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Linux API block

Block API requests passing through a target Linux machine for a configurable duration by returning a configured status code, so you can test how callers handle a sudden API outage.

Linux API block is a chaos fault that starts a local proxy on the target Linux machine, redirects traffic to/from TARGET_SERVICE_PORT through the proxy on PROXY_PORT, and responds to matching API calls with STATUS_CODE for DURATION. Match filters include PATH_FILTER, HEADERS_FILTERS, METHODS, SOURCE_HOSTS/SOURCE_IPS (ingress) and DESTINATION_HOSTS/DESTINATION_IPS/DESTINATION_PORTS (egress), with SERVICE_DIRECTION choosing ingress or egress. HTTPS interception is enabled with HTTPS_ENABLED and the supplied certificate material. The fault runs through the Linux Chaos Infrastructure (LCI) systemd service installed on the target VM.

Use this fault to test how callers behave when a specific API endpoint suddenly returns errors: whether retries amplify the failure, whether circuit breakers fire, whether fallback responses kick in, and whether monitoring detects the elevated error rate within the alerting SLA.

RUN YOUR FIRST EXPERIMENT

If you have not installed the Linux Chaos Infrastructure yet, go to Linux Chaos Infrastructure to install the agent and connect the VM to the control plane.

Use cases

Run this fault when you want to answer concrete questions like:

  • Caller error handling: When PATH_FILTER returns STATUS_CODE, do callers surface a clean error or retry indefinitely?

  • Circuit breakers: Does the service-mesh or application circuit breaker open within the configured threshold?

  • Fallback paths: Do fallback code paths or cached responses kick in correctly?

  • Monitoring fidelity: Do alerts on 5xx error rate, retry counters, and end-to-end p99 fire within the alerting SLA?

Prerequisites

  • Linux Chaos Infrastructure installed: The linux-chaos-infrastructure systemd service is active on the target VM and the infrastructure is in CONNECTED state. Go to Linux Chaos Infrastructure to install it.

  • Target service port reachable: TARGET_SERVICE_PORT is the port the application listens on (ingress) or the upstream service port (egress).

  • Proxy port free: PROXY_PORT (default 20000) is not bound by another process on the target VM.

  • HTTPS material if HTTPS_ENABLED=true: Provide a CA bundle in HTTPS_ROOT_CERT_FILE and additional certificates in CUSTOM_CERTIFICATES so the proxy can decrypt egress HTTPS traffic.

  • /tmp is exec-mountable: The proxy binary executes from /tmp. Verify with findmnt -l | grep noexec | grep /tmp. If /tmp is mounted noexec, remount with sudo mount /tmp -o remount,exec.

Supported environments

The fault has been tested on the following Linux distributions. Go to Linux fault requirements to see the full compatibility matrix.

Platform
Support status

Ubuntu 16+, Debian 10+

Supported

CentOS 7+, RHEL 7+, Fedora 30+

Supported

openSUSE LEAP 15.4+ / SUSE Linux Enterprise 15+

Supported

Permissions required

This fault is classified as an Advanced Linux fault. It requires the Linux Chaos Infrastructure systemd service to run with the root user and root user group on the target VM so it can install iptables redirects and bind the proxy port. No cloud credentials are needed.

Fault tunables

Configure the following fault parameters when you add Linux API block to an experiment in Chaos Studio. Defaults are shown for reference.

Chaos parameters

Tunable
Description
Default

DURATION

Total duration of the fault. Accepts [hours]h[minutes]m[seconds]s format.

30s

STATUS_CODE

HTTP status code returned for blocked API calls.

404

TARGET_SERVICE_PORT

Port of the application (ingress) or upstream service (egress) to intercept.

80

PROXY_PORT

Port on which the local proxy listens.

20000

NETWORK_INTERFACE

Network interface used for ingress redirection.

eth0

RAMP_TIME

Wait period in seconds before and after the fault. Go to ramp time to read how it is applied.

0

Request filters (combine as needed)

Tunable
Description
Default

PATH_FILTER

API path glob to match (for example, /api/v1/orders/*). Leave empty to match all paths.

""

HEADERS_FILTERS

HTTP headers filter (comma-separated Header=value pairs).

""

METHODS

Comma-separated HTTP methods to match (for example, GET,POST). Leave empty to match all methods.

""

Direction and source/destination filters

Tunable
Description
Default

SERVICE_DIRECTION

Direction of service traffic. Accepts ingress (block requests arriving at the VM) or egress (block requests leaving the VM).

ingress

SOURCE_HOSTS

Comma-separated source host names to match (ingress only).

