[Bug 103010] Re: qemu no tun/tap networking

Chris Bainbridge chris.bainbridge at gmail.com
Tue Mar 9 13:02:36 GMT 2010

Kees: I know about the wiki page - I'm the one who edited it to note
this issue. It is not true that file capabilities only work in Lucid - I
am using Karmic and it works fine. libcap2-bin is also not a PAM
package. It is also not true that you need to manually add users to
/etc/security/capability.conf. I will corrrect these points in the Wiki.

It is true that with CAP_NET_ADMIN capability any user could boot a VM
and gain access to a virtual ethernet device. That's the whole point. I
would assume that the majority of people installing qemu would actually
want to be able to create bridged virtual ethernet devices. VirtualBox
allows normal users to create bridged ethernet devices that could be
used to "disrupt networking". Why should qemu be different? If you are
concerned about users directly abusing the capability with their own
software then this is not possible - giving the qemu binary the
capability means that only that binary gets the capability - other
binaries executed by the same user do not get the capability.  Access to
/dev/net/tun can still be controlled using standard file permissions as

Using a file capability would obviously be preferable as it would not
require individual users to be manually assigned the capability, and
could be done in postinst and would survive qemu package upgrades. The
only way someone could disrupt the network in this way is if they:

1) Were allowed to run qemu
2) Had rw access to /dev/net/tun
3) Had some exploit for qemu to allow them to run some arbitrary network disrupting code

Basically, whatever potential problems there are, the same problems are
also present in VirtualBox, and yet that ships with working network
bridging for VMs (the mechanism is different, but the fundamental
problems are the same). Why can't qemu have working bridged networking?
Why not  create a "tun" group that has rw access to /dev/net/tun if that
is the problem you are trying to avoid?

And if making the existing package work is unacceptable, then why not
create a "qemu-kvm-working-bridged-networking" package and recommend
that users who want bridged network use that instead?

qemu no tun/tap networking
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