Monitoring tools often use SNMP to query another system's information and status. For that to work on a Red Hat Enterprise Linux system, that system will have to have SNMP configured. And to allow a remote (monitoring) system to query SNMP information of a Red Hat Enterprise Linux system, one has to complete the following 3 items:

• Set up SNMP.
• Configure SNMP to use a non-public community name.
• Allow access through the firewall, if configured.

For the configuration of SNMP, you'll need to install the following 2 packages:

# yum -y install net-snmp net-snmp-utils

Next, start and enable (at boot time) the SNMP daemon to run on the system:

# systemctl enable snmpd
# systemctl start snmpd

Now you can test if you can query SNMP infomation -locally- on the system, by using the snmpwalk command:

# snmpwalk -v2c -c public localhost | head -5

The community string used above ("public") is a well-known SNMP community string, and this can be (and probably "is") utilized by hackers or other unfriendly people to obtain information about the system remotely, and as such, it's best practice to change the public community name into something a littlebit different, preferably something that can't be guessed very easily. For the sake of this tutorial, we'll change it to "kermit".

Basically, you'll have to update this line in /etc/snmp/snmpd.conf from "public" to "kermit":

Before:

com2sec notConfigUser  default       public

After:

com2sec notConfigUser  default       kermit

Then, restart the SNMP daemon, so it picks up the changes to configuration file /etc/snmp/snmpd.conf:

# systemctl restart snmpd

Now test again with the snmpwalk command but this time by using the "kermit" community name:

# snmpwalk -v2c -c kermit localhost

That should give you quite a bit of output. If it doesn't, you've made a mistake, and you'll have to re-trace your steps.

The final step is to allow remote access. That will be needed if a remote system is being used to monitor the server, for example by a tool like Solarwinds. By default, remote access will be blocked by the firewall daemon on the system. To allow remote access, open up UDP port 161 on the client:

# firewall-cmd --zone=public --add-port=161/udp --permanent
# firewall-cmd --reload

Now log in to a remote system and run a similar snmpwalk command, but this time, specify the hostname of the server that you're querying (instead of "localhost"). For example, if the name of the host is "myserver", run:

# snmpwalk -v2c -c kermit myserver

And that's it. You can now remotely monitor a Linux server using SNMP, and you've secured it by changing the community name.



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