvious that this requires a special driver, but I suspect
that in most cases it will.  You'd think you could do it via a raw
socket, for instance, but just try...  (If anyone knows how, I'd 
like to see the code.)  I wonder if the Ultrix driver would work
on many other Unices.

It also seems that a streams-based system should be able to do this,
but I haven't yet succeeded in decrypting the manuals, so I can't
say for sure.  Anyone know for sure?

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