High-availability for DHCP

This section describes how to use the agent management (alias agent) and scheduler (alias agent_scheduler) extensions for DHCP agents scalability and HA.

Note

Use the openstack extension list command to check if these extensions are enabled. Check agent and agent_scheduler are included in the output.

$ openstack extension list --network -c Name -c Alias
+-------------------------------------------------------------+---------------------------+
| Name                                                        | Alias                     |
+-------------------------------------------------------------+---------------------------+
| Default Subnetpools                                         | default-subnetpools       |
| Network IP Availability                                     | network-ip-availability   |
| Network Availability Zone                                   | network_availability_zone |
| Auto Allocated Topology Services                            | auto-allocated-topology   |
| Neutron L3 Configurable external gateway mode               | ext-gw-mode               |
| Port Binding                                                | binding                   |
| Neutron Metering                                            | metering                  |
| agent                                                       | agent                     |
| Subnet Allocation                                           | subnet_allocation         |
| L3 Agent Scheduler                                          | l3_agent_scheduler        |
| Neutron external network                                    | external-net              |
| Neutron Service Flavors                                     | flavors                   |
| Network MTU                                                 | net-mtu                   |
| Availability Zone                                           | availability_zone         |
| Quota management support                                    | quotas                    |
| HA Router extension                                         | l3-ha                     |
| Provider Network                                            | provider                  |
| Multi Provider Network                                      | multi-provider            |
| Address scope                                               | address-scope             |
| Neutron Extra Route                                         | extraroute                |
| Subnet service types                                        | subnet-service-types      |
| Resource timestamps                                         | standard-attr-timestamp   |
| Neutron Service Type Management                             | service-type              |
| Router Flavor Extension                                     | l3-flavors                |
| Neutron Extra DHCP opts                                     | extra_dhcp_opt            |
| Resource revision numbers                                   | standard-attr-revisions   |
| Pagination support                                          | pagination                |
| Sorting support                                             | sorting                   |
| security-group                                              | security-group            |
| DHCP Agent Scheduler                                        | dhcp_agent_scheduler      |
| Router Availability Zone                                    | router_availability_zone  |
| RBAC Policies                                               | rbac-policies             |
| standard-attr-description                                   | standard-attr-description |
| Neutron L3 Router                                           | router                    |
| Allowed Address Pairs                                       | allowed-address-pairs     |
| project_id field enabled                                    | project-id                |
| Distributed Virtual Router                                  | dvr                       |
+-------------------------------------------------------------+---------------------------+

Demo setup

../_images/demo_multiple_dhcp_agents.png

There will be three hosts in the setup.

Host

Description

OpenStack controller host - controlnode

Runs the Networking, Identity, and Compute services that are required to deploy VMs. The node must have at least one network interface that is connected to the Management Network. Note that nova-network should not be running because it is replaced by Neutron.

HostA

Runs nova-compute, the Neutron L2 agent and DHCP agent

HostB

Same as HostA

Configuration

controlnode: neutron server

  1. Neutron configuration file /etc/neutron/neutron.conf:

    [DEFAULT]
core_plugin = linuxbridge
rabbit_host = controlnode
host = controlnode
agent_down_time = 5
dhcp_agents_per_network = 1

    Note

    In the above configuration, we use dhcp_agents_per_network = 1 for this demonstration. In usual deployments, we suggest setting dhcp_agents_per_network to more than one to match the number of DHCP agents in your deployment. See Enabling DHCP high availability by default.

