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Host machine exposed to tenant networks via IPv6


New interfaces created by Neutron in the default namespace, were done so without disabling IPv6 link-local addresses. This resulted in instances gaining the ability to directly access the host OS, therefore breaking guest isolation.

In Linux, link-local IPv6 addresses are assigned to all active interfaces, unlike IPv4 addresses where an administrator must configure each interface explicitly. This leads to a time window between when the interface is enabled and when it is attached to bridge device. Within this time window the host could be accessed from a tenant network.

IPv6 is now disabled automatically by both Neutron and Nova, prior to bringing any links up, however operators should still be aware of the security risks associated with re-enabling IPv6 link-local addresses.

Affected Services / Software

Nova, Neutron, networking-midonet, Kilo, Liberty


Linux assigns link-local IPv6 addresses to all the active interfaces, which is different to that of IPv4 addresses, where an administrator must configure each interface explicitly. Once an interface is enslaved in a bridge, all addresses assigned to it are ignored and only the addresses on the bridge are active. They are exposed via LinuxBridgeManager calls to `ensure_vlan` and `ensure_vxlan` where a new VLAN or VXLAN interface is created prior to enslaving the interfaces in the bridge.

Both Neutron and Nova now disable IPv6 on all interfaces before bringing the interface up. This avoids exposing a time window between when the interface is enabled and when it is attached to a bridge device, during which time the host could be accessible from a tenant network.

Recommended Actions

IPv6 should remain disabled for each interface, before the interface is brought to link up. If an operator, for any given reason, needs to re-enable link-local IPv6 adreses, they should be aware of the security implications of allowing tenant networks access of the host.

IPv6 is now disabled by default using root_dev.disable_ipv6() in interface.py, which calls the method common.ip_utils.is_enabled()

We can verify the value of the IPv6 parameters within sysctl.conf by using the following command:

 $ sysctl -a | grep disable | grep ipv6

A value of `0` represents IPv6 enabled, with `1` as disabled.

 net.ipv6.conf.default.disable_ipv6 = 1
 net.ipv6.conf.$IFACE.disable_ipv6 = 1

Here $IFACE refers to the interface for which IPv6 is disabled by default in Neutron.

Note: The value set at /proc/sys/net/ipv6/conf/$IFACE/disable_ipv6 is equivalent to net.ipv6.conf.$IFACE.disable_ipv6 in sysctl.conf

Contacts / References

Author: Vinay Potluri, Intel & Luke Hinds, Red Hat

This OSSN : https://wiki.openstack.org/wiki/OSSN/OSSN-0069

Original LaunchPad Bug : https://bugs.launchpad.net/ossn/+bug/1534652

This issue was referenced in https://bugs.launchpad.net/Neutron/+bug/1459856

Related issue addressed in Nova: https://review.openstack.org/#/c/313070/

OpenStack Security ML : openstack-dev@lists.openstack.org

OpenStack Security Group : https://launchpad.net/~openstack-ossg