Monitoring Blocked and Passthrough External Service Traffic

How can you use Istio to monitor blocked and passthrough external traffic.

Sep 28, 2019 | By Neeraj Poddar - Aspen Mesh

Understanding, controlling and securing your external service access is one of the key benefits that you get from a service mesh like Istio. From a security and operations point of view, it is critical to monitor what external service traffic is getting blocked as they might surface possible misconfigurations or a security vulnerability if an application is attempting to communicate with a service that it should not be allowed to. Similarly, if you currently have a policy of allowing any external service access, it is beneficial to monitor the traffic so you can incrementally add explicit Istio configuration to allow access and better secure your cluster. In either case, having visibility into this traffic via telemetry is quite helpful as it enables you to create alerts and dashboards, and better reason about your security posture. This was a highly requested feature by production users of Istio and we are excited that the support for this was added in release 1.3.

To implement this, the Istio default metrics are augmented with explicit labels to capture blocked and passthrough external service traffic. This blog will cover how you can use these augmented metrics to monitor all external service traffic.

The Istio control plane configures the sidecar proxy with predefined clusters called BlackHoleCluster and Passthrough which block or allow all traffic respectively. To understand these clusters, let’s start with what external and internal services mean in the context of Istio service mesh.

External and internal services

Internal services are defined as services which are part of your platform and are considered to be in the mesh. For internal services, Istio control plane provides all the required configuration to the sidecars by default. For example, in Kubernetes clusters, Istio configures the sidecars for all Kubernetes services to preserve the default Kubernetes behavior of all services being able to communicate with other.

External services are services which are not part of your platform i.e. services which are outside of the mesh. For external services, Istio provides two options, first to block all external service access (enabled by setting global.outboundTrafficPolicy.mode to REGISTRY_ONLY) and second to allow all access to external service (enabled by setting global.outboundTrafficPolicy.mode to ALLOW_ANY). The default option for this setting (as of Istio 1.3) is to allow all external service access. This option can be configured via mesh configuration.

This is where the BlackHole and Passthrough clusters are used.

What are BlackHole and Passthrough clusters?

BlackHoleCluster - The BlackHoleCluster is a virtual cluster created in the Envoy configuration when global.outboundTrafficPolicy.mode is set to REGISTRY_ONLY. In this mode, all traffic to external service is blocked unless service entries are explicitly added for each service. To implement this, the default virtual outbound listener at 0.0.0.0:15001 which uses original destination is setup as a TCP Proxy with the BlackHoleCluster as the static cluster. The configuration for the BlackHoleCluster looks like this:
```
{
  "name": "BlackHoleCluster",
  "type": "STATIC",
  "connectTimeout": "10s"
}
```
As you can see, this cluster is static with no endpoints so all the traffic will be dropped. Additionally, Istio creates unique listeners for every port/protocol combination of platform services which gets hit instead of the virtual listener if the request is made to an external service on the same port. In that case, the route configuration of every virtual route in Envoy is augmented to add the BlackHoleCluster like this:
```
{
  "name": "block_all",
  "domains": [
    "*"
  ],
  "routes": [
    {
      "match": {
        "prefix": "/"
      },
      "directResponse": {
        "status": 502
      }
    }
  ]
}
```
The route is setup as direct response with 502 response code which means if no other routes match the Envoy proxy will directly return a 502 HTTP status code.
PassthroughCluster - The PassthroughCluster is a virtual cluster created in the Envoy configuration when global.outboundTrafficPolicy.mode is set to ALLOW_ANY. In this mode, all traffic to any external service external is allowed. To implement this, the default virtual outbound listener at 0.0.0.0:15001 which uses SO_ORIGINAL_DST, is setup as a TCP Proxy with the PassthroughCluster as the static cluster. The configuration for the PassthroughCluster looks like this:
```
{
  "name": "PassthroughCluster",
  "type": "ORIGINAL_DST",
  "connectTimeout": "10s",
  "lbPolicy": "ORIGINAL_DST_LB",
  "circuitBreakers": {
    "thresholds": [
      {
        "maxConnections": 102400,
        "maxRetries": 1024
      }
    ]
  }
}
```
This cluster uses the original destination load balancing policy which configures Envoy to send the traffic to the original destination i.e. passthrough.
Similar to the BlackHoleCluster, for every port/protocol based listener the virtual route configuration is augmented to add the PassthroughCluster as the default route:
```
{
  "name": "allow_any",
  "domains": [
    "*"
  ],
  "routes": [
    {
      "match": {
        "prefix": "/"
      },
      "route": {
        "cluster": "PassthroughCluster"
      }
    }
  ]
}
```

Prior to Istio 1.3, there were no metrics reported or if metrics were reported there were no explicit labels set when traffic hit these clusters, resulting in lack of visibility in traffic flowing through the mesh.

The next section covers how to take advantage of this enhancement as the metrics and labels emitted are conditional on whether the virtual outbound or explicit port/protocol listener is being hit.

Using the augmented metrics

To capture all external service traffic in either of the cases (BlackHole or Passthrough), you will need to monitor istio_requests_total and istio_tcp_connections_closed_total metrics. Depending upon the Envoy listener type i.e. TCP proxy or HTTP proxy that gets invoked, one of these metrics will be incremented.

Additionally, in case of a TCP proxy listener in order to see the IP address of the external service that is blocked or allowed via BlackHole or Passthrough cluster, you will need to add the destination_ip label to the istio_tcp_connections_closed_total metric. In this scenario, the host name of the external service is not captured. This label is not added by default and can be easily added by augmenting the Istio configuration for attribute generation and Prometheus handler. You should be careful about cardinality explosion in time series if you have many services with non-stable IP addresses.

