Domain Resolution and URL Processing

The DNS module provides DNS resolution using c-ares, a C library that provides asynchronous DNS requests. It’s used by Node with some of its other modules, and can be useful for applications that need to discover domains or IP addresses.

To discover the IP address given a domain, use the dns.lookup method and print out the returned IP address:

var dns = require('dns');
dns.lookup('burningbird.net', function (err, ip) {
    if (err) throw err;
    console.log(ip);
});

The dns.reverse method returns an array of domain names for a given IP address:

dns.reverse('173.255.206.103', function (err, domains) {
    domains.forEach(function (domain) {
        console.log(domain);
    });
});

The dns.resolve method returns an array of record types by a given type, such as A, MX, NS, and so on. In the following code, I’m looking for the name server domains for my domain name, burningbird.net:

var dns = require('dns');

dns.resolve('burningbird.net', 'NS', function (err, domains) {
    domains.forEach(function (domain) {
        console.log(domain);
    });
});

This returns:

ns1.linode.com

ns3.linode.com

ns5.linode.com

ns4.linode.com

We used the URL module in Example 1-3 in Chapter 1. This simple module provides a way of parsing a URL and returning an object with all of the URL components. Passing in the following URL:

var url = require('url');

var urlObj = url.parse('http://examples.burningbird.net:8124/?file=main');

returns the following JavaScript object:

{ protocol: 'http:',

slashes: true,

host: 'examples.burningbird.net:8124',

port: '8124',

hostname: 'examples.burningbird.net',

href: 'http://examples.burningbird.net:8124/?file=main',

search: '?file=main',

query: 'file=main',

pathname: '/',

path: '/?file=main' }

Each of the components can then be discretely accessed like so:

var qs = urlObj.query; // get the query string

Calling the URL.format method performs the reverse operation:

console.log(url.format(urlObj)); // returns original URL

The URL module is often used with the Query String module. The latter module is a simple utility module that provides functionality to parse a received query string, or prepare a string for use as a query string.

To chunk out the key/value pairs in the query string, use the querystring.parse method. The following:

var vals = querystring.parse('file=main&file=secondary&type=html");

results in a JavaScript object that allows for easy access of the individual query string values:

{ file: [ 'main', 'secondary' ], type: 'html' }

Since file is given twice in the query string, both values are grouped into an array, each of which can be accessed individually:

console.log(vals.file[0]); // returns main

You can also convert an object into a query string, using querystring.stringify:

var qryString = querystring.stringify(vals)