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List:       user-mode-linux-user
Subject:    Re: [uml-user] Fine tuning for UML's
From:       David C <davidc () hotpop ! com>
Date:       2003-12-30 13:37:15
Message-ID: 200312301337.15582.davidc () hotpop ! com
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On Saturday 27 December 2003 4:00 pm, Nikolaus Hammler wrote:
> Thank you, my machine itself works now very well
> with network but now I have a few questions about
> "fine tuning":
>
> 1.) For networking I use a bridge, based on David's
> tutorial. But which MAC Address does the eth0 interface
> inside the UML have? Is this a random one?
> Is it possible to set this address for myself?

Apologies Nikolaus, I seem to have missed your mail.  I don't think I've 
replied to this one but if I have, please disregard.

Yes, the MAC address is random but you can specify it yourself on the 
commandline.  I forget the exact syntax but see the networking section on 
http://user-mode-linux.sf.net.

> Wouldn't it be then very simple to "fake" MAC addresses
> in order to pass MAC filter and so on?

Not if you only allow outgoing frames from the MAC address the UML should 
have.  You can do this using ebtables.  If you do this, even if the UML 
fakes its MAC address (or configures itself with another, etc) the frames 
wont pass your filter.

Also remember that whilst MAC spoofing is theoretically (and practically) 
possible and also quite easy, without some form of ARP spoofing or 
spoiling you wont be able to pick up any replies.  I don't honestly know 
how the bridging code in the kernel would deal with a situation where a 
UML on one port has a MAC address and another UML on a different port 
fakes the MAC of the other machine.  I say this because bridges are 
"intelligent" and "learn" what is plugged into where.  Whether they do so 
from ARP replies or outgoing frames, I don't know.  I would imagine the 
former.

> 2.) Goes *every* traffic through the bridge? (I mean
> broadcasts and similar). Could I configure my UML with
> DHCP when in the other network outside would be a DHCP
> server?

Certainly.  DHCP is just a broadcast which should go through the bridge 
fine.

> Could the UML serve a DHCP from inside the machine for
> the whole Ethernet network outside?

Again, I'm fairly certain this should be possible.  Give it a go, please 
let me know your success.

David



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