HTTPDNS:Write user-defined resolution functions

Resolution mechanism of compute policies for a user-defined resolution function

The HTTPDNS resolution process consists of several stages. Software-Defined Name System (SDNS) allows you to call resolution functions that you create in Function Compute among these stages. This way, additional stages are included in the process, and the resolution process of HTTPDNS is changed to implement

The following table describes the capabilities that the user-defined resolution functions must deliver by additional stage.

Input and output of SDNS functions

HTTPDNS passes the runtime context to a Function Compute function that is defined based on your business requirements.Then, the function can process the runtime context. The precessed results are merged back into the HTTPDNS resolution process.

Format of input parameters

HTTPDNS passes the runtime context as input parameters to a function in Function Compute. The runtime context is an event object of the function and is in the JSON format.

The following table describes the fields.

Example of input parameters

{
  "domainName": "www.aliyun.com", // The domain name that you want to resolve.
  "clientIp": "192.168.1.4", // The IP address of the client.
  "location": {
    "continent": "asia", // The continent to which the IP address of the client belongs.
    "country": "china", // The country to which the IP address of the client belongs.
    "isp": "bgp", // The ISP to which the IP address of the client belongs. 
    "province": "zhejiang" // The province to which the IP address of the client belongs.
  },
  "ips": ["192.168.1.3"], // The IP addresses to which the domain name is resolved. This field is invalid in the BEFORE_READ_CACHE stage.
  "ttl": 60, // The TTL value of the resolution result. This field is invalid in the BEFORE_READ_CACHE stage.
  "hookType": "BEFORE_WRITE_CACHE", // The resolution stage of the function.
  "parameters":{ // The parameter objects during the resolution.
    "param1":"p1", // The sdns-param1=p1 parameter in the URL.
    "param2":"p2"
   }
}

Format of output parameters

The functions in Function Compute return the processed runtime context to HTTPDNS in the following format to facilitate resolution in HTTPDNS. All fields are optional. If a field is not returned or null is returned for a field, HTTPDNS considers that SDNS does not need to modify the field.

Sample output parameter

{
    "ips": event.ips.concat(['192.168.1.2']),
    "ttl": event.ttl * 2,
    "extra": "some-thing-send-to-user"
    // ,"domainName": "www.alibabacloud.com" This field is valid only in the BEFORE_READ_CACHE stage.
    // ,"cacheKey": "cache-key-001" This field is valid only in the BEFORE_READ_CACHE and BEFORE_WRITE_CACHE stages.
 }

Function capabilities

You can call resolution functions that you create in Function Compute at specified stages of the HTTPDNS resolution process. You can also use the following capabilities:

1. Obtain the information about the region and ISP that correspond to the IP address of the client.

2. Modify the DNS resolution result and the TTL value.

3. Add custom output and return the custom output together with the resolution result.

Function demo

The programming language of the demo is Node.js 6 or Node.js 8.

'use strict';
exports.handler = (event, context, callback) => {
  // Format the input parameters as objects.
  const eventObj = JSON.parse(event.toString());
  const {
    location, // The region.
    ips, // The resolution result returned.
    ttl, // The original TTL.
  } = eventObj;

  if (location.province === 'zhejiang' && location.isp === 'chinanet') {
    // When a user of China Telecom in Zhejiang province accesses the domain name, the specified IP address is returned.
    callback(null, {
      ips: ips.concat(['1.1.1.1']), // Add the specified IP address to the returned array of IP addresses.
      ttl: event.ttl * 2, // Modify the TTL.
      extra: "", // Additional parameters carried to the client.
    });
  } else {
    // Returned by default.
    callback(null, {
      ips,
      ttl,
      extra: "",
    });
  }
};

For more information about how to create a Function Compute service, see Add a Function Compute policy.