network: allow guest to guest IPv6 without gateway definition

This patch adds the capability for virtual guests to do IPv6
communication via a virtual network interface with no IPv6 (gateway)
addresses specified.  This capability has always been enabled by
default for IPv4, but disabled for IPv6 for security concerns, and
because it requires the ip6tables command to be operational (which
isn't the case on a system with the ipv6 module completely disabled).

This patch adds a new attribute "ipv6" at the toplevel of a <network>
object.  If ipv6='yes', the extra ip6tables rules required to permite
inter-guest communications are added when the network is started. If
it is 'no', or not present, those rules will not be added; thus the
default behavior doesn't change, so there should be no compatibility
issues with any existing installations.

Note that virtual guests cannot communication with the virtualization
host via this interface, because the following kernel tunable has
been set:

   net.ipv6.conf.<bridge_interface_name>.disable_ipv6 = 1

This assures that the bridge interface will not have an IPv6
link-local (fe80::) address.

To control this behavior so that it is not enabled by default, the parameter
ipv6='yes' on the <network> statement has been added.

Documentation related to this patch has been updated.
The network schema has also been updated.

docs/formatnetwork.html.in

@@ -33,7 +33,7 @@
     </p>
 
     <pre>
-      &lt;network&gt;
+      &lt;network ipv6='yes'&gt;
         &lt;name&gt;default&lt;/name&gt;
         &lt;uuid&gt;3e3fce45-4f53-4fa7-bb32-11f34168b82b&lt;/uuid&gt;
         ...</pre>
@@ -52,6 +52,12 @@
         The format must be RFC 4122 compliant, eg <code>3e3fce45-4f53-4fa7-bb32-11f34168b82b</code>.
         If omitted when defining/creating a new network, a random
         UUID is generated. <span class="since">Since 0.3.0</span></dd>
+      <dt><code>ipv6='yes'</code></dt>
+      <dd>The new, optional parameter <code>ipv6='yes'</code> enables
+        a network definition with no IPv6 gateway addresses specified
+        to have guest-to-guest communications.  For further information,
+        see the example below for the example with no gateway addresses.
+        <span class="since">Since 1.0.1</span></dd>
     </dl>
 
     <h3><a name="elementsConnect">Connectivity</a></h3>
@@ -773,5 +779,25 @@
         &lt;/forward&gt;
       &lt;/network&gt;</pre>
 
+    <h3><a name="examplesNoGateway">Network config with no gateway addresses</a></h3>
+
+    <p>
+    A valid network definition can contain no IPv4 or IPv6 addresses.  Such a definition
+    can be used for a "very private" or "very isolated" network since it will not be
+    possible to communicate with the virtualization host via this network.  However,
+    this virtual network interface can be used for communication between virtual guest
+    systems.  This works for IPv4 and <span class="since">(Since 1.0.1)</span> IPv6.
+    However, the new ipv6='yes' must be added for guest-to-guest IPv6
+    communication.
+    </p>
+
+    <pre>
+      &lt;network ipv6='yes'&gt;
+        &lt;name&gt;nogw&lt;/name&gt;
+        &lt;uuid&gt;7a3b7497-1ec7-8aef-6d5c-38dff9109e93&lt;/uuid&gt;
+        &lt;bridge name="virbr2" stp="on" delay="0" /&gt;
+        &lt;mac address='00:16:3E:5D:C7:9E'/&gt;
+      &lt;/network&gt;</pre>
+
   </body>
 </html>

docs/schemas/network.rng

@@ -17,6 +17,16 @@
           <data type="unsignedInt"/>
         </attribute>
       </optional>
+      <!-- Enables IPv6 guest-to-guest communications on a network
+           with no gateways addresses specified -->
+      <optional>
+        <attribute name="ipv6">
+         <choice>
+          <value>yes</value>
+          <value>no</value>
+          </choice>
+         </attribute>
+       </optional>
       <interleave>
 
         <!-- The name of the network, used to refer to it through the API

src/conf/network_conf.c

@@ -1226,6 +1226,7 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
     xmlNodePtr virtPortNode = NULL;
     xmlNodePtr forwardNode = NULL;
     int nIps, nPortGroups, nForwardIfs, nForwardPfs, nForwardAddrs;
+    char *ipv6nogwStr = NULL;
     char *forwardDev = NULL;
     char *forwardManaged = NULL;
     char *type = NULL;
@@ -1264,6 +1265,22 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
         def->uuid_specified = true;
     }
 
+    /* check if definitions with no IPv6 gateway addresses is to
+     * allow guest-to-guest communications.
+     */
+    ipv6nogwStr = virXPathString("string(./@ipv6)", ctxt);
+    if (ipv6nogwStr) {
+        if (STREQ(ipv6nogwStr, "yes")) {
+            def->ipv6nogw = true;
+        } else if (STRNEQ(ipv6nogwStr, "no")) {
+            virReportError(VIR_ERR_XML_ERROR,
+                           _("Invalid ipv6 setting '%s' in network '%s'"),
+                           ipv6nogwStr, def->name);
+            goto error;
+        }
+        VIR_FREE(ipv6nogwStr);
+    }
+
     /* Parse network domain information */
     def->domain = virXPathString("string(./domain[1]/@name)", ctxt);
 
