Cumulus Linux uses the open source Net-SNMP agent snmpd, version 5.7, which provides support for most of the common industry-wide MIBs, including interface counters and TCP/UDP IP stack data.

Contents

 This topic describes ...

History

SNMP is an IETF standards-based network management architecture and protocol that traces its roots back to Carnegie-Mellon University in 1982. Since then, it has been modified by programmers at the University of California. In 1995, this code was also made publicly available as the UCD project. After that, ucd-snmp was extended by work done at the University of Liverpool as well as later in Denmark. In late 2000, the project name changed to net-snmp and became a fully-fledged collaborative open source project. The version used by Cumulus Networks is based on the latest net-snmp 5.7 branch with added custom MIBs and pass-through and pass-persist scripts (see below for more information on pass persist scripts).

Introduction to Simple Network Management Protocol

SNMP Management servers gather information from different systems in a consistent manner and the paths to the relevant information are standardized in IETF RFCs. SNMPs longevity is due to the fact that it standardizes the objects collected from devices, the protocol used for transport, and architecture of the management systems. The most widely used, and most insecure, versions of SNMP are versions 1 and 2c and their popularity is largely due to implementations that have been in use for decades. SNMP version 3 is the recommended version because of its advanced security features. In general, a network being profiled by SNMP Management Stations mainly consist of devices containing SNMP agents. The agent running on Cumulus Linux switches and routers is the snmpd daemon.

SNMP Managers

An SNMP Network Management System (NMS) is a computer that is configured to poll SNMP agents (in this case, Cumulus Linux switches and routers) to gather information and present it. This manager can be any machine that can send query requests to SNMP agents with the correct credentials. This NMS can be a large set of monitoring suite or as simple as some scripts that collect and display data. The managers generally poll the agents and the agents respond with the data. There are a variety of polling command-line tools (snmpget, snmpgetnext, snmpwalk, snmpbulkget, snmpbulkwalk, and so on).  SNMP agents can also send unsolicited Traps/Inform messages to the SNMP Manager based on predefined criteria (like link changes).

SNMP Agents

The SNMP agents (snmpd) running on the switches do the bulk of the work and are responsible for gathering information about the local system and storing data in a format that can be queried updating an internal database called the management information base, or MIB. The MIB is a standardized, hierarchical structure that stores information that can be queried. Parts of the MIB tree are available and provided to incoming requests originating from an NMS host that has authenticated with the correct credentials. You can configure the Cumulus Linux switch with usernames and credentials to provide authenticated and encrypted responses to NMS requests. The snmpd  agent can also proxy requests and act as a master agent to sub-agents running on other daemons (FRR, LLDP).

Management Information Base (MIB)

The MIB is a database that is implemented on the daemon (or agent) and follows IETF RFC standards to which the manager and agents adhere. It is a hierarchical structure that, in many areas, is globally standardized, but also flexible enough to allow vendor-specific additions.  Cumulus Networks implements a number of custom enterprise MIB tables and these are defined in text files located on the switch and in files named /usr/share/snmp/mibs/Cumulus*. The MIB structure is best understood as a top-down hierarchical tree. Each branch that forks off is labeled with both an identifying number (starting with 1) and an identifying string that is unique for that level of the hierarchy. These strings and numbers can be used interchangeably. A specific node of the tree can be traced from the unnamed root of the tree to the node in question. The parent IDs (numbers or strings) are strung together, starting with the most general to form an address for the MIB Object. Each junction in the hierarchy is represented by a dot in this notation so that the address ends up being a series of ID strings or numbers separated by dots. This entire address is known as an object identifier (OID).

Hardware vendors that embed SNMP agents in their devices sometimes implement custom branches with their own fields and data points. However, there are standard MIB branches that are well defined and can be used by any device. The standard branches discussed here are all under the same parent branch structure. This branch defines information that adheres to the MIB-2 specification, which is a revised standard for compliant devices. You can use various online and command-line tools to translate between numbers and string and to also provide definitions for the various MIB Objects. For example, you can view the sysLocation object in the system table with either a string of numbers 1.3.6.1.2.1.1.6 or the string representation iso.org.dod.internet.mgmt.mib-2.system.sysLocation. You can view the definition with the snmptranslate (1) command (found in the snmp Debian package).

snmptranslate Command
/home/cumulus# snmptranslate -Td -On SNMPv2-MIB::sysLocation                                                                                                                                                                                                                                                                                      
.1.3.6.1.2.1.1.6
sysLocation OBJECT-TYPE
  -- FROM       SNMPv2-MIB
  -- TEXTUAL CONVENTION DisplayString
  SYNTAX        OCTET STRING (0..255) 
  DISPLAY-HINT  "255a"
  MAX-ACCESS    read-write
  STATUS        current
  DESCRIPTION   "The physical location of this node (e.g., 'telephone
            closet, 3rd floor').  If the location is unknown, the
            value is the zero-length string."
::= { iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1) system(1) 6 }

/home/cumulus# snmptranslate  -Tp -IR   system
+--system(1)
   |
   +-- -R-- String    sysDescr(1)
   |        Textual Convention: DisplayString
   |        Size: 0..255
   +-- -R-- ObjID     sysObjectID(2)
   +-- -R-- TimeTicks sysUpTime(3)
   |  |
   |  +--sysUpTimeInstance(0)
   |
   +-- -RW- String    sysContact(4)
   |        Textual Convention: DisplayString
   |        Size: 0..255
   +-- -RW- String    sysName(5)
   |        Textual Convention: DisplayString
   |        Size: 0..255
   +-- -RW- String    sysLocation(6)
   |        Textual Convention: DisplayString
   |        Size: 0..255
   +-- -R-- INTEGER   sysServices(7)
   |        Range: 0..127
   +-- -R-- TimeTicks sysORLastChange(8)
   |        Textual Convention: TimeStamp

The section 1.3.6.1 or  iso.org.dod.internet is the OID that defines internet resources. The 2 or mgmt that follows is for a management subcategory. The 1 or mib-2 under that defines the MIB-2 specification.    And finally, the 1 or system is the parent for a number of child objects (sysDescr, sysObjectID, sysUpTime, sysContact, sysName, sysLocation, sysServices, and so on).

Getting Started

The simplest use case for using SNMP consists of creating a readonly community password and enabling a listening address for the loopback address (this is the default listening-address provided).  This allows for testing functionality of snmpd before extending the listening addresses to IP addresses reachable from outside the switch or router. This first sample configuration adds a listening address on the loopback interface (this is not a change from the default so we get a message stating that the configuration has not changed), sets a simple community password (SNMPv2) for testing, changes the system-name object in the system table, commits the change, checks the status of snmpd, and gets the first MIB object in the system table:

cumulus@router1:~$ net add snmp-server listening-address localhost

Configuration has not changed 

cumulus@router1:~$ net add snmp-server readonly-community mynotsosecretpassword access any

cumulus@router1:~$ net add snmp-server system-name my little router

cumulus@router1:~$ net commit

 

cumulus@router1:~$ net show snmp-server status

 

Simple Network Management Protocol (SNMP) Daemon.

---------------------------------  ----------------

Current Status                     active (running)

Reload Status                      enabled

Listening IP Addresses             localhost

Main snmpd PID                     13669

Version 1 and 2c Community String  Configured

Version 3 Usernames                Not Configured

---------------------------------  ---------------- 

 

cumulus@router1:~$ snmpgetnext -v 2c -c mynotsosecretpassword localhost SNMPv2-MIB::sysName

SNMPv2-MIB::sysName.0 = STRING: my little router

Configure SNMP

For external SNMP NMS systems to poll Cumulus Linux switches and routers, you must configure the SNMP agent (snmpd) running on the switch with one or more IP addresses (with net add snmp-server listening-address <ip>) on which the agent listens. You must configure these IP addresses on interfaces that have link state UP. By default, the SNMP configuration has a listening address of localhost (or 127.0.0.1), which allows the daemon to respond to SNMP requests originating on the switch itself. This is a useful method of checking the configuration for SNMP without exposing the switch to attacks from the outside. The only other required configuration is a readonly community password (configured with net add snmp-server readonly-community <password> access <ip | any>), that allows polling of the various MIB objects on the device itself. SNMPv3 is recommended since SNMPv2c (with a community string) exposes the password in the GetRequest and GetResponse packets. SNMPv3 does not expose the username passwords and has the option of encrypting the packet contents.

