From: Ard van Breemen <ard@telegraafnet.nl>
To: vlan@Scry.WANfear.com
Subject: Re: [VLAN] same ip in all vlan interfaces possible?
Message-ID: <20020110192832.GI25507@telegraafnet.nl>
References: <15421.44456.756906.695146@hoggar.fisica.ufpr.br>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <15421.44456.756906.695146@hoggar.fisica.ufpr.br>
User-Agent: Mutt/1.3.24i
Sender: vlan-admin@Scry.WANfear.com
Errors-To: vlan-admin@Scry.WANfear.com
Precedence: bulk
Reply-To: vlan@Scry.WANfear.com
Date: Thu, 10 Jan 2002 20:28:32 +0100

On Thu, Jan 10, 2002 at 01:05:12PM -0200, Carlos Carvalho wrote:
> I'm about to try ethernet tags for the first time. I need to use the
> same ip in all vlan interfaces. Is this possible?
> 
> I can't put each vlan in a different IP subnet because there will be
> tens of them.
I am not sure of what you are trying to do, but this is possible:

Give all the networks the idea of a virtual 192.168.0.0/16 f.i. .
You can divide your network in c classes by f.i.:

vconfig add eth0 2
ip addr add 127.0.0.1/8 dev vlan2
ip route add 192.168.2.0/24 dev vlan2
echo 1 > /proc/sys/net/ipv4/conf/vlan2/proxy_arp

vconfig add eth0 3
ip addr add 127.0.0.1/8 dev vlan3
ip route add 192.168.3.0/24 dev vlan3
echo 1 > /proc/sys/net/ipv4/conf/vlan2/proxy_arp

vconfig add eth0 4
ip addr add 127.0.0.1/8 dev vlan4
ip route add 192.168.4.0/24 dev vlan4
echo 1 > /proc/sys/net/ipv4/conf/vlan2/proxy_arp

vconfig add eth0 5
ip addr add 127.0.0.1/8 dev vlan5
ip route add 192.168.5.0/24 dev vlan5
echo 1 > /proc/sys/net/ipv4/conf/vlan2/proxy_arp

ip addr add 192.168.0.1/32 dev lo
echo 1 > /proc/sys/net/ipv4/ip_forward

If 192.168.5.2 tries to ping 192.168.3.9, it will arp for it, since it
thinks it is in the same subnet (/16). Your box knows how to route it,
and therefore will reply to the arp. 192.168.5.2 will then send the
packet to your box, which will happily forward it.
The default gw in this example is 192.168.0.1, put on lo.

You need to have at least 1 ip address on a device to get the ip stack
bound to that device. If you don't have an ip address on a device (no
matter what ip address), you will not be able to use the kernel-ip stack
on that device. Not binding the kernel ip-stack to the device is a good
idea if you want to do some network sniffing for intrusion detection
etc...

(Thanks to Pauline Middelink for pointing me at the proxy_arp idea)

DISCLAIMER: I already tested the concept using 127.0.0.1 addresses and
proxy-arp in a sort of production environment. I did not test the example
above :)
-- 
<ard@telegraafnet.nl> Telegraaf Elektronische Media  http://wwwijzer.nl
http://leerquoten.monster.org/ http://www.faqs.org/rfcs/rfc1855.html 
Let your government know you value your freedom. Sign the petition:
http://petition.eurolinux.org/
_______________________________________________
VLAN mailing list  -  VLAN@Scry.WANfear.com
http://www.WANfear.com/mailman/listinfo/vlan
VLAN Page:  http://scry.wanfear.com/~greear/vlan.html