> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/ecs-container-http-modify-body.md).

# ECS container HTTP modify body

ECS container HTTP modify body is an AWS chaos fault that replaces the HTTP response body of every request served on `TARGET_SERVICE_PORT` inside a percentage of running ECS tasks (EC2 launch type) with the value of `RESPONSE_BODY` for a configurable duration. The fault interposes a transparent HTTP proxy on the container instance, scoped to the target container via ECS container metadata and dispatched via AWS Systems Manager Run Command. The status code is preserved; only the body is rewritten.

Use this fault to test how HTTP clients behave when they receive a syntactically or semantically wrong body (corrupt JSON, missing fields, unexpected content type): whether deserialization fails cleanly, whether contract tests catch the discrepancy, and whether downstream consumers degrade gracefully.

{% hint style="info" %}
**RUN YOUR FIRST EXPERIMENT**

If you have not configured the chaos infrastructure yet, go to [Quickstart](/resilience-testing/chaos-engineering/new-to-chaos-engineering/quickstart.md) to install the chaos infrastructure and run an experiment end to end.
{% endhint %}

***

### Use cases <a href="#use-cases" id="use-cases"></a>

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

* **Deserialization safety:** When the body is malformed, does the client return a clear error to its caller, or does it crash on a parse exception?
* **Schema contract tests:** Do contract or schema-validation gates in your CI detect the same break?
* **Downstream consumer behaviour:** Does the modified body break a downstream consumer that assumes specific fields, and is the failure surfaced via logs and metrics?
* **Cache poisoning risk:** Do downstream caches store the chaos body, and is that risk understood?
* **Recovery time:** When the chaos ends, do clients clear stale state and recover within SLO?

***

### Prerequisites <a href="#prerequisites" id="prerequisites"></a>

* **Kubernetes version:** 1.21 or later for the chaos infrastructure cluster. Go to [What's supported](/resilience-testing/chaos-engineering/new-to-chaos-engineering/whats-supported.md) to confirm distribution support.
* **Target ECS service or cluster:** `CLUSTER_NAME` exists in `REGION` and uses the EC2 launch type.
* **Container instances are SSM-managed.**
* **ECS container metadata enabled.**
* **AWS credentials available:** Either an AWS credentials file uploaded as a **File Secret in Harness Secret Manager** (see Authentication below) or an IAM role for service accounts (IRSA) bound to the chaos infrastructure service account.
* **IAM permissions granted:** The credentials or role include the permissions listed below.

***

### Supported environments <a href="#supported-environments" id="supported-environments"></a>

| Platform                                             | Support status                                                    |
| ---------------------------------------------------- | ----------------------------------------------------------------- |
| Amazon ECS on EC2 launch type                        | Supported                                                         |
| Amazon ECS on Fargate launch type                    | Not supported                                                     |
| HTTP/1.1 and HTTP/2 traffic on `TARGET_SERVICE_PORT` | Supported                                                         |
| HTTPS terminated inside the container                | Not supported                                                     |
| Linux container instances                            | Supported                                                         |
| Windows container instances                          | Not supported                                                     |
| AWS regions                                          | Supported in every commercial region; pass the region in `REGION` |

***

### Permissions required <a href="#permissions-required" id="permissions-required"></a>

The IAM principal that the chaos pod uses (the credentials mounted from the Harness Secret Manager file secret, the IRSA role on the chaos service account, or the role assumed via `ASSUME_ROLE_ARN`) needs the following AWS actions.

```json
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ecs:DescribeClusters",
        "ecs:DescribeServices",
        "ecs:DescribeTasks",
        "ecs:ListTasks",
        "ecs:ListContainerInstances",
        "ecs:DescribeContainerInstances"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ssm:SendCommand",
        "ssm:CancelCommand",
        "ssm:GetCommandInvocation",
        "ssm:DescribeInstanceInformation"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ec2:DescribeInstances",
        "ec2messages:AcknowledgeMessage",
        "ec2messages:DeleteMessage",
        "ec2messages:FailMessage",
        "ec2messages:GetEndpoint",
        "ec2messages:GetMessages",
        "ec2messages:SendReply"
      ],
      "Resource": "*"
    }
  ]
}
```

Go to [common policy for all AWS faults](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/security-configurations/policy-for-all-aws-faults.md) to use a single superset IAM policy across every AWS fault.

***

### Authentication <a href="#authentication" id="authentication"></a>

The fault supports three credential delivery models. Pick one based on how your chaos infrastructure is deployed.

| Method                                | When to use it                                                                 | How to configure                                                                                                                                                                                                                    |
| ------------------------------------- | ------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Harness Secret Manager file secret    | Chaos infrastructure runs outside EKS, or you want explicit static credentials | Upload the AWS credentials file as a **File Secret** in Harness Secret Manager and reference its identifier via `AWS_AUTHENTICATION_SECRET`                                                                                         |
| IAM Roles for Service Accounts (IRSA) | Chaos infrastructure runs in EKS and uses an OIDC-bound service account        | No tunable changes; the chaos pod inherits the role automatically. Go to [AWS IAM integration](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/security-configurations/aws-iam-integration.md) to set it up |
| Assume role                           | The fault needs to act in a different account or with elevated permissions     | Set `ASSUME_ROLE_ARN` to the role ARN; the chaos pod assumes the role on top of its base credentials                                                                                                                                |

