Policy Primer via Examples

Policy Primer via Examples Edit

This page covers how to write policies for the content of the requests that are passed to OPA by Envoy’s External Authorization filter.

Writing Policies

Let’s start with an example policy that restricts access to an endpoint based on a user’s role and permissions.

package envoy.authz

import input.attributes.request.http

default allow := false

allow if {
	is_token_valid
	action_allowed
}

is_token_valid if {
	token.valid
	now := time.now_ns() / 1000000000
	token.payload.nbf <= now
	now < token.payload.exp
}

action_allowed if {
	http.method == "GET"
	token.payload.role == "guest"
	glob.match("/people/*", ["/"], http.path)
}

action_allowed if {
	http.method == "GET"
	token.payload.role == "admin"
	glob.match("/people/*", ["/"], http.path)
}

action_allowed if {
	http.method == "POST"
	token.payload.role == "admin"
	glob.match("/people", ["/"], http.path)
	lower(input.parsed_body.firstname) != base64url.decode(token.payload.sub)
}

token := {"valid": valid, "payload": payload} if {
	[_, encoded] := split(http.headers.authorization, " ")
	[valid, _, payload] := io.jwt.decode_verify(encoded, {"secret": "secret"})
}

The first line package envoy.authz declaration gives the (hierarchical) name envoy.authz to the rules in the remainder of the policy. If the OPA-Envoy configuration does not specify the path field, envoy/authz/allow will be considered as the default policy decision path. data.envoy.authz.allow will be the name of the policy decision to query in the default case.

The above policy uses the io.jwt.decode_verify builtin function to parse and verify the JWT containing information about the user making the request. It uses other builtins like glob.match, lower, base64url.decode etc. OPA has 150+ builtins detailed at openpolicyagent.org/docs/policy-reference.

The dot notation seen in multiple places in the policy for ex. input.parsed_body.firstname simply descends through the hierarchy to access the requested value. The dot (.) operator never throws any errors; if the path does not exist the value of the expression is undefined.

data.envoy.authz.allow

Sample input received by OPA is shown below:

{
  "attributes": {
    "request": {
      "http": {
        "method": "GET",
        "path": "/people/",
        "headers": {
          "authorization": "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoiZ3Vlc3QiLCJzdWIiOiJZV3hwWTJVPSIsIm5iZiI6MTUxNDg1MTEzOSwiZXhwIjoxNjQxMDgxNTM5fQ.K5DnnbbIOspRbpCr2IKXE9cPVatGOCBrBQobQmBmaeU"
        }
      }
    }
  }
}

With the input value above, the answer is:

false

Example Policy with Additional Controls

The allow variable in the above policy returns a boolean decision to indicate whether a request should be allowed or not. If you want, you can also control the HTTP status sent to the upstream or downstream client, along with the response body, and the response headers. Response metadata can also be set for consumption by the next Envoy filter. To do that, you can write rules like the ones below to fill in values for variables with the following types:

headers is an object whose keys are strings and values are strings. In case the request is denied, the object represents the HTTP response headers to be sent to the downstream client. If the request is allowed, the object represents additional request headers to be sent to the upstream.
response_headers_to_add is an object whose keys are strings and values are strings. It defines the HTTP response headers to be sent to the downstream client when a request is allowed.
request_headers_to_remove is an array of strings which describes the HTTP headers to remove from the original request before dispatching it to the upstream when a request is allowed.
body is a string which represents the response body data sent to the downstream client when a request is denied.
status_code is a number which represents the HTTP response status code sent to the downstream client when a request is denied.
dynamic_metadata is an object whose keys are strings and values can be booleans, strings, numbers, arrays, or objects. It will set the DynamicMetadata in the CheckResponse returned by the opa-envoy-plugin and can be consumed elsewhere in the envoy filter chain.