""

SOURCE_IPS

Comma-separated source IPs to match (ingress only).

""

DESTINATION_HOSTS

Comma-separated destination host names to match (egress only).

""

DESTINATION_IPS

Comma-separated destination IPs to match (egress only).

""

DESTINATION_PORTS

Comma-separated destination ports for egress traffic.

""

HTTPS interception (egress)

Tunable
Description
Default

HTTPS_ENABLED

Enable HTTPS interception. Required for matching HTTPS traffic by PATH_FILTER, headers, or methods.

false

HTTPS_ROOT_CERT_FILE

Path to the root CA bundle used by the proxy to terminate egress HTTPS.

""

CUSTOM_CERTIFICATES

Base64-encoded custom certificates added to the proxy trust store.

""

Tunables that apply to every fault are documented in common tunables for all faults.

Fault execution in brief

Adds an iptables redirect on NETWORK_INTERFACE that routes TARGET_SERVICE_PORT traffic (in SERVICE_DIRECTION) to the local proxy on PROXY_PORT for DURATION. Requests matching the filters return STATUS_CODE; non-matching requests are forwarded to the original destination. On exit, the iptables redirect and proxy are removed.

Expected behavior during fault execution

  • Matching API calls receive STATUS_CODE instead of the real response.

  • Non-matching calls pass through to the original destination.

  • Callers see immediate failures with the configured status code; their retry and circuit-breaker logic engages.

  • After the duration ends, the iptables redirect is removed and the proxy stops; traffic resumes its original path.

WHEN THE FAULT ENDS

The chaos pod removes the iptables redirect and stops the local proxy. Traffic resumes its original path on the next packet; clients may need to reconnect for long-lived sessions.

Signals to watch

Attach resilience probes to assert each layer:

  • Caller error rate: Use a Prometheus probe on caller 5xx counters.

  • Circuit-breaker state: Use a Prometheus probe on circuit-breaker open/close transitions.

  • End-to-end availability: Use an HTTP probe on a user-visible endpoint that exercises the blocked API.

Verify the fault execution effect

  1. Send a matching request from the VM.

    The response should report STATUS_CODE during the chaos window and the original response afterwards.

  2. Inspect iptables redirects.

    The redirect rule should be present during the chaos window and removed afterwards.

  3. Inspect Linux Chaos Infrastructure logs.

Recovery and cleanup

  • End of duration: The chaos pod removes the iptables redirect and stops the proxy when DURATION elapses.

  • Abort the experiment: Stopping the experiment from Chaos Studio also removes the redirect and stops the proxy.

  • Manual recovery: If the redirect survives an abort, inspect with sudo iptables -t nat -L -n -v and delete the rules added by the fault; kill the proxy with sudo lsof -i :<PROXY_PORT> followed by sudo kill <pid>.

  • Workload recovery: Clients that hit retries during the chaos window may continue retrying briefly; rate-limited or circuit-broken callers recover per their own policy.

Limitations

  • HTTP/1.1 and HTTP/2 over TLS: The proxy supports common HTTP semantics. HTTP/3 (QUIC) is not intercepted.

  • HTTPS requires certificates: Without HTTPS_ROOT_CERT_FILE and a trusted CA, HTTPS traffic cannot be matched on path/header/method.

  • Single VM scope: Each fault run targets one VM (the VM hosting the selected Linux Chaos Infrastructure).

  • Long-lived connections: Streaming or WebSocket connections established before the redirect may not be intercepted until the next reconnect.

  • /tmp exec required: A noexec mount on /tmp prevents the proxy from starting.

Troubleshooting

Linux API block fault did not block any request in Harness Chaos Engineering

Verify PATH_FILTER, METHODS, and HEADERS_FILTERS match your test traffic. For HTTPS endpoints, set HTTPS_ENABLED=true and provide HTTPS_ROOT_CERT_FILE. Check sudo iptables -t nat -L -n -v during the chaos window to confirm the redirect is present.

Proxy port already in use

PROXY_PORT (default 20000) conflicted with another listener. Change PROXY_PORT to a free port and re-run. Confirm availability with sudo ss -lntp | grep .

/tmp mounted noexec prevents the proxy from starting

Remount /tmp with exec permissions for the duration of the experiment: sudo mount /tmp -o remount,exec. Restore the original mount options after the experiment if required by your security policy.

Requests still blocked after the experiment ended

If the iptables redirect was not removed, inspect with sudo iptables -t nat -L -n -v and delete the rule added by the fault. Kill any orphan proxy with sudo lsof -i : followed by sudo kill .

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