Cumulus Linux 3.4 and later releases support configuring SNMP with NCLU. While NCLU does not provide functionality to configure every single snmpd feature, it is the recommended method of configuring snmpd. You are not restricted to using NCLU for configuration and can edit the /etc/snmp/snmpd.conf file and control snmpd with systemctl commands. For Cumulus Linux versions earlier than 3.0, snmpd has a default configuration that listens to incoming requests on all interfaces.

Cumulus Linux 3.6 and later releases support configuring VRFs for listening-addresses as well as Trap/Inform support. If management VRF is enabled on the switch, this places the eth0 interface in the management VRF. When configuring the listening-address for snmp-server, you must specify an additional parameter to enable listening on the eth0 interface with the following command:

cumulus@router1:~$ net add snmp-server listening-address 10.10.10.10 vrf mgmt

These additional parameters are described in detail below.

You must add a default community string for v1 or v2c environments or the snmpd daemon does not respond to any requests. For security reasons, the default configuration configures snmpd to listen to SNMP requests on the loopback interface so access to the switch is restricted to requests originating from the switch itself. The only required commands for snmpd to function are a listening-address and either a username or a readonly-community string.

Configure SNMP with NCLU

The table below highlights the structure of NCLU commands available for configuring SNMP. An example command set is provided below the table. NCLU restarts the snmpd daemon after configuration changes are made and committed.

CommandSummary
net del all or net del snmp-server allRemoves all entries in the /etc/snmp/snmpd.conf file and replaces them with defaults. The defaults remove all SNMPv3 usernames, readonly-communities, and a listening-address of localhost is configured.
net add snmp-server listening-address (localhost|localhost-v6)

For security reasons, the localhost is set to a listening address 127.0.0.1 by default so that the SNMP agent only responds to requests originating on the switch itself. You can also configure listening only on the IPv6 localhost address with localhost-v6. When using IPv6 addresses or localhost, you can use a readonly-community-v6 for v1 and v2c requests. For v3 requests, you can use the username command to restrict access.

net add snmp-server listening-address localhost
net add snmp-server listening-address localhost-v6
net add snmp-server listening-address (all|all-v6)

Configures the snmpd agent to listen on all interfaces for either IPv4 or IPv6 UDP port 161 SNMP requests. This command removes all other individual IP addresses configured.

Note: This command does not allow snmpd to cross VRF table boundaries. To listen on IP addresses in different VRF tables, use multiple listening-address commands each with a VRF name, as shown below.

net add snmp-server listening-address all
net add snmp-server listening-address all-v6
net add snmp-server listening-address IP_ADDRESS IP_ADDRESS ...

Sets snmpd to listen to a specific IPv4 or IPv6 address, or a group of addresses with space separated values, for incoming SNMP queries. If VRF tables are used, be sure to specify an IP address with an associated VRF name, as shown below. If you omit a VRF name, the default VRF is used.

net add snmp-server listening-address 10.10.10.10
net add snmp-server listening-address 10.10.10.10 44.44.44.44

net add snmp-server listening-address IP_ADDRESS vrf VRF_NAME

Sets snmpd to listen to a specific IPv4 or IPv6 address on an interface within a particular VRF. With VRFs, identical IP addresses can exist in different VRF tables. This command restricts listening to a particular IP address within a particular VRF. If the VRF name is not given, the default VRF is used.

net add snmp-server listening-address 10.10.10.10 vrf mgmt


net add snmp-server username [user name] (auth-none|auth-md5|auth-sha) <authentication password> [(encrypt-des|encrypt-aes) <encryption password>] (oid <OID>|view <view name>)

Creates an SNMPv3 username and the necessary credentials for access. You can restrict a user to a particular OID tree or predefined view name if these are specified. If you specify auth-none, no authentication password is required. Otherwise, an MD5 or SHA password is required for access to the MIB objects. If specified, an encryption password is used to hide the contents of the request and response packets.

net add snmp-server username testusernoauth  auth-none
net add snmp-server username testuserauth    auth-md5  myauthmd5password 
net add snmp-server username testuserboth    auth-md5  mynewmd5password   encrypt-aes  myencryptsecret 
net add snmp-server username limiteduser1    auth-md5  md5password1       encrypt-aes  myaessecret       oid 1.3.6.1.2.1.1
net add snmp-server viewname [view name] (included | excluded) [OID or name]

Creates a view name that is used in readonly-community to restrict MIB tree exposure. By itself, this view definition has no effect; however, when linked to an SNMPv3 username or community password, and a host from a restricted subnet, any SNMP request with that username and password must have a source IP address within the configured subnet.

Note: OID can be either a string of period separated decimal numbers or a unique text string that identifies an SNMP MIB object. Some MIBs are not installed by default; you must install them either by hand or with the latest Debian package called snmp-mibs-downloader. You can remove specific view name entries with the delete command or with just a view name to remove all entries matching that view name. You can define a specific view name multiple times and fine tune to provide or restrict access using the included or excluded command to specify branches of certain MIB trees.

net add snmp-server viewname cumulusOnly included .1.3.6.1.4.1.40310
net add snmp-server viewname cumulusCounters included .1.3.6.1.4.1.40310.2

net add snmp-server readonly-community simplepassword access any view cumulusOnly
net add snmp-server username testusernoauth  auth-none view cumulusOnly
net add snmp-server username limiteduser1    auth-md5  md5password1 encrypt-aes  myaessecret  view cumulusCounters
net add snmp-server (readonly-community | readonly-community-v6) [password] access (any | localhost | [network]) [(view [view name]) or [oid [oid or name])

This command defines the password required for SNMP version 1 or 2c requests for GET or GETNEXT.  By default, this provides access to the full OID tree for such requests, regardless of from where they were sent. There is no default password set, so snmpd does not respond to any requests that arrive. Users often specify a source IP address token to restrict access to only that host or network given. You can specify a view name to restrict the subset of the OID tree.

Examples of readonly-community commands are shown below. The first command sets the read only community string to simplepassword for SNMP requests and this restricts requests to those sourced from hosts in the 10.10.10.0/24 subnet and restricts viewing to the mysystem view name defined with the viewname command. The second example creates a read-only community password showitall that allows access to the entire OID tree for requests originating from any source IP address.

net add snmp-server viewname mysystem included 1.3.6.1.2.1.1
net add snmp-server readonly-community simplepassword access 10.10.10.0/24 view mysystem

net add snmp-server readonly-community showitall access any
net add snmp-server trap-destination (localhost | [ipaddress]) [vrf vrf name] community-password [password] [version [1 | 2c]]

For SNMP versions 1 and 2C, this command sets the SNMP Trap destination IP address. Multiple destinations can exist, but you must set up at least one to enable SNMP Traps to be sent. Removing all settings disables SNMP traps. The default version is 2c, unless otherwise configured. You must include a VRF name with the IP address to force Traps to be sent in a non-default VRF table.

net add snmp-server trap-destination 10.10.10.10 community-password mynotsosecretpassword version 1
net add snmp-server trap-destination 20.20.20.20 vrf mgmt community-password mymanagementvrfpassword version 2c
 

net add snmp-server trap-destination (localhost | [ipaddress]) [vrf vrf name] username <v3 username> (auth-md5|auth-sha) <authentication password> [(encrypt-des|encrypt-aes) <encryption password>] engine-id <text> [inform]

For SNMPv3 Trap and Inform messages, this command configures the trap destination IP address (with an optional VRF name). You must define the authentication type and password. The encryption type and password are optional. You must specify the engine ID/user name pair. The inform keyword is used to specify an Inform message where the SNMP agent waits for an acknowledgement.

