aserto-dev/go-aserto

Package go-aserto implements clients and middleware for Aserto services.

• Docs: https://docs.aserto.com/docs/
• API Reference: https://aserto.readme.io/

Install

go get -u github.com/aserto-dev/go-aserto

Authorizer

The Authorizer service is is an open source authorization engine which uses the Open Policy Agent (OPA) to make decisions by computing authorization policies.

AuthorizerClient

The AuthorizerClient interface, defined in github.com/aserto-dev/go-authorizer/aserto/authorizer/v2, describes the operations exposed by the Aserto authorizer service.

Two implementation of AuthorizerClient are available:

1. authorizer/grpc provides a client that communicates with the authorizer using gRPC.

2. authorizer/http provides a client that communicates with the authorizer over its REST HTTP endpoints.

Create a new client using New() in either package.

The snippet below creates an authorizer client that talks to Aserto's hosted authorizer over gRPC:

import (
	"github.com/aserto-dev/go-aserto/client"
	"github.com/aserto-dev/go-aserto/authorizer/grpc"
)
...
authorizer, err := grpc.New(
	ctx,
	client.WithAPIKeyAuth("<API Key>"),
)

Connection Options

The options below can be specified to override default behaviors:

WithAddr() - sets the server address and port. Default: "authorizer.prod.aserto.com:8443".

WithAPIKeyAuth() - sets an API key for authentication.

WithTokenAuth() - sets an OAuth2 token to be used for authentication.

WithTenantID() - sets the aserto tenant ID.

WithInsecure() - enables/disables TLS verification. Default: false.

WithCACertPath() - adds the specified PEM certificate file to the connection's list of trusted root CAs.

Connection Timeout

Connection timeout can be set on the specified context using context.WithTimeout. If no timeout is set on the context, the default connection timeout is 5 seconds. For example, to increase the timeout to 10 seconds:

ctx := context.Background()

authorizer, err := grpc.New(
	context.WithTimeout(ctx, time.Duration(10) * time.Second),
	aserto.WithAPIKeyAuth("<API Key>"),
)

Making Authorization Calls

Use the client's Is() method to request authorization decisions from the Aserto authorizer service.

import (
	authz "github.com/aserto-dev/go-authorizer/aserto/authorizer/v2"
	"github.com/aserto-dev/go-authorizer/aserto/authorizer/v2/api"
)

resp, err := authorizer.Is(c.Context, &authz.IsRequest{
	PolicyContext: &api.PolicyContext{
		Path:      		"peoplefinder.GET.users.__id",
		Decisions: 		"allowed",
	},
	IdentityContext: &api.IdentityContext{
		Identity: "<user name>",
		Type:     api.IdentityType_IDENTITY_TYPE_SUB,
	},
})

Directory

The Directory stores information required to make authorization decisions.

Directory Client

The directory client provides access to the directory services:

1. Reader - provides functions to query the directory.
2. Writer - provides functions to mutate or delete directory data.
3. Exporter - provides bulk export of data from the directory.
4. Importer - provides bulk import of data into the directory.

To create a directory client:

import (
	"context"
	"github.com/aserto-dev/go-aserto/client"
	"github.com/aserto-dev/go-aserto/client/directory/v3"
)

...

dir, err := directory.New(context.Background(), client.WithAPIKeyAuth('<api key>'))

Connection options are the same as those for the authorizer client. If WithAddr() is not provided, the default address is directory.prod.aserto.com:8443.

Configuration

The hosted Aserto directory exposes all services on the same address (directory.prod.aserto.com:8443). However, with Topaz or in self-hosted environments, it is possible to configure the services individually and to disable selected services entirely.

The directory.Config structs allows for customization of connection options for directory services.

// Config provides configuration for connecting to the Aserto Directory service.
type Config struct {
	// Base configuration. If non-nil, this configuration is used for any client that doesn't have its own configuration.
	// If nil, only clients that have their own configuration will be created.
	*client.Config

	// Reader configuration.
	Reader *client.Config `json:"reader"`

	// Writer configuration.
	Writer *client.Config `json:"writer"`

	// Importer configuration.
	Importer *client.Config `json:"importer"`

	// Exporter configuration.
	Exporter *client.Config `json:"exporter"`
}

The embedded *client.Config acts as a fallback. If no configuration is provided for a specific service, the fallback configuration is used. If no fallback is provided, the client for that service is nil.

