Using 31-Bit and 32-Bit prefixes for IPv4 reasonably

When needing to create a point to point connection via a subnet,
generally and /31 is the recommended cidr. Neutron supports /31
disabling dhcp and gateway on a subnet. /32 is also supported in
openstack.

Closes-Bug: #1580927
Change-Id: I3bfa3efb9fb8076656b16c89d2f35d74efde12b7

neutron/db/ipam_backend_mixin.py

@@ -310,9 +310,17 @@ class IpamBackendMixin(db_base_plugin_common.DbBasePluginCommon):
         Finally, verify that each range fall within the subnet's CIDR.
         """
         subnet = netaddr.IPNetwork(subnet_cidr)
-        subnet_first_ip = netaddr.IPAddress(subnet.first + 1)
-        # last address is broadcast in v4
-        subnet_last_ip = netaddr.IPAddress(subnet.last - (subnet.version == 4))
+        if subnet.version == const.IP_VERSION_4:
+            if subnet.prefixlen <= 30:
+                subnet_first_ip = netaddr.IPAddress(subnet.first + 1)
+                # last address is broadcast in v4
+                subnet_last_ip = netaddr.IPAddress(subnet.last - 1)
+            else:
+                subnet_first_ip = netaddr.IPAddress(subnet.first)
+                subnet_last_ip = netaddr.IPAddress(subnet.last)
+        else:  # IPv6 case
+            subnet_first_ip = netaddr.IPAddress(subnet.first + 1)
+            subnet_last_ip = netaddr.IPAddress(subnet.last)
 
         LOG.debug("Performing IP validity checks on allocation pools")
         ip_sets = []

neutron/ipam/utils.py

@@ -62,12 +62,17 @@ def generate_pools(cidr, gateway_ip):
     if first == last:
         # handle single address subnet case
         return [netaddr.IPRange(first, last)]
-    first_ip = first + 1
-    # last address is broadcast in v4
-    last_ip = last - (ip_version == 4)
-    if first_ip >= last_ip:
-        # /31 lands here
-        return []
+    if ip_version == constants.IP_VERSION_4:
+        if net.prefixlen <= 30:
+            first_ip = first + 1
+            # last address is broadcast in v4
+            last_ip = last - (ip_version == 4)
+        else:
+            first_ip = first
+            last_ip = last
+    else:  # IPv6 case
+        first_ip = first + 1
+        last_ip = last
     ipset = netaddr.IPSet(netaddr.IPRange(first_ip, last_ip))
     if gateway_ip:
         ipset.remove(netaddr.IPAddress(gateway_ip, ip_version))

neutron/tests/fullstack/resources/machine.py

@@ -129,7 +129,10 @@ class FakeFullstackMachine(machine_fixtures.FakeMachineBase):
     def _configure_static_ipaddress(self):
         self.port.addr.add(self.ip_cidr)
         if self.gateway_ip:
-            net_helpers.set_namespace_gateway(self.port, self.gateway_ip)
+            net = netaddr.IPNetwork(self.ip_cidr)
+            gateway_ip = netaddr.IPAddress(self.gateway_ip)
+            if gateway_ip in net:
+                net_helpers.set_namespace_gateway(self.port, self.gateway_ip)
 
     def _configure_ipaddress_via_dhcp(self):
         self._start_async_dhclient()

neutron/tests/fullstack/test_subnet.py

@@ -20,6 +20,7 @@ from oslo_utils import uuidutils
 from neutron.tests.common.exclusive_resources import ip_network
 from neutron.tests.fullstack import base
 from neutron.tests.fullstack.resources import environment
+from neutron.tests.fullstack.resources import machine
 
 
 class TestSubnet(base.BaseFullStackTestCase):
@@ -132,3 +133,32 @@ class TestSubnet(base.BaseFullStackTestCase):
         subnet = self._show_subnet(subnet['id'])
         self.assertEqual(subnet['subnet']['cidr'], str(subnets[2].cidr))
         self.assertEqual(subnet['subnet']['gateway_ip'], str(gateway_ip))
+
+    def test_subnet_with_prefixlen_31_connectivity(self):
+        network = self._create_network(self._project_id)
+        self.safe_client.create_subnet(
+            self._project_id, network['id'],
+            cidr='10.14.0.20/31',
+            gateway_ip='10.14.0.19',
+            name='subnet-test',
+            enable_dhcp=False)
+
+        vms = self._prepare_vms_in_net(self._project_id, network, False)
+        vms.ping_all()
+
+    def test_subnet_with_prefixlen_32_vm_spawn(self):
+        network = self._create_network(self._project_id)
+        self.safe_client.create_subnet(
+            self._project_id, network['id'],
+            cidr='10.14.0.20/32',
+            gateway_ip='10.14.0.19',
+            name='subnet-test',
+            enable_dhcp=False)
+
+        vm = self.useFixture(
+                machine.FakeFullstackMachine(
+                    self.environment.hosts[0],
+                    network['id'],
+                    self._project_id,
+                    self.safe_client))
+        vm.block_until_boot()

