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Re: [Qemu-devel] Making qemu use 10.0.3.x not 10.0.2.x

From: Ian Jackson
Subject: Re: [Qemu-devel] Making qemu use 10.0.3.x not 10.0.2.x
Date: Wed, 6 Feb 2008 10:40:55 +0000

Warner Losh writes ("Re: [Qemu-devel] Making qemu use 10.0.3.x not 10.0.2.x"):
> I think that the suggestion is that qemu picks, one time, a new
> default.  This new default would be selected at random, and would be
> the same on all new versions of qemu.


> I don't think that the suggestion is to pick a random address every
> time qemu starts.

Indeed, that would be insane.

Ben Taylor writes ("Re: [Qemu-devel] Making qemu use 10.0.3.x not 10.0.2.x"):
> It seems to me that there is a corner case where the local host has
> a 10.0.2.x or 10.0.x.x address which would cause a qemu guest
> problems that has a address (for -net user only).

Yes, that's exactly the problem.

Using a (once) randomly-chosen default greatly reduces the odds of
that happening.  Many many people foolishly choose 10.0.{0,1,2,3}.x.
Many fewer choose (say) 172.30.206.x.  So the fixed qemu default
should be 172.30.206.x, or some other range also chosen at random.

(This is why it's worth changing: of course if you choose randomly you
sometimes get 10.0.2.x.  But if you knew you were trying to choose
randomly and your RNG gave you 10.0.2.x you'd probably roll the dice
again - because 10.0.2.x is already overused..)

> I think the default should be left at 10.0.2.x, and if the localhost has
> a 10.0.x.x address, then one of the other ranges (172.16.x.x or
> 192.168.x.x) could be used.

This is a bad idea.  That makes the behaviour very difficult to
predict and debug.  For example, the addresses used by qemu might
depend on whether the boot scripts which start a guest happen to run
before or after an external dhcp server manages to give the host an

This kind of `helpful' behaviour is a recipe for pain.  The addresses
used should be fixed in each particular installation, but configurable,
with a well-chosen default.

andrzej zaborowski writes ("Re: [Qemu-devel] Making qemu use 10.0.3.x not 
> This rfc talks about organisations and networks that are real, not
> about the network inside qemu which doesn't have connectivity with
> another qemu network.

Address clashes are still a problem even if the two networks don't
exchange packets, if there is any system which needs to be on both
networks.  And of course in the qemu case the host is on both

So the addresses used by the guest networks must be distinct from any
addresses of other systems outside the host that the host might need
to talk to.


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