  2. Update the plug-in configuration file /etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini:

    [vlans]
tenant_network_type = vlan
network_vlan_ranges = physnet1:1000:2999
[database]
connection = mysql+pymysql://root:root@127.0.0.1:3306/neutron_linux_bridge
retry_interval = 2
[linux_bridge]
physical_interface_mappings = physnet1:eth0

HostA and HostB: L2 agent

  1. Neutron configuration file /etc/neutron/neutron.conf:

    [DEFAULT]
rabbit_host = controlnode
rabbit_password = openstack
# host = HostB on hostb
host = HostA

  2. Update the plug-in configuration file /etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini:

    [vlans]
tenant_network_type = vlan
network_vlan_ranges = physnet1:1000:2999
[database]
connection = mysql://root:root@127.0.0.1:3306/neutron_linux_bridge
retry_interval = 2
[linux_bridge]
physical_interface_mappings = physnet1:eth0

  3. Update the nova configuration file /etc/nova/nova.conf:

    [DEFAULT]
use_neutron=True
firewall_driver=nova.virt.firewall.NoopFirewallDriver

[neutron]
admin_username=neutron
admin_password=servicepassword
admin_auth_url=http://controlnode:35357/v2.0/
auth_strategy=keystone
admin_tenant_name=servicetenant
url=http://203.0.113.10:9696/

HostA and HostB: DHCP agent

  • Update the DHCP configuration file /etc/neutron/dhcp_agent.ini:

    [DEFAULT]
interface_driver = neutron.agent.linux.interface.BridgeInterfaceDriver

Prerequisites for demonstration

Admin role is required to use the agent management and scheduler extensions. Ensure you run the following commands under a project with an admin role.

To experiment, you need VMs and a neutron network:

$ openstack server list
+--------------------------------------+-----------+--------+----------------+--------+----------+
| ID                                   | Name      | Status | Networks       | Image  | Flavor   |
+--------------------------------------+-----------+--------+----------------+--------+----------+
| c394fcd0-0baa-43ae-a793-201815c3e8ce | myserver1 | ACTIVE | net1=192.0.2.3 | cirros | m1.tiny  |
| 2d604e05-9a6c-4ddb-9082-8a1fbdcc797d | myserver2 | ACTIVE | net1=192.0.2.4 | ubuntu | m1.small |
| c7c0481c-3db8-4d7a-a948-60ce8211d585 | myserver3 | ACTIVE | net1=192.0.2.5 | centos | m1.small |
+--------------------------------------+-----------+--------+----------------+--------+----------+

$ openstack network list
+--------------------------------------+------+--------------------------------------+
| ID                                   | Name | Subnets                              |
+--------------------------------------+------+--------------------------------------+
| ad88e059-e7fa-4cf7-8857-6731a2a3a554 | net1 | 8086db87-3a7a-4cad-88c9-7bab9bc69258 |
+--------------------------------------+------+--------------------------------------+

Managing agents in neutron deployment

  1. List all agents:

    $ openstack network agent list
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+
| ID                                   | Agent Type         | Host  | Availability Zone | Alive | State | Binary                    |
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+
| 22467163-01ea-4231-ba45-3bd316f425e6 | Linux bridge agent | HostA | None              | True  | UP    | neutron-linuxbridge-agent |
| 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b | DHCP agent         | HostA | None              | True  | UP    | neutron-dhcp-agent        |
| 3066d20c-9f8f-440c-ae7c-a40ffb4256b6 | Linux bridge agent | HostB | nova              | True  | UP    | neutron-linuxbridge-agent |
| 55569f4e-6f31-41a6-be9d-526efce1f7fe | DHCP agent         | HostB | nova              | True  | UP    | neutron-dhcp-agent        |
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+

    Every agent that supports these extensions will register itself with the neutron server when it starts up.

    The output shows information for four agents. The alive field shows True if the agent reported its state within the period defined by the agent_down_time option in the neutron.conf file. Otherwise the alive is False.

  2. List DHCP agents that host a specified network:

    $ openstack network agent list --network net1
+--------------------------------------+---------------+----------------+-------+
| ID                                   | Host          | Admin State Up | Alive |
+--------------------------------------+---------------+----------------+-------+
| 22467163-01ea-4231-ba45-3bd316f425e6 | HostA         | UP             | True  |
+--------------------------------------+---------------+----------------+-------+

  3. List the networks hosted by a given DHCP agent:

    This command is to show which networks a given dhcp agent is managing.

