Monitoring NixOS with SNMP in Nagios XI: A Step-by-Step Guide

NixOS is a distinctive Linux distribution focused on reproducibility and declarative configuration, utilizing the Nix package manager. Unlike traditional distributions, NixOS doesn’t follow conventional package management workflows, but it supports SNMP (Simple Network Management Protocol) for agentless monitoring. This guide details installing SNMP on NixOS, configuring SNMP v2c or v3, and integrating it with Nagios XI for real-time monitoring of system metrics like CPU, memory, disk, and network usage as of March 2025.

Prerequisites

Ensure:

• A NixOS system (updated with sudo nix-channel --update && sudo nixos-rebuild switch) with sudo or root access.
• A running Nagios XI instance (latest version recommended).
• Network connectivity between NixOS and Nagios XI (UDP port 161 open).
• Access to edit /etc/nixos/configuration.nix.

Step 1: Install SNMP Packages

Option 1: Imperative Installation (For Testing)

nix-env -iA nixos.net-snmp

nix-env -iA nixos.net-snmp

Verify:

snmpd --version

snmpd --version

Option 2: Declarative Installation (Recommended)

Add to /etc/nixos/configuration.nix:

environment.systemPackages = with pkgs; [ net-snmp ];

environment.systemPackages = with pkgs; [ net-snmp ];

Apply changes:

sudo nixos-rebuild switch

sudo nixos-rebuild switch

Verify:

/run/current-system/sw/bin/snmpd --version

/run/current-system/sw/bin/snmpd --version

Step 2: Configure SNMP Access

Option 1: SNMP v2c

sudo mkdir -p /etc/snmp
sudo cp /etc/snmp/snmpd.conf /etc/snmp/snmpd.bak 2>/dev/null || true
echo "rocommunity Str0ngC0mmunity <Nagios-XI-IP>" | sudo tee /etc/snmp/snmpd.conf

sudo mkdir -p /etc/snmp
sudo cp /etc/snmp/snmpd.conf /etc/snmp/snmpd.bak 2>/dev/null || true
echo "rocommunity Str0ngC0mmunity <Nagios-XI-IP>" | sudo tee /etc/snmp/snmpd.conf

Replace <Nagios-XI-IP> with your Nagios XI server IP (e.g., 10.25.5.12).

Option 2: SNMP v3 (Secure)

sudo systemctl stop snmpd.service
sudo net-snmp-create-v3-user -ro -a SHA -A Str0ng@uth3ntic@ti0n -x AES -X Str0ngPriv@cy nagios
sudo mkdir -p /etc/snmp
echo "rouser nagios authPriv" | sudo tee /etc/snmp/snmpd.conf

sudo systemctl stop snmpd.service
sudo net-snmp-create-v3-user -ro -a SHA -A Str0ng@uth3ntic@ti0n -x AES -X Str0ngPriv@cy nagios
sudo mkdir -p /etc/snmp
echo "rouser nagios authPriv" | sudo tee /etc/snmp/snmpd.conf

Step 3: Configure Firewall

Declarative Method (Recommended)

Add to /etc/nixos/configuration.nix:

networking.firewall.allowedUDPPorts = [ 161 ];

networking.firewall.allowedUDPPorts = [ 161 ];

Apply changes:

sudo nixos-rebuild switch

sudo nixos-rebuild switch

Verify:

sudo iptables -L -n | grep 161

sudo iptables -L -n | grep 161

Imperative Method (Temporary)

sudo iptables -I INPUT -p udp --dport 161 -j ACCEPT
sudo mkdir -p /etc/iptables
sudo iptables-save > /etc/iptables/rules.v4

sudo iptables -I INPUT -p udp --dport 161 -j ACCEPT
sudo mkdir -p /etc/iptables
sudo iptables-save > /etc/iptables/rules.v4

Step 4: Enable SNMP Daemon on Boot

Add to /etc/nixos/configuration.nix:

systemd.services.snmpd = {
  description = "SNMP Daemon";
  after = [ "network.target" ];
  wantedBy = [ "multi-user.target" ];
  serviceConfig = {
    ExecStart = "${pkgs.net-snmp}/bin/snmpd -c /etc/snmp/snmpd.conf";
    Restart = "on-failure";
  };
};

systemd.services.snmpd = {
  description = "SNMP Daemon";
  after = [ "network.target" ];
  wantedBy = [ "multi-user.target" ];
  serviceConfig = {
    ExecStart = "${pkgs.net-snmp}/bin/snmpd -c /etc/snmp/snmpd.conf";
    Restart = "on-failure";
  };
};

Apply changes:

sudo nixos-rebuild switch

sudo nixos-rebuild switch

Verify:

sudo systemctl status snmpd.service

sudo systemctl status snmpd.service

Step 5: Test SNMP Communication

Install SNMP tools on Nagios XI (if needed):

sudo dnf install net-snmp-utils -y

sudo dnf install net-snmp-utils -y

SNMP v2c

snmpwalk -v2c -c Str0ngC0mmunity <NixOS-IP>

snmpwalk -v2c -c Str0ngC0mmunity <NixOS-IP>

SNMP v3

snmpwalk -v3 -u nagios -l authPriv -a SHA -A Str0ng@uth3ntic@ti0n -x AES -X Str0ngPriv@cy <NixOS-IP>

snmpwalk -v3 -u nagios -l authPriv -a SHA -A Str0ng@uth3ntic@ti0n -x AES -X Str0ngPriv@cy <NixOS-IP>

Expected output:

iso.3.6.1.2.1.1.1.0 = STRING: "Linux nixos-<hostname> <version> x86_64"

iso.3.6.1.2.1.1.1.0 = STRING: "Linux nixos-<hostname> <version> x86_64"

Step 6: Add Host to Nagios XI Using SNMP Wizard

1. Log into Nagios XI
2. Navigate to Configure > Configuration Wizards
3. Select Linux SNMP
4. Configure Host:
   • IP: 192.168.0.31
   • SNMP Version: v2c or v3
   • v2c Community: Str0ngC0mmunity
   • v3 Username: nagios, Auth Protocol: SHA, Auth Password: Str0ng@uth3ntic@ti0n, Privacy Protocol: AES, Privacy Password: Str0ngPriv@cy
5. Click Next, select metrics (CPU, memory, disk, etc.)
6. Click Finish and apply
7. Verify monitoring in Monitoring > Hosts

Troubleshooting

SNMP Not Responding

sudo systemctl status snmpd
ps aux | grep snmpd
sudo systemctl restart snmpd

sudo systemctl status snmpd
ps aux | grep snmpd
sudo systemctl restart snmpd

Firewall Issue

sudo iptables -L -n
sudo nc -zu <NixOS-IP> 161

sudo iptables -L -n
sudo nc -zu <NixOS-IP> 161

Log SNMP Output

sudo /run/current-system/sw/bin/snmpd -f -Lo -c /etc/snmp/snmpd.conf

sudo /run/current-system/sw/bin/snmpd -f -Lo -c /etc/snmp/snmpd.conf

Conclusion

You’ve configured SNMP on NixOS and integrated it with Nagios XI for agentless, real-time monitoring as of March 28, 2025. NixOS’s declarative nature requires explicit setup for SNMP, but this approach ensures reliability. For long-term stability, maintain all configurations (packages, firewall, services) in /etc/nixos/configuration.nix. Explore further customization in the Nagios SNMP Guide, Net-SNMP Documentation, or NixOS Manual.