To create a directory client from configuration, call Connect(context.Context) on the config struct:

import (
	"context"

	"github.com/aserto-dev/go-aserto/client/directory/v3"
	"github.com/aserto-dev/go-directory/aserto/directory/common/v2"
	"github.com/aserto-dev/go-directory/aserto/directory/reader/v2"
)

...

// Use the same address for all services.
cfg := &directory.Config{Address: "localhost:9292"}

dir, err := cfg.Connect(context.Background())
if err != nil {
	panic(err)
}

resp, err := dir.Reader.GetObjects(&reader.GetObjectsRequest{})

Examples

All services use the same configuration:

{
	"address": "directory.prod.aserto.com:8443",
	"api_key": "<API-KEY>",
	"tenant_id": "<TENANT-ID>"
}

All services use the same configuration except for the writer, that uses a different address:

{
	"address": "localhost:9292",
	"writer": {
		"address": "localhost:9293"
	}
}

Only a reader and writer are configured. Client.Importer and Client.Exporter are nil:

{
	"reader": {
		"address": "localhost:9292"
	},
	"writer": {
		"address": "localhost:9293"
	}
}

Middleware

Two middleware implementations are available in subpackages:

• middleware/grpc provides middleware for gRPC servers.
• middleware/http provides middleware for HTTP REST servers.

When authorization middleware is configured and attached to a server, it examines incoming requests, extracts authorization parameters like the caller's identity, calls the Aserto authorizers, and rejects messages if their access is denied.

Both gRPC and HTTP middleware are created from an AuthorizerClient and a Policy with parameters that can be shared by all authorization calls.

// Policy holds global authorization options that apply to all requests.
type Policy struct {
	// Name is the Name of the aserto policy being queried for authorization.
	Name string

	// Path is the package name of the rego policy to evaluate.
	// If left empty, a policy mapper must be attached to the middleware to provide
	// the policy path from incoming messages.
	Path string

	// Decision is the authorization rule to use.
	Decision string

	// Label name of the aserto policy's instance being queried for authorization.
	InstanceLabel string
}

The value of several authorization parameters often depends on the content of incoming requests. Those are:

• Identity - the identity (subject or JWT) of the caller.
• Policy Path - the name of the authorization policy package to evaluate. A default value can be set in Policy.Path when creating the middleware, but the path is often dependent on the details of the request being authorized.
• Resource Context - Additional data sent to the authorizer as JSON.

Identity

Middleware offer control over the identity used in authorization calls:

// Use the subject name "george@acmecorp.com".
middleware.Identity.Subject().ID("george@acmecorp.com")

// Use a JWT from the Authorization header.
middleware.Identity.JWT().FromHeader("Authorization")

// Use subject name from the "identity" metadata key in the request `Context`.
middleware.Identity.Subject().FromMetadata("identity")

// Read identity from the context value "user". Middleware infers the identity type from the value.
middleware.Identity.FromContext("user")

// Manually pass the identity to the authorizer without resolving it to a user.
// Manual identities are availabe in the authorizer's policy language through the "input.identity" variable.
middleware.Manual().ID("object_id")

In addition, it is possible to provide custom logic to specify the callers identity. For example, in HTTP middleware:

middleware.Identity.Mapper(func(r *http.Request, identity middleware.Identity) {
	username := getUserFromRequest(r) // custom logic to get user identity

	identity.Subject().ID(username) // set it on the middleware
})

In all cases, if a value cannot be retrieved from the specified source (header, context, etc.), the authorization call checks for unauthenticated access.

Policy

The authorization policy's ID and the decision to be evaluated are specified when creating authorization Middleware, but the policy path is often derived from the URL or method being called.

