> 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/ec2-network-loss.md).

# EC2 network loss

EC2 network loss is an AWS chaos fault that drops a configurable percentage of outbound packets on a target EC2 instance for a configurable duration. The loss can be scoped to specific destination hosts or IPs, and applied on a specific network interface. The fault dispatches the shaping rules via AWS Systems Manager Run Command.

Use this fault to test how a workload reacts when network reliability degrades: do timeouts trigger correctly, do TCP retransmissions absorb the loss transparently, does the application fall back to a healthy replica, do connection pools recover after a churn event?

{% 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:

* **Partial-loss tolerance:** When 10-20% of packets to a critical destination are dropped, does the application stay within SLO?
* **Full-loss behaviour:** When 100% of packets are dropped, does the application treat the destination as down and fall back?
* **TCP retransmission cost:** How much does throughput drop when packet loss is added, and does the application's connection pool absorb it?
* **Replica failover:** When loss to one replica is high, does the client steer traffic to a healthy replica?
* **Observability:** Does the loss show up in network metrics (packets dropped, TCP retransmits) and alerts at the right cadence?

***

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

* **Kubernetes version:** 1.21 or later for the chaos infrastructure cluster.
* **Target instance is reachable via SSM:** The instance has the SSM Agent running and an instance profile with `AmazonSSMManagedInstanceCore`.
* **Selector provided:** Either `EC2_INSTANCE_ID` or `EC2_INSTANCE_TAG` is set.
* **Network shaping tools installable:** `tc` / `iproute2` must be installable on the target. Set `INSTALL_NETEM=true` (default) to install them if missing.
* **AWS credentials available:** Either an AWS credentials file uploaded as a **File Secret in Harness Secret Manager** (see Authentication below) or IRSA on the chaos infrastructure service account.

***

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

| Platform                                    | Support status                             |
| ------------------------------------------- | ------------------------------------------ |
| Amazon EC2 (Linux instances with SSM Agent) | Supported                                  |
| Amazon EKS managed worker nodes             | Supported (if SSM Agent is installed)      |
| Amazon EKS self-managed worker nodes        | Supported (if SSM Agent is installed)      |
| Targeting by tag                            | Supported via `EC2_INSTANCE_TAG`           |
| Targeting by ID                             | Supported via `EC2_INSTANCE_ID`            |
| Windows instances                           | Not supported (Linux-only network shaping) |