@@ -1591,6 +1608,7 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
     VIR_FREE(portGroupNodes);
     VIR_FREE(forwardIfNodes);
     VIR_FREE(forwardPfNodes);
+    VIR_FREE(ipv6nogwStr);
     VIR_FREE(forwardDev);
     ctxt->node = save;
     return NULL;
@@ -1839,6 +1857,8 @@ char *virNetworkDefFormat(const virNetworkDefPtr def, unsigned int flags)
     if (!(flags & VIR_NETWORK_XML_INACTIVE) && (def->connections > 0)) {
         virBufferAsprintf(&buf, " connections='%d'", def->connections);
     }
+    if (def->ipv6nogw)
+        virBufferAddLit(&buf, " ipv6='yes'");
     virBufferAddLit(&buf, ">\n");
     virBufferAdjustIndent(&buf, 2);
     virBufferEscapeString(&buf, "<name>%s</name>\n", def->name);

src/conf/network_conf.h

@@ -184,6 +184,11 @@ struct _virNetworkDef {
     virMacAddr mac; /* mac address of bridge device */
     bool mac_specified;
 
+    /* specified if ip6tables rules added
+     * when no ipv6 gateway addresses specified.
+     */
+    bool ipv6nogw;
+
     int forwardType;    /* One of virNetworkForwardType constants */
     int managed;        /* managed attribute for hostdev mode */
 

src/network/bridge_driver.c

@@ -1593,14 +1593,22 @@ networkRemoveRoutingIptablesRules(struct network_driver *driver,
     }
 }
 
-/* Add all once/network rules required for IPv6 (if any IPv6 addresses are defined) */
+/* Add all once/network rules required for IPv6.
+
+ * If no IPv6 addresses are defined and <network ipv6='yes'> is
+ * specified, then allow IPv6 commuinications between guests connected
+ * to this network. If any IPv6 addresses are defined, then add all
+ * rules for regular operation (including inter-guest communication).
+ */
 static int
 networkAddGeneralIp6tablesRules(struct network_driver *driver,
                                virNetworkObjPtr network)
 {
 
-    if (!virNetworkDefGetIpByIndex(network->def, AF_INET6, 0))
+    if (!virNetworkDefGetIpByIndex(network->def, AF_INET6, 0) &&
+        !network->def->ipv6nogw) {
         return 0;
+    }
 
     /* Catch all rules to block forwarding to/from bridges */
 
@@ -1629,6 +1637,10 @@ networkAddGeneralIp6tablesRules(struct network_driver *driver,
         goto err3;
     }
 
+    /* if no IPv6 addresses are defined, we are done. */
+    if (!virNetworkDefGetIpByIndex(network->def, AF_INET6, 0))
+        return 0;
+
     /* allow DNS over IPv6 */
     if (iptablesAddTcpInput(driver->iptables, AF_INET6,
                             network->def->bridge, 53) < 0) {
@@ -1665,11 +1677,17 @@ static void
 networkRemoveGeneralIp6tablesRules(struct network_driver *driver,
                                   virNetworkObjPtr network)
 {
-    if (!virNetworkDefGetIpByIndex(network->def, AF_INET6, 0))
+    if (!virNetworkDefGetIpByIndex(network->def, AF_INET6, 0) &&
+                !network->def->ipv6nogw)
         return;
+    if (virNetworkDefGetIpByIndex(network->def, AF_INET6, 0)) {
+        iptablesRemoveUdpInput(driver->iptables, AF_INET6, network->def->bridge, 53);
+        iptablesRemoveTcpInput(driver->iptables, AF_INET6, network->def->bridge, 53);
+    }
 
-    iptablesRemoveUdpInput(driver->iptables, AF_INET6, network->def->bridge, 53);
-    iptablesRemoveTcpInput(driver->iptables, AF_INET6, network->def->bridge, 53);
+    /* the following rules are there if no IPv6 address has been defined
+     * but network->def->ipv6nogw == true
+     */
     iptablesRemoveForwardAllowCross(driver->iptables, AF_INET6, network->def->bridge);
     iptablesRemoveForwardRejectIn(driver->iptables, AF_INET6, network->def->bridge);
     iptablesRemoveForwardRejectOut(driver->iptables, AF_INET6, network->def->bridge);

tests/networkxml2xmlin/empty-allow-ipv6.xml

+<network ipv6='yes'>
+  <name>empty</name>
+  <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9c</uuid>
+  <bridge name='virbr7' />
+  <mac address='52:54:00:17:3F:47'/>
+</network>

tests/networkxml2xmlin/isolated-network.xml

-<network>
+<network ipv6='no'>
   <name>private</name>
   <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9b</uuid>
   <bridge name="virbr2" />

tests/networkxml2xmlout/empty-allow-ipv6.xml

+<network ipv6='yes'>
+  <name>empty</name>
+  <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9c</uuid>
+  <bridge name='virbr7' stp='on' delay='0' />
+  <mac address='52:54:00:17:3F:47'/>
+</network>

tests/networkxml2xmltest.c

@@ -92,6 +92,7 @@ mymain(void)
     } while (0)
 #define DO_TEST(name) DO_TEST_FULL(name, 0)
 
+    DO_TEST("empty-allow-ipv6");
     DO_TEST("isolated-network");
     DO_TEST("routed-network");
     DO_TEST("nat-network");