In some email I received from NetBSD Mailing-List, sie wrote:
> I have searched high and low, but cannot find the answer to my question...
> 
> Can NetBSD pass GRE packets through IPNAT?  I found out that ipchains in
> L*nux can do it.... are there any plans to do this, or is this one of
> those philosophical things that I don't understand?

What do you want it to do with them?

an ipnat.conf line without portmap/tcp/udp will map all protocols.