Sending an SNMP Trap From the Command Line in Linux

Introduction to SNMP Traps

Simple Network Management Protocol (SNMP) is a widely used protocol for monitoring and managing network devices and systems. SNMP traps are asynchronous notifications sent by managed devices (agents) to a central network management system (NMS) when specific events or conditions occur. SNMP traps allow administrators to receive real-time alerts about critical events, such as system failures, high CPU usage, or interface status changes.

In this article, we will explore how to send an SNMP trap from the command line in Linux using the snmptrap utility.

Prerequisites

Before proceeding, make sure you have the following components installed on your Linux machine:

1. snmptrap utility: This tool comes with the Net-SNMP package, which is commonly available in most Linux distributions.

Sending an SNMP Trap

To send an SNMP trap from the command line, follow these steps:

Step 1: Configure SNMP

Before sending an SNMP trap, ensure that SNMP is properly configured on your Linux machine. You will need the SNMP community string, which acts as a simple password for SNMP communication.

To configure SNMP, open the SNMP configuration file /etc/snmp/snmpd.conf using a text editor:

sudo nano /etc/snmp/snmpd.conf

Look for the line containing the rocommunity directive. It should resemble the following:

rocommunity public default -V systemonly

The public in this example is the default SNMP community string. You can use the default value or change it to a more secure string. Save and close the file after making any changes.

Step 2: Send the SNMP Trap

Now that SNMP is configured, you can use the snmptrap utility to send an SNMP trap. The snmptrap command has the following syntax:

snmptrap -v VERSION -c COMMUNITY HOST OID TRAP_TYPE VAR_TYPE VALUE [OID TRAP_TYPE VAR_TYPE VALUE]...

• -v VERSION: Specify the SNMP version (1, 2c, or 3).
• -c COMMUNITY: Specify the SNMP community string.
• HOST: Specify the target NMS hostname or IP address.
• OID: Specify the object identifier for the trap.
• TRAP_TYPE: Specify the type of trap (e.g., “linkDown” or “linkUp”).
• VAR_TYPE: Specify the variable type (e.g., “i” for integer or “s” for string).
• VALUE: Specify the value for the corresponding OID.

Let’s send a sample SNMP trap for demonstration purposes. We’ll use SNMP version 2c and the default community string “public.”

snmptrap -v 2c -c public localhost '' 1.3.6.1.6.3.1.1.5.1 0.1.3.6.1.2.1.2.2.1.7.2 i 2

In this example, we send an SNMP trap with the following details:

• SNMP version 2c
• Community string: “public”
• Target NMS: localhost (the same machine)
• OID for the trap: 1.3.6.1.6.3.1.1.5.1 (coldStart trap)
• Interface index: 2
• Link status: 2 (down)

Verifying the Trap Reception

After sending the SNMP trap, you can verify its reception in your network management system or SNMP trap receiver. The SNMP trap receiver should be configured to listen for incoming traps on the specified host.

Customizing the SNMP Trap

In the previous example, we sent a predefined SNMP trap (coldStart) with specific values. However, SNMP traps can be much more powerful when customized to convey meaningful information about the events occurring in your network or systems.

Let’s take a look at how we can customize an SNMP trap and send more relevant information.

snmptrap -v 2c -c public localhost '' 1.3.6.1.4.1.12345.1 192.168.1.101 6 2 '' 1.3.6.1.2.1.2.2.1.7.2 i 2

In this example, we send a customized SNMP trap with the following details:

• SNMP version 2c
• Community string: “public”
• Target NMS: localhost (the same machine)
• OID for the trap: 1.3.6.1.4.1.12345.1 (a custom OID to represent our organization-specific trap)
• Source IP address: 192.168.1.101 (the IP address of the device or interface generating the trap)
• Trap type: 6 (enterpriseSpecific trap)
• Specific trap number: 2 (a specific trap number defined by the organization)

In this custom SNMP trap, we use a custom OID (1.3.6.1.4.1.12345.1) to represent a trap specific to our organization. Additionally, we include the source IP address and a specific trap number to provide more context about the event that triggered the trap.

Handling SNMP Traps in Network Management System

To effectively handle SNMP traps in your network management system (NMS), you need to configure the NMS to receive and process incoming traps. SNMP trap receivers listen for traps on a specified UDP port (usually 162) and process the information contained in the traps. The configuration process varies depending on the NMS software being used.

Most NMS solutions allow you to define trap handlers, which are scripts or actions executed when a specific trap is received. In these trap handlers, you can perform various actions, such as sending alerts, generating notifications, or logging the events.

Conclusion

Customizing and sending SNMP traps from the command line in Linux enables network administrators to provide more context-rich information about critical events and conditions occurring in their network or systems. SNMP traps play a crucial role in real-time monitoring and event management, allowing administrators to promptly respond to issues and ensure the smooth operation of their infrastructure. Understanding the SNMP trap format and utilizing the snmptrap utility empowers administrators to effectively use SNMP as part of their network management toolkit.