What Is Agentless Monitoring and Why It Breaks Down

What Is Agentless Monitoring?

Agentless monitoring collects metrics from servers and devices without installing any software on them. Instead, it queries systems remotely using network protocols.

Common agentless methods include:

• SNMP (Simple Network Management Protocol)
• SSH with remote commands
• WMI (Windows Management Instrumentation)
• Cloud APIs (AWS CloudWatch, Azure Monitor)
• HTTP endpoints (health checks, metrics endpoints)

The monitoring server reaches out to targets, collects data, and stores it centrally.

How Agentless Monitoring Works

The Polling Model

1. Monitoring server has a list of targets
2. On a schedule (e.g., every minute), it connects to each target
3. It runs queries or commands to collect metrics
4. Results are stored and analysed

Example: SSH-Based Collection

Monitoring Server → SSH → Target Server
                    → Run: "free -m"
                    → Parse output
                    → Store memory metrics

Example: SNMP Collection

Monitoring Server → SNMP GET → Network Switch
                    → OID: 1.3.6.1.2.1.2.2.1.10 (interface bytes in)
                    → Store network metrics

When Agentless Monitoring Works Well

Network Devices

Routers, switches, and firewalls typically support SNMP but can't run custom agents. Agentless is the only option.

Legacy Systems

Old servers or specialised equipment may not support modern agents. SSH or SNMP provides visibility without changes.

Quick Audits

Need a one-time inventory of your infrastructure? Agentless discovery scans can map your environment without deploying anything.

Compliance Restrictions

Some environments have strict change control. Using existing protocols (SSH, SNMP) may be easier to approve than installing new software.

Cloud Resources

Cloud providers expose metrics via APIs. There's no server to install an agent on—agentless is the native approach.

Why Agentless Monitoring Breaks Down

1. Network Dependency

Agentless monitoring only works when the network works.

The problem: If the network is congested, firewalled, or down, you lose visibility exactly when you need it most.

With agents: Agents buffer data locally and send when connectivity returns.

2. Credential Management

Every agentless connection needs credentials.

The problem: Managing SSH keys or SNMP community strings across hundreds of servers becomes a security and operational burden.

With agents: Agents authenticate once during installation. Ongoing collection doesn't need stored credentials.

3. Firewall Complexity

Agentless requires inbound connections to monitored systems.

The problem: Opening SSH or SNMP ports across network segments creates security exposure. Cloud instances behind NAT are unreachable.

With agents: Agents make outbound connections. No inbound firewall rules needed.

4. Limited Metrics

Remote protocols can only collect what they expose.

The problem: SNMP provides basic metrics. SSH commands are slow and limited. You miss detailed application metrics.

With agents: Agents have direct access to the system—processes, files, application internals.

5. Polling Overhead

The monitoring server does all the work.

The problem: Polling 500 servers every minute means 500 connections per minute. The monitoring server becomes a bottleneck.

With agents: Each agent handles its own collection. The central server just receives data.

6. Timing and Accuracy

Polling happens on a schedule, missing events between polls.

The problem: A CPU spike that lasts 20 seconds might be missed if you poll every 60 seconds.

With agents: Agents can sample more frequently or report events as they happen.

Agentless vs Agent-Based: Quick Comparison

The Hybrid Approach

Most mature monitoring setups use both:

• Agents for servers and applications (deep visibility)
• Agentless for network devices and cloud APIs (where agents aren't possible)

This gives you comprehensive coverage without forcing a single approach.

When to Choose Each Approach

Use Agentless When:

• Monitoring network devices (routers, switches)
• Working with legacy systems that can't run agents
• Using cloud provider metrics (CloudWatch, Azure Monitor)
• Running quick, one-time audits
• Change control prevents software installation

Use Agent-Based When:

• Monitoring servers (physical or virtual)
• You need detailed system metrics
• Monitoring across network boundaries
• Reliability during network issues matters
• You're monitoring at scale (100+ systems)

Summary

Agentless monitoring is useful for specific scenarios—network devices, legacy systems, cloud APIs. It's quick to set up and doesn't require installing software.

But it breaks down for serious infrastructure monitoring because:

• Network problems blind your monitoring
• Credential and firewall management becomes complex
• Metrics are limited compared to agents
• The polling model doesn't scale well

For server monitoring, agent-based approaches provide better visibility, reliability, and scalability. Use agentless where it makes sense, but don't rely on it as your primary strategy.