    $ openstack network list --agent 22467163-01ea-4231-ba45-3bd316f425e6
+--------------------------------+------------------------+---------------------------------+
| ID                             | Name                   | Subnets                         |
+--------------------------------+------------------------+---------------------------------+
| ad88e059-e7fa-                 | net1                   | 8086db87-3a7a-4cad-             |
| 4cf7-8857-6731a2a3a554         |                        | 88c9-7bab9bc69258               |
+--------------------------------+------------------------+---------------------------------+

  4. Show agent details.

    The openstack network agent show command shows details for a specified agent:

    $ openstack network agent show 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b
+---------------------+--------------------------------------------------+
| Field               | Value                                            |
+---------------------+--------------------------------------------------+
| admin_state_up      | UP                                               |
| agent_type          | DHCP agent                                       |
| alive               | True                                             |
| availability_zone   | nova                                             |
| binary              | neutron-dhcp-agent                               |
| configurations      | dhcp_driver='neutron.agent.linux.dhcp.Dnsmasq',  |
|                     | dhcp_lease_duration='86400',                     |
|                     | log_agent_heartbeats='False', networks='1',      |
|                     | notifies_port_ready='True', ports='3',           |
|                     | subnets='1'                                      |
| created_at          | 2016-12-14 00:25:54                              |
| description         | None                                             |
| last_heartbeat_at   | 2016-12-14 06:53:24                              |
| host                | HostA                                            |
| id                  | 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b             |
| started_at          | 2016-12-14 00:25:54                              |
| topic               | dhcp_agent                                       |
+---------------------+--------------------------------------------------+

    In this output, last_heartbeat_at is the time on the neutron server. You do not need to synchronize all agents to this time for this extension to run correctly. configurations describes the static configuration for the agent or run time data. This agent is a DHCP agent and it hosts one network, one subnet, and three ports.

    Different types of agents show different details. The following output shows information for a Linux bridge agent:

    $ openstack network agent show 22467163-01ea-4231-ba45-3bd316f425e6
+---------------------+--------------------------------------+
| Field               | Value                                |
+---------------------+--------------------------------------+
| admin_state_up      | UP                                   |
| agent_type          | Linux bridge agent                   |
| alive               | True                                 |
| availability_zone   | nova                                 |
| binary              | neutron-linuxbridge-agent            |
| configurations      | {                                    |
|                     |      "physnet1": "eth0",             |
|                     |      "devices": "4"                  |
|                     | }                                    |
| created_at          | 2016-12-14 00:26:54                  |
| description         | None                                 |
| last_heartbeat_at   | 2016-12-14 06:53:24                  |
| host                | HostA                                |
| id                  | 22467163-01ea-4231-ba45-3bd316f425e6 |
| started_at          | 2016-12-14T06:48:39.000000           |
| topic               | N/A                                  |
+---------------------+--------------------------------------+

    The output shows bridge-mapping and the number of virtual network devices on this L2 agent.

Managing assignment of networks to DHCP agent

A single network can be assigned to more than one DHCP agents and one DHCP agent can host more than one network. You can add a network to a DHCP agent and remove one from it.

  1. Default scheduling.

    When you create a network with one port, the network will be scheduled to an active DHCP agent. If many active DHCP agents are running, select one randomly. You can design more sophisticated scheduling algorithms in the same way as nova-schedule later on.

    $ openstack network create net2
$ openstack subnet create --network net2 --subnet-range 198.51.100.0/24 subnet2
$ openstack port create port2 --network net2
$ openstack network agent list --network net2
+--------------------------------------+---------------+----------------+-------+
| ID                                   | Host          | Admin State Up | Alive |
+--------------------------------------+---------------+----------------+-------+
| 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b | HostA         | UP             | True  |
+--------------------------------------+---------------+----------------+-------+

    It is allocated to DHCP agent on HostA. If you want to validate the behavior through the dnsmasq command, you must create a subnet for the network because the DHCP agent starts the dnsmasq service only if there is a DHCP.