neutron/tests/unit/db/test_db_base_plugin_v2.py

@@ -1693,16 +1693,6 @@ fixed_ips=ip_address%%3D%s&fixed_ips=ip_address%%3D%s&fixed_ips=subnet_id%%3D%s
                 self.assertEqual(data['port']['fixed_ips'],
                                  res['port']['fixed_ips'])
 
-    def test_no_more_port_exception(self):
-        with self.subnet(cidr='10.0.0.0/31', enable_dhcp=False,
-                         gateway_ip=None) as subnet:
-            id = subnet['subnet']['network_id']
-            res = self._create_port(self.fmt, id)
-            data = self.deserialize(self.fmt, res)
-            msg = str(lib_exc.IpAddressGenerationFailure(net_id=id))
-            self.assertEqual(data['NeutronError']['message'], msg)
-            self.assertEqual(webob.exc.HTTPConflict.code, res.status_int)
-
     def test_create_ports_native_quotas(self):
         quota = 1
         cfg.CONF.set_override('quota_port', quota, group='QUOTAS')
@@ -5091,6 +5081,43 @@ class TestSubnetsV2(NeutronDbPluginV2TestCase):
                 res = req.get_response(self.api)
                 self.assertEqual(webob.exc.HTTPConflict.code, res.status_int)
 
+    def test_create_subnet_allocation_pools_with_prefixlen_31(self):
+        with self.network() as network:
+            with self.subnet(network=network,
+                             enable_dhcp=False,
+                             gateway_ip='10.14.0.19',
+                             cidr='10.14.0.20/31'):
+                res = self._create_port(self.fmt, network['network']['id'])
+                self.assertEqual(webob.exc.HTTPCreated.code, res.status_int)
+                res = self._create_port(self.fmt, network['network']['id'])
+                self.assertEqual(webob.exc.HTTPCreated.code, res.status_int)
+
+    def test_update_subnet_allocation_pools_with_prefixlen_31(self):
+        with self.network() as network:
+            with self.subnet(network=network,
+                             enable_dhcp=False,
+                             gateway_ip='10.14.0.19',
+                             cidr='10.14.0.20/31') as subnet:
+                data = {'subnet': {'allocation_pools': [
+                    {'start': '10.14.0.20', 'end': '10.14.0.21'}]}}
+                req = self.new_update_request('subnets', data,
+                                              subnet['subnet']['id'])
+                res = req.get_response(self.api)
+                self.assertEqual(webob.exc.HTTPOk.code, res.status_int)
+                res = self._create_port(self.fmt, network['network']['id'])
+                self.assertEqual(webob.exc.HTTPCreated.code, res.status_int)
+                res = self._create_port(self.fmt, network['network']['id'])
+                self.assertEqual(webob.exc.HTTPCreated.code, res.status_int)
+
+    def test_create_subnet_allocation_pools_with_prefixlen_32(self):
+        with self.network() as network:
+            with self.subnet(network=network,
+                             enable_dhcp=False,
+                             gateway_ip='10.14.0.19',
+                             cidr='10.14.0.20/32'):
+                res = self._create_port(self.fmt, network['network']['id'])
+                self.assertEqual(webob.exc.HTTPCreated.code, res.status_int)
+
     def test_create_subnets_native_quotas(self):
         quota = 1
         cfg.CONF.set_override('quota_subnet', quota, group='QUOTAS')

neutron/tests/unit/ipam/test_utils.py

@@ -89,7 +89,7 @@ class TestIpamUtils(base.BaseTestCase):
 
     def test_generate_pools_v4_31(self):
         cidr = '192.168.0.0/31'
-        expected = []
+        expected = [netaddr.IPRange('192.168.0.0', '192.168.0.1')]
         self.assertEqual(expected, utils.generate_pools(cidr, None))
 
     def test_generate_pools_v4_gateway_middle(self):

releasenotes/notes/create-subnet-with-prefixlen-31-32-4cf8c9417325721a.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Neutron supports creating IPv4 subnet with prefixlen /31 and /32,
+    via disabling dhcp on a subnet.
+    For more information, see bug
+    `1580927 <https://bugs.launchpad.net/neutron/+bug/1580927>`_.