***

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

```json
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:DescribeInstances",
        "ec2:DescribeInstanceStatus"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ssm:SendCommand",
        "ssm:CancelCommand",
        "ssm:GetCommandInvocation",
        "ssm:DescribeInstanceInformation",
        "ssm:GetDocument",
        "ssm:DescribeDocument"
      ],
      "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.

***

### 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 File Secret should contain an 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`.

***

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

**Required parameters**

| Tunable                                   | Description                                                | Default    |
| ----------------------------------------- | ---------------------------------------------------------- | ---------- |
| `REGION`                                  | AWS region that hosts the target instance.                 | (required) |
| `EC2_INSTANCE_ID` *or* `EC2_INSTANCE_TAG` | One of these must be set to select the target instance(s). | `""`       |

**Chaos parameters**

| Tunable                          | Description                                                                                              | Default    |
| -------------------------------- | -------------------------------------------------------------------------------------------------------- | ---------- |
| `NETWORK_PACKET_LOSS_PERCENTAGE` | Percentage of outbound packets to drop (0-100).                                                          | `100`      |
| `NETWORK_INTERFACE`              | Network interface to apply shaping on.                                                                   | `eth0`     |
| `DESTINATION_HOSTS`              | Comma-separated list of destination hostnames to target. Leave empty to affect all destinations.         | `""`       |
| `DESTINATION_IPS`                | Comma-separated list of destination IPs/CIDRs to target. Leave empty to affect all destinations.         | `""`       |
| `TOTAL_CHAOS_DURATION`           | Duration of the fault in seconds.                                                                        | `30`       |
| `INSTANCE_AFFECTED_PERC`         | Percentage of matching instances to target (only with `EC2_INSTANCE_TAG`). `0` targets one instance.     | `0`        |
| `INSTALL_DEPENDENCIES`           | Install dependencies inside the target instance if missing. Set to `False` to skip.                      | `True`     |
| `INSTALL_NETEM`                  | Install the `tc` / netem networking tools if missing.                                                    | `true`     |
| `PROXY`                          | HTTP/HTTPS proxy used by the in-instance installer (for example `https_proxy=http://proxy.server:3128`). | `""`       |
| `SEQUENCE`                       | Order in which multiple instances are processed: `parallel` or `serial`.                                 | `parallel` |
| `RAMP_TIME`                      | Wait period in seconds before and after the fault.                                                       | `0`        |

**Authentication**

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

{% hint style="info" %}
**SCOPE THE BLAST RADIUS WITH DESTINATIONS**

Without `DESTINATION_HOSTS` or `DESTINATION_IPS`, packet loss applies to all outbound traffic, including the SSM control channel back to AWS. Scope to specific destinations whenever possible.
{% endhint %}

***

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

Sends an SSM Run Command to the selected instance(s) in `REGION` that adds a network-shaping rule on `NETWORK_INTERFACE` to drop `NETWORK_PACKET_LOSS_PERCENTAGE` percent of outbound packets to `DESTINATION_HOSTS`/`DESTINATION_IPS` for `TOTAL_CHAOS_DURATION` seconds, then removes the rule.

***

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

* Outbound packets to the targeted destinations are dropped at the specified percentage.
* TCP connections absorb small loss percentages (1-5%) through retransmits, with reduced throughput.
* Larger loss percentages (≥20%) cause noticeable slowdown and connection drops.
* 100% loss makes destinations appear unreachable; existing connections eventually time out and new ones fail to establish.
* Network counters on the target instance (`netstat -s`, kernel TCP retransmits) reflect the loss.

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

The chaos pod removes the network-shaping rule. Traffic returns to baseline immediately, but connections that failed during the fault must be re-established by the caller.
{% endhint %}

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

* **End-to-end availability:** Use an [HTTP probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/http-probe.md) against an endpoint served by the affected destination.
* **Synthetic packet loss:** Use a [command probe](/resilience-testing/chaos-testing/probes/command-probe.md) that runs `ping -c 20 <destination>` via SSM during the fault and asserts loss is close to `NETWORK_PACKET_LOSS_PERCENTAGE`.
* **TCP retransmissions:** Use a Prometheus probe on `node_netstat_Tcp_RetransSegs` to confirm the loss surfaces as retransmits.

***

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

While the experiment is running:

1. **Confirm the shaping rule is in place via SSM.**

   ```bash
   aws ssm send-command \
     --region <region> \
     --instance-ids <id> \
     --document-name AWS-RunShellScript \
     --parameters 'commands=["tc qdisc show dev <NETWORK_INTERFACE>"]'
   ```
2. **Measure loss to the destination.**

   ```bash
   aws ssm send-command \
     --region <region> \
     --instance-ids <id> \
     --document-name AWS-RunShellScript \
     --parameters 'commands=["ping -c 20 <destination-host>"]'
   ```

***

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

* **End of duration:** The chaos pod removes the network-shaping rule.
* **Abort the experiment:** Stopping the experiment from Chaos Studio cancels the in-flight SSM command and removes the rule.
* **Manual cleanup:** If a rule is left behind, remove it via SSM: `tc qdisc del dev <NETWORK_INTERFACE> root`.

***

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

* **Linux-only payload:** This fault shapes traffic on Linux instances. For Windows hosts, use `windows-ec2-network-loss`.
* **SSM Agent required:** Instances without the SSM Agent online cannot be targeted.
* **Outbound only:** Loss is applied to traffic leaving `NETWORK_INTERFACE`. Inbound loss is not directly controllable from the receiver side.
* **Control-channel impact:** Without `DESTINATION_HOSTS`/`DESTINATION_IPS`, the rule affects all outbound traffic including the SSM control channel; very high loss values may delay cleanup.
* **DNS resolution timing:** Hosts in `DESTINATION_HOSTS` are resolved once at the start of the fault.

***

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

<details>

<summary>EC2 network loss experiment fails with InvalidInstanceId in Harness Chaos Engineering</summary>

The SSM Agent is not online for the target instance. Confirm with aws ssm describe-instance-information --filters 'Key=InstanceIds,Values='. If missing, install the SSM Agent and attach an instance profile that includes AmazonSSMManagedInstanceCore.

</details>

<details>

<summary>EC2 network loss runs but no packet loss is observed</summary>

The most common causes are: tc/iproute2 failed to install (set INSTALL\_NETEM=true and verify network egress, or use PROXY); NETWORK\_INTERFACE is wrong (use 'ip link show' to confirm); or DESTINATION\_HOSTS resolved to IPs that the target reaches via a different interface. Run 'tc qdisc show dev ' via SSM during the fault and use 'ping' to measure loss directly.

</details>

<details>

<summary>EC2 network loss disrupted my SSM connection and the fault hangs</summary>

DESTINATION\_HOSTS / DESTINATION\_IPS were left empty, so loss applied to all outbound traffic including the SSM control channel. Wait for TOTAL\_CHAOS\_DURATION to elapse and the cleanup command to land, then rerun with explicit DESTINATION\_HOSTS or DESTINATION\_IPS that exclude the SSM endpoint.

</details>

***

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

* [EC2 network latency](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/ec2-network-latency.md): Add latency instead of dropping packets.
* [EC2 DNS chaos](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/ec2-dns-chaos.md): Block DNS resolution for specific hostnames.
* [Windows EC2 network loss](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/windows-ec2-network-loss.md): Packet loss for Windows instances.
* [Common AWS fault tunables](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/aws/aws-fault-tunables.md): Shared environment variables for AWS faults.