package envoy.authz

import input.attributes.request.http

default allow := false

allow if {
	is_token_valid
	action_allowed
}

headers["x-ext-auth-allow"] := "yes"
headers["x-validated-by"] := "security-checkpoint"

request_headers_to_remove := ["one-auth-header", "another-auth-header"]

response_headers_to_add["x-foo"] := "bar"

status_code := 200 if {
	allow
} else := 401 if {
	not is_token_valid
} else := 403

body := "Authentication Failed" if status_code == 401
body := "Unauthorized Request"  if status_code == 403

dynamic_metadata := {"foo": "bar"}

is_token_valid if {
	token.valid
	now := time.now_ns() / 1000000000
	token.payload.nbf <= now
	now < token.payload.exp
}

action_allowed if {
	http.method == "GET"
	token.payload.role == "guest"
	glob.match("/people/*", ["/"], http.path)
}

action_allowed if {
	http.method == "GET"
	token.payload.role == "admin"
	glob.match("/people/*", ["/"], http.path)
}

action_allowed if {
	http.method == "POST"
	token.payload.role == "admin"
	glob.match("/people", ["/"], http.path)
	lower(input.parsed_body.firstname) != base64url.decode(token.payload.sub)
}

token := {"valid": valid, "payload": payload} if {
	[_, encoded] := split(http.headers.authorization, " ")
	[valid, _, payload] := io.jwt.decode_verify(encoded, {"secret": "secret"})
}

data.envoy.authz

Sample input received by OPA is shown below:

{
  "attributes": {
    "request": {
      "http": {
        "method": "GET",
        "path": "/people",
        "headers": {
          "authorization": "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoiZ3Vlc3QiLCJzdWIiOiJZV3hwWTJVPSIsIm5iZiI6MTUxNDg1MTEzOSwiZXhwIjoxNjQxMDgxNTM5fQ.K5DnnbbIOspRbpCr2IKXE9cPVatGOCBrBQobQmBmaeU"
        }
      }
    }
  }
}

With the input value above, the value of all the variables in the package are:

{
  "allow": false,
  "body": "Authentication Failed",
  "dynamic_metadata": {
    "foo": "bar"
  },
  "headers": {
    "x-ext-auth-allow": "yes",
    "x-validated-by": "security-checkpoint"
  },
  "request_headers_to_remove": [
    "one-auth-header",
    "another-auth-header"
  ],
  "response_headers_to_add": {
    "x-foo": "bar"
  },
  "status_code": 401,
  "token": {
    "payload": {},
    "valid": false
  }
}

Output Document

When Envoy receives a policy decision, it expects a JSON object with the following fields:

allowed (required): a boolean deciding whether or not the request is allowed
headers (optional): an object mapping a string header name to a string header value (e.g. key “x-ext-auth-allow” has value “yes”)
response_headers_to_add (optional): an object mapping a string header name to a string header value
request_headers_to_remove (optional): is an array of string header names
http_status (optional): a number representing the HTTP status code
body (optional): the response body
dynamic_metadata (optional): an object representing dynamic metadata to be consumed by the next Envoy filter.

To construct that output object using the policies demonstrated in the last section, you can use the following Rego snippet. Notice that we are using partial object rules so that any variables with undefined values simply have no key in the result object.

result["allowed"] := allow
result["headers"] := headers
result["response_headers_to_add"] := response_headers_to_add
result["request_headers_to_remove"] := request_headers_to_remove
result["body"] := body
result["http_status"] := status_code
result["dynamic_metadata"] := dynamic_metadata

For a single user, including this snippet in your normal policy is fine, but when you have multiple teams writing policies, you will typically pull this bit of boilerplate into a wrapper package, so your teams can focus on writing the policies shown in the previous sections.

Input Document

In OPA, input is a reserved, global variable whose value is the request sent by the Envoy External Authorization filter to OPA. The OPA-Envoy plugin supports both v2 and v3 versions of the CheckRequest which is used to pass the request to OPA.

For v3 requests, the specified JSON mapping for protobuf is used for making the incoming envoy.service.auth.v3.CheckRequest available in input. It differs from the encoding used for v2 requests. In v3, all keys are lower camelcase. Also, needless nesting of oneof values is removed.

