JMeter Socket Closed Connection Reset

Socket closed / connection reset in JMeter

Symptom

Samples fail mid-run or under higher concurrency with messages such as:

• Connection reset / Connection reset by peer
• Socket closed / SocketException
• Broken pipe
• Often shown as Non HTTP response code

Unlike pure connection refused, some samples may succeed first; failures increase with load or longer tests.

Common causes

Cause	Detail
Server or load balancer limits	Max connections, rate limits, or worker exhaustion
Idle timeouts	LB/proxy closes keep-alive connections JMeter tries to reuse
Injector ephemeral port / file descriptor exhaustion	Many short connections from one host
TLS session / middlebox interference	Intermittent handshake or reset under load
SUT crash or restart	Bursts of resets when the app dies
Aggressive timeouts	Client gives up; appears as socket errors depending on stack
Network path instability	VPN, NAT, or multi-hop firewalls

Fix (ordered)

1. Capture error rate vs active threads on the dashboard and SUT metrics (connections, 5xx, restarts).
2. Reproduce with moderate threads; note whether failures are immediate or after idle.
3. Align keep-alive behaviour with the environment (HTTP client settings / connection reuse). If a proxy closes idle sockets quickly, reduce reuse or match idle timeouts.
4. Check SUT and LB max connections and idle timeout documentation for your stack.
5. On the injector: watch open files, TIME_WAIT, and CPU; split load across engines if the client is saturated (distributed testing, Thread Calculator).
6. Distinguish from ConnectException (never connected) vs reset (peer closed an established or half-open connection).
7. Review TLS separately if resets cluster around HTTPS only (SSLHandshakeException).
8. Ensure you are not melting the SUT with zero think time and huge threads (best practices).

Related tools and topics

Resource	Use when
Throughput stuck	Resets plus flat RPS
OutOfMemoryError heap	Injector death causing drops
Coordinated Omission	Throughput below target while latency looks fine
Grafana / Backend Listener	Live error charts

Frequently asked questions

Is connection reset always a JMeter bug?

Rarely. It usually means something on the path or server closed the TCP connection. Validate with server and load-balancer metrics.

Why only under load?

Connection pools and OS limits appear only when many sockets are open. One-thread tests often hide them.

Should I disable keep-alive?

Sometimes as a diagnostic. Prefer fixing idle timeouts and server capacity; disabling keep-alive increases handshakes and can worsen injector load.

Continue Learning

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Next Practical Step

Correlate JMeter error timestamps with SUT connection counts and load-balancer idle timeout settings.

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Related Reference

• Non HTTP response code
• Best practices
• Distributed testing