John wrote:
I'm going to jump in here, because this question was my reason for having
joined the Freebsd-questions list in the first place.  Of all the time I've
been running FreeBSD, this is my first post to this list... :P

I have a similar situation.  Firewall/NAT machine with 3 nics.  Only rather
than using the two external interfaces for different services, I would like
to use two nic's on the external subnet (using the FreeBSD machine as a
NAT/Firewall) for the following purpose:
--I would like one interface to be used for external IPF/NAT connectivity
for my network computers, allowing my network connectivity to my ISP.
--I would like a second interface to acquire a SECOND ip address to be set
up as bimap in NAT, to allow a second machine (my workstation) to be the
only machine to utilize the second external IP.  Similar to being in a DMZ,
but it would still use an internal address, as well as be subject to the
firewall rules in IPF.
I don't understand:
a) Why you need 3 NICs to do this?
b) Why you need 3 IPs to do this?
Just put an internal and external IP (2 NICs) and if you have a specific
machine within the network that you want treated specially, write special
ipfw rules for it.  Why the need for 3 IPs/NICs?

Again, I have read that this is available on Linux.  My searches have shown
that there are ways to do this on RedHat w/ ipchains (etc.).. ... but I
That's fine.  I'm sure there are lots of systems that have spiffy (or maybe
not so spiffy) things that you can do that you can't in FreeBSD (or other
spiffy system).

My only question I have is why do you need it?  There are other ways to get
the end result.

I have tried putting two nics in and having dhclient obtain addresses for
both on the same subnet.  dhclient will get both addresses (shown in
dhclient.leases), but fails to assign an ip to the second interface, failing
with the error "file already exists".  I'm sure this is a different (but
related) issue.
Sounds very related.

In my situation, another solution might be to use an alias on a single
external interface.. only I'm not sure how to get dhclient to obtain the
second IP address and assign it to the alias, nor how to get IPF to
recognize the alias'd interface properly.
That sure seems to be beyond what the software was designed to do.  You
could probably write some fancy scripts or something, but I ask my original
question: What are you trying to accomplish in the end?  Because it sure
seems like you're trying to use a wrench to hammer nails.

Bridging also comes to mind, but I'm not certain that if I bridge the
interface to my workstation computer it would correctly handle having an
internal as well as external address (other software application
complications would arise as well, I'm sure).  That's not my intent anyway,
so I have not and likely will not persue bridging as an option.
If you need NAT to get out, then bridging won't work.

Maybe I should have posted this on a diff. thread?  :P  But I believe the
resolution to this issue is the same as the originally posted issue.
Hopefully something will come out of it.
I could be wrong, but I suspect the "resolution" of your problem is to determine
what you want to accomplish, and then use FreeBSD in the manner it was intended
to achieve your goal.

Addtn'l info:  I have a FreeBSD 4.7 Stable #2 (updated yesterday).

----- Original Message -----
From: "Bill Moran" <[EMAIL PROTECTED]>
To: "Anand Buddhdev" <[EMAIL PROTECTED]>
Sent: Monday, January 13, 2003 8:31 AM
Subject: Re: Multiple network cards with IP addresses in the same network

Anand Buddhdev wrote:

On Mon, Jan 13, 2003 at 07:53:08AM -0500, Bill Moran wrote:

I have a FreeBSD 4.7 system, with 3 ethernet cards. The first two
are recognised as fxp0 and fxp1 and the second as em0 (intel gigabit
card). I configured the em0 with address I then wanted
to configure fxp0 with the address, and also connect it
to the switch so that I can connect to the server via both addresses.
However, FreeBSD's ifconfig command fails, and won't let me add the


address to the fxp0 interface. I read the manual page about ifconfig,
and read about aliases, where it said that for aliases, I must use the
netmask /32. When I do try to add the second address with a netmask of
/32, it works, but it doesn't make sense to me. How is that interface
going to to know that it is part of a /24 network if I use a /32


Would anyone be kind enough to explain why:

1. For aliases, I need the /32 mask
I didn't know that you did.  I've certainly had aliases that weren't /32
I have been using linux for about 2 years now. Let me explain why I
don't understand.
Well, first off, I misunderstood your original question.

I have addresses and I want to run different
services on the 2 different IP addresses. In a linux system, I do:

ifconfig eth0 netmask
ifconfig eth0:0 netmask

So that I have 2 different addresses bound to the same interface.

On FreeBSD, if I do:

ifconfig fxp0 netmask
ifconfig fxp0 netmask alias

That fails.
It should, it creates an ambiguous network situation.
Also, I don't understand why you would want to do this.

The ifconfig manpage states that a nonconflicting netmaks must be used
for the alias, and suggests to use 0xffffffff. I don't understand why,
because I don't see why one network interface cannot have more than one
address bound to it within the same network. If I use a /32 netmask for
the alias address, how will the kernel respond to arp requests for that
alias address?
What would you expect it to do that would be unusual?
If you really want two IPs on the same subnet on the same NIC, then use
a /32 subnet for one.  What's it going to hurt?

2. Adding a second IP to a *different* network card in the same server
does not work if the second IP is within the network of the first one.
Because it breaks routing and the basic concept of IP addys and


If you have two NICs on the same network, how is the kernel supposed to
route packets?
I still don't understand. In a linux system I can do:

route add -net netmask gw dev eth0
route add -net netmask gw dev eth1
Just because you can do it on a Linux system doesn't mean it's right.

This adds 2 default routes in the linux kernel, telling it to route
packets to the outside world using either network interface, where eth0
has address and eth1 has address What's
wrong with that?
It's ambiguous.  Where does the kernel route to when there are two

routes?  There's really no reason for such a thing.

If you want this setup as a failover solution, there are other ways.
There's a program in the ports (I can't remember the name, you'll have


do some research) that will monitor an interface, and if it becomes non-
responsive, run a script of your choosing.  Thus, you can have it start
up the other network card if the first fails.
Ok, I understand that, and it may be a very useful program, *if* you
want to bring up the other interface with perhaps the same IP address.
Well, it's useful for other reasons as well, but it doesn't apply to your
situation.  I suggested it because I didn't understand what you were
asking before.

All I want to do is to have 2 different IP addresses on each of the
different interfaces in the server, where the addresses are in the
same network. I can do it in linux. Why can't I do it in FreeBSD?
Because you shouldn't do it.

If failover isn't what you're looking for, then I'd reconsider your
network topology.  It doesn't really make sense to have 2 NICs with the
same network number in one machine.

Why not? I haven't seen any such warning in my IP networking books
or courses.
My answer to your question is:
1. Why would you WANT to do that?  I don't care if Linux, Windows and

   but FreeBSD _allows_ you to, the reason for it escapes me.
2. If you reall _do_ want to do that, use a /32 netmask as required.  If

   doesn't work for you for some reason, then the answer to your question

   beyond my expertise.

Bill Moran
Potential Technologies

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Potential Technologies

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