TUT:Configuring snmptrapd to receive SNMPv3 notifications

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SNMPv3 background

Before you can begin to understand how to use snmptrapd with SNMPv3 protected notifications you need to understand some basic concepts. Specifically, please read:

  • SNMPv3 Options -- Documents how to use Net-SNMP with SNMPv3 in general
  • TUT:snmptrap -- Discussing SNMP notifications and sending them using snmptrap
  • TUT:snmptrap SNMPv3 -- Discussing SNMPv3 notifications and sending them using snmptrap

SNMP INFORMs vs SNMP TRAPs Notifications

SNMP supports two types of notifications: TRAPs and INFORMs. (In SNMPv1, there was only TRAPs; SNMPv2c and SNMPv3 support INFORMs too). There is one fundamental difference between SNMP INFORMs and TRAPs:

TRAPs
Sent by an application or daemon but no response is sent or expected by the notification receiver.
INFORMs
INFORMs are nothing more than an acknowledged TRAP. I.E., when the notification receiver receives an INFORM it sends a response back that indicates "I got it". (An application may be configured to send more than one INFORM message if it failed to receive an acknowledgment)

SNMPv3 INFORMs vs SNMP TRAPs

SNMPv3 with the User-Based Security Model (USM) makes use of an EngineID identifier for the SNMPv3 application that is authoratative (meaning the one who controls the flow of information).

  • With SNMPv3 TRAPs, the authoratative engine is the engine that sends the trap
  • With SNMPv3 INFORMs, the authoratative engine is the engine that receives the trap.

SNMPv3 USM users are uniquely defined by a combination of the authoratative EngineID and the user name.

Configuring snmptrapd for receiving SNMPv3 TRAPs

Since the application sending the TRAP is authoratative, that means the user created within the snmptrapd must be tied to the EngineID sending the trap. You do this by creating a line like the following in your /var/net-snmp/snmptrapd.conf file: 

 createUser -e ENGINEID myuser SHA "my authentication pass" AES "my encryption pass"

In the above line, the following things need to be set:

ENGINEID; the EngineID of the application that is going to be sending the trap. (see below)
myuser; the USM username that is going to be sending the trap.
SHA; the authentication type (SHA or MD5, with SHA being better)
"my authentication pass"; The authentication pass-phrase to use to generate the secret authentication key. Enclose it in quotation marks if it contains spaces.
AES; the encryption type to use (AES or DES, with AES being better)
"my encryption pass"; The encryption pass-phrase to use to generate the secret encyrption key. Enclose it in quotation marks if it contains spaces. If you leave it off, it will be set to the same pass-phrase as the authentication pass-phrase.

Authorizing your user to do things with the received notifications

Now that your user has been properly created, you still need to allow snmptrapd to do things with the traps and INFORMs that get sent. EG, just because the request has been received and (cryptographically) verified that it was authentic, snmptrapd still won't do anything with the notification if it isn't allowed to.

In your /usr/local/share/snmp/snmaptrapd.conf file put: 

 authUser log,execute,net myuser

This line lets snmptrapd receive traps authenticated with the myuser passwords log, execute commands and forward them. (By default, snmptrapd only logs received notifications but it can be setup to execute commands and to forward notifications to somewhere else). The snmptrapd.conf manual page describes this configuration directive in greater detail. Please refer to it for details. for the other options that are available.

Real World Examples

A SNMPv3 TRAP

(for this, I'm making up an engineID to use: 0x8000000001020304)

In a /tmp/snmptrapd.conf file put: 

 createUser -e 0x8000000001020304 traptest SHA mypassword AES
 authuser log traptest

Then start snmptrapd pointing to that file (runs in the foreground, uses only that config file and logs to stderr): 

 snmptrapd -f -C -c /tmp/snmptrapd.conf -Le

Then run snmptrap (in another window) to send a linkup trap: 

 snmptrap -v 3 -a SHA -A mypassword -x AES -X mypassword -l authPriv -u traptest -e 0x8000000001020304 localhost 0 linkUp.0

You should see this in the output of the window running snmptrapd: 

 2007-10-10 10:19:11 localhost [UDP: [127.0.0.1]:46380]: DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (0) 0:00:00.00 SNMPv2-MIB::snmpTrapOID.0 = OID: IF-MIB::linkUp.0

Success!

A SNMPv3 INFORM

In a /tmp/snmptrapd.conf file put: 

 createUser informtest SHA mypassword AES
 authuser log informtest

Then start snmptrapd pointing to that file (runs in the foreground, uses only that config file and logs to stderr): 

 snmptrapd -f -C -c /tmp/snmptrapd.conf -Le

Then run snmptrap (in another window) to send a linkup inform (the -Ci switch makes snmptrap send an inform): 

 snmptrap -Ci -v 3 -a SHA -A mypassword -x AES -X mypassword -l authPriv -u informtest localhost 0 linkUp.0

You should see this in the output of the window running snmptrapd: 

 2007-10-10 10:19:11 localhost [UDP: [127.0.0.1]:46380]: DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (0) 0:00:00.00 SNMPv2-MIB::snmpTrapOID.0 = OID: IF-MIB::linkUp.0

Success!

Left To The Reader

  • Turn on the -d switch for both snmptrap and snmptrapd to watch how packets traverse the applications. Note that the INFORMs require more packets since the snmptrap application first has to probe the snmptrapd daemon for it's engineID, then send the inform and get the response that gets sent back. However, INFORMs are more robust because of this acknowledgment.

Tutorial Sections

About the SNMP Protocol

These tutorial links talk about SNMP generically and how the protocol itself works. They are good introductory reading material and the concepts are important to understand before diving into the later tutorials about Net-SNMP itself.

Net-SNMP Command Line Applications

These tutorial pages discuss the command line tools provided in the Net-SNMP suite of tools. Nearly all the example commands in these tutorials works if you try it yourself, as they're all examples that talk to our online Net-SNMP test agent. Given them a shot!

Application Configuration

All of our applications support configuration to allow you to customize how they behave.

Net-SNMP Daemons

Net-SNMP comes with two long-running daemons: a SNMP agent (snmpd) for responding to management requests and a notification receiver (snmptrapd) for receiving SNMP notifications.

Coding Tutorials

Net-SNMP comes with a highly flexible and extensible API. The API allows you to create your own commands, add extensions to the agent to support your own MIBs and perform specialized processing of notifications.

Debugging SNMP Applications and Agents

All our tools and applications have extensive debugging output. These tutorials talk about how the debugging system works and how you can add your own debugging statements to you code:

Operating System Specific Tutorials

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