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Pod API modify header

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Pod API modify header is a Kubernetes pod-level chaos fault that overrides headers on selected API requests or responses on the target pod for a configurable duration. You provide a map of header keys to values that the proxy then injects, replaces, or removes (when the value is empty). It accepts the same rich filter set as other API faults (path, method, headers, query, source, destination) and supports HTTPS through user-supplied TLS certificates. When the fault ends, headers return to normal immediately.

Use this fault to test how a client or server behaves when a header it depends on is missing, malformed, or carries an unexpected value, scoped to one specific endpoint or upstream rather than every call on the pod.

Run your first experiment

If you have not configured the chaos infrastructure yet, go to Quickstart to install the chaos infrastructure and run an experiment end to end.

Use cases​

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

  • Auth header tampering on one path: Remove Authorization only on PATH_FILTER=/v2/admin to verify whether the admin endpoint returns a clean 401 rather than crashing.
  • Cache-control flips: Change Cache-Control to max-age=3600 on responses from a specific microservice to see whether intermediate caches store data they should not.
  • Tracing continuity: Strip X-Request-ID or traceparent only on calls to a particular upstream to expose tracing gaps in your observability pipeline.
  • Tenant-scoped header chaos: Use HEADERS_FILTERS to match a tenant header and HEADERS_MAP to tamper with another header only for that tenant.
  • Security header removal: Strip Content-Security-Policy from a specific endpoint's responses to test browser fallback behavior.

Prerequisites​

  • Kubernetes version: 1.21 or later. Go to What's supported to confirm distribution support.
  • Target pods are Running: The application pods you intend to target are in the Running state before the fault is launched.
  • Privileged pods allowed: The cluster lets you schedule privileged pods in the chaos namespace. GKE Autopilot supports this fault but requires the one-time setup in Chaos on GKE Autopilot; other locked-down distributions may need similar exemptions.
  • Container runtime access: The chaos pod can reach the container runtime socket on the target node (/run/containerd/containerd.sock, /var/run/docker.sock, or /var/run/crio/crio.sock).
  • API service on a known port: The target container serves HTTP, HTTPS, or gRPC traffic on a port you can specify with TARGET_SERVICE_PORT.
  • TLS material for HTTPS targets: When HTTPS_ENABLED=true, you provide CA, server, and (optionally) client certificate secrets so the proxy can terminate TLS and apply filters.
  • Workload selector defined: The chaos experiment knows the target workload by kind, namespace, and either names or labels.

Supported environments​

PlatformSupport status
Amazon EKSSupported
Azure AKSSupported
Google GKESupported
Red Hat OpenShiftSupported
RancherSupported
VMware TanzuSupported
Self-managed Kubernetes (CNCF-certified)Supported
GKE AutopilotSupported with Autopilot setup
EKS Fargate, ACI virtual nodesNot supported (no access to container runtime sockets)

Permissions required​

The fault runs under the chaos infrastructure's service account.

Resource (apiGroup)VerbsWhy it is needed
pods ("")get, list, create, delete, deletecollection, patch, updateDiscover target pods and run the chaos pod on the same node
pods/log ("")get, list, watchStream chaos pod logs for status and debugging
deployments, statefulsets, replicasets, daemonsets (apps)get, listResolve the target workload to the pods it owns
events ("")get, list, create, patch, updateRecord fault progress as Kubernetes events
jobs (batch)get, list, create, delete, deletecollectionRun the chaos job that drives the fault
secrets ("")get, listRead TLS certificate secrets when HTTPS_ENABLED=true

The default Harness chaos infrastructure service account already includes these permissions.

Fault tunables​

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

Chaos parameters

TunableDescriptionDefault
HEADERS_MAPJSON object of header key-value pairs to inject or replace. Set a value to "" to remove that header. Example: {"X-Auth-Token":"deadbeef","X-Trace-Id":""}.""
DATA_DIRECTIONWhich direction to modify: request (headers sent to the server), response (headers returned to the client), or both.both
TRANSACTION_PERCENTAGEPercentage of matched API calls whose headers are modified, between 0 and 100.0
TARGET_SERVICE_PORTPort the target container listens on for API traffic.80
TOTAL_CHAOS_DURATIONDuration of the fault in seconds.60

Filters (API matching)

All filters are optional. Leave empty to match everything in that dimension. Combining filters narrows the match (AND across dimensions).

