TCP Connection Check Reference

TCP Connection Check Reference

The TCP connection check counts active TCP connections by state and returns the worst result across three independently evaluated axes: total connections, TIME_WAIT connections, and CLOSE_WAIT connections. Each axis fires when its count is greater than or equal to the configured Warning or Critical threshold. The Established, Listening, and FinWait counts are collected and reported but are not evaluated against thresholds.

Configuration fields

Collected metrics

• Total Connections — total number of active TCP connections (listening sockets are counted separately under Listening)
• Established — connections in the ESTABLISHED state
• TIME_WAIT — connections in the TIME_WAIT state
• CLOSE_WAIT — connections in the CLOSE_WAIT state
• Listening — sockets in the LISTEN state
• FIN_WAIT — combined count of FIN_WAIT_1 and FIN_WAIT_2 connections

Platform support

Notes

• CLOSE_WAIT accumulation typically indicates a connection leak in an application — the remote side has closed the connection, but the local application never called close.
• TIME_WAIT is a normal TCP state after connection teardown. High numbers are expected on busy web servers.
• If a Warning threshold is configured higher than its matching Critical threshold for the same metric, the check silently swaps the two values before evaluation.
• If the connection counts cannot be collected, the check returns a Failure result, shown as an "Error" badge in the check detail view.
• The check runs every 5 minutes by default.

See also: Configure a TCP Connection Check