By default, the policy path is derived from the URL path in HTTP middleware and the grpc.Method in gRPC middleware.

To provide custom logic, use middleware.WithPolicyPathMapper(). For example, in gRPC middleware:

middleware.WithPolicyPathMapper(func(ctx context.Context, req interface{}) string {
	path := getPolicyPath(ctx, req) // custom logic to retrieve a JWT token
	return path
})

Resource

A resource can be any structured data that the authorization policy uses to evaluate decisions. By default, middleware do not include a resource in authorization calls.

To add resource data, use Middleware.WithResourceMapper() to attach custom logic. For example, in HTTP middleware:

middleware.WithResourceMapper(func(r *http.Request) *structpb.Struct {
	return structFromBody(r.Body) // custom logic
})

In addition to these, each middleware has built-in mappers that can handle common use-cases.

gRPC Middleware

The gRPC middleware is available in the sub-package middleware/grpc. It implements unary and stream gRPC server interceptors in its .Unary() and .Stream() methods.

import (
	"github.com/aserto-dev/go-aserto/middleware"
	grpcmw "github.com/aserto-dev/go-aserto/middleware/grpc"
	"google.golang.org/grpc"
)
...
middleware, err := grpcmw.New(
	client,
	middleware.Policy{
		Decision: 	   "allowed",
	},
)

server := grpc.NewServer(
	grpc.UnaryInterceptor(middleware.Unary),
	grpc.StreamInterceptor(middleware.Stream),
)

Mappers

gRPC mappers take as their input the incoming request's context and the message.

type (
	// StringMapper functions are used to extract string values like identity and policy-path from incoming messages.
	StringMapper func(context.Context, interface{}) string

	// ResourceMapper functions are used to extract structured data from incoming message.
	ResourceMapper func(context.Context, interface{}, map[string]interface{})
)

In addition to the general WithIdentityMapper, WithPolicyPathMapper, and WithResourceMapper, the gRPC middleware provides methods to help construct resource contexts from incoming messages.

WithResourceFromFields(fields ...string) selects a specified set of fields from the incoming message to be included in the authorization resource.

WithResourceFromMessageByPath(fieldsByPath map[string][]string, defaults ...string) is similar to WithResourceFromFields but can select different sets of fields depending on which service method is called.

WithResourceFromContextValue(ctxKey interface{}, field string) reads a value from the incoming request context and adds it as a field to the resource context.

Default Mappers

The default behavior of the gRPC middleware is:

• Identity is pulled form the "authorization" metadata field (i.e. middleware.Identity.FromMetadata("authorization")).
• Policy path is constructed from grpc.Method() with dots (.) replacing path delimiters (/).
• No Resource Context is included in authorization calls by default.

HTTP Middleware

The HTTP middleware are available under the sub-package middleware/http.

Two flavors are implemented:

• Standard net/http middleware with support for gorilla/mux is implemented in middleware/http/std.
• Gin middleware is implemented in middleware/http/gin.

Both are constructed and configured in a similar way. They differ in the signature of their Handler() function, which is used to attach them to HTTP routes, and in the signatures of their mapper functions.

net/http Middleware

import (
	"github.com/aserto-dev/go-aserto/middleware"
	"github.com/aserto-dev/go-aserto/middleware/http/std"
)
...
mw := std.New(
	client,
	middleware.Policy{
		Decision:	   "allowed",
	},
)

Adding the created authorization middleware to a basic net/http server may look something like this:

http.Handle("/foo", mw.Handler(fooHandler))

The popular gorilla/mux package provides a powerful and flexible HTTP router. Attaching the standard authorization middleware to a gorilla/mux server is as simple as:

router := mux.NewRouter()
router.Use(mw.Handler)

router.HandleFunc("/foo", fooHandler).Methods("GET")

Mappers

HTTP mappers take the incoming http.Request as their sole parameter.

type (
	StringMapper func(*http.Request) string
	StructMapper func(*http.Request) *structpb.Struct
)

In addition to the general WithIdentityMapper, WithPolicyMapper, and WithResourceMapper, the HTTP middleware provides WithIdentityFromHeader() to extract identity information from HTTP headers, and WithNoResourceContext() to omit a resource context from authorization calls.

