Developing a Custom Authorizer with FunctionGraph

Overview

In addition to IAM and app authentication, APIG also supports custom authentication with your own authentication system, which can better adapt to your business capabilities.

Custom authentication is implemented using the FunctionGraph service. You can create a FunctionGraph function so that APIG can invoke it to authenticate requests for your API. This section uses basic authentication as an example to describe how to implement custom authentication with FunctionGraph.

Developing a Custom Authentication Function

Create a function on the FunctionGraph console by referring to section "Creating a Function for Frontend Custom Authentication" in the API Gateway Developer Guide.

Specify the runtime as Python 3.6.

After the function is created, go to the function details page. On the Code tab page, copy the following code to index.py (if you are using a dedicated gateway, for which the authorizer_context_support_num_bool parameter has been enabled, the type of value in context can be boolean or number).

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# -*- coding:utf-8 -*-
import json
def handler(event, context):
# If the authentication information is correct, the username is returned.
    if event["headers"]["authorization"]=='Basic dXN****cmQ=':
        return {
            'statusCode': 200,
            'body': json.dumps({
                "status":"allow",
                "context":{
                    "user_name":"user1"
                }
            })
        }
    else:
        return {
            'statusCode': 200,
            'body': json.dumps({
                "status":"deny",
                "context":{
                    "code": "1001",  
                    "message":"incorrect username or password",
                    "authorizer_success": "false"  
                }
            })
        }

Creating a Custom Authorizer

On the APIG console, go to the Create Custom Authorizer page, set Type to Frontend, select the function created in the preceding section, and click OK.

Creating a Custom Authentication API

Create an API by referring to section "Creating an API" in the API Gateway User Guide. Set the authentication mode to Custom, and select the custom authorizer created in the preceding section. After modifying the API, publish it.

Setting the Error Response

If incorrect authentication information is carried in a request for the API, the response is displayed as follows:

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{"error_msg":"Incorrect authentication information: frontend authorizer","error_code":"APIG.0305","request_id":"36e42b3019077c2b720b6fc847733ce9"}

To return the field in the function's context as the API response (if you are using a dedicated gateway, for which the authorizer_context_support_num_bool parameter has been enabled, the type of value in context can be boolean or number), modify the gateway response template. On the details page of the group to which the API belongs, navigate to the Gateway Responses area on the Gateway Information tab, and click Edit. Change the status code to 401, modify the response template with the following code, and click OK (no need to add double quotes for variables of the boolean or number type):

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{"code":"$context.authorizer.frontend.code","message":"$context.authorizer.frontend.message", "authorizer_success": "$context.authorizer.frontend.authorizer_success"}

After the modification, if incorrect authentication is transferred when calling the API, the status code 401 is returned and the response result is as follows:

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 {"code":"1001","message":"incorrect username or password","authorizer_success": "false"}

Mapping Frontend Authentication Parameters to Backend Parameters

If the authentication is successful, the context information returned by the function can be transferred to the backend of the API. To do this, perform the following configurations:

On the APIs page, choose More > Edit in the row that contains the API, and go to the Define Backend Request page. Add a system parameter, specify the parameter type as Frontend authentication parameter, set the parameter name to the content of the context field in the function response, and set the name and location of the backend parameter to which you want to the map the frontend authentication parameter.

After modifying the API, publish it again. If the authentication information carried in a request for the API is correct, the response result contains the X-User-Name header field whose value is the same as that of user_name in the context field of the authentication function.