  2. Assign a network to a given DHCP agent.

    To add another DHCP agent to host the network, run this command:

    $ openstack network agent add network --dhcp \
  55569f4e-6f31-41a6-be9d-526efce1f7fe net2
$ openstack network agent list --network net2
+--------------------------------------+-------+----------------+--------+
| ID                                   | Host  | Admin State Up | Alive  |
+--------------------------------------+-------+----------------+--------+
| 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b | HostA | UP             | True   |
| 55569f4e-6f31-41a6-be9d-526efce1f7fe | HostB | UP             | True   |
+--------------------------------------+-------+----------------+--------+

    Both DHCP agents host the net2 network.

  3. Remove a network from a specified DHCP agent.

    This command is the sibling command for the previous one. Remove net2 from the DHCP agent for HostA:

    $ openstack network agent remove network --dhcp \
  2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b net2
$ openstack network agent list --network net2
+--------------------------------------+-------+----------------+-------+
| ID                                   | Host  | Admin State Up | Alive |
+--------------------------------------+-------+----------------+-------+
| 55569f4e-6f31-41a6-be9d-526efce1f7fe | HostB | UP             | True  |
+--------------------------------------+-------+----------------+-------+

    You can see that only the DHCP agent for HostB is hosting the net2 network.

HA of DHCP agents

Boot a VM on net2. Let both DHCP agents host net2. Fail the agents in turn to see if the VM can still get the desired IP.

  1. Boot a VM on net2:

    $ openstack network list
+--------------------------------------+------+--------------------------------------+
| ID                                   | Name | Subnets                              |
+--------------------------------------+------+--------------------------------------+
| ad88e059-e7fa-4cf7-8857-6731a2a3a554 | net1 | 8086db87-3a7a-4cad-88c9-7bab9bc69258 |
| 9b96b14f-71b8-4918-90aa-c5d705606b1a | net2 | 6979b71a-0ae8-448c-aa87-65f68eedcaaa |
+--------------------------------------+------+--------------------------------------+
$ openstack server create --image tty  --flavor 1 myserver4 \
  --nic net-id=9b96b14f-71b8-4918-90aa-c5d705606b1a
...
$ openstack server list
+--------------------------------------+-----------+--------+-------------------+---------+----------+
| ID                                   | Name      | Status | Networks          | Image   | Flavor   |
+--------------------------------------+-----------+--------+-------------------+---------+----------+
| c394fcd0-0baa-43ae-a793-201815c3e8ce | myserver1 | ACTIVE | net1=192.0.2.3    | cirros  | m1.tiny  |
| 2d604e05-9a6c-4ddb-9082-8a1fbdcc797d | myserver2 | ACTIVE | net1=192.0.2.4    | ubuntu  | m1.small |
| c7c0481c-3db8-4d7a-a948-60ce8211d585 | myserver3 | ACTIVE | net1=192.0.2.5    | centos  | m1.small |
| f62f4731-5591-46b1-9d74-f0c901de567f | myserver4 | ACTIVE | net2=198.51.100.2 | cirros1 | m1.tiny  |
+--------------------------------------+-----------+--------+-------------------+---------+----------+

  2. Make sure both DHCP agents hosting net2:

    Use the previous commands to assign the network to agents.

    $ openstack network agent list --network net2
+--------------------------------------+-------+----------------+-------+
| ID                                   | Host  | Admin State Up | Alive |
+--------------------------------------+-------+----------------+-------+
| 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b | HostA | UP             | True  |
| 55569f4e-6f31-41a6-be9d-526efce1f7fe | HostB | UP             | True  |
+--------------------------------------+-------+----------------+-------+

To test the HA of DHCP agent:

  1. Log in to the myserver4 VM, and run udhcpc, dhclient or other DHCP client.

  2. Stop the DHCP agent on HostA. Besides stopping the neutron-dhcp-agent binary, you must stop the dnsmasq processes.

  3. Run a DHCP client in VM to see if it can get the wanted IP.

  4. Stop the DHCP agent on HostB too.

  5. Run udhcpc in the VM; it cannot get the wanted IP.

  6. Start DHCP agent on HostB. The VM gets the wanted IP again.

No HA for metadata service on isolated networks

All Neutron backends using the DHCP agent can also provide metadata service in isolated networks (i.e. networks without a router). In this case the DHCP agent manages the metadata service (see config option enable_isolated_metadata).

