In this guide I just install configure SNMP daemon on Centos 7.5 (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

Before we can start the service, I like to have SNMP a little less verbose, since 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"

When this has been done, lets open up the Iptables to allow for requests.  (change the -s if you have another range!)

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

Starting the daemon

systemctl start snmpd
systemctl enable snmpd

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)

FirewallD

If you wish to use firewalld you need to open the :

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

 

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