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Date:      Tue, 11 Jun 2002 16:40:01 -0700
From:      Lars Eggert <larse@ISI.EDU>
To:        John Nielsen <hackers@jnielsen.net>
Cc:        hackers@FreeBSD.ORG
Subject:   Re: gif(4) tunnel through MSN DSL modem
Message-ID:  <3D068A51.1060302@isi.edu>
References:  <015301c2117d$0db539c0$0900a8c0@max> <3D06761F.7000904@isi.edu> <01c601c2119f$e3e9ae50$0900a8c0@max>

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John Nielsen wrote:
>>># "Public" interface -- 192.168.1.2 netmask 255.255.255.252"
>>>ifconfig_ed0="DHCP"
>>>gif_interfaces="gif0"
>>>gifconfig_gif0="DSL.public.ip myend.public.ip"
>>>ifconfig_gif0="192.168.6.1 192.168.0.1"
>>>static_routes="john"
>>>route_john="-net 192.168.0 -interface gif0"
>>
>>The problem (one part, at least) is that you use the same IP address
>>(192.168.6.1) on your xl0 and gif0 interfaces (on both ends). You'll
>>want the tunnel addresses to be in a different subnet.
> 
> I have another tunnel set up this way and it works fine.  Why should the
> tunnel addresses be on a different subnet?

Because your routing table will have an entry that says "to reach net X 
use gateway Y", and there will appear to be multiple ways to reach 
gateway Y if you have multiple addresses attached to the same subnet.

Also, assigning the same IP address to multiple interfaces is usually a 
bad idea. (It is useful in some setups, but this ain't one.) Add 
encapsulation, and you've a fine example of black hole due to infinite 
encapsulation.

>>Also, the netmask in the infconfig_xl0 line doesn't match the comment,
>>which one is wrong?
> 
> The public interface (ed0) always gets the same address from the DSL modem,
> even though it's using DHCP.  I think you associated the comment with the
> wrong ifconfig line (I've added a break between them to clarify).

Oh, you're right, sorry. But then you're assigning the same IP address 
to THREE interfaces!

> I'm starting to think that it would be easier to use ppp/tun and ssh rather
> than gif in this instance, even though I'm less familiar with that
> arrangement.

I'm willing to bet a beer that these problems will dissappear if you 
pick different subnets and IP addresses for your interfaces. This is a 
pretty straightforward setup.

Lars
-- 
Lars Eggert <larse@isi.edu>           USC Information Sciences Institute

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