TunableDescriptionDefault
PATH_FILTERURL path the fault matches. Empty matches all paths.""
METHODSComma-separated HTTP methods to match (for example GET,POST). Empty matches all methods.""
QUERY_PARAMSQuery-parameter filter. Empty matches all query strings.""
HEADERS_FILTERSHeader filter used to match incoming calls (distinct from HEADERS_MAP, which is used to inject). Empty matches all headers.""

Filters (traffic source and destination)

TunableDescriptionDefault
SERVICE_DIRECTIONWhether to filter ingress traffic (received by the pod) or egress traffic (sent from the pod).ingress
SOURCE_HOSTSHostnames of the calling client (ingress only). Empty matches any source.""
SOURCE_IPSSource IPs of the calling client (ingress only). Empty matches any source.""
DESTINATION_HOSTSDestination hostnames for the call (egress only). Empty matches any destination.""
DESTINATION_IPSDestination IPs for the call (egress only). Empty matches any destination.""
DESTINATION_PORTSComma-separated destination ports (egress only). Empty matches any port.""

TLS (for HTTPS targets)

TunableDescriptionDefault
HTTPS_ENABLEDSet to true when the target serves HTTPS so the proxy terminates TLS to apply filters.false
CA_CERTIFICATESKubernetes secret holding the Base64-encoded CA certificate (ca.key, ca.crt).""
SERVER_CERTIFICATESSecret holding the Base64-encoded server certificate (server.key, server.crt).""
CLIENT_CERTIFICATESSecret holding the Base64-encoded client certificate (client.key, client.crt) for mTLS upstreams.""

Proxy and interface

TunableDescriptionDefault
PROXY_PORTPort the chaos proxy listens on inside the container's network namespace.20000
NETWORK_INTERFACENetwork interface inside the target container's namespace. Almost always eth0 for standard CNI plugins.eth0

Targeting

TunableDescriptionDefault
TARGET_PODSComma-separated list of pod names to target. Empty selects from the workload's pods using POD_AFFECTED_PERCENTAGE.""
TARGET_CONTAINERContainer in the pod whose network namespace to enter. Empty targets the first container in the pod spec.""
NODE_LABELLabel selector to filter target pods by the node they run on. Empty disables node-based filtering.""
POD_AFFECTED_PERCENTAGEPercentage of the workload's pods to target. 0 means one pod.0
SEQUENCEWhen multiple pods are targeted, inject parallel (all at once) or serial (one after another).parallel

Runtime and helper

TunableDescriptionDefault
CONTAINER_RUNTIMEContainer runtime on the target nodes. One of containerd, docker, crio.containerd
SOCKET_PATHPath to the container runtime socket on the target node. Set to match CONTAINER_RUNTIME./run/containerd/containerd.sock
RAMP_TIMEWait period in seconds before and after the fault. Go to ramp time to read how it is applied.0

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

Remove a header

Set the header's value in HEADERS_MAP to an empty string to remove the header. For example, {"Authorization":""} removes Authorization rather than setting it to an empty value.

Configure for your container runtime​

Set CONTAINER_RUNTIME and SOCKET_PATH to match the runtime on the target node:

CONTAINER_RUNTIMESOCKET_PATH
containerd (default)/run/containerd/containerd.sock
docker/var/run/docker.sock
crio/var/run/crio/crio.sock

Fault execution in brief​

Intercepts API traffic on TARGET_SERVICE_PORT inside the container's network namespace and rewrites headers on the request, response, or both (as set by DATA_DIRECTION) for the configured percentage of calls that match the path, method, header, query, and source or destination filters, optionally terminating TLS to apply the same logic to HTTPS calls.

Expected behavior during fault execution​

  • API calls that match every configured filter have their headers rewritten according to HEADERS_MAP. Existing headers with matching keys are replaced; new headers are added; headers with an empty value in the map are removed.
  • When DATA_DIRECTION=request, the server sees the modified headers. When =response, the client sees them. When =both, both sides are modified the same way.
  • Only the percentage selected by TRANSACTION_PERCENTAGE is modified; the rest pass through unchanged. Bodies, status codes, and other headers are preserved.
  • For HTTPS targets, the proxy terminates TLS using the supplied certificates so it can read and rewrite headers.
  • Clients or servers that fail validation on missing or unexpected headers surface their own error responses (typically 400 or 401); ones that silently fall back may continue to operate, which is itself a finding.
When the fault ends

After TOTAL_CHAOS_DURATION, the proxy is torn down and headers return to their original values within a couple of seconds.