For Traps, the engine ID/user name is for the CL switch sending the traps. This can be found at the end of the /var/lib/snmp/snmpd.conf file labelled oldEngineID. Configure this same engine ID/user name (with authentication and encryption passwords) for the Trap daemon receiving the trap to validate the received Trap.

net add snmp-server trap-destination 10.10.10.10 username myv3userrsion auth-md5 md5password1 encrypt-aes myaessecret engine-id  0x80001f888070939b14a514da5a00000000
net add snmp-server trap-destination 20.20.20.20 vrf mgmt username mymgmtvrfusername auth-md5 md5password2 encrypt-aes myaessecret2 engine-id  0x80001f888070939b14a514da5a00000000

For Inform messages (Informs are acknowledged version 3 Traps), the engine ID/user name is the one used to create the username on the receiving Trap daemon server. The Trap receiver sends the response for the Trap message using its own engine ID/user name. In practice, the trap daemon generates the usernames with its own engine ID and after these are created, the SNMP server (or agent) needs to use these engine ID/user names when configuring the Inform messages so that they are correctly authenticated and the correct response is sent to the snmpd agent that sent it.

net add snmp-server trap-destination 10.10.10.10 username myv3userrsion auth-md5 md5password1 encrypt-aes myaessecret engine-id  0x80001f888070939b14a514da5a00000000 inform
net add snmp-server trap-destination 20.20.20.20 vrf mgmt username mymgmtvrfusername auth-md5 md5password2 encrypt-aes myaessecret2 engine-id  0x80001f888070939b14a514da5a00000000 inform
net add snmp-server trap-link-up [check-frequency [seconds]]

Enables notifications for interface link-up to be sent to SNMP Trap destinations.

net add snmp-server trap-link-up check-frequency 15
net add snmp-server trap-link-down [check-frequency [seconds]]

Enables notifications for interface link-down to be sent to SNMP Trap destinations.

net add snmp-server trap-link-down check-frequency 10
net add snmp-server trap-snmp-auth-failures

Enables SNMP Trap notifications to be sent for every SNMP authentication failure.

net add snmp-server trap-snmp-auth-failures

net add snmp-server trap-cpu-load-average one-minute [threshold] five-minute [5-min-threshold]

fifteen-minute [15-min-threshold]

Enables a trap when the cpu-load-average exceeds the configured threshold. You can only use integers or floating point numbers.

net add snmp-server trap-cpu-load-average one-minute 4.34 five-minute 2.32 fifteen-minute 6.5

This table describes system setting configuration commands for SNMPv2-MIB.

CommandSummary
net add snmp-server system-location [string]

Sets the system physical location for the node in the SNMPv2-MIB system table.

net add snmp-server system-location  My private bunker
net add snmp-server system-contact [string]

Sets the identification of the contact person for this managed node, together with information on how to contact this person.

net add snmp-server system-contact user X at myemail@example.com
net add snmp-server system-name [string]

Sets an administratively-assigned name for the managed node. By convention, this is the fully-qualified domain name of the node.

net add snmp-server system-name CumulusBox number 1,543,567

The example commands below enable an SNMP agent to listen on all IPv4 addresses with a community string password, set the trap destination host IP address, and create four types of SNMP traps.

cumulus@switch:~$ net add snmp-server listening-address all
cumulus@switch:~$ net add snmp-server readonly-community tempPassword access any
cumulus@switch:~$ net add snmp-server trap-destination 1.1.1.1 community-password mypass version 2c
cumulus@switch:~$ net add snmp-server trap-link-up check-frequency 15
cumulus@switch:~$ net add snmp-server trap-link-down check-frequency 10
cumulus@switch:~$ net add snmp-server trap-cpu-load-average one-minute 7.45 five-minute 5.14
cumulus@switch:~$ net add snmp-server trap-snmp-auth-failures

Configure SNMP Manually

If you need to manually edit the SNMP configuration; for example, if the necessary option has not been implemented in NCLU, you need to edit the configuration directly, which is stored in the /etc/snmp/snmpd.conf file.

Use caution when editing this file. The next time you use NCLU to update your SNMP configuration, if NCLU is unable to correctly parse the syntax, some of the options might be overwritten. 

Make sure you do not delete the snmpd.conf file; this can cause issues with the package manager the next time you update Cumulus Linux.

The SNMP daemon, snmpd, uses the /etc/snmp/snmpd.conf configuration file for most of its configuration. The syntax of the most important keywords are defined in the following table.

SyntaxMeaning
agentaddress

Required. This command sets the protocol, IP address, and the port for snmpd to listen for incoming requests. The IP address must exist on an interface that has link UP on the switch where snmpd is being used. By default, this is set to udp:127.0.0.1:161, which means snmpd listens on the loopback interface and only responds to requests (snmpwalk, snmpget, snmpgetnext) originating from the switch. A wildcard setting of udp:161,udp6:161 forces snmpd to listen on all IPv4 and IPv6 interfaces for incoming SNMP requests. You can configure multiple IP addresses as comma-separated values; for example, udp:66.66.66.66:161,udp:77.77.77.77:161,udp6:[2001::1]:161. You can use multiple lines to define listening addresses. To bind to a particular IP address within a particular VRF table, follow the IP address with a @ and the name of the VRF table (for example, 10.10.10.10@mgmt).

rocommunity

Required. This command defines the password that is required for SNMP version 1 or 2c requests for GET or GETNEXT.  By default, this provides access to the full OID tree for such requests, regardless of from where they were sent. There is no default password set, so snmpd does not respond to any requests that arrive. Specify a source IP address token to restrict access to only that host or network given. Specify a view name (as defined above) to restrict the subset of the OID tree.

Examples of rocommunity commands are shown below. The first command sets the read only community string to simplepassword for SNMP requests sourced from the 10.10.10.0/24 subnet and restricts viewing to the systemonly view name defined previously with the view command. The second example creates a read-only community password that allows access to the entire OID tree from any source IP address.

rocommunity simplepassword 10.10.10.0/24 -V systemonly

rocommunity cumulustestpassword
view

This command defines a view name that specifies a subset of the overall OID tree. You can reference this restricted view by name in the rocommunity command to link the view to a password that is used to see this restricted OID subset. By default, the snmpd.conf file contains numerous views with the systemonly view name.

view   systemonly  included   .1.3.6.1.2.1.1 

view   systemonly  included   .1.3.6.1.2.1.2

view   systemonly  included   .1.3.6.1.2.1.3 

The systemonly view is used by rocommunity to create a password for access to only these branches of the OID tree.

trapsink

trap2sink

This command defines the IP address of the notification (or trap) receiver for either SNMPv1 traps or SNMPv2 traps. If you specify several sink directives, multiple copies of each notification (in the appropriate formats) are generated. You must configure a trap server to receive and decode these trap messages (for example, snmptrapd). You can configure the address of the trap receiver with a different protocol and port but this is most often left out. The defaults are to use the well-known UDP packets and port 162.

createuser

iquerysecName

rouser

These three commands define an internal SNMPv3 username that is required for snmpd to send traps. This username is required to authorize the DisMan service even though SNMPv3 is not being configured for use. The example snmpd.conf configuration shown below creates snmptrapusernameX as the username (this is just an example username) using the createUser command. iquerysecname defines the default SNMPv3 username to be used when making internal queries to retrieve monitored expressions. rouser specifies the username for these SNMPv3 queries. All three are required for snmpd to retrieve information and send built-in traps or for those configured with the monitor command shown below in the examples.

createuser snmptrapusernameX

iquerysecname snmptrapusernameX

rouser snmptrapusernameX

linkUpDownNotifications yes

This command enables link up and link down trap notifications, assuming the other trap configurations settings are set. This command configures the Event MIB tables to monitor the ifTable for network interfaces being taken up or down, and triggering a linkUp or linkDown notification as appropriate. This is equivalent to the following configuration:

notificationEvent  linkUpTrap    linkUp   ifIndex ifAdminStatus ifOperStatus

notificationEvent  linkDownTrap  linkDown ifIndex ifAdminStatus ifOperStatus 

monitor  -r 60 -e linkUpTrap   "Generate linkUp" ifOperStatus != 2

monitor  -r 60 -e linkDownTrap "Generate linkDown" ifOperStatus == 2

defaultMonitors yes

This command configures the Event MIB tables to monitor the various UCD-SNMP-MIB tables for problems (as indicated by the appropriate xxErrFlag column objects) and send a trap. This assumes you have downloaded the snmp-mibs-downloader Debian package and commented out mibs from the /etc/snmp/snmp.conf file (#mibs). This command is exactly equivalent to the following configuration:

monitor   -o prNames -o prErrMessage "process table" prErrorFlag != 0

monitor   -o memErrorName -o memSwapErrorMsg "memory" memSwapError != 0

monitor   -o extNames -o extOutput "extTable" extResult != 0

monitor   -o dskPath -o dskErrorMsg "dskTable" dskErrorFlag != 0

monitor   -o laNames -o laErrMessage  "laTable" laErrorFlag != 0

monitor   -o fileName -o fileErrorMsg  "fileTable" fileErrorFlag != 0

Start the SNMP Daemon

Use the recommended process described below to start snmpd and monitor it using systemctl.

To start the SNMP daemon:

  1. Start the snmpd daemon:

    cumulus@switch:~$ sudo systemctl start snmpd.service
  2. Configure the snmpd daemon to start automatically after reboot:

    cumulus@switch:~$ sudo systemctl enable snmpd.service
  3. To enable snmpd to restart automatically after failure:
    1. Create a file called /etc/systemd/system/snmpd.service.d/restart.conf.
    2. Add the following lines:

      [Service]
      Restart=always
      RestartSec=60
    3. Run sudo systemctl daemon-reload.

