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fck-nat Features

High-availability Mode

fck-nat can operate on a single instance, or withing an autoscaling group for improved availability. When running in an autoscaling group, fck-nat can be configured to always attach a specific ENI at start-up, allowing fck-nat to maintain a consistent internal-facing IP address. Additionally, it is also possible to configure an already allocated EIP address that would be carried through instance refreshs.

Those features are controlled by eni_id and eip_id directive in the configuration file.

IAM requirements: ec2:AttachNetworkInterface, ec2:ModifyNetworkInterfaceAttribute on * for ha-mode, plus ec2:AssociateAddress, ec2:DisassociateAddress on * when using a static EIP.


One of the objectives of fck-nat is to offer as close as possible metric parity with Managed NAT Gateway. While the project supports various metrics similar to the managed NAT Gateway via Cloudwatch agent, each provider is responsible for passing their configuration to the agent via fck-nat's cwagent_enabled, and cwagent_cfg_param_name directives within its configuration file.

As an example, you might use the following configuration file which have Cloudwatch agent report most of metrics provided in the managed NAT Gateway:

  "agent": {
    "metrics_collection_interval": 60,
    "run_as_user": "root",
    "usage_data": false
  "metrics": {
    "namespace": "fck-nat",
    "metrics_collected": {
      "net": {
        "resources": ["eth0", "eth1"],
        "measurement": [
          { "name": "bytes_recv", "rename": "BytesIn",  "unit": "Bytes" },
          { "name": "bytes_sent", "rename": "BytesOut",  "unit": "Bytes" },
          { "name": "packets_sent", "rename": "PacketsOutCount",  "unit": "Count" },
          { "name": "packets_recv", "rename": "PacketsInCount",  "unit": "Count" },
          { "name": "drop_in", "rename": "PacketsDropInCount",  "unit": "Count" },
          { "name": "drop_out", "rename": "PacketsDropOutCount",  "unit": "Count" }
      "netstat": {
        "measurement": [
          { "name": "tcp_syn_sent", "rename": "ConnectionAttemptOutCount",  "unit": "Count" },
          { "name": "tcp_syn_recv", "rename": "ConnectionAttemptInCount",  "unit": "Count" },
          { "name": "tcp_established", "rename": "ConnectionEstablishedCount",  "unit": "Count" }
      "ethtool": {
        "interface_include": ["eth0", "eth1"],
        "metrics_include": [
      "mem": {
        "measurement": [
          { "name": "used_percent", "rename": "MemoryUsed",  "unit": "Percent" }
    "append_dimensions": {
      "InstanceId": "$${aws:InstanceId}"

Ensure you are aware of Cloudwatch metrics costs before enabling Cloudwatch agent. The above configuration would cost you about $17/monthly, excluding free tier.

IAM requirements: ssm:GetParameter on the SSM Parameter ARN, and cloudwatch:PutMetricData on *.