Features/HelperNetworking

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Revision as of 21:37, 21 December 2011 by Coreyb (talk | contribs)


Summary

Introduce infrastructure to allowed QEMU network backends to be implemented outside of QEMU in a generic way.

Owner

Detailed Summary

Infrastructure is introduced to enable a network helper to be executed by QEMU. This also allows third parties to implement user-visible network backends without having to introduce them into QEMU itself.

A default network helper is introduced that implements the same functionality as the common qemu-ifup script. It creates a tap file descriptor, attaches it to a bridge, and passes it back to QEMU. This helper runs with higher privileges and allows QEMU to be invoked as a non-privileged user. (The helper runs as setuid root and privileges are immediately dropped to cap_net_admin.)

The default network helper uses it's own ACL mechanism for access control. Administrators can restrict the bridges that an unprivileged user can put a guest on (ie. what should be isolated networks). A future network helper could be developed to support PolicyKit for access control.

Setup

The setuid attribute needs to be turned on for the default network helper:

   sudo chmod u+s /usr/local/libexec/qemu-bridge-helper

ACLs must be implemented for the default network helper. The ACL mechanism that is enforced by qemu-bridge-helper is a fairly simple whitelist/blacklist mechanisms with a wildcard of 'all'. All users are blacklisted by default, and deny takes precedence over allow.

As an example:

/etc/qemu/bridge.conf root:qemu 0640

allow br0
include /etc/qemu/alice.conf
include /etc/qemu/bob.conf
include /etc/qemu/charlie.conf

/etc/qemu/alice.conf root:alice 0640

allow br1

/etc/qemu/bob.conf root:bob 0640

allow br2

/etc/qemu/charlie.conf root:charlie 0640

deny all

This ACL pattern allows any user in the qemu group to get a tap device connected to br0 (which is bridged to the physical network).

Users in the alice group can additionally get a tap device connected to br1. This allows br1 to act as a private bridge for the alice group.

Users in the bob group can additionally get a tap device connected to br2. This allows br2 to act as a private bridge for the bob group.

Users in the charlie group cannot get a tap device connected to any bridge.

Under no circumstance can the bob group get access to br1 or can the alice group get access to br2. And under no cicumstance can the charlie group get access to any bridge.

Execution

The following examples run Qemu with the default network helper, attaching a tap device to the default br0 bridge:

   qemu linux.img -net bridge -net nic,model=virtio
   qemu linux.img -net tap,helper=/usr/local/libexec/qemu-bridge-helper -net nic,model=virtio
   qemu linux.img -netdev bridge,id=hn0 -device virtio-net-pci,netdev=hn0,id=nic1
   qemu linux.img -netdev tap,helper=/usr/local/libexec/qemu-bridge-helper,id=hn0 -device virtio-net-pci,netdev=hn0,id=nic1

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