After the service starts, you can use SNMP to manage various components on the switch.

Configure SNMP with Management VRF (used prior to Cumulus Linux 3.6)

When you configure Management VRF, you need to be aware of the interface IP addresses on which SNMP is listening. If you set listening-address to all, the snmpd daemon responds to incoming requests on all interfaces that are in the default VRF. If you prefer to listen on a limited number of IP addresses, Cumulus Networks recommends that you run only one instance of the snmpd daemon and specify the VRF name along with the listening-address. You can configure IP addresses in different VRFs and a single SNMP daemon listens on multiple IP addresses each with its own VRF. Because SNMP has native VRF awareness, using systemctl commands to manage snmpd in different VRFs is no longer necessary.

SNMP configuration in NCLU is VRF aware so you can configure the snmpd daemon to listen to incoming SNMP requests on a particular IP address within particular VRFs. Because interfaces in a particular VRF (routing table) are not aware of interfaces in a different VRF, the snmpd daemon only responds to polling requests and sends traps on the interfaces of the VRF on which it is configured. 

When management VRF is configured, configure the listening-address with a VRF name as shown above.  This allows snmpd to receive and respond to SNMP polling requests on eth0. 

Prior to CL 3.6, you could not configure a VRF name in the listening-address or the trap-destination commands. To manually handle VRF functionality, you had to do the following:

  1. Configure all the required SNMP settings with NCLU. Pay particular attention to the listening-address configuration setting, which should contain one or more IP addresses that belong to interfaces within a single VRF (if management VRF is configured, this is typically the IP address of eth0 ). You can use IP addresses other than eth0, but the interfaces for these IP addresses must be in the same VRF (typically the management VRF).
  2. Commit the changes to start the snmpd daemon in the default VRF.

  3. Manually stop the snmpd daemon from running in the default VRF.

  4. Manually restart the snmpd  daemon in the management VRF.

Running Multiple Instances of snmpd

Prior to CL 3.6, more complex configurations may have been needed; for example, you can run more than one snmpd daemon (one in each VRF designed to receive SNMP polling requests). Cumulus Networks does not recommend this for memory and cpu resource reasons. However, if this is required, you must use a separate configuration file with each instance of the snmpd daemon. You can use a copy of the /etc/snmp/snmpd.conf file. When you use this file, start an snmpd daemon with the following command:

cumulus@switch:~$ sudo /usr/sbin/snmpd -y -LS 0-4 d -Lf /dev/null -u snmp -g snmp -I -smux -p /run/snmpd.pid -C -c <new snmp config filename> (edited)

To use management VRF, you need to configure the IP address of eth0 as the listening-address. In the example below, eth0 IP address is 10.10.10.10. You can also add other snmp-server configurations, then commit the changes.

cumulus@switch:~$ net add snmp-server listening-address 10.10.10.10
cumulus@switch:~$ net add snmp-server readonly-community tempPassword access any
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

This restarts the snmpd daemon in the default VRF. Then, to run snmpd in the correct VRF, stop the daemon in the default VRF (or stop any other snmpd daemons that happen to be running), then restart snmpd in the management VRF so that it can respond to requests on interfaces only in that VRF. Make sure that only one instance of the snmpd daemon is running and that it is running in the desired VRF. Assuming the Management VRF has been enabled, the following example shows how to stop snmpd and restart it in the management VRF.

cumulus@switch:mgmt-vrf:~$ systemctl stop snmpd.service
cumulus@switch:mgmt-vrf:~$ systemctl disable snmpd.service
cumulus@switch:mgmt-vrf:~$ ps aux | grep snmpd
cumulus@switch:mgmt-vrf:~$ 
cumulus@switch:mgmt-vrf:~$ systemctl start snmpd@mgmt.service
cumulus@switch:mgmt-vrf:~$ systemctl enable snmpd.service
cumulus@switch:mgmt-vrf:~$ systemctl status snmpd@mgmt.service
root@switch:mgmt-vrf:/home/cumulus# systemctl status snmpd@mgmt.service                                                                                                                                                                                                                                                                               ● snmpd@mgmt.service - Simple Network Management Protocol (SNMP) Daemon.                                                                                                                                                                                                                                                                                       Loaded: loaded (/lib/systemd/system/snmpd.service; disabled)                                                                                                                                                                                                                                                                                               Drop-In: /run/systemd/generator/snmpd@.service.d                                                                                                                                                                                                                                                                                                                     └─vrf.conf
   Active: active (running) since Thu 2017-12-07 20:05:41 UTC; 2min 22s ago                                                                                                                                                                                                                                                                                  Main PID: 30880 (snmpd)
   CGroup: /system.slice/system-snmpd.slice/snmpd@mgmt.service                                                                                                                                                                                                                                                                                                         └─30880 /usr/sbin/snmpd -y -LS 0-4 d -Lf /dev/null -u snmp -g snmp -I -smux -p /run/snmpd.pid -f
                                                                                                                                                                                                                                                                                                                                                            Dec 07 20:05:41 cel-redxp-01 systemd[1]: Started Simple Network Management Protocol (SNMP) Daemon..
 
cumulus@switch:mgmt-vrf:~$ ps aux | grep snmpd
snmp     30880  0.4  0.3  57176 12276 ?        Ss   20:05   0:00 /usr/sbin/snmpd -y -LS 0-4 d -Lf /dev/null -u snmp -g snmp -I -smux -p /run/snmpd.pid -f

Set up the Custom Cumulus Networks MIBs

No changes are required in the /etc/snmp/snmpd.conf file on the switch to support the custom Cumulus Networks MIBs. The following lines are already included by default and provide support for both the Cumulus Counters and the Cumulus Resource Query MIBs.

sysObjectID 1.3.6.1.4.1.40310
pass_persist .1.3.6.1.4.1.40310.1 /usr/share/snmp/resq_pp.py
pass_persist .1.3.6.1.4.1.40310.2 /usr/share/snmp/cl_drop_cntrs_pp.py

However, you need to copy several files to the NMS server for the custom Cumulus MIB to be recognized on NMS server.

  • /usr/share/snmp/mibs/Cumulus-Snmp-MIB.txt

  • /usr/share/snmp/mibs/Cumulus-Counters-MIB.txt

  • /usr/share/snmp/mibs/Cumulus-Resource-Query-MIB.txt

Set the Community String

The snmpd authentication for versions 1 and 2 is disabled by default in Cumulus Linux. You can enable this password (called a community string) by setting rocommunity (for read-only access) or rwcommunity (for read-write access). Setting a community string is required. 

To enable read-only querying by a client:

  1. Open the /etc/snmp/snmpd.conf file in a text editor.

  2. To allow read-only access, uncomment the following line, then save the file:

    rocommunity public default -V systemonly
    KeywordMeaning
    rocommunityRead-only community; rwcommunity is for read-write access.
    public

    Plain text password/community string.

    Cumulus Networks strongly recommends you change this password to something else.

    defaultThe default keyword allows connections from any system. The localhost keyword allows requests only from the local host. A restricted source can either be a specific hostname (or address), or a subnet, represented as IP/MASK (like 10.10.10.0/255.255.255.0), or IP/BITS (like 10.10.10.0/24), or the IPv6 equivalents.
    systemonlyThe name of this particular SNMP view. This is a user-defined value.
  3. Restart snmpd:

    cumulus@switch:~$ sudo systemctl restart snmpd.service

Enable SNMP Support for FRRouting

SNMP supports Routing MIBs in FRRouting. To enable SNMP support for FRRouting, you need to:

  • Configure AgentX (ASX) access in FRRouting
  • The default /etc/snmp/snmpd.conf configuration already enables agentx and sets the correct permissions

Enabling FRRouting includes support for BGP. However, if you plan on using the BGP4 MIB, be sure to provide access to the MIB tree 1.3.6.1.2.1.15.

At this time, SNMP does not support monitoring BGP unnumbered neighbors.

If you plan on using the OSPFv2 MIB, provide access to 1.3.6.1.2.1.14 and to 1.3.6.1.2.1.191 for the OSPv3 MIB.

To enable SNMP support for FRRouting:

  1. Configure AgentX access in FRRouting:

    cumulus@switch:~$ net add routing agentx
    cumulus@switch:~$ net pending
    cumulus@switch:~$ net commit
  2. Update the SNMP configuration to enable FRRouting to respond to SNMP requests. Open the /etc/snmp/snmpd.conf file in a text editor and verify that the following configuration exists:

    agentxsocket /var/agentx/master 
    agentxperms 777 777 snmp snmp
    master agentx

    Make sure that the /var/agentx directory is world-readable and world-searchable (octal mode 755).