When using the Harness Secret Manager method, the contents of the File Secret should be the AWS credentials file in the standard `~/.aws/credentials` format:

```ini
[default]
aws_access_key_id = REPLACE_WITH_ACCESS_KEY_ID
aws_secret_access_key = REPLACE_WITH_SECRET_ACCESS_KEY
```

Upload this file as a **File Secret** in Harness Secret Manager (Project Setup → Secrets → New File Secret), and pass the secret identifier in `AWS_AUTHENTICATION_SECRET` when configuring the fault.

Go to [AWS named profile for chaos](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/security-configurations/aws-switch-profile.md) to switch between profiles inside a single credentials file.

***

### Fault tunables <a href="#fault-tunables" id="fault-tunables"></a>

Configure the following fault parameters when you add ECS container HTTP modify body to an experiment in Chaos Studio. Defaults are shown for reference.

**Required parameters**

| Tunable        | Description                                                      | Default    |
| -------------- | ---------------------------------------------------------------- | ---------- |
| `CLUSTER_NAME` | Name of the target ECS cluster.                                  | (required) |
| `REGION`       | AWS region that hosts the ECS cluster (for example `us-east-1`). | (required) |

**Targeting parameters**

| Tunable                      | Description                                                                                                              | Default |
| ---------------------------- | ------------------------------------------------------------------------------------------------------------------------ | ------- |
| `SERVICE_NAME`               | Name of the target ECS service. When set, the fault selects `TASK_REPLICA_AFFECTED_PERC` of the service's running tasks. | `""`    |
| `TASK_REPLICA_ID`            | ID of a specific task replica to target.                                                                                 | `""`    |
| `TASK_REPLICA_AFFECTED_PERC` | Percentage of running tasks to affect when `SERVICE_NAME` is set.                                                        | `100`   |

**Chaos parameters**

| Tunable                | Description                                                                                                                                                                                                      | Default                                     |
| ---------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------- |
| `RESPONSE_BODY`        | String the proxy returns as the HTTP response body for every intercepted request.                                                                                                                                | `This is a modified response body by chaos` |
| `TARGET_SERVICE_PORT`  | TCP port the affected container serves on.                                                                                                                                                                       | `80`                                        |
| `PROXY_PORT`           | Port the chaos proxy listens on inside the container.                                                                                                                                                            | `20000`                                     |
| `NETWORK_INTERFACE`    | Network interface inside the container on which to install the redirect rules. `auto` discovers the primary interface.                                                                                           | `auto`                                      |
| `TOTAL_CHAOS_DURATION` | Duration of the fault in seconds.                                                                                                                                                                                | `60`                                        |
| `INSTALL_DEPENDENCIES` | Install the proxy and networking tooling on each container instance if missing.                                                                                                                                  | `true`                                      |
| `SEQUENCE`             | Order in which multiple tasks are affected: `parallel` installs the proxy on all selected tasks at once; `serial` does so one at a time.                                                                         | `parallel`                                  |
| `RAMP_TIME`            | Wait period in seconds before and after the fault. Go to [ramp time](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/common-tunables-for-all-faults.md#ramp-time) to read how it is applied. | `0`                                         |

**Authentication**

| Tunable                     | Description                                                                                                                       | Default |
| --------------------------- | --------------------------------------------------------------------------------------------------------------------------------- | ------- |
| `ASSUME_ROLE_ARN`           | ARN of an IAM role to assume on top of the base credentials. Leave empty to use the base credentials directly.                    | `""`    |
| `AWS_AUTHENTICATION_SECRET` | Identifier of the **File Secret in Harness Secret Manager** that contains the AWS credentials file. Not required when using IRSA. | `""`    |

Tunables that apply to every fault are documented in [common tunables for all faults](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/common-tunables-for-all-faults.md). AWS-specific shared tunables are documented in [common AWS fault tunables](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/aws-fault-tunables.md).

***

### Fault execution in brief <a href="#fault-execution-in-brief" id="fault-execution-in-brief"></a>

Resolves the running tasks for `SERVICE_NAME` (or the explicit `TASK_REPLICA_ID`), picks `TASK_REPLICA_AFFECTED_PERC` of them, and dispatches a transparent HTTP proxy via AWS Systems Manager Run Command into the affected container. New HTTP requests on `TARGET_SERVICE_PORT` are redirected through the proxy on `PROXY_PORT`, the proxy forwards the request to the application, then replaces the response body with `RESPONSE_BODY` (preserving status code and content length recalculation) before returning to the client. After `TOTAL_CHAOS_DURATION` seconds the proxy is stopped and the redirect rules are removed.

***

### Expected behavior during fault execution <a href="#expected-behavior-during-fault-execution" id="expected-behavior-during-fault-execution"></a>

* HTTP clients of the affected containers receive responses with the original status code and headers, but the body is replaced by `RESPONSE_BODY`.
* Clients that expect a specific schema (JSON, XML, Protobuf) likely fail deserialization and surface errors to their callers.
* Downstream caches that key on URL may cache the chaos body until they expire.
* Application logs on the server side are unaffected; the chaos is invisible from inside the application.

