Re: Multiple DHCPed IPs on the same device

[Nick Holland <nhollan@home.com>]

A few days ago, I invited people to tell me why I was a fool for something I
posted here, someone said they didn't think I was, so I'm going to try again.
(I don't learn much when people agree with me, and I'm too new at OpenBSD to
permit myself to think I am an expert!)

I don't think it is possible to do exactly what you are wanting to do -- have
one NIC respond as multiple IPs using DHCP.  However, I bet you can accomplish
what you want in other ways...

First, why I think what you desire to do is impossible:  DHCP (typically?)
allocates IP addresses by machine name, or at least a machine name broadcast
by the DHCP client software.  One card will only broadcast one machine name, I
would think...

But there are probably other ways...

1) Do you REALLY need DHCP?  *MOST* of the Cable Internet systems I have seen
use DHCP for configuration, but the IP address(es) assigned to you is yours,
and stays that way.  In other words, static addresses are often quite
acceptable (This is NOT true for many DSL connections, apparently!).  There is
some question in my mind as to whether or not you would really want to access
a DHCP server outside your building -- it strikes me that it might be possible
to do a "man-in-the-middle" interseption of all traffic in and out of your
machine, but then, I might be full of it...  I'm also trying to remember how I
set up one of my machines.  You see how scared of this I am 8-)

2) Do you REALLY need FOUR IPs going through one machine?  If your system is
like mine, you pay for each additional IP (about $8U.S. here, if you care).  I
use my second IP for testing and configuration (although, I could probably
live without it).  My second IP doesn't go through my OpenBSD box, it is kept
separate for a different box/different applications.  NAT will give you as
many IPs internally as you need.

3) Multiple NICs in the one server.  I thought this was possible until I
started researching it, now I'm not so sure, but you might be able to stick
several NICs in the system, and have each one broadcast a different DHCP
client name, and thus, each getting a different DHCP setting.  (I think this
is where I might just convince people I am a fool 8-)  Of course, you would
need a fifth card for your internal network, and now you have to find yourself
a box which can actually handle FIVE network cards.  I think this is a pretty
bad answer.

4)  Multiple machines.  I've found OpenBSD runs darned well on very modest
hardware, hardware that many of us have stacked up in the basement.  As
inefficient as this may seem, this is the only thing which would make sense
for my needs (i.e., I want to be able to pull something out, put something
else in, and yet have my "work" port still 100% functional).  Not sure why you
want four IPs, so I don't know how usefull/idiotic this suggestion is to you.

Hope this helps!
NICk.


Alexander Funcke wrote:

> Hi,
>
> I've got a OpenBSD box serving NAT to my LAN from a cable modem. The thing
> is that my ISP gives me 4 IPs, but I don't want to have the cablemodem
> connected directly to my LAN...
>
> So I wonder if their is someway to run dhcp multiple times on the same
> device to use all the IPs?
>
> regards.
> //Alexander Funcke