  3. Optionally, you might need to expose various MIBs:

    • For the BGP4 MIB, allow access to 1.3.6.1.2.1.15
    • For the OSPF MIB,  allow access to 1.3.6.1.2.1.14 
    • For the OSPFV3 MIB, allow access to 1.3.6.1.2.1.191 

To verify the configuration, run snmpwalk. For example, if you have a running OSPF configuration with routes, you can check this OSPF-MIB first from the switch itself with:

cumulus@switch:~$ sudo snmpwalk -v2c -cpublic localhost 1.3.6.1.2.1.14

Enable the .1.3.6.1.2.1 Range

Some MIBs, including storage information, are not included by default in snmpd.conf in Cumulus Linux. This results in some default views on common network tools (like librenms) to return less than optimal data. You can include more MIBs by enabling all the .1.3.6.1.2.1 range. This simplifies the configuration file, removing concern that any required MIBs will be missed by the monitoring system. Various MIBs were added to version 3.0 and include the following: ENTITY and ENTITY-SENSOR MIB and parts of the BRIDGE-MIB and Q-BRIDGE-MIBs. These are included in the default configuration.

This configuration grants access to a large number of MIBs, including all SNMPv2-MIB, which might reveal more data than expected. In addition to being a security vulnerability, it might consume more CPU resources.

To enable the .1.3.6.1.2.1 range, make sure the view name commands include the required MIB objects.

Configure SNMPv3

SNMPv3 is often used to enable authentication and encryption, as community strings in versions 1 and 2c are sent in plaintext. SNMPv3 usernames are added to the /etc/snmp/snmpd.conf file, along with plaintext authentication and encryption pass phrases.

Warning

Cumulus Networks recommends that you configure SNMPv3 usernames and passwords with NCLU. However, if you prefer to edit the /etc/snmp/snmpd.conf manually instead, be aware that snmpd caches SNMPv3 usernames and passwords in the /var/lib/snmp/snmpd.conf file. Make sure you stop snmpd and remove the old entries when making changes. Otherwise, Cumulus Linux uses the old usernames and passwords in the /var/lib/snmp/snmpd.conf file instead of the ones in the /etc/snmp/snmpd.conf file.

The NCLU command structures for configuring SNMP user passwords are:

cumulus@switch:~$ net add snmp-server username <username> [auth-none] | [(auth-md5 | auth-sha) <auth-password>]
cumulus@switch:~$ net add snmp-server username <username> auth-(none | sha | md5)  (oid <OID> | view <view>)

The example below defines five users, each with a different combination of authentication and encryption:

cumulus@switch:~$ net add snmp-server username user1 auth-none
cumulus@switch:~$ net add snmp-server username user2 auth-md5 user2password
cumulus@switch:~$ net add snmp-server username user3 auth-md5 user3password encrypt-des user3encryption
cumulus@switch:~$ net add snmp-server username user666 auth-sha user666password encrypt-aes user666encryption
cumulus@switch:~$ net add snmp-server username user999 auth-md5 user999password encrypt-des user999encryption
cumulus@switch:~$ net add snmp-server username user1 auth-none oid 1.3.6.1.2.1
cumulus@switch:~$ net add snmp-server username user1 auth-none oid system

cumulus@switch:~$ net add snmp-server username user2 auth-md5 test1234 view testview
 oid 1.3.6.1.2.1
cumulus@switch:~$ net add snmp-server username user3 auth-sha testshax encrypt-aes testaesx oid 1.3.6.1.2.1
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit
# simple no auth user
#createuser user1

# user with MD5 authentication
#createuser user2 MD5 user2password

# user with MD5 for auth and DES for encryption
#createuser user3 MD5 user3password DES user3encryption

# user666 with SHA for authentication and AES for encryption
createuser user666 SHA user666password AES user666encryption

# user999 with MD5 for authentication and DES for encryption
createuser user999 MD5 user999password DES user999encryption

# restrict users to certain OIDs
# (Note: creating rouser or rwuser will give
# access regardless of the createUser command above. However,
# createUser without rouser or rwuser will not provide any access).
rouser user1 noauth 1.3.6.1.2.1
rouser user2 auth 1.3.6.1.2.1
rwuser user3 priv 1.3.6.1.2.1
rwuser user666
rwuser user999

After configuring user passwords and restarting the snmpd daemon, you can check user access with a client.

The snmp Debian package contains snmpget, snmpwalk, and other programs that are useful for checking daemon functionality from the switch itself or from another workstation.

The following commands check the access for each user defined above from the localhost:

# check user1 which has no authentication or encryption (NoauthNoPriv)
snmpget -v 3 -u user1 -l NoauthNoPriv localhost 1.3.6.1.2.1.1.1.0
snmpwalk -v 3 -u user1 -l NoauthNoPriv localhost 1.3.6.1.2.1.1

# check user2 which has authentication but no encryption (authNoPriv)
snmpget -v 3 -u user2 -l authNoPriv -a MD5 -A user2password localhost 1.3.6.1.2.1.1.1.0
snmpget -v 3 -u user2 -l authNoPriv -a MD5 -A user2password localhost 1.3.6.1.2.1.2.1.0
snmpwalk -v 3 -u user2 -l authNoPriv -a MD5 -A user2password localhost 1.3.6.1.2.1
  
# check user3 which has both authentication and encryption (authPriv)
snmpget -v 3 -u user3 -l authPriv -a MD5 -A user3password -x DES -X user3encryption localhost .1.3.6.1.2.1.1.1.0
snmpwalk -v 3 -u user3 -l authPriv -a MD5 -A user3password -x DES -X user3encryption localhost .1.3.6.1.2.1
snmpwalk -v 3 -u user666 -l authPriv -a SHA -x AES -A user666password -X user666encryption localhost 1.3.6.1.2.1.1
snmpwalk -v 3 -u user999 -l authPriv -a MD5 -x DES -A user999password -X user999encryption localhost 1.3.6.1.2.1.1

The following procedure shows a slightly more secure method of configuring SNMPv3 users without creating cleartext passwords:

  1. Install the net-snmp-config script that is in libsnmp-dev package:

    cumulus@switch:~$ sudo -E apt-get update
    cumulus@switch:~$ sudo -E apt-get install libsnmp-dev
  2. Stop the daemon:

    cumulus@switch:~$ sudo systemctl stop snmpd.service
  3. Use the net-snmp-config command to create two users, one with MD5 and DES, and the next with SHA and AES. 

    The minimum password length is eight characters and the arguments -a and -x have different meanings in net-snmp-config than snmpwalk.

    cumulus@switch:~$ sudo net-snmp-config --create-snmpv3-user -a md5authpass -x desprivpass -A MD5 -X DES userMD5withDES 
    cumulus@switch:~$ sudo net-snmp-config --create-snmpv3-user -a shaauthpass -x aesprivpass -A SHA -X AES userSHAwithAES
    cumulus@switch:~$ sudo systemctl start snmpd.service

This adds a createUser command in /var/lib/snmp/snmpd.conf. Do not edit this file by hand unless you are removing usernames. You can edit this file and restrict access to certain parts of the MIB by adding noauth, auth or priv to allow unauthenticated access, require authentication, or to enforce use of encryption.

The snmpd daemon reads the information from the /var/lib/snmp/snpmd.conf file and then the line is removed (eliminating the storage of the master password for that user) and replaced with the key that is derived from it (using the EngineID). This key is a localized key, so that if it is stolen, it cannot be used to access other agents. To remove the two users userMD5withDES and userSHAwithAES, stop the snmpd daemon and edit the /var/lib/snmp/snmpd.conf file. Remove the lines containing the username, then restart the snmpd daemon as in step 3 above.

From a client, you access the MIB with the correct credentials. (The roles of -x, -a and -X and -A are reversed on the client side as compared with the net-snmp-config command used above.)

snmpwalk -v 3 -u userMD5withDES -l authPriv -a MD5 -x DES -A md5authpass -X desprivpass localhost 1.3.6.1.2.1.1.1
snmpwalk -v 3 -u userSHAwithAES -l authPriv -a SHA -x AES -A shaauthpass -X aesprivpass localhost 1.3.6.1.2.1.1.1

Manually Configure SNMP Traps (Non-NCLU)

Generate Event Notification Traps

The Net-SNMP agent provides a method to generate SNMP trap events using the Distributed Management (DisMan) Event MIB for various system events, including:

  • Link up/down.
  • Exceeding the temperature sensor threshold, CPU load, or memory threshold.
  • Other SNMP MIBs.

