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Linux API modify header

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Linux API modify header 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 replaces header values in matching API calls with the keys/values defined in HEADERS_MAP 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 altered headers: whether authentication breaks with invalid tokens, whether content-type changes affect parsing, whether cache headers cause stale or missed cache lookups, and whether monitoring detects the resulting errors 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:

  • Authentication failures: When Authorization is replaced with a stale token, do upstream services return clean 401s?
  • Content negotiation: Replace Accept or Content-Type to drive different code paths in the dependent service.
  • Cache headers: Override Cache-Control or ETag to validate cache-validation behavior.
  • Monitoring fidelity: Do alerts on authentication failures, content-type mismatches, and downstream errors 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.
  • /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.

PlatformSupport 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 header to an experiment in Chaos Studio. Defaults are shown for reference.

Required parameters

TunableDescriptionDefault
HEADERS_MAPJSON map of header keys to override and the new values (for example, {"Authorization":"Bearer stale","Cache-Control":"no-cache"}).(required)

Chaos parameters

TunableDescriptionDefault
DURATIONTotal duration of the fault. Accepts [hours]h[minutes]m[seconds]s format.30s
TARGET_SERVICE_PORTPort of the application (ingress) or upstream service (egress) to intercept.80
PROXY_PORTPort on which the local proxy listens.20000
NETWORK_INTERFACENetwork interface used for ingress redirection.eth0
DATA_DIRECTIONDirection of data to modify. Accepts request, response, or both.both
RAMP_TIMEWait period in seconds before and after the fault. Go to ramp time to read how it is applied.0

Request filters (combine as needed)

TunableDescriptionDefault
PATH_FILTERAPI path glob to match.""
HEADERS_FILTERSHTTP headers filter (used to match traffic, distinct from HEADERS_MAP).""
METHODSComma-separated HTTP methods to match.""

Direction and source/destination filters

TunableDescriptionDefault
SERVICE_DIRECTIONingress or egress.ingress
SOURCE_HOSTSComma-separated source host names (ingress only).""
SOURCE_IPSComma-separated source IPs (ingress only).""
DESTINATION_HOSTSComma-separated destination host names (egress only).""
DESTINATION_IPSComma-separated destination IPs (egress only).""
DESTINATION_PORTSComma-separated destination ports (egress only).""

HTTPS interception (egress)

TunableDescriptionDefault
HTTPS_ENABLEDEnable HTTPS interception.false
HTTPS_ROOT_CERT_FILEPath to the root CA bundle.""
CUSTOM_CERTIFICATESBase64-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 header values replaced per HEADERS_MAP (in DATA_DIRECTION) before being forwarded. On exit, the iptables redirect and proxy are removed.

Expected behavior during fault execution​

  • Headers listed in HEADERS_MAP are replaced with the configured values on matching API calls.
  • Headers not in HEADERS_MAP pass through unchanged.
  • The status code and body are unchanged unless paired with Linux API status code or Linux API modify body.
  • After the duration ends, the iptables redirect is removed and the proxy stops; headers 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 headers.

Signals to watch​

Attach resilience probes to assert each layer:

  • Authentication failures: Use a Prometheus probe on 401/403 counters.
  • Cache hit ratio: Use a Prometheus probe on cache-hit-ratio metrics when overriding cache headers.
  • End-to-end behavior: Use an HTTP probe on a user-visible endpoint.

Verify the fault execution effect​

  1. Send a matching request from the VM.

    curl -i http://localhost:<TARGET_SERVICE_PORT><PATH_FILTER-example>

    The response headers should reflect the overrides in HEADERS_MAP during the chaos window.

  2. Inspect iptables redirects.

    sudo iptables -t nat -L -n -v | grep <PROXY_PORT>

  3. Inspect Linux Chaos Infrastructure logs.

    sudo journalctl -u linux-chaos-infrastructure -n 100 --no-pager

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 on PROXY_PORT.

Limitations​

  • Header replacement, not addition: HEADERS_MAP replaces existing headers. To add headers that do not yet exist, the proxy still injects them as the new value.
  • Authentication contracts: Replacing Authorization may break TLS-bound or signed requests if the upstream verifies the signature against original headers.
  • HTTPS requires certificates for matching and rewriting HTTPS traffic.
  • Single VM scope: Each fault run targets one VM.
  • /tmp exec required.

Troubleshooting​

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

Verify HEADERS_MAP is valid JSON and PATH_FILTER/METHODS match your test traffic. For HTTPS endpoints, set HTTPS_ENABLED=true and provide HTTPS_ROOT_CERT_FILE.

HEADERS_MAP parsing error

HEADERS_MAP must be a single-line valid JSON object such as {"Header-Name":"value"}. Validate with jq before running the fault.

Header overrides 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.