For example, source address data that looks like this in v2,

"source": {
  "address": {
    "Address": {
      "SocketAddress": {
        "PortSpecifier": {
          "PortValue": 59052
        },
        "address": "127.0.0.1"
      }
    }
  }
}

becomes, in v3,

"source": {
  "address": {
    "socketAddress": {
      "address": "127.0.0.1",
      "portValue": 59052
    }
  }
}

The following table shows the rego code for common data, in v2 and v3:

information	rego v2	rego v3
`source address`	`input.attributes.source.address.Address.SocketAddress.address`	`input.attributes.source.address.socketAddress.address`
`source port`	`input.attributes.source.address.Address.SocketAddress.PortSpecifier.PortValue`	`input.attributes.source.address.socketAddress.portValue`
`destination address`	`input.attributes.destination.address.Address.SocketAddress.address`	`input.attributes.destination.address.socketAddress.address`
`destination port`	`input.attributes.destination.address.Address.SocketAddress.PortSpecifier.PortValue`	`input.attributes.destination.address.socketAddress.portValue`
`dynamic metadata`	`input.attributes.metadata_context.filter_metadata`	`input.attributes.metadataContext.filterMetadata`

Due to those differences, it’s important to know which version is used when writing policies. Thus, this information is passed into the OPA evaluation under input.version, where you’ll either find, for v2,

input.version == { "ext_authz": "v2", "encoding": "encoding/json" }

or, for v3,

input.version == { "ext_authz": "v3", "encoding": "protojson" }

To have Envoy use the v3 version of the service, the http_filters entry in the Envoy configuration should look like below (minimal version):

http_filters:
- name: envoy.ext_authz
  typed_config:
    '@type': type.googleapis.com/envoy.extensions.filters.http.ext_authz.v3.ExtAuthz
    transport_api_version: V3
    grpc_service:
      google_grpc: # or envoy_grpc
        target_uri: "127.0.0.1:9191"

Example Input

Example v3 Input

{
  "attributes": {
    "source": {
      "address": {
        "socketAddress": {
          "address": "172.17.0.1",
          "portValue": 61402
        }
      }
    },
    "destination": {
      "address": {
        "socketAddress": {
          "address": "172.17.06",
          "portValue": 8000
        }
      }
    },
    "request": {
      "time": "2020-11-20T09:47:47.722473Z",
      "http": {
        "id":"13519049518330544501",
        "method": "POST",
        "headers": {
          ":authority":"192.168.99.206:30164",
          ":method":"POST",
          ":path":"/people?lang=en",
          "accept": "*/*",
          "authorization":"Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoiYWRtaW4iLCJzdWIiOiJZbTlpIiwibmJmIjoxNTE0ODUxMTM5LCJleHAiOjE2NDEwODE1Mzl9.WCxNAveAVAdRCmkpIObOTaSd0AJRECY2Ch2Qdic3kU8",
          "content-length":"41",
          "content-type":"application/json",
          "user-agent":"curl/7.54.0",
          "x-forwarded-proto":"http",
          "x-request-id":"7bca5c86-bf55-432c-b212-8c0f1dc999ec"
        },
        "host":"192.168.99.206:30164",
        "path":"/people?lang=en",
        "protocol":"HTTP/1.1",
        "body":"{\"firstname\":\"Charlie\", \"lastname\":\"Opa\"}",
        "size":41
      }
    },
    "metadataContext": {}
  },
  "parsed_body":{"firstname": "Charlie", "lastname": "Opa"},
  "parsed_path":["people"],
  "parsed_query": {"lang": ["en"]},
  "truncated_body": false,
  "version": {
    "encoding":"protojson",
    "ext_authz":"v3"
  }
}

