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This one needs to be tested:

--- a/nova/virt/libvirt/firewall.py
+++ b/nova/virt/libvirt/firewall.py
@@ -76,6 +76,28 @@ class NWFilterFirewall(base_firewall.FirewallDriver):
                   </filter>'''
 
     @staticmethod
+    def nova_dhcp_isolate_filter():
+        """This filter will disallow all traffic toward the gateway of the guests.
+        """
+
+        return '''<filter name='nova-isolate-dhcp-server' chain='ipv4'>
+                    <uuid>891e4787-e5c0-d59b-cda6-41bc3c6b36fc</uuid>
+                    <rule action='allow' direction='in'
+                          priority='100'>
+                      <tcp dstipaddr='$DHCPSERVER'
+                           dstportstart='8776'/>
+                    </rule>
+                    <rule action='drop' direction='in'
+                          priority='100'>
+                      <tcp dstipaddr='$DHCPSERVER' />
+                    </rule>
+                    <rule action='drop' direction='in'
+                          priority='100'>
+                      <udp dstipaddr='$DHCPSERVER' />
+                    </rule>
+                  </filter>'''
+
+    @staticmethod
     def nova_dhcp_filter():
         """The standard allow-dhcp-server filter is an <ip> one, so it uses
            ebtables to allow traffic through. Without a corresponding rule in
@@ -221,6 +243,7 @@ class NWFilterFirewall(base_firewall.FirewallDriver):
         self._define_filter(self._filter_container('nova-vpn',
                                                    ['allow-dhcp-server']))
         self._define_filter(self.nova_dhcp_filter)
+        self._define_filter(self.nova_dhcp_isolate_filter)
 
         self.static_filters_configured = True

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https://bugs.launchpad.net/bugs/1316271

Title:
  Network Security: VM hosts can SSH to compute node

Status in OpenStack Compute (Nova):
  New
Status in OpenStack Security Advisories:
  Incomplete

Bug description:
  Hi guys,

      We're still using nova-network and we'll be using it for a while
  and we noticed that the VM guests can contact the compute nodes on all
  ports ... The one we're the most preoccupied with is SSH.   We've
  written the following patch in order to isolate the VM guests from the
  VM hosts.

  --- linux_net.py.orig   2014-05-05 17:25:10.171746968 +0000
  +++ linux_net.py        2014-05-05 18:42:54.569209220 +0000
  @@ -805,6 +805,24 @@

  
   @utils.synchronized('lock_gateway', external=True)
  +def isolate_compute_from_guest(network_ref):
  +    if not network_ref:
  +        return
  +
  +    iptables_manager.ipv4['filter'].add_rule('INPUT',
  +                                             '-p tcp -d %s --dport 8775 '
  +                                             '-j ACCEPT' % network_ref['dhcp_server'])
  +    iptables_manager.ipv4['filter'].add_rule('FORWARD',
  +                                             '-p tcp -d %s --dport 8775 '
  +                                             '-j ACCEPT' % network_ref['dhcp_server'])
  +    iptables_manager.ipv4['filter'].add_rule('INPUT',
  +                                             '-d %s '
  +                                             '-j DROP' % network_ref['dhcp_server'])
  +    iptables_manager.ipv4['filter'].add_rule('FORWARD',
  +                                             '-d %s '
  +                                             '-j DROP' % network_ref['dhcp_server'])
  +    iptables_manager.apply()
  +
   def initialize_gateway_device(dev, network_ref):
       if not network_ref:
           return
  @@ -1046,6 +1064,7 @@
               try:
                   _execute('kill', '-HUP', pid, run_as_root=True)
                   _add_dnsmasq_accept_rules(dev)
  +                isolate_compute_from_guest(network_ref)
                   return
               except Exception as exc:  # pylint: disable=W0703
                   LOG.error(_('Hupping dnsmasq threw %s'), exc)
  @@ -1098,6 +1117,7 @@

       _add_dnsmasq_accept_rules(dev)

  +    isolate_compute_from_guest(network_ref)

   @utils.synchronized('radvd_start')
   def update_ra(context, dev, network_ref):

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