Note however that the metadata service is only redundant for IPv4, and not IPv6, even when the DHCP service is configured to be highly available (config option dhcp_agents_per_network > 1). This is because the DHCP agent will insert a route to the well known metadata IPv4 address (169.254.169.254) via its own IP address, so it will be reachable as long as the DHCP service is available at that IP address. This also means that recovery after a failure is tied to the renewal of the DHCP lease, since that route will only change if the DHCP server for a VM changes.

With IPv6, the well known metadata IPv6 address (fe80::a9fe:a9fe) is used, but directly configured in the DHCP agent network namespace. Due to the enforcement of duplicate address detection (DAD), this address can only be configured in at most one DHCP network namespaces at any time. See RFC 4862 for details on the DAD process.

For this reason, even when you have multiple DHCP agents, an arbitrary one (where the metadata IPv6 address is not in dadfailed state) will serve all metadata requests over IPv6. When that metadata service instance becomes unreachable there is no failover and the service will become unreachable.

Disabling and removing an agent

An administrator might want to disable an agent if a system hardware or software upgrade is planned. Some agents that support scheduling also support disabling and enabling agents, such as L3 and DHCP agents. After the agent is disabled, the scheduler does not schedule new resources to the agent.

After the agent is disabled, you can safely remove the agent. Even after disabling the agent, resources on the agent are kept assigned. Ensure you remove the resources on the agent before you delete the agent.

Disable the DHCP agent on HostA before you stop it:

$ openstack network agent set 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b --disable
$ openstack network agent list
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+
| ID                                   | Agent Type         | Host  | Availability Zone | Alive | State | Binary                    |
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+
| 22467163-01ea-4231-ba45-3bd316f425e6 | Linux bridge agent | HostA | None              | True  | UP    | neutron-linuxbridge-agent |
| 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b | DHCP agent         | HostA | None              | True  | DOWN  | neutron-dhcp-agent        |
| 3066d20c-9f8f-440c-ae7c-a40ffb4256b6 | Linux bridge agent | HostB | nova              | True  | UP    | neutron-linuxbridge-agent |
| 55569f4e-6f31-41a6-be9d-526efce1f7fe | DHCP agent         | HostB | nova              | True  | UP    | neutron-dhcp-agent        |
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+

After you stop the DHCP agent on HostA, you can delete it by the following command:

$ openstack network agent delete 2444c54d-0d28-460c-ab0f-cd1e6b5d3c7b
$ openstack network agent list
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+
| ID                                   | Agent Type         | Host  | Availability Zone | Alive | State | Binary                    |
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+
| 22467163-01ea-4231-ba45-3bd316f425e6 | Linux bridge agent | HostA | None              | True  | UP    | neutron-linuxbridge-agent |
| 3066d20c-9f8f-440c-ae7c-a40ffb4256b6 | Linux bridge agent | HostB | nova              | True  | UP    | neutron-linuxbridge-agent |
| 55569f4e-6f31-41a6-be9d-526efce1f7fe | DHCP agent         | HostB | nova              | True  | UP    | neutron-dhcp-agent        |
+--------------------------------------+--------------------+-------+-------------------+-------+-------+---------------------------+

After deletion, if you restart the DHCP agent, it appears on the agent list again.

Enabling DHCP high availability by default

You can control the default number of DHCP agents assigned to a network by setting the following configuration option in the file /etc/neutron/neutron.conf.

dhcp_agents_per_network = 3