To enable specific types of traps, you need to create the following configurations in /etc/snmp/snmpd.conf.

Define Access Credentials

An SNMPv3 username is required to authorize the DisMan service even though you are not configuring SNMPv3 here. The example snmpd.conf configuration shown below creates trapusername as the username using the createUser command. iquerySecName defines the default SNMPv3 username to be used when making internal queries to retrieve monitored expressions. rouser specifies which username to use for these SNMPv3 queries.  All three are required for snmpd to retrieve information and send traps (even with the monitor command shown below in the examples). Add the following lines to your /etc/snmp/snmpd.conf configuration file: 

createuser trapusername
iquerysecname trapusername
rouser trapusername

iquerysecname specifies the default SNMPv3 username to be used when making internal queries to retrieve any necessary information — either for evaluating the monitored expression or building a notification payload. These internal queries always use SNMPv3, even if normal querying of the agent is done using SNMPv1 or SNMPv2c. Note that this user must also be explicitly created via createUser and given appropriate access rights, for rouser, for example. The iquerysecname directive is purely concerned with defining which user should be used, not with actually setting this user up.

Define Trap Receivers

The following configuration defines the trap receiver IP address where SNMPv2 traps are sent:

trap2sink 192.168.1.1 public
# For SNMPv1 Traps, use
# trapsink  192.168.1.1  public 

Although the traps are sent to an SNMPV2 receiver, the SNMPv3 user is still required. Starting with Net-SNMP 5.3, snmptrapd no longer accepts all traps by default. snmptrapd must be configured with authorized SNMPv1/v2c community strings and/or SNMPv3 users. Non-authorized traps/informs are dropped. Refer to the snmptrapd.conf(5) manual page for details.

It is possible to define multiple trap receivers and to use the domain name instead of an IP address in the trap2sink directive.

Restart the snmpd service to apply the changes.

cumulus@switch:~$ sudo systemctl restart snmpd.service

SNMP Version 3 Trap and Inform Messages

You can configure SNMPv3 trap and inform messages with the trapsess configuration command. Inform messages are traps that are acknowledged by the receiving trap daemon. You configure inform messages with the -Ci parameter. You must specify the EngineID of the receiving trap server with the -e field.

SNMPv3 TRAP/INFORM
trapsess -Ci -e 0x80ccff112233445566778899 -v3 -l authPriv  -u trapuser1 -a MD5 -A trapuser1password -x DES -X trapuser1encryption 192.168.1.1

The SNMP trap receiving daemon must have usernames, authentication passwords, and encryption passwords created with its own EngineID. You must configure this trap server EngineID in the switch snmpd daemon sending the trap and inform messages. You specify the level of authentication and encryption for SNMPv3 trap and inform messages with -l (NoauthNoPriv, authNoPriv, or authPriv).

You can define multiple trap receivers and use the domain name instead of an IP address in the trap2sink directive.

After you complete the configuration, restart the snmpd service to apply the changes:

cumulus@switch:~$ sudo systemctl restart snmpd.service

Source Traps from a Different Source IP Address

When client SNMP programs (such as snmpget, snmpwalk, or snmptrap) are run from the command line, or when snmpd is configured to send a trap (based on snmpd.conf), you can configure a clientaddr in snmp.conf that allows the SNMP client programs or snmpd (for traps) to source requests from a different source IP address.

snmptrap, snmpget, snmpwalk and snmpd itself must be able to bind to this address.

For more information, read the the snmp.conf man page:

clientaddr [<transport-specifier>:]<transport-address>
              specifies the source address to be used by command-line applica‐
              tions when sending SNMP requests. See snmpcmd(1) for more infor‐
              mation about the format of addresses.
              This value is also used by snmpd when generating notifications.

Monitor Fans, Power Supplies, and Transformers

An SNMP agent (snmpd) waits for incoming SNMP requests and responds to them. If no requests are received, an agent does not initiate any actions. However, various commands can configure snmpd to send traps based on preconfigured settings (load, file, proc, disk, or swap commands), or customized monitor commands.

From the snmpd.conf man page, the monitor command is defined this way:

monitor [OPTIONS] NAME EXPRESSION

              defines  a  MIB  object to monitor.  If the EXPRESSION condition holds then 
              this will trigger the corresponding event, and either send a notification or
              apply a SET assignment (or both).  Note that the event will only be triggered once,
              when the expression first matches.  This monitor entry will not fire again until the
              monitored condition first becomes false, and then matches again.  NAME is an administrative
              name for this expression, and is used for indexing the mteTriggerTable (and related tables).
              Note also that such monitors use an internal SNMPv3 request to retrieve the values
              being monitored (even  if  normal  agent  queries  typically  use SNMPv1 or SNMPv2c).
              See the iquerySecName token described above.

       EXPRESSION
              There are three types of monitor expression supported by the Event MIB - existence, boolean and threshold tests.

              OID | ! OID | != OID

                     defines  an  existence(0)  monitor  test.  A bare OID specifies a present(0) test,
                     which will fire when (an instance of) the monitored OID is created.  An expression
                     of the form ! OID specifies an absent(1) test, which will fire when the monitored
                     OID is delected.  An expression of the form != OID specifies a changed(2) test,
                     which will fire whenever the monitored value(s) change.  Note that there must be
                     whitespace before the OID token.

              OID OP VALUE

                     defines a boolean(1) monitor test.  OP should be one of the defined comparison operators
                     (!=, ==, <, <=, >, >=) and VALUE should be an integer value to compare against.  Note that
                     there must be whitespace around the OP token.  A comparison such as OID !=0 will not be
                     handled correctly.

              OID MIN MAX [DMIN DMAX]

                     defines a threshold(2) monitor test.  MIN and MAX are integer values, specifying
                     lower and upper thresholds.  If the value of the monitored OID falls below the lower
                     threshold (MIN) or rises above the upper threshold (MAX), then the monitor entry will
                     trigger the corresponding event.

                     Note that the rising threshold event will only be re-armed when the monitored value
                     falls below the lower threshold (MIN).  Similarly, the falling threshold event will
                     be re-armed by the upper threshold (MAX).

                     The optional parameters DMIN and DMAX configure a pair of similar threshold tests,
                     but working with the delta differences between successive sample values.

       OPTIONS

              There are various options to control the behaviour of the monitored expression.  These include:
              -D     indicates that the expression should be evaluated using delta differences between sample
                     values (rather than the values themselves).
              -d OID  or  -di OID
                     specifies a discontinuity marker for validating delta differences.  A -di object instance
                     will be used exactly as given.  A -d object will have the instance subidentifiers from
                     the corresponding (wildcarded) expression object appended.  If the -I flag is specified,
                     then there is no difference between these two options. This option also implies -D.
              -e EVENT
                     specifies the event to be invoked when this monitor entry is triggered.  If this option
                     is not given, the monitor entry will generate one of the standard notifications defined
                     in the DISMAN-EVENT-MIB.
              -I     indicates that the monitored expression should be applied to the specified OID as a
                     single instance.  By default, the OID will be treated as a wildcarded object, and the
                     monitor expanded to cover all matching instances.
              -i OID or -o OID
                     define additional varbinds to be added to the notification payload when this monitor
                     trigger fires.  For a wildcarded expression, the suffix of the matched instance will be
                     added to any OIDs specified using -o, while OIDs specified using -i will be treated
                     as exact instances.  If the -I flag is specified,  then  there  is  no difference between
                     these two options.
                     See strictDisman for details of the ordering of notification payloads.
              -r FREQUENCY
                     monitors the given expression every FREQUENCY, where FREQUENCY is in seconds or optionally
                     suffixed by one of s (for seconds), m (for minutes), h (for hours), d (for days),
                     or w (for weeks).  By default, the expression will be evaluated every 600s (10 minutes).
              -S     indicates that the monitor expression should not be evaluated when the agent first starts up.
                     The first evaluation will be done once the first repeat interval has expired.
              -s     indicates that the monitor expression should be evaluated when the agent first starts up.
                     This is the default behaviour.
                     Note:  Notifications triggered by this initial evaluation will be sent before the coldStart trap.
              -u SECNAME
                     specifies a security name to use for scanning the local host, instead of the default
                     iquerySecName.  Once again, this user must be explicitly created and given suitable access rights.

