SNMP
----

Simple Network Management Protocol (SNMP_) is an Internet Standard protocol
for collecting and organizing information about managed devices on IP networks
and for modifying that information to change device behavior. Devices that
typically support SNMP include cable modems, routers, switches, servers,
workstations, printers, and more.

SNMP is widely used in network management for network monitoring. SNMP exposes
management data in the form of variables on the managed systems organized in
a management information base (MIB_) which describe the system status and
configuration. These variables can then be remotely queried (and, in some
circumstances, manipulated) by managing applications.

Three significant versions of SNMP have been developed and deployed. SNMPv1 is
the original version of the protocol. More recent versions, SNMPv2c and SNMPv3,
feature improvements in performance, flexibility and security.

SNMP is a component of the Internet Protocol Suite as defined by the Internet
Engineering Task Force (IETF). It consists of a set of standards for network
management, including an application layer protocol, a database schema, and a
set of data objects.

Overview and basic concepts
^^^^^^^^^^^^^^^^^^^^^^^^^^^

In typical uses of SNMP, one or more administrative computers called managers
have the task of monitoring or managing a group of hosts or devices on a
computer network. Each managed system executes a software component called an
agent which reports information via SNMP to the manager.

An SNMP-managed network consists of three key components:

* Managed devices

* Agent – software which runs on managed devices

* Network management station (NMS) – software which runs on the manager

A managed device is a network node that implements an SNMP interface that
allows unidirectional (read-only) or bidirectional (read and write) access to
node-specific information. Managed devices exchange node-specific information
with the NMSs. Sometimes called network elements, the managed devices can be
any type of device, including, but not limited to, routers, access servers,
switches, cable modems, bridges, hubs, IP telephones, IP video cameras,
computer hosts, and printers.

An agent is a network-management software module that resides on a managed
device. An agent has local knowledge of management information and translates
that information to or from an SNMP-specific form.

A network management station executes applications that monitor and control
managed devices. NMSs provide the bulk of the processing and memory resources
required for network management. One or more NMSs may exist on any managed
network.

.. figure:: /_static/images/service_snmp_communication_principles_diagram.png
   :scale: 20 %
   :alt: Principle of SNMP Communication

   Image thankfully borrowed from
   https://en.wikipedia.org/wiki/File:SNMP_communication_principles_diagram.PNG
   which is under the GNU Free Documentation License

.. note:: VyOS SNMP supports both IPv4 and IPv6.

SNMP protocol versions
^^^^^^^^^^^^^^^^^^^^^^

VyOS itself supports SNMPv2_ (version 2) and SNMPv3_ (version 3) where the
later is recommended because of improved security (optional authentication and
encryption).

SNMPv2
^^^^^^

SNMPv2 is the original and most commonly used version. For authorizing clients,
SNMP uses the concept of communities. Communities may have authorization set
to read only (this is most common) or to read and write (this option is not
actively used in VyOS).

SNMP can work synchronously or asynchronously. In synchronous communication,
the monitoring system queries the router periodically. In asynchronous, the
router sends notification to the "trap" (the monitoring host).

SNMPv2 does not support any authentication mechanisms, other than client source
address, so you should specify addresses of clients allowed to monitor the
router. Note that SNMPv2 also supports no encryption and always sends data in
plain text.

Example
*******

.. code-block:: sh

  # Define a community
  set service snmp community routers authorization ro

  # Allow monitoring access from the entire network
  set service snmp community routers network 192.0.2.0/24
  set service snmp community routers network 2001::db8:ffff:eeee::/64

  # Allow monitoring access from specific addresses
  set service snmp community routers client 203.0.113.10
  set service snmp community routers client 203.0.113.20

  # Define optional router information
  set service snmp location "UK, London"
  set service snmp contact "admin@example.com"

  # Trap target if you want asynchronous communication
  set service snmp trap-target 203.0.113.10

  # Listen only on specific IP addresses (port defaults to 161)
  set service snmp listen-address 172.16.254.36 port 161
  set service snmp listen-address 2001:db8::f00::1


SNMPv3
^^^^^^

SNMPv3 is an updated version that, among other things, supports encryption and
cryptographic authentication of clients.

Example
*******

.. code-block:: sh

  set service snmp v3 engineid '0x0aa0d6c6f450'
  set service snmp v3 group defaultgroup mode 'ro'
  set service snmp v3 group defaultgroup seclevel 'priv'
  set service snmp v3 group defaultgroup view 'defaultview'
  set service snmp v3 view defaultview oid '1'

  set service snmp v3 user testUser1 auth plaintext-key testUserKey1
  set service snmp v3 user testUser1 auth type 'md5'
  set service snmp v3 user testUser1 engineid '0x0aa0d6c6f450'
  set service snmp v3 user testUser1 group 'defaultgroup'
  set service snmp v3 user testUser1 mode 'ro'
  set service snmp v3 user testUser1 privacy type aes
  set service snmp v3 user testUser1 privacy plaintext-key testUserKey1

After commit the resulting configuration will look like:

.. note:: SNMPv3 keys won't we stored in plaintext. On ``commit`` the keys
   will be encrypted and the encrypted key is based on the engineid!

.. code-block:: sh

  vyos@vyos# show service snmp
   v3 {
       engineid 0x0aa0d6c6f450
       group defaultgroup {
           mode ro
           seclevel priv
           view defaultview
       }
       user testUser1 {
           auth {
               encrypted-key 0x3b68d4162c2c817b8e9dfb6f08583e5d
               type md5
           }
           engineid 0x0aa0d6c6f450
           group defaultgroup
           mode ro
           privacy {
               encrypted-key 0x3b68d4162c2c817b8e9dfb6f08583e5d
               type aes
           }
       }
       view defaultview {
           oid 1 {
           }
       }
   }


.. _SNMP: https://en.wikipedia.org/wiki/Simple_Network_Management_Protocol
.. _SNMPv2: https://en.wikipedia.org/wiki/Simple_Network_Management_Protocol#Version_2
.. _SNMPv3: https://en.wikipedia.org/wiki/Simple_Network_Management_Protocol#Version_3
.. _MIB: https://en.wikipedia.org/wiki/Management_information_base