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Linux API modify body

Replace API request or response bodies passing through a target Linux machine for a configurable duration so you can test how callers handle unexpected payloads.

Linux API modify body 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 overwrites the request or response body with RESPONSE_BODY for matching API calls for DURATION. DATA_DIRECTION chooses whether to modify the request, response, or both. Filters and HTTPS interception controls mirror Linux API block. The fault runs through the Linux Chaos Infrastructure (LCI) systemd service installed on the target VM.

Use this fault to test how callers handle unexpected payloads: whether body validation catches malformed data, whether response parsers crash or recover, whether PII redaction works correctly, and whether monitoring detects the unexpected data 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:

  • Body validation: When RESPONSE_BODY substitutes an unexpected payload, do callers validate it cleanly or crash?

  • PII redaction: Substitute redacted strings to verify the application does not leak sensitive data through logs or downstream calls.

  • Stub responses: Inject a fixed payload to exercise downstream paths without changing the upstream service.

  • Monitoring fidelity: Do alerts on schema-validation failures and business-metric anomalies 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.

  • HTTPS material if HTTPS_ENABLED=true: Provide a CA bundle in HTTPS_ROOT_CERT_FILE and additional certificates in CUSTOM_CERTIFICATES.

  • /tmp is exec-mountable.

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 modify body to an experiment in Chaos Studio. Defaults are shown for reference.

Required parameters

Tunable
Description
Default

RESPONSE_BODY

Body string used to overwrite matching request or response payloads.

(required)

Chaos parameters

Tunable
Description
Default

DURATION

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

30s

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

DATA_DIRECTION

Direction of data to modify. Accepts request, response, or both.

both

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.

""

HEADERS_FILTERS

HTTP headers filter.

""

METHODS

Comma-separated HTTP methods to match.

""

Direction and source/destination filters

Tunable
Description
Default

SERVICE_DIRECTION

ingress or egress.

ingress

SOURCE_HOSTS

Comma-separated source host names (ingress only).

""

SOURCE_IPS

Comma-separated source IPs (ingress only).

""

DESTINATION_HOSTS

Comma-separated destination host names (egress only).

""

DESTINATION_IPS

Comma-separated destination IPs (egress only).

""

DESTINATION_PORTS

Comma-separated destination ports (egress only).

""

HTTPS interception (egress)

Tunable
Description
Default

HTTPS_ENABLED

Enable HTTPS interception.

false

HTTPS_ROOT_CERT_FILE

Path to the root CA bundle.

""

CUSTOM_CERTIFICATES

Base64-encoded custom certificates.

""

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

Fault execution in brief

Adds an iptables redirect that routes TARGET_SERVICE_PORT traffic through the local proxy on PROXY_PORT for DURATION. Matching API calls have their body (per DATA_DIRECTION) overwritten with RESPONSE_BODY before being forwarded. On exit, the iptables redirect and proxy are removed.

Expected behavior during fault execution

  • Matching API calls deliver RESPONSE_BODY in place of the real body (or in addition to the real body, depending on DATA_DIRECTION).

  • Body schema validators in dependent code fire if the new body does not match the expected schema.

  • Status code and headers are unchanged unless paired with Linux API status code or Linux API modify header.

  • After the duration ends, the iptables redirect is removed and the proxy stops; bodies pass through unchanged.

WHEN THE FAULT ENDS

The chaos pod removes the iptables redirect and stops the local proxy. The next request flows directly with the original body.

Signals to watch

Attach resilience probes to assert each layer:

  • Validation errors: Use a Prometheus probe on body-validation or schema-mismatch counters.

  • End-to-end behavior: Use an HTTP probe on a user-visible endpoint.

  • Business metrics: Use a Prometheus probe on a business-level success metric (orders accepted, payments completed).

Verify the fault execution effect

  1. Send a matching request from the VM.

    The response body should equal RESPONSE_BODY during the chaos window.

  2. Inspect iptables redirects.

  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 rule; kill the proxy with sudo lsof -i :<PROXY_PORT> followed by sudo kill <pid>.

Limitations

  • Body replacement, not patch: RESPONSE_BODY overwrites the body entirely; partial JSON patches are not supported.

  • Content-Length: The proxy updates Content-Length to match RESPONSE_BODY. Chunked encodings are handled but very large bodies may incur overhead.

  • HTTPS requires certificates for matching by path/header/method.

  • Single VM scope: Each fault run targets one VM.

  • /tmp exec required.

Troubleshooting

Linux API modify body fault did not modify any body in Harness Chaos Engineering

Verify PATH_FILTER and METHODS match your test traffic. For HTTPS endpoints, set HTTPS_ENABLED=true and provide HTTPS_ROOT_CERT_FILE. Confirm the iptables redirect is in place with sudo iptables -t nat -L -n -v.

Content-Length mismatch errors in callers

The proxy updates Content-Length to match RESPONSE_BODY. If callers still see length errors, they may be parsing chunked responses; ensure RESPONSE_BODY is valid for the target Content-Type.

Body modification persisted after the experiment ended

If the iptables redirect was not removed, inspect with sudo iptables -t nat -L -n -v and delete the rule. Kill any orphan proxy on PROXY_PORT.

Related faults

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