how to install SNMP service on Centos 8.2

In this guide I just install configure SNMP daemon on Centos 8.2 (latest at this writing). SNMP is used to read out the state of the machine, this includes CPU, network (-traffic, -settings,…), memory, … I can suggest LibreNMS, for the monitor. Back to setting SNMP daemon up !

Install
Install the service using our loved yum.

yum install net-snmp net-snmp-utils

Config
To the configuration; which can be found here : /etc/snmp/snmpd.conf
I however remove the original one -most of the time- and just copy/paste the one I use on all servers; Keep the original for reference, although its very verbose (IMHO)

mv /etc/snmp/snmpd.conf /etc/snmp/snmpd.conf.ori

This is the config I use and why in the comments :

# this create a  SNMPv1/SNMPv2c community named "my_servers"
# and restricts access to LAN adresses 192.168.0.0/16 (last two 0's are ranges)
rocommunity my_servers 192.168.0.0/16

# setup info
syslocation  "rack 1, room 3, Antwerpen serverrroom"
syscontact  "Svenn"

# open up
agentAddress  udp:161

# run as
agentuser  root

# dont log connection from UDP:
dontLogTCPWrappersConnects yes

# fix for larger then 2TB disks (raid!)
realStorageUnits 0

Network security

Since this is a network tool, we need to allow incoming connections through the firewall; For Centos 6-7 this is mostly IPTABLES, if you are using Centos 8 then FirewallD is the one to config;

IPTABLES

iptables -A INPUT -s 192.168.0.0/16 -p udp --dport 161 -j ACCEPT

Be sure to change the ip range!

FirewallD

firewall-cmd --permanent --add-service=snmp
firewall-cmd --reload

Note I did not add the range here (you could add a zone).

Starting the daemon

systemctl start snmpd
systemctl enable snmpd

Reduce verbose

In older versions, snmpd was really verbose. By default EVERY connection is logged in rsyslog. Considering I poll every 5 minutes, this adds up in /var/log/messages. So lets down it a bit, this can be done in /etc/sysconfig/snmpd add the following, to reduce the verbosity :

OPTIONS=”-Ls3d”

Extending snmp

Librenms can track multiple snmp extends, most relevant for me are zfs, apc ups, nfs server (my own creation), these can generally be added using a small executable script adding this in snmp.conf

extend nfs-server /etc/snmp/nfs-server.sh
extend zfs /etc/snmp/zfs-linux
extend ups-apcups /etc/snmp/ups-apcups.sh

For more info on these, I would refer to the documentation of LibreNMS.

Testing … 123
On the machine you are installing LibreNMS or any other NMS package try :

snmpwalk -c my_servers -v1 servername SNMPv2-MIB::sysDescr.0

This should return the system description. (only change -c COMMUNITY and -v1 SERVERNAME/IP)

 

Happy monitoring!

changes :

• added the option dontLogTCPWrappersConnects for less verbose setup
• added the option realStorageUnits for a bug work around on large disks (+32TB)
• tested for Centos 7.3 (08/03/2017)
• tested for Centos 7.5, added info on extend option (07/08/2018)
• On Centos 8, or 7, added instructions for firewalld
• rewritten the post to focus on Centos 8.