{% hint style="info" %}
**WHEN THE FAULT ENDS**

The chaos pod stops the proxy and removes the redirect rules. New requests get the real body again. Downstream caches keep the chaos body until they expire or are invalidated.
{% endhint %}

#### Signals to watch <a href="#signals-to-watch" id="signals-to-watch"></a>

Attach [resilience probes](/resilience-testing/chaos-engineering/use-chaos-engineering/probes.md) to assert each layer:

* **Response body assertion:** Use an [HTTP probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/http-probe.md) and assert the body matches the expected schema; the probe should fail during the chaos window and pass afterwards.
* **Client deserialization errors:** Use a [Prometheus probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/apm-probes.md) on client-side parse-error counters.
* **Downstream error rate:** Use a Prometheus probe on application-level error counters to confirm the chaos surfaces as visible errors.

***

### Verify the fault execution effect <a href="#verify-the-fault-execution-effect" id="verify-the-fault-execution-effect"></a>

While the experiment is running, confirm the modified body is returned:

1. **Curl the endpoint.**

   ```bash
   curl -i http://<service-endpoint>:<port>/some-endpoint
   ```

   The body should match `RESPONSE_BODY` during the chaos window; the status code is unchanged.
2. **Inspect the redirect rules on the host (via SSM).**

   ```bash
   aws ssm send-command \
     --region <region> \
     --document-name AWS-RunShellScript \
     --instance-ids <container-instance-id> \
     --parameters 'commands=["iptables -t nat -L PREROUTING -n"]'
   ```
3. **Inspect SSM command status.**

   ```bash
   aws ssm list-command-invocations --region <region> --details --filters "key=Status,value=InProgress"
   ```

***

### Recovery and cleanup <a href="#recovery-and-cleanup" id="recovery-and-cleanup"></a>

* **End of duration:** The chaos pod stops the proxy and removes the redirect rules on each host.
* **Abort the experiment:** Stopping the experiment from Chaos Studio cancels the SSM command and removes the proxy.
* **Manual recovery:** If the fault exits before cleanup, remove the redirect rules and kill the proxy process via SSM.
* **Workload recovery:** Downstream caches that stored the chaos body may need to be flushed explicitly.

***

### Limitations <a href="#limitations" id="limitations"></a>

* **EC2 launch type only.**
* **Container metadata must be enabled.**
* **SSM-managed hosts only.**
* **Linux-only.**
* **HTTPS terminated inside the container is not supported.**
* **One port per experiment:** The proxy intercepts only `TARGET_SERVICE_PORT`.
* **Cross-region targeting:** A single experiment targets one region (the value of `REGION`).
* **Downstream cache pollution:** Caches that key on URL may cache the chaos body for their TTL; account for this when planning the experiment.

***

### Troubleshooting <a href="#troubleshooting" id="troubleshooting"></a>

<details>

<summary>ECS container HTTP modify body fails with AccessDeniedException in Harness Chaos Engineering</summary>

The credentials supplied to the chaos pod do not have the required ECS or SSM permissions. Confirm the IAM policy attached to the user, role, or IRSA service account includes ecs:DescribeServices, ecs:DescribeTasks, ssm:SendCommand, and ssm:GetCommandInvocation.

</details>

<details>

<summary>Body modification not observed by client</summary>

The most common causes are: the client connects on a port other than TARGET\_SERVICE\_PORT (set the correct port); the client speaks HTTPS to the container and the proxy cannot decrypt; the request bypassed the affected tasks because the load balancer routed elsewhere; or the proxy could not install the iptables redirect rule (check NETWORK\_INTERFACE). Inspect the chaos pod logs for proxy startup errors.

</details>

<details>

<summary>Downstream cache returned the chaos body after the experiment</summary>

CDNs and reverse proxies cache responses keyed on URL. Even after the experiment ends, the chaos body remains cached until the cache entry expires. Flush the affected cache entries explicitly, or design the experiment to use cache-busting query parameters.

</details>

<details>

<summary>Redirect rules remain after the chaos window</summary>

If the cleanup SSM command failed, the iptables PREROUTING redirect rules may persist on the container instance. Send an SSM AWS-RunShellScript command to the affected container instance that flushes the iptables NAT PREROUTING chain. Be cautious if the host has other iptables rules in PREROUTING. The exact command is recorded in the chaos pod logs.

</details>

***

### Related faults <a href="#related-faults" id="related-faults"></a>

* [ECS container HTTP latency](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/ecs-container-http-latency.md): Delay the response instead of modifying the body.
* [ECS container HTTP status code](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/ecs-container-http-status-code.md): Return a chaos status code instead of modifying the body.
* [ECS container HTTP reset peer](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/ecs-container-http-reset-peer.md): Reset the connection instead of returning a chaos body.
* [Common AWS fault tunables](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/aws-fault-tunables.md): Shared environment variables for AWS faults.