Default Mappers

The default behavior of the HTTP middleware is:

• Identity is retrieved from the "Authorization" HTTP Header, if present.
• Policy path is retrieved from the request URL and method to form a path of the form METHOD.path.to.endpoint. If the server uses gorilla/mux and the route contains path parameters (e.g. "api/products/{id}"), the surrounding braces are replaced with a double-underscore prefix. For example, with policy root "myApp", a request to GET api/products/{id} gets the policy path myApp.GET.api.products.__id.
• Any path parameters defined using gorilla/mux are included in the resource context. For example, if the route is defined as "api/products/{id}" and the incoming request URL path is "api/products/123" then the resource context will be {"id": "123"}.

Gin Middleware

The gin middleware looks and behaves just like the net/http middleware with the following differences:

• Its Handler function is a gin.HandlerFunc which can be used with IRoutes.Use(...HandlerFunc).
• Its mappers take *gin.Context instead of *http.Request:

    type (
    	StringMapper func(*gin.Context) string
    	StructMapper func(*gin.Context) *structpb.Struct
    )

Check Middleware (ReBAC)

In addition to the pattern described above, in which each route is authorized by its own policy module, the HTTP middleware can be used to implement Relation-Based Access Control (rebac) in which authorization decisions are made by checking if a given subject has the necessary permission or relation to the object being accessed.

This is achieved using the Check function on http.Middleware.

A check call needs three pieces of information:

• The type and key of the object.
• The name of the relation or permission to look for.
• The type and key of the subject. When omitted, the subject is derived from the middleware's Identity with type "user".

Example:

router := mux.NewRouter()
router.Handle(
	"/items/{id}",
	mw.Check(
		std.WithObjectType("item"),
		std.WithObjectIDFromVar("id"),
		std.WithRelation("read"),
	).HandlerFunc(GetItem),
).Methods("GET")

GetItem() is an http handler function that serves GET request to the /items/{id} route. The mw.Check call only authorizes requests if the calling user has the read permission on an object of type item with the object ID extracted from the route's {id} parameter.

Check Options

The Check() function accepts options that configure the object, subject, and relation sent to the authorizer.

WithIdentityMapper(IdentityMapper) can be used to override the identity context sent to the authorizer. The mapper is a function that takes an http.Request and a middleware.Identity and can set options on the Identity object based on information from the request. If an identity mapper isn't provided, the check call uses the identity configured on the middleware object on which the Check call is made.

WithRelation(string) sets the relation name sent to the authorizer.

WithRelationMapper(StringMapper) can be used in cases where the relation to be checked isn't known ahead of time. It receives a function that takes an http.Request object and returns the name of the relation.

WithObjectType(string) sets the object type sent to the authorizer.

WithObjectID(string) sets the object ID sent to the authorizer.

WithObjectIDMapper(StringMapper) is used to determine the object ID sent to the authorizer at runtime. It receives a function that takes an http.Request object and returns an object ID.

WithObjectIDFromVar(string) configures the check call to use the value of a path parameter as the object ID sent to the authorizer.

WithObjectMapper(ObjectMapper) can be used to set both the object type and ID at runtime. It receives a function that takes an http.Request and returns a (objectType string, objectID string) pair.

WithPolicyPath(string) sets the name of the policy module to evaluate in check calls. It defaults to check. If the Policy object used to construct the middleware contains the Root field, the root is used as a prefix. For example, if the root is set to "myPolicy", the Check call looks for a policy module named myPolicy.check.

Other Aserto Services

In addition to the authorizer service, go-aserto provides gRPC clients for Aserto's administrative services, allowing users to programmatically manage their aserto account.

client/authorizer defines a client for services run at the edge and used to serve authorization requests.

An API client is created using New() with the same connection options as the authorizer client.

Edge Client

// Client provides access to Aserto edge services.
type Client struct {
	// Authorizer provides methods for performing authorization requests.
	Authorizer authorizer.AuthorizerClient
}