Practical guide

SNMP, MIBs, OIDs, and Net-SNMP

How the identifiers in this browser relate to devices, and how to query them using the same MIB directory layout used by Observium.

The pieces

SNMP: The protocol used by a manager such as Observium to read data from, and receive notifications from, an agent on a device.
OID: A numeric identifier such as .1.3.6.1.2.1.2.2.1.8. An OID identifies a definition or a particular indexed instance.
MIB: A definition file that gives OIDs names, types, descriptions, table structure, and enumerated values. A MIB does not contain live device data.
Net-SNMP: The command-line tools Observium invokes to communicate with SNMP agents and translate named MIB objects.

Observium MIB paths

A standard Observium installation stores MIBs below /opt/observium/mibs. RFC and Net-SNMP definitions are normally in:

/opt/observium/mibs/rfc
/opt/observium/mibs/net-snmp

Vendor definitions use directories such as /opt/observium/mibs/cisco. The generated commands put the directory selected in this browser first. This matters when a vendor and an RFC directory contain different modules with the same name.

Net-SNMP’s -M option supplies the ordered directory list; -m selects the module to load. If Observium is installed somewhere other than /opt/observium, replace that prefix with the configured installation directory.

Which command should I use?

Command	Use it for
`snmptranslate`	Translate a named object to its numeric OID or inspect its MIB definition. It does not contact a device.
`snmpget`	Read one complete instance, such as a scalar ending in `.0` or a table column followed by its indexes.
`snmpbulkwalk`	Read the instances beneath a table, column, row, or subtree efficiently with SNMP v2c or v3.
`snmpwalk`	Walk a subtree without GETBULK, chiefly for SNMP v1 or devices whose agents have broken bulk-walk support.

Examples

Translate an object

/usr/bin/snmptranslate -Pud -Ir -On -m 'IF-MIB' -M '/opt/observium/mibs/rfc:/opt/observium/mibs/net-snmp' 'IF-MIB::ifOperStatus'

Read a scalar instance

/usr/bin/snmpget -v2c -c '<community>' -Pud -Ir -OQUv -m 'SNMPv2-MIB' -M '/opt/observium/mibs/rfc:/opt/observium/mibs/net-snmp' 'udp:<hostname>:161' 'SNMPv2-MIB::sysUpTime.0'

Walk a table

/usr/bin/snmpbulkwalk -v2c -c '<community>' -Pud -Ir -OQUs -m 'IF-MIB' -M '/opt/observium/mibs/rfc:/opt/observium/mibs/net-snmp' 'udp:<hostname>:161' 'IF-MIB::ifTable'

Replace the angle-bracket placeholders before running a device query. The examples use SNMP v2c for brevity; use your device’s configured version, transport, credentials, timeout, and retry settings.

Scalars and table indexes

A scalar definition is not itself an instance. Its single value uses the .0 suffix: SNMPv2-MIB::sysUpTime.0.

A table column can have many instances. Append every index in the order declared by its row—for example IF-MIB::ifOperStatus.<ifIndex>—or walk the column first to discover available indexes.

Notifications are different

A notification or trap is sent by an agent to a receiver; it is not a stored value that can be fetched with snmpget or enumerated with snmpwalk.

Use snmptranslate -Td to inspect its definition and varbind list. Receiving notifications requires a configured receiver such as snmptrapd; this differs from Observium’s normal polling of device OIDs.

SNMP versions and security

SNMPv1 is legacy and cannot use GETBULK.
SNMPv2c is widely supported, but its community is transmitted without encryption.
SNMPv3 can authenticate and encrypt requests and should be preferred where devices support it.
Treat community strings, SNMPv3 authentication data, and privacy keys as secrets. Do not paste real credentials into public URLs or support tickets.
Restrict SNMP access with device ACLs and firewalls; do not expose UDP port 161 broadly to the Internet.

Common errors

`Cannot find module`	The required MIB file or one of its imports is absent from the directories passed with `-M`.
`Unknown Object Identifier`	The module was not loaded, the wrong variant won directory precedence, or the object name is not declared by that module.
`No Such Object`	The device agent does not implement that definition in the requested context.
`No Such Instance`	The scalar lacks `.0`, a table index is incomplete, or that particular row is absent.
Timeout	Check reachability, UDP port 161, ACLs, credentials, SNMP version, context, timeout, and retry settings.