You can configure snmpd to monitor the operational status of an Entity MIB or Entity-Sensor MIB. You can determine the operational status, given as a value of ok(1), unavailable(2) or nonoperational(3), by adding the following example configuration to /etc/snmp/snmpd.conf and adjusting the values:

  • Using the entPhySensorOperStatus integer:

    # without installing extra MIBS we can check the check Fan1 status
    # if the Fan1 index is 100011001, monitor this specific OID (-I) every 10 seconds (-r), and defines additional information to be included in the trap (-o).
    monitor -I -r 10  -o 1.3.6.1.2.1.47.1.1.1.1.7.100011001 "Fan1 Not OK"  1.3.6.1.2.1.99.1.1.1.5.100011001 > 1
    # Any Entity Status non OK (greater than 1)
    monitor  -r 10  -o 1.3.6.1.2.1.47.1.1.1.1.7  "Sensor Status Failure"  1.3.6.1.2.1.99.1.1.1.5 > 1
  • Using the OID name:

    # for a specific fan called Fan1 with an index 100011001
    monitor -I -r 10  -o entPhysicalName.100011001 "Fan1 Not OK"  entPhySensorOperStatus.100011001 > 1
    # for any Entity Status not OK ( greater than 1)
    monitor  -r 10  -o entPhysicalName  "Sensor Status Failure"  entPhySensorOperStatus > 1

    You can use the OID name if the snmp-mibs-downloader package is installed.

    The entPhySensorOperStatus integer can be found by walking the entPhysicalName table.

  • To get all sensor information, run snmpwalk on the entPhysicalName table. For example:

    cumulus@leaf01:~$ snmpwalk -v 2c -cpublic localhost .1.3.6.1.2.1.47.1.1.1.1.7
    iso.3.6.1.2.1.47.1.1.1.1.7.100000001 = STRING: "PSU1Temp1"
    iso.3.6.1.2.1.47.1.1.1.1.7.100000002 = STRING: "PSU2Temp1"
    iso.3.6.1.2.1.47.1.1.1.1.7.100000003 = STRING: "Temp1"
    iso.3.6.1.2.1.47.1.1.1.1.7.100000004 = STRING: "Temp2"
    iso.3.6.1.2.1.47.1.1.1.1.7.100000005 = STRING: "Temp3"
    iso.3.6.1.2.1.47.1.1.1.1.7.100000006 = STRING: "Temp4"
    iso.3.6.1.2.1.47.1.1.1.1.7.100000007 = STRING: "Temp5"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011001 = STRING: "Fan1"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011002 = STRING: "Fan2"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011003 = STRING: "Fan3"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011004 = STRING: "Fan4"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011005 = STRING: "Fan5"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011006 = STRING: "Fan6"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011007 = STRING: "PSU1Fan1"
    iso.3.6.1.2.1.47.1.1.1.1.7.100011008 = STRING: "PSU2Fan1"
    iso.3.6.1.2.1.47.1.1.1.1.7.110000001 = STRING: "PSU1"
    iso.3.6.1.2.1.47.1.1.1.1.7.110000002 = STRING: "PSU2"

Enable MIB to OID Translation

MIB names can be used instead of OIDs, by installing the snmp-mibs-downloader, to download SNMP MIBs to the switch prior to enabling traps. This greatly improves the readability of the snmpd.conf file.

  1. Open /etc/apt/sources.list in a text editor.

  2. Add the non-free repository, then save the file:

    cumulus@switch:~$ sudo deb http://ftp.us.debian.org/debian/ jessie main non-free
  3. Update the switch:

    cumulus@switch:~$ sudo -E apt-get update
  4. Install the snmp-mibs-downloader:

    cumulus@switch:~$ sudo -E apt-get install snmp-mibs-downloader
  5. Open the /etc/snmp/snmp.conf file to verify that the mibs : line is commented out:

    #
    # As the snmp packages come without MIB files due to license reasons, loading
    # of MIBs is disabled by default. If you added the MIBs you can reenable
    # loading them by commenting out the following line.
    #mibs :
  6. Open the /etc/default/snmpd file to verify that the export MIBS= line is commented out:

    # This file controls the activity of snmpd and snmptrapd
    
    # Don't load any MIBs by default.
    # You might comment this lines once you have the MIBs Downloaded.
    #export MIBS=
  7. After you confirm the configuration, remove or comment out the non-free repository in /etc/apt/sources.list.

    #deb http://ftp.us.debian.org/debian/ jessie main non-free

Configure Link Up/Down Notifications

The linkUpDownNotifications directive is used to configure link up/down notifications when the operational status of the link changes.

linkUpDownNotifications yes

The default frequency for checking link up/down is 60 seconds. You can change the default frequency using the monitor directive directly instead of the linkUpDownNotifications directive. See man snmpd.conf for details.

Configure Temperature Notifications

Temperature sensor information for each available sensor is maintained in the the lmSensors MIB. Each platform can contain a different number of temperature sensors. The example below generates a trap event when any temperature sensor exceeds a threshold of 68 degrees (centigrade). It monitors each lmTempSensorsValue. When the threshold value is checked and exceeds the lmTempSensorsValue, a trap is generated. The -o lmTempSenesorsDevice option is used to instruct SNMP to also include the lmTempSensorsDevice MIB in the generated trap. The default frequency for the monitor directive is 600 seconds. You can change the default frequency with the -r option.:

monitor lmTemSensor -o lmTempSensorsDevice lmTempSensorsValue > 68000

To monitor the sensors individually, first use the sensors command to determine which sensors are available to be monitored on the platform.

cumulus@switch:~$ sudo sensors
  
CY8C3245-i2c-4-2e
Adapter: i2c-0-mux (chan_id 2)
fan5: 7006 RPM (min = 2500 RPM, max = 23000 RPM)
fan6: 6955 RPM (min = 2500 RPM, max = 23000 RPM)
fan7: 6799 RPM (min = 2500 RPM, max = 23000 RPM)
fan8: 6750 RPM (min = 2500 RPM, max = 23000 RPM)
temp1: +34.0 C (high = +68.0 C)
temp2: +28.0 C (high = +68.0 C)
temp3: +33.0 C (high = +68.0 C)
temp4: +31.0 C (high = +68.0 C)
temp5: +23.0 C (high = +68.0 C)

Configure a monitor command for the specific sensor using the -I option. The -I option indicates that the monitored expression is applied to a single instance. In this example, there are five temperature sensors available. Use the following directive to monitor only temperature sensor 3 at 5 minute intervals.

monitor -I -r 300 lmTemSensor3 -o lmTempSensorsDevice.3 lmTempSensorsValue.3 > 68000

Configure Free Memory Notifications

You can monitor free memory using the following directives. The example below generates a trap when free memory drops below 1,000,000KB. The free memory trap also includes the amount of total real memory:

monitor MemFreeTotal -o memTotalReal memTotalFree <  1000000

Configure Processor Load Notifications

To monitor CPU load for 1, 5, or 15 minute intervals, use the load directive with the monitor directive. The following example generates a trap when the 1 minute interval reaches 12%, the 5 minute interval reaches 10%, or the 15 minute interval reaches 5%.

load 12 10 5

Configure Disk Utilization Notifications

To monitor disk utilization for all disks, use the includeAllDisks directive together with the monitor directive. The example code below generates a trap when a disk is 99% full:

includeAllDisks 1%
monitor -r 60 -o dskPath -o DiskErrMsg "dskTable" diskErrorFlag !=0

Configure Authentication Notifications

To generate authentication failure traps, use the authtrapenable directive:

authtrapenable 1

snmptrapd.conf

Use the Net-SNMP trap daemon to receive SNMP traps. The /etc/snmp/snmptrapd.conf file is used to configure how incoming traps are processed. Starting with Net-SNMP release 5.3, you must specify who is authorized to send traps and informs to the notification receiver (and what types of processing these are allowed to trigger). You can specify three processing types:

  • log logs the details of the notification in a specified file to standard output (or stderr), or through syslog (or similar).
  • execute passes the details of the trap to a specified handler program, including embedded Perl.
  • net forwards the trap to another notification receiver.

Typically, this configuration is log,execute,net to cover any style of processing for a particular category of notification. But it is possible (even desirable) to limit certain notification sources to selected processing only.

authCommunity TYPES COMMUNITY [SOURCE [OID | -v VIEW ]] authorizes traps and SNMPv2c INFORM requests with the specified community to trigger the types of processing listed. By default, this allows any notification using this community to be processed. You can use the SOURCE field to specify that the configuration only applies to notifications received from particular sources. For more information about specific configuration options within the file, look at the snmpd.conf(5) man page with the following command:

cumulus@switch:~$ man 5 snmptrapd.conf
###############################################################################
#
# EXAMPLE-trap.conf:
#   An example configuration file for configuring the Net-SNMP snmptrapd agent.
#
###############################################################################
#
# This file is intended to only be an example.  If, however, you want
# to use it, it should be placed in /etc/snmp/snmptrapd.conf.
# When the snmptrapd agent starts up, this is where it will look for it.
#
# All lines beginning with a '#' are comments and are intended for you
# to read.  All other lines are configuration commands for the agent.