Signals to watch​

Attach resilience probes to assert each layer:

  • Client error rate: Use an HTTP probe against the calling service to detect 4xx responses driven by missing or unexpected headers.
  • Authentication and authorization metrics: Use a Prometheus probe on auth-failure counters when modifying Authorization or session headers.
  • Trace continuity: Use a command probe against your tracing system for orphan spans during the experiment window.

Verify the fault execution effect​

While the experiment is running, confirm the headers are being rewritten:

  1. Inspect headers on a matched call.

    For DATA_DIRECTION=response, run:

    kubectl run -n <namespace> tester --image=curlimages/curl --rm -it -- \
      curl -i -X <METHOD> "http://<target-pod-ip>:<TARGET_SERVICE_PORT><PATH_FILTER>"

    The response headers should reflect HEADERS_MAP: added or modified keys present with the configured values, removed keys absent.

  2. Inspect headers on the server side (when DATA_DIRECTION=request).

    Have the target application log incoming request headers (or use an echo endpoint) and confirm the modifications appear there. Status codes, body, and headers outside HEADERS_MAP should be unchanged.

Recovery and cleanup​

  • End of duration: The proxy is removed automatically and API headers return to their original values.
  • Abort the experiment: Stopping the experiment from Chaos Studio triggers the same cleanup path.
  • Failed cleanup: If automated cleanup did not complete, restart the target pod to reset its network state.

Limitations​

  • Serverless Kubernetes (EKS Fargate, ACI virtual nodes): These platforms do not expose container runtime sockets and reject the privileged access the fault needs. GKE Autopilot is supported once the one-time setup in Chaos on GKE Autopilot is in place.
  • Windows containers: This fault is supported on Linux pods only.
  • HTTPS without certificates: When HTTPS_ENABLED=true, the proxy must terminate TLS. If the supplied certificates do not chain to one the client trusts, the client refuses the connection before any filter is applied.
  • Pseudo-headers (HTTP/2 :method, :path, :status, :authority): These cannot be modified through HEADERS_MAP. Use other faults (such as Pod API status code) to control them.
  • Hop-by-hop headers: Headers like Connection and Transfer-Encoding are owned by the proxy itself and are not safe to override through HEADERS_MAP.
  • Port already bound: If PROXY_PORT collides with a port the target container is already using, the fault fails to start.

Troubleshooting​

Pod API modify header experiment stays Pending or never starts in Harness Chaos Engineering

Inspect the chaos pods in the experiment namespace with kubectl describe pod -n <chaos-namespace>. The most common causes are taints on the target node that the chaos pods do not tolerate, insufficient resources, or a PodSecurity admission policy blocking privileged pods. Add the required tolerations to the experiment or run in a namespace with privileged Pod Security level.

Headers are unchanged during pod-api-modify-header

The most common causes are: TRANSACTION_PERCENTAGE is 0 (default) so no calls are modified; DATA_DIRECTION is set to a side you are not inspecting; HEADERS_MAP is not valid JSON; filters are over-specified; HTTPS_ENABLED is false but the target serves HTTPS; or the supplied TLS certificates do not chain correctly. Re-run with TRANSACTION_PERCENTAGE=100 and a minimal one-entry HEADERS_MAP to confirm the path.

TLS handshake fails for pod-api-modify-header in Harness Chaos Engineering

Verify the secrets referenced by CA_CERTIFICATES, SERVER_CERTIFICATES, and CLIENT_CERTIFICATES exist in the chaos namespace and contain Base64-encoded key/crt pairs (ca.key/ca.crt, server.key/server.crt, client.key/client.crt). The server certificate must include the target pod's service name in its SAN list.

Headers remain modified after pod-api-modify-header ends

Automated cleanup did not complete. Restart the target pod to reset its network state. If the issue recurs, capture the chaos pod logs from the experiment namespace before the next run and share them with Harness support.