Example v2 Input

{
  "attributes":{
     "source":{
        "address":{
           "Address":{
              "SocketAddress":{
                 "PortSpecifier":{
                    "PortValue":61402
                 },
                 "address":"172.17.0.1"
              }
           }
        }
     },
     "destination":{
        "address":{
           "Address":{
              "SocketAddress":{
                 "PortSpecifier":{
                    "PortValue":8000
                 },
                 "address":"172.17.0.6"
              }
           }
        }
     },
     "request":{
        "http":{
           "id":"13519049518330544501",
           "method":"POST",
           "headers":{
              ":authority":"192.168.99.206:30164",
              ":method":"POST",
              ":path":"/people?lang=en",
              "accept":"*/*",
              "authorization":"Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoiYWRtaW4iLCJzdWIiOiJZbTlpIiwibmJmIjoxNTE0ODUxMTM5LCJleHAiOjE2NDEwODE1Mzl9.WCxNAveAVAdRCmkpIObOTaSd0AJRECY2Ch2Qdic3kU8",
              "content-length":"41",
              "content-type":"application/json",
              "user-agent":"curl/7.54.0",
              "x-forwarded-proto":"http",
              "x-request-id":"7bca5c86-bf55-432c-b212-8c0f1dc999ec"
           },
           "host":"192.168.99.206:30164",
           "path":"/people?lang=en",
           "protocol":"HTTP/1.1",
           "body":"{\"firstname\":\"Charlie\", \"lastname\":\"Opa\"}",
           "size":41
        }
     }
  },
  "parsed_body":{"firstname": "Charlie", "lastname": "Opa"},
  "parsed_path":["people"],
  "parsed_query": {"lang": ["en"]},
  "truncated_body": false,
  "version": {
    "encoding":"encoding/json",
    "ext_authz":"v2"
  }
}

The parsed_path field in the input is generated from the path field in the HTTP request which is included in the Envoy External Authorization CheckRequest message type. This field provides the request path as a string array which can help policy authors perform pattern matching on the HTTP request path. The below sample policy allows anyone to access the path /people.

package envoy.authz

default allow := false

allow if input.parsed_path == ["people"]

The parsed_query field in the input is also generated from the path field in the HTTP request. This field provides the HTTP URL query as a map of string array. The below sample policy allows anyone to access the path /people?lang=en&id=1&id=2.

package envoy.authz

default allow := false

allow if {
	input.parsed_path == ["people"]
	input.parsed_query.lang == ["en"]
	input.parsed_query.id == ["1", "2"]
}

The parsed_body field in the input is generated from the body field in the HTTP request which is included in the Envoy External Authorization CheckRequest message type. This field contains the deserialized JSON request body which can then be used in a policy as shown below.

package envoy.authz

default allow := false

allow if {
	input.parsed_body.firstname == "Charlie"
	input.parsed_body.lastname == "Opa"
}

The truncated_body field in the input represents if the HTTP request body is truncated. The body is considered to be truncated, if the value of the Content-Length header exceeds the size of the request body.

If skip-request-body-parse: true is specified in the OPA-Envoy configuration, then the parsed_body and truncated_body fields will be omitted from the input.

Example with JWT payload passed from Envoy

Envoy can be configured to pass validated JWT payload data into the ext_authz filter with metadata_context_namespaces and payload_in_metadata.

Example Envoy Configuration

http_filters:
- name: envoy.filters.http.jwt_authn
  typed_config:
  "@type": type.googleapis.com/envoy.config.filter.http.jwt_authn.v2alpha.JwtAuthentication
  providers:
    example:
      payload_in_metadata: verified_jwt
      <...>
- name: envoy.ext_authz
  config:
    metadata_context_namespaces:
    - envoy.filters.http.jwt_authn
    <...>

Example OPA Input

This will result in something like the following dictionary being added to input.attributes (some common fields have been excluded for brevity):

  "metadata_context": {
    "filter_metadata": {
      "envoy.filters.http.jwt_authn": {
        "verified_jwt": {
          "email": "alice@example.com",
          "exp": 1569026124,
          "name": "Alice"
        }
      }
    }
  }

Feedback

Was this page helpful?

Glad to hear it! Please tell us how we can improve.

Sorry to hear that. Please tell us how we can improve.