#
# PLEASE: read the snmptrapd.conf(5) manual page as well!
#
# this is the default (port 162) and defines the listening
# protocol and address  (e.g.  udp:10.10.10.10)
snmpTrapdAddr localhost
# 
# defines the actions and the community string 
authCommunity log,execute,net public

Supported MIBs

Below are the MIBs supported by Cumulus Linux, as well as suggested uses for them. The overall Cumulus Linux MIB is defined in the /usr/share/snmp/mibs/Cumulus-Snmp-MIB.txt file.

MIB NameSuggested Uses

BGP4-MIB,

OSPFv2-MIB,

OSPFv3-MIB,

RIPv2-MIB

You can enable FRRouting SNMP support to provide support for OSPF-MIB (RFC-1850), OSPFV3-MIB (RFC-5643), and BGP4-MIB (RFC-1657). See the FRRouting section above.

CUMULUS-COUNTERS-MIB

Discard counters: Cumulus Linux also includes its own counters MIB, defined in /usr/share/snmp/mibs/Cumulus-Counters-MIB.txt. It has the OID .1.3.6.1.4.1.40310.2

CUMULUS-POE-MIB

The Cumulus Networks custom Power over Ethernet PoE MIB defined in the /usr/share/snmp/mibs/Cumulus-POE-MIB.txt file. For devices that provide PoE, this provides users with the system wide power information in poeSystemValues as well as per interface PoeObjectsEntry values for the poeObjectsTable. Most of this information comes from the poectl command. To enable this MIB, uncomment the following line in /etc/snmp/snmpd.conf:

#pass_persist .1.3.6.1.4.1.40310.3 /usr/share/snmp/cl_poe_pp.py
CUMULUS-RESOURCE-QUERY-MIB

Cumulus Linux includes its own resource utilization MIB, which is similar to using cl-resource-query. This MIB monitors layer 3 entries by host, route, nexthops, ECMP groups, and layer 2 MAC/BDPU entries.The MIB is defined in /usr/share/snmp/mibs/Cumulus-Resource-Query-MIB.txt and has the OID .1.3.6.1.4.1.40310.1.

CUMULUS-SNMP-MIB

SNMP counters. For information on exposing CPU and memory information with SNMP, see this knowledge base article.
DISMAN-EVENT-MIBTrap monitoring
ENTITY-MIBFrom RFC 4133, the temperature sensors, fan sensors, power sensors, and ports are covered.
ENTITY-SENSOR-MIBPhysical sensor information (temperature, fan, and power supply) from RFC 3433.
HOST-RESOURCES-MIB

Users, storage, interfaces, process info, run parameters

IEEE8021-BRIDGE-MIB

IEEE8021-Q-BRIDGE-MIB

The dot1dBasePortEntry and dot1dBasePortIfIndex tables in the BRIDGE-MIB and dot1qBase, dot1qFdbEntry, dot1qTpFdbEntry, dot1qTpFdbStatus, and dot1qVlanStaticName tables in the Q-BRIDGE-MIB tables. You must uncomment the bridge_pp.py pass_persist script in /etc/snmp/snmpd.conf.
IEEE8023-LAG-MIB

Implementation of the IEEE 8023-LAG-MIB includes the dot3adAggTable and dot3adAggPortListTable tables. To enable this, edit /etc/snmp/snmpd.conf and uncomment or add the following lines:

view systemonly included .1.2.840.10006.300.43
pass_persist .1.2.840.10006.300.43 /usr/share/snmp/ieee8023_lag_pp.py

IF-MIB

Interface description, type, MTU, speed, MAC, admin, operation status, counters

The IF-MIB cache is disabled by default. The non-caching code path in the IF-MIB treats 64-bit counters like 32-bit counters (a 64-bit counter rolls over after the value increments to a value that extends beyond 32 bits). To enable the counter to reflect traffic statistics using 64-bit counters, remove the -y option from the SNMPDOPTS line in the /etc/default/snmpd file. The example below first shows the original line, commented out, then the modified line without the -y option:

cumulus@switch:~$ cat /etc/default/snmpd
# SNMPDOPTS='-y -LS 0-4 d -Lf /dev/null -u snmp -g snmp -I -smux -p /run/snmpd.pid'
SNMPDOPTS='-LS 0-4 d -Lf /dev/null -u snmp -g snmp -I -smux -p /run/snmpd.pid'
IP-FORWARD-MIB  IP routing table

IP-MIB (includes ICMP)

IPv4, IPv4 addresses, counters, netmasks
IPv6-MIBIPv6 counters
LLDP-MIBLayer 2 neighbor information from lldpd (you need to enable the SNMP subagent in LLDP). You need to start lldpd with the -x option to enable connectivity to snmpd (AgentX).

LM-SENSORS MIB

Fan speed, temperature sensor values, voltages. This is deprecated since the ENTITY-SENSOR MIB has been added.
NET-SNMP-AGENT-MIBAgent timers, user, group config

NET-SNMP-EXTEND-MIB

See this knowledge base article on extending NET-SNMP in Cumulus Linux to include data from power supplies, fans, and temperature sensors.

NET-SNMP-VACM-MIB

Agent timers, user, group config

NOTIFICATION-LOG-MIB

Local logging

SNMP-FRAMEWORK-MIB

Users, access

SNMP-MPD-MIB

Users, access
SNMP-TARGET-MIB 

SNMP-USER-BASED-SM-MIB

Users, access

SNMP-VIEW-BASED-ACM-MIB

Users, access

TCP-MIB

TCP-related information
UCD-SNMP-MIBSystem memory, load, CPU, disk IO

UDP-MIB

UDP-related information

The ENTITY MIB does not show the chassis information in Cumulus Linux.

Pass Persist Scripts

The pass persist scripts in Cumulus Linux use the pass_persist extension to Net-SNMP. The scripts are stored in /usr/share/snmp and include:

  • bgp4_pp.py
  • bridge_pp.py
  • cl_drop_cntrs_pp.py
  • cl_poe_pp.py
  • entity_pp.py
  • entity_sensor_pp.py
  • ieee8023_lag_pp.py
  • resq_pp.py
  • snmpifAlias_pp.py
  • sysDescr_pass.py

All the scripts are enabled by default in Cumulus Linux, except for:

  • bgp4_pp.py, which is now handled by FRRouting instead of Quagga, so monitoring has changed accordingly.
  • cl_poe_pp.py, which is disabled by default as only certain platforms that Cumulus Linux supports are capable of doing Power over Ethernet.

Troubleshooting

Use the following commands to troubleshoot potential SNMP issues:

cumulus@switch:~$ net show snmp-server status                               
Simple Network Management Protocol (SNMP) Daemon.
---------------------------------  ------------------------------------------------------------------------------------
Current Status                     failed (failed)
Reload Status                      enabled
Listening IP Addresses             localhost 9.9.9.9
Main snmpd PID                     0
Version 1 and 2c Community String  Configured
Version 3 Usernames                Not Configured
Last Logs (with Errors)            -- Logs begin at Thu 2017-08-03 16:23:05 UTC, end at Fri 2017-08-04 18:17:24 UTC. --
                                   Aug 04 18:17:19 cel-redxp-01 snmpd[8389]: Error opening specified endpoint "9.9.9.9"
                                   Aug 04 18:17:19 cel-redxp-01 snmpd[8389]: Server Exiting with code 1
---------------------------------  ------------------------------------------------------------------------------------
cumulus@switch:~$ net show configuration snmp-server                                                            
snmp-server                                         
  listening-address 127.0.0.1                
  readonly-community public access default
  readonly-community allpass access any
  readonly-community temp2 access 1.1.1.1
  readonly-community temp2 access 2.2.2.2
  trap-destination 1.1.1.1 community-password public version 2c
  trap-link-up check-frequency 10
  trap-snmp-auth-failures
cumulus@switch:~$ net show configuration commands
...
net add snmp-server listening-address all
net add snmp-server readonly-community allpass access any
net add snmp-server readonly-community temp2 access 1.1.1.1
net add snmp-server readonly-community temp2 access 2.2.2.2
net add snmp-server trap-destination 1.1.1.1 community-password public version 2c
net add snmp-server trap-link-up check-frequency 10
net add snmp-server trap-snmp-auth-failures
...