Subject: if_null
To: None <>
From: Steven M. Bellovin <>
List: tech-net
Date: 01/11/2005 16:27:46
As I mentioned several months ago, I've implemented a null network 
interface for NetBSD.  The problem I have is that packets sent to the 
interface address are not discarded; instead, they're treated as 
destined for the machine itself.

I don't see any obvious way to fix the problem.  One thought I had was 
a per-interface flag saying "no local addresses here".  It seems kind 
of clumsy; besides, there are no free flags in struct ifnet.  (I 
actually grepped the kernel to see if IFF_NOTRAILERS and IFF_NOARP are 
still used.  To my surprise, they are.)  There is a pad short 
following if_flags; maybe the two fields could be coalesced, though I 
worry about breaking binary compatibility on some architectures.

A slightly better place for a flag is in struct ifaddr, where it could 
be per-address.  There's a u_int flag field there; the comment says 
"mostly rt_flags for cloning", but only one flag is defined in if.h.

Even with these problems, it's still possible to use ifnull: assign the 
interfaces non-routable addresses, and simply route desired netblocks 
to the interface.  But that's clumsy and not the semantics I really 

Any suggestions?

		--Prof. Steven M. Bellovin,