Net-SNMP version 3 and OpenNMS

September 22, 2017

To monitor your systems you rely heavily on SNMP, it gives out of the box a lot of possibilities getting important performance and status information.

The main topic security is often not considered. SNMP version 1 and 2c transmit everything in plain text over the wire. There is also no user, password authentication method, just a shared community string which gives access to the information. To address these problems SNMP v3 was introduced.

The Linux Net-SNMP agent supports SNMP v3 and OpenNMS does as well, so nothing prevents us to use encryption and user authentication.

WARNING: I assume Net-SNMP uses SHA-1 which is secure anymore. As far I know today, there is no implementation for Net-SNMP available which supports SHA-2 with a 256-bit hash.

Nevertheless here is the way to configure SNMP v3. It is still better than sending everything over the wire in plain text. In critical environments, I would definitely consider adding mechanisms to isolate and protect the management network from the rest of the world on network layers to reduce the attack vector.

Make your Net-SNMP configuration modular

Today, people running configuration management tools rolling out configurations to a lot of systems. Net-SNMP gives you the possibility to use an include drop-in folder to extend the default configuration, which is very handy to include device dependent configuration snippets.

All you have to do is to add the following line in your snmpd.conf

includeDir /etc/snmp/conf.d

All files ending with .conf will now be added to your Net-SNMP configuration. This makes it using configuration management tools to add device dependent disk, process or log monitoring directives without mangling one large snmpd.conf with variables.

How to configure Net-SNMP with SNMP v3

The first step, create a user with password and tell the agent what methods for encryption and signature should be used with:

createUser monitor SHA 0p3nnm5423 AES opennmsopennms rouser monitor priv .1.3.6.1.2.1

The command creates a user named monitor and uses SHA as Message Authentication Code. For encryption you have the choice between DES and AES , I would recommend the newer AES encryption method. I can recommend using something like apg to create better passwords.

Once you added the configuration you have to restart the Net-SNMP daemon and you can test it with the following command:

snmpget -v 3 -u monitor -l authPriv -a SHA -A 0p3nnm5423 -x AES -X opennmsopennms localhost .1.3.6.1.2.1.1.6.0

You should be able to get the system location. Next, you can configure OpenNMS to use SNMP v3 for your IP address or a whole range in the Web UI by going to "Admin -> Configure SNMP Community by IP".

That's it – happy monitoring.

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