HTTPDNS:Write custom resolution function code

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

The HTTPDNS resolution process consists of several stages. Software-Defined Name System (SDNS) lets you 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 as needed.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 to the event input parameter object of an FC function. The context is passed as a JSON object with a fixed structure.

The fields are described as follows:

Example of input parameters

{
  "domainName": "www.aliyun.com", // The domain name that is being resolved.
  "clientIp": "192.168.1.4", // The client IP.
  "location": {
    "continent": "asia", // The continent where the client IP is located.
    "region": "cn", // The country or region where the client IP is located.
    "isp": "bgp", // The carrier line for the client IP.
    "province": "zhejiang" // The province where the client IP is located.
  },
  "ips": ["192.168.1.3"], // A list of IP addresses from the resolution result. This field is not available in the BEFORE_READ_CACHE stage.
  "ttl": 60, // The TTL of the resolution result. This field is not available in the BEFORE_READ_CACHE stage.
  "hookType": "BEFORE_WRITE_CACHE", // The execution stage of the function.
  "parameters":{ // The parameter object carried in the request.
    "param1":"p1",   // Corresponds to the "sdns-param1=p1" parameter in the URL.
    "param2":"p2"
   }
}

Format of output parameters

An FC function must return the processed result to HTTPDNS in the following format. All fields are optional. If a field is not returned or its value is null, HTTPDNS assumes that the SDNS function does not need to modify that 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

This example uses the nodejs6 or nodejs8 runtime.

'use strict';
exports.handler = (event, context, callback) => {
  // Format the input parameters into an object.
  const eventObj = JSON.parse(event.toString());
  const {
    location, // Region
    ips, // The resolution result returned by the authoritative DNS server.
    ttl, // The original TTL.
  } = eventObj;

  if (location.province === 'zhejiang' && location.isp === 'chinanet') {
    // When a China Telecom user in Zhejiang accesses the domain name, return the specified IP address.
    callback(null, {
      ips: ips.concat(['1.1.X.X']), // Add the specified IP address to the returned IP address array.
      ttl: event.ttl * 2, // Modify the TTL.
      extra: "", // Additional parameters to be sent to the client.
    });
  } else {
    // Default return.
    callback(null, {
      ips,
      ttl,
      extra: "",
    });
  }
};

For more information about how to create a service in Function Compute, see Configure custom DNS resolution using Function Compute.