From OpenNMS

Monitoring (also known as polling) is where OpenNMS periodically polls the status of nodes by ICMP ping, and services on those nodes by protocol specific pollers or basic TCP/IP port connections which perform a simple test to see if the resource is responding correctly. If not, events are generated and passed into the OpenNMS event system.

Node status monitoring

Nodes are monitored by basic ICMP ping. If a response is not received within a configurable timeout, with a configurable number of retries, then the node is considered "down", and appropriate events passed into the OpenNMS event system.

Path Outages

For any node, the IP address of an upstream device can be specified - if a node is found to be "down", this "path outage" node is checked as well. If the upstream node is also down, then the "Node Down" event parms identify it as a path outage, the Node Down notification is suppressed, and a "Path outage" event is created. Chains of devices can be configured to represent the path between the OpenNMS server and the monitored nodes. Careful configuration of this feature ensures that, for example, if a link to a remote site goes down, your NOC operators will not be flooded with numerous unnecessary "Node Down" events.

Service monitoring

Services are monitored slightly differently to nodes. Custom pollers check specific features of the protocol they're designed to monitor, and ensure that the service is responding correctly. A more generic TCP/IP poller can simply check that some process is listening on a given TCP port, which is often sufficient to test less common services without a specific custom poller. As of version 1.3.2, the folowing custom protocols can be monitored:

• Citrix
• DHCP
• DNS
• Domino IIOP
• FTP
• HTTP
• HTTPS
• IMAP
• JBoss
• JDBC
• JMX
• LDAP
• MX4J
• NRPE
• NSClient
• OMSAStorage
• POP3
• Radius
• SMTP
• SNMP
• SSH

In addition, a General Purpose poller can be used to run an arbitrary external script or program. This external function does whatever is necessary to check for service availability and outputs a 1 (for service up) or 0 (for service down).

When a service is found to be down, the node is rechecked (by ICMP ping). If the node is subsequently found to be down, then the Service Down event is suppressed, and a Node Down sent instead.