Set Up the SNMP Agent (snmpd) on CentOS and Ubuntu

SNMP (Simple Network Management Protocol) lets monitoring tools read data from a server: CPU load, memory, disk and network traffic. The program on the server is the SNMP agent (snmpd, from the net-snmp package).

Step 1: Update the system

CentOS and compatible systems:

sudo yum update -y

Ubuntu and Debian:

sudo apt update && sudo apt upgrade -y

Step 2: Install the SNMP packages

CentOS and compatible systems:

sudo yum install net-snmp net-snmp-utils -y

Ubuntu and Debian:

sudo apt install snmpd snmp -y

Step 3: Configure the agent

Back up the configuration, then edit it:

sudo cp /etc/snmp/snmpd.conf /etc/snmp/snmpd.conf.bak
sudo nano /etc/snmp/snmpd.conf

A simple, safe configuration:

# Listen on all interfaces, UDP port 161
agentaddress udp:161

# Read-only community, only from your monitoring server
rocommunity MyStrongCommunity 203.0.113.10

# System information
sysLocation  Data Center 1
sysContact   admin@example.com

• Replace MyStrongCommunity with a long random string. The default public is well known and unsafe.
• Replace 203.0.113.10 with the IP of your monitoring server. localhost limits it to the server itself.
• On Ubuntu, agentaddress may be set in /etc/default/snmpd or the config file. If it only listens on localhost, change it.

SNMP v1 and v2c send the community string in clear text. For better security use SNMP v3 with a user and encryption: run sudo net-snmp-create-v3-user -ro -A AuthPassword -a SHA -X PrivPassword -x AES monitoruser (stop snmpd first).

Step 4: Start the service

sudo systemctl enable snmpd
sudo systemctl restart snmpd
sudo systemctl status snmpd

Step 5: Open the firewall

firewalld (CentOS family):

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="203.0.113.10" port protocol="udp" port="161" accept'
sudo firewall-cmd --reload

UFW (Ubuntu):

sudo ufw allow from 203.0.113.10 to any port 161 proto udp

Step 6: Test

On the server:

snmpwalk -v2c -c MyStrongCommunity localhost system

You should see the system description and uptime. From the monitoring server, use the server IP instead of localhost.

Get one value:

snmpget -v2c -c MyStrongCommunity localhost 1.3.6.1.2.1.1.1.0

Troubleshooting

• Timeout: the firewall blocks UDP 161, or the agent listens only on localhost.
• No such object: the community string or the allowed source IP is wrong.
• Look at the logs with journalctl -u snmpd.

Quick recap

• Install net-snmp (or snmpd), set a strong community and a source IP.
• Open UDP 161 only for the monitoring server and test with snmpwalk.