PassthroughCluster metrics

This section explains the metrics and the labels emitted based on the listener type invoked in Envoy.

HTTP proxy listener: This happens when the port of the external service is same as one of the service ports defined in the cluster. In this scenario, when the PassthroughCluster is hit, istio_requests_total will get increased like this:

{
  "metric": {
    "__name__": "istio_requests_total",
    "connection_security_policy": "unknown",
    "destination_app": "unknown",
    "destination_principal": "unknown",
    "destination_service": "httpbin.org",
    "destination_service_name": "PassthroughCluster",
    "destination_service_namespace": "unknown",
    "destination_version": "unknown",
    "destination_workload": "unknown",
    "destination_workload_namespace": "unknown",
    "instance": "100.96.2.183:42422",
    "job": "istio-mesh",
    "permissive_response_code": "none",
    "permissive_response_policyid": "none",
    "reporter": "source",
    "request_protocol": "http",
    "response_code": "200",
    "response_flags": "-",
    "source_app": "sleep",
    "source_principal": "unknown",
    "source_version": "unknown",
    "source_workload": "sleep",
    "source_workload_namespace": "default"
  },
  "value": [
    1567033080.282,
    "1"
  ]
}

Note that the destination_service_name label is set to PassthroughCluster to indicate that this cluster was hit and the destination_service is set to the host of the external service.

TCP proxy virtual listener - If the external service port doesn’t map to any HTTP based service ports within the cluster, this listener is invoked and istio_tcp_connections_closed_total is the metric that will be increased:

{
  "status": "success",
  "data": {
    "resultType": "vector",
    "result": [
      {
        "metric": {
          "__name__": "istio_tcp_connections_closed_total",
          "connection_security_policy": "unknown",
          "destination_app": "unknown",
          "destination_ip": "52.22.188.80",
          "destination_principal": "unknown",
          "destination_service": "unknown",
          "destination_service_name": "PassthroughCluster",
          "destination_service_namespace": "unknown",
          "destination_version": "unknown",
          "destination_workload": "unknown",
          "destination_workload_namespace": "unknown",
          "instance": "100.96.2.183:42422",
          "job": "istio-mesh",
          "reporter": "source",
          "response_flags": "-",
          "source_app": "sleep",
          "source_principal": "unknown",
          "source_version": "unknown",
          "source_workload": "sleep",
          "source_workload_namespace": "default"
        },
        "value": [
          1567033761.879,
          "1"
        ]
      }
    ]
  }
}

In this case, destination_service_name is set to PassthroughCluster and the destination_ip is set to the IP address of the external service. The destination_ip label can be used to do a reverse DNS lookup and get the host name of the external service. As this cluster is passthrough, other TCP related metrics like istio_tcp_connections_opened_total, istio_tcp_received_bytes_total and istio_tcp_sent_bytes_total are also updated.

BlackHoleCluster metrics

Similar to the PassthroughCluster, this section explains the metrics and the labels emitted based on the listener type invoked in Envoy.

HTTP proxy listener: This happens when the port of the external service is same as one of the service ports defined in the cluster. In this scenario, when the BlackHoleCluster is hit, istio_requests_total will get increased like this:

{
  "metric": {
    "__name__": "istio_requests_total",
    "connection_security_policy": "unknown",
    "destination_app": "unknown",
    "destination_principal": "unknown",
    "destination_service": "httpbin.org",
    "destination_service_name": "BlackHoleCluster",
    "destination_service_namespace": "unknown",
    "destination_version": "unknown",
    "destination_workload": "unknown",
    "destination_workload_namespace": "unknown",
    "instance": "100.96.2.183:42422",
    "job": "istio-mesh",
    "permissive_response_code": "none",
    "permissive_response_policyid": "none",
    "reporter": "source",
    "request_protocol": "http",
    "response_code": "502",
    "response_flags": "-",
    "source_app": "sleep",
    "source_principal": "unknown",
    "source_version": "unknown",
    "source_workload": "sleep",
    "source_workload_namespace": "default"
  },
  "value": [
    1567034251.717,
    "1"
  ]
}

Note the destination_service_name label is set to BlackHoleCluster and the destination_service to the host name of the external service. The response code should always be 502 in this case.

{
  "metric": {
    "__name__": "istio_tcp_connections_closed_total",
    "connection_security_policy": "unknown",
    "destination_app": "unknown",
    "destination_ip": "52.22.188.80",
    "destination_principal": "unknown",
    "destination_service": "unknown",
    "destination_service_name": "BlackHoleCluster",
    "destination_service_namespace": "unknown",
    "destination_version": "unknown",
    "destination_workload": "unknown",
    "destination_workload_namespace": "unknown",
    "instance": "100.96.2.183:42422",
    "job": "istio-mesh",
    "reporter": "source",
    "response_flags": "-",
    "source_app": "sleep",
    "source_principal": "unknown",
    "source_version": "unknown",
    "source_workload": "sleep",
    "source_workload_namespace": "default"
  },
  "value": [
    1567034481.03,
    "1"
  ]
}

Note the destination_ip label represents the IP address of the external service and the destination_service_name is set to BlackHoleCluster to indicate that this traffic was blocked by the mesh. Is is interesting to note that for the BlackHole cluster case, other TCP related metrics like istio_tcp_connections_opened_total are not increased as there’s no connection that is ever established.

Monitoring these metrics can help operators easily understand all the external services consumed by the applications in their cluster.