Hello,

> I'm considering throwing away my old 56k and switching to DSL, and I 
> would
> like to have a UNIX machine set up as a router for my mini-LAN. I've 
> got an
> old LC475 which has several pros (quiet, very low power consumption, 
> quite
> stable with NetBSD-softfloat), but one big problem: it can have (and 
> has)
> only one network card. I know that what I want to accomplish is 
> possible
> using interface aliases,
You don't need aliases or anything - I did exactly this for ages with a 
little Sun ( running Solaris though ) with only one network interface, 
the DSL modem plugged into the switch - runs without problems, and even 
2x DSL traffic doesn't come anywhere near saturating a 10MBit network. 
Although - if you have fast DSL ( like 2MBit or more ) the Mac could 
become a bottleneck - last time I checked they maxed out at 
~400-500kB/s. If you use excessive filtering it will be a lot less - my 
Sun ( 110MHz MicroSPARC II ) ran at ~30% CPU with full DSL load ( I had 
768/128kBit ) - I doubt the 68040 will manage more than that at all, 
but that was Solaris, not NetBSD and it did some logging so your 
mileage will (greatly) vary.
It's a common misconception that you need a separate ethernet interface 
for DSL - it's nice to have but by no means necessary. Your box will 
create a ppp interface for it, the rest will just continue working.

>  but my question is, are there any serious drawbacks in doing that? I 
> remember having read somewhere that having a router with only one 
> network interface is not secure -- is that true?
Depends - how much do you trust the rest of your own network? And of 
course you can filter things - the DSL stuff will go through ppp0 or 
something like that on the router box, no matter which ethernet 
interface it uses to talk to the modem. Since the traffic between the 
modem and the router is only pppoe there isn't any harm in piping it 
through your switch.

have fun
Michael