Project: Open Policy Agent (OPA) (Provisioning / Security & Compliance)
File: envoy-primer.md

title: Policy Primer via Examples kind: envoy weight: 2 This page covers how to write policies for the content of the requests that are passed to OPA by Envoy's [External Authorization filter](https://www.envoyproxy.io/docs/envoy/latest/intro/archoverview/security/extauthz_filter.html). Let's start with an example policy that restricts access to an endpoint based on a user's role and permissions. ```live:bool_example:module:openable package envoy.authz import rego.v1 import input.attributes.request.http default allow := false allow if { istokenvalid action_allowed } istokenvalid 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.decodeverify(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 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 . 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`. ```live:bool_example:query:hidden data.envoy.authz.allow ``` Sample input received by OPA is shown below: ```live:bool_example:input { "attributes": { "request": { "http": { "method": "GET", "path": "/people/", "headers": { "authorization": "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoiZ3Vlc3QiLCJzdWIiOiJZV3hwWTJVPSIsIm5iZiI6MTUxNDg1MTEzOSwiZXhwIjoxNjQxMDgxNTM5fQ.K5DnnbbIOspRbpCr2IKXE9cPVatGOCBrBQobQmBmaeU" } } } } } ``` With the input value above, the answer is: ```live:bool_example:output ``` 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. `responseheadersto_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. `requestheadersto_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

```live:obj_example:module:openable package envoy.authz import rego.v1 import input.attributes.request.http default allow := false allow if { istokenvalid action_allowed } headers["x-ext-auth-allow"] := "yes" headers["x-validated-by"] := "security-checkpoint" requestheadersto_remove := ["one-auth-header", "another-auth-header"] responseheadersto_add["x-foo"] := "bar" status_code := 200 if { allow } else := 401 if { not istokenvalid } else := 403 body := "Authentication Failed" if status_code == 401 body := "Unauthorized Request" if status_code == 403 dynamic_metadata := {"foo": "bar"} istokenvalid 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.decodeverify(encoded, {"secret": "secret"}) } ``` ```live:obj_example:query:hidden data.envoy.authz ``` Sample input received by OPA is shown below: ```live:obj_example:input { "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: ```live:obj_example:output ``` 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") `responseheadersto_add` (optional): an object mapping a string header name to a string header value `requestheadersto_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. ```rego result["allowed"] := allow result["headers"] := headers result["responseheaderstoadd"] := responseheaderstoadd result["requestheaderstoremove"] := requestheaderstoremove result["body"] := body result["httpstatus"] := statuscode result["dynamicmetadata"] := dynamicmetadata ``` 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. 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 and versions of the `CheckRequest` which is used to pass the request to OPA. For v3 requests, the 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.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, ```live:v2sample:module:readonly input.version == { "ext_authz": "v2", "encoding": "encoding/json" } ``` or, for v3, ```live:v3sample:module:readonly 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): ```yaml http_filters: name: envoy.ext_authz typed_config: '@type': type.googleapis.com/envoy.extensions.filters.http.ext_authz.v3.ExtAuthz transportapiversion: V3 grpc_service: googlegrpc: # or envoygrpc target_uri: "127.0.0.1:9191" ``` {{<detail-tag "Example v3 Input">}} ```json { "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" } } ``` {{</detail-tag>}} {{<detail-tag "Example v2 Input">}} ```json { "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" } } ``` {{</detail-tag>}} 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`. ```live:parsedpathexample:module:read_only package envoy.authz import rego.v1 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`. ```live:parsedqueryexample:module:read_only package envoy.authz import rego.v1 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. ```live:parsedbodyexample:module:read_only package envoy.authz import rego.v1 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 , then the `parsedbody` and `truncatedbody` fields will be omitted from the input. Envoy can be configured to pass validated JWT payload data into the `extauthz` filter with `metadatacontext_namespaces` and `payloadinmetadata`. ```yaml http_filters: name: envoy.filters.http.jwt_authn typed_config: "@type": type.googleapis.com/envoy.config.filter.http.jwt_authn.v2alpha.JwtAuthentication providers: example: payloadinmetadata: verified_jwt <...> name: envoy.ext_authz config: metadatacontextnamespaces: envoy.filters.http.jwt_authn <...> ``` 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" } } } } ```