Merge "DHCP agent: allow using gateway IPs instead of uniquely allocated"

diff --cc neutron/agent/linux/dhcp.py
Simple merge

diff --cc neutron/agent/linux/interface.py
index cf37fcfc976f27c12b136399c3c7ffaec59b53ab,5ac117e67d27b50e554d4189e76bca6675af0c8f..090b62def7c6dfbefa8edc8c0191fe0ad7313fca
--- 1/neutron/agent/linux/interface.py
--- 2/neutron/agent/linux/interface.py
+++ b/neutron/agent/linux/interface.py
@@@ -52,18 -51,47 +52,58 @@@ class LinuxInterfaceDriver(object)
  
      def __init__(self, conf):
          self.conf = conf
 +        if self.conf.network_device_mtu:
 +            self._validate_network_device_mtu()
 +
 +    def _validate_network_device_mtu(self):
 +        if (ipv6_utils.is_enabled() and
 +            self.conf.network_device_mtu < n_const.IPV6_MIN_MTU):
 +            LOG.error(_LE("IPv6 protocol requires a minimum MTU of "
 +                          "%(min_mtu)s, while the configured value is "
 +                          "%(current_mtu)s"), {'min_mtu': n_const.IPV6_MIN_MTU,
 +                          'current_mtu': self.conf.network_device_mtu})
 +            raise SystemExit(1)
  
+     @property
+     def use_gateway_ips(self):
+         """Whether to use gateway IPs instead of unique IP allocations.
+ 
+         In each place where the DHCP agent runs, and for each subnet for
+         which DHCP is handling out IP addresses, the DHCP port needs -
+         at the Linux level - to have an IP address within that subnet.
+         Generally this needs to be a unique Neutron-allocated IP
+         address, because the subnet's underlying L2 domain is bridged
+         across multiple compute hosts and network nodes, and for HA
+         there may be multiple DHCP agents running on that same bridged
+         L2 domain.
+ 
+         However, if the DHCP ports - on multiple compute/network nodes
+         but for the same network - are _not_ bridged to each other,
+         they do not need each to have a unique IP address.  Instead
+         they can all share the same address from the relevant subnet.
+         This works, without creating any ambiguity, because those
+         ports are not all present on the same L2 domain, and because
+         no data within the network is ever sent to that address.
+         (DHCP requests are broadcast, and it is the network's job to
+         ensure that such a broadcast will reach at least one of the
+         available DHCP servers.  DHCP responses will be sent _from_
+         the DHCP port address.)
+ 
+         Specifically, for networking backends where it makes sense,
+         the DHCP agent allows all DHCP ports to use the subnet's
+         gateway IP address, and thereby to completely avoid any unique
+         IP address allocation.  This behaviour is selected by running
+         the DHCP agent with a configured interface driver whose
+         'use_gateway_ips' property is True.
+ 
+         When an operator deploys Neutron with an interface driver that
+         makes use_gateway_ips True, they should also ensure that a
+         gateway IP address is defined for each DHCP-enabled subnet,
+         and that the gateway IP address doesn't change during the
+         subnet's lifetime.
+         """
+         return False
+ 
      def init_l3(self, device_name, ip_cidrs, namespace=None,
                  preserve_ips=[], gateway_ips=None,
                  clean_connections=False):

diff --cc neutron/tests/unit/agent/linux/test_dhcp.py
index d4d8e86e891bf0d268d1303d37b8adf47907ae31,5508b01d21d33908acd3cc88f3d816a4a469ec16..d82c02f125a58af07051463806bdf2ad67b23aaa
--- 1/neutron/tests/unit/agent/linux/test_dhcp.py
--- 2/neutron/tests/unit/agent/linux/test_dhcp.py
+++ b/neutron/tests/unit/agent/linux/test_dhcp.py
@@@ -500,20 -499,22 +500,27 @@@ class FakeV6Network(object)
  class FakeDualNetwork(object):
      id = 'cccccccc-cccc-cccc-cccc-cccccccccccc'
      subnets = [FakeV4Subnet(), FakeV6SubnetDHCPStateful()]
 -    ports = [FakePort1(), FakeV6Port(), FakeDualPort(), FakeRouterPort()]
 +    # ports = [FakePort1(), FakeV6Port(), FakeDualPort(), FakeRouterPort()]
      namespace = 'qdhcp-ns'
  
 +    def __init__(self, domain='openstacklocal'):
 +        self.ports = [FakePort1(domain=domain), FakeV6Port(domain=domain),
 +                      FakeDualPort(domain=domain),
 +                      FakeRouterPort(domain=domain)]
 +
  
  class FakeDeviceManagerNetwork(object):
-     id = 'cccccccc-cccc-cccc-cccc-cccccccccccc'
-     subnets = [FakeV4Subnet(), FakeV6SubnetDHCPStateful()]
-     ports = [FakePort1(), FakeV6Port(), FakeDualPort(), FakeRouterPort()]
-     namespace = 'qdhcp-ns'
+     # Use instance rather than class attributes here, so that we get
+     # an independent set of ports each time FakeDeviceManagerNetwork()
+     # is used.
+     def __init__(self):
+         self.id = 'cccccccc-cccc-cccc-cccc-cccccccccccc'
+         self.subnets = [FakeV4Subnet(), FakeV6SubnetDHCPStateful()]
+         self.ports = [FakePort1(),
+                       FakeV6Port(),
+                       FakeDualPort(),
+                       FakeRouterPort()]
+         self.namespace = 'qdhcp-ns'
  
  
  class FakeDualNetworkReserved(object):