Due to my domain name and site title I get a fair few visitors who get directed to this site looking for information about community strings, so I thought its about time to write some information on the topic SNMP community strings.

Simple Network Management Protocol (SNMP) is a set of standards for managing network devices, network devices are monitored by a SNMP manager which connects to an SNMP agent on network devices. Data which the SNMP agent can access is stored in a database called Management Information Base (MIB), MIBs are sometimes called MIB trees and small pieces of information (variables) are stored on MIB leaves.

A community string is a password for accessing the SNMP agent and separate community strings are usually used for systems which require read only or read/write access.

There are 4 version of SNMP;

SNMPv1 – Basic authentication through the use of community strings using SMIv1, the community string is sent in plain text

SNMPv2 – Does not community strings to authenticate. Mandates the use of SMIv2 and allows the use of a new message GetBulk and Inform

SNMPv2c – Uses SNMP version 1 style community strings sent in plain text but operates more similarly to to SNMPv2

SNMPv3 – Similar to SNMPv2 but improvements made for security and access control.

There aren’t that many SNMP message types and its useful to know them all, the SNMP message types are;

Get - Requests a single single variable from a MIB

GetNext - Requests the next MIB leaf in the MIB tree

GetBulk – Requests a sequential list of MIB leaves in a single request, GetBulk is commonly used to extract complex MIB structures

Set - this message changes the value of a MIB variable

Response - Sent in response to a set, get or inform type messages

Trap - This message is sent in an unsolicited fashion and does not require confirmation

Inform - Sent between SNMP managers to inform each other about MIB data