This page is intended to provide a troubleshooting and validation process to the SNMP configuration on the servers where NMIS is installed.

What is SNMP?

SNMP stands for Simple Network Management Protocol and consists of three key components: managed devices, agents and network management systems. The protocol is a set of standards for communicating with devices on a TCP/IP network. It can be defined as an application-level protocol designed to monitor network infrastructure and provides administrators with device-centric visibility. SNMP monitoring is useful for anyone responsible for servers and network devices such as servers, routers, hubs, switches, ups, etc.

How to troubleshoot SNMP communication issues

There are a number of reasons may not be able to communicate with a device during discovery, or communication could be lost some time later. There are things you can check to verify proper SNMP communication.

Device Troubleshooting Process

General Troubleshooting

Start with these basic checks:

• If you’re using SNMPv1 or v2: Is the device configured with the correct community string in LogicMonitor (either at the global, group or device level)? If no community string is set, LogicMonitor defaults to using public. Note: Some Linux distributions significantly restrict which metrics are exposed if the community string is set to “public”. Therefore, we recommend you set your community string to something else.  See the section below to verify that your device has the correct community string set.
• If you’re using SNMPv3: Is the device configured with the correct authpass, privpass and username (either at the global, group or device level)? See the section below to verify that your device has the correct v3 credentials set.
• Can queries from the collector device reach the monitored device? You can check this by running tcpdump on the monitored host. If the queries are not reaching the device, there may be a firewall issue.
• Is the monitored device replying to the queries from the collector?

If the queries are reaching the host, but the host is not replying, things to check are:

• The access restrictions in snmpd.conf may not allow queries from the collector, or the community string is wrong.
  • The simplest SNMPd v1/v2 configuration would be the single line: rocommunity [community]
  • Note that SNMPd must be restarted after changing the configuration file contents. (/etc/init.d/snmpd restart)
• SNMPd may only be listening on a loopback address.
  • On some distributions of Debian and Redhat, by default SNMPd only listens on 127.0.0.1. You can correct this in /etc/default/snmpd or /etc/syconfig/snmpd.options and restart SNMPd.
  • If you see this line: agentAddress  udp:127.0.0.1:161, it means the host is only listening on the loopback address for SNMP queries. Please comment that line.
• IP Access restrictions may be blocking the SNMP requests from being accepted.
  • /etc/hosts.allow may be restricting the IP addresses that SNMP will respond to (you will see syslog messages about “Connection Refused”). Ensure the collector is listed in this file for SNMP access, if the file exists.
  • IPTables rules may be preventing the reception of SNMP packets from the collector.

Lexicographic order issues:

• If you are receiving the common error message “Agent did not return variable bindings in lexicographic order”, set the snmp.ignore.lexicographic.order Collector setting to TRUE. As discussed in Editing the Collector Config Files, this setting must be updated from the Collector’s agent.conf file.

Ports/rules required by the snmpd service.

SNMP operates at the application layer of the Internet protocol suite (layer 7 of the OSI model).

The ports commonly used for SNMP are as follows:

for more references click here

snmpd daemon status validation

Procedure to validate if the snmpd daemon is correctly found on the NMIS server.

NMIS server snmp configuration

Tutorial on how to configure SNMP to monitor our server, we will focus on CentOS as it is one of the most widespread distributions for servers. Except for the installation, the rest is similar in other distributions.

configuration steps.

Snmp queries to devices

The most widely used SNMP versions are SNMP version 1 (SNMPv1) and SNMP version 2 (SNMPv2). SNMP version 3 (SNMPv3) includes important changes with respect to previous versions, especially in security issues; however, its acceptance has been very low due to some implementation problems and incompatibilities.
The snmpwalk command will be used for these queries.

Examples of command execution.