lookup-dns-cache - DNS cache to replace NodeJS dns.lookup standard method

Super simple to use

``js
const request = require('request');
const {lookup} = require('lookup-dns-cache');

// With "request" module

request({
url: 'http://google.com',
method: 'GET',
lookup: lookup
}, (error, response, body) => {
// ...
});

// Direct usage

lookup('google.com', {}, (error, address, family) => {
// ...
});
`

Table of contents

- Motivation
- How to use
- Inner implementation
- Examples

---

Motivation

The main idea behind this package is eliminate NodeJS event loop usage when you do network request.
See NodeJS DNS implementation to understand the problem with dns.lookup.


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---

How to use

This module supports almost the same params as dns.lookup does. Concretely, you can pass options object as a second param, and
set:
- family to 4 or 6
- all flag to true/false if you want/don't want get all IP addresses at once.

Because this implementation does not use getaddrinfo method, the hints param is not supported.

The verbatim param is not supported for now. If you will not specify any family you will get IPv4 addresses first and IPv6 addresses second.

The callback function works the same way as a standard method.

The error object would have all fields the standard implementation's error object has.

NodeJS dns.lookup:
`javascript
> const dns = require('dns');
> dns.lookup('host-doesnot-support-ipv6', {family: 6}, console.log)

> { Error: getaddrinfo ENOTFOUND vss-cc-ha.hwtool.net
at errnoException (dns.js:55:10)
at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:97:26)
code: 'ENOTFOUND',
errno: 'ENOTFOUND',
syscall: 'getaddrinfo',
hostname: 'vss-cc-ha.hwtool.net' }

`

lookup-dns-cache
`javascript
> const {lookup} = require('lookup-dns-cache');
> lookup('host-doesnot-support-ipv6', {family: 6}, console.log)

> { Error: queryAaaa ENOTFOUND vss-cc-ha.hwtool.net
at makeNotFoundError (/path/lookup-dns-cache/src/Lookup.js:182:19)
at ipv6AddressesTable.resolve (/path/lookup-dns-cache/src/Lookup.js:147:37)
at Immediate.setImmediate [as _onImmediate] (/path/lookup-dns-cache/src/IpAddressesTable.js:70:48)
at runCallback (timers.js:773:18)
at tryOnImmediate (timers.js:734:5)
at processImmediate [as _immediateCallback] (timers.js:711:5)
hostname: 'vss-cc-ha.hwtool.net',
syscall: 'queryAaaa',
code: 'ENOTFOUND',
errno: 'ENOTFOUND' }
`

If you are looking for IPv4 addresses only, explicitly specify param {family: 4}. In that case, you will avoid
spending time on useless searching for IPv6. Apply the same technique if you are looking for IPv6 addresses only.

Under the hood, lookup method has Round-robin algorithm. It means that if particular hostname resolves to several addresses
it will return new address every time you call that function. For example:
`javascript
// hostname: example.com
// resolves to: [1.2.3.4, 5.6.7.8, 9.10.11.12]

lookup('example.com', {family: 4}, (error, address, family) => {
// address === "1.2.3.4"
// family === 4
});

lookup('example.com', {family: 4}, (error, address, family) => {
// address === "5.6.7.8"
// family === 4
});

lookup('example.com', {family: 4}, (error, address, family) => {
// address === "9.10.11.12"
// family === 4
});

lookup('example.com', {family: 4}, (error, address, family) => {
// address === "1.2.3.4"
// family === 4
});
`

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---


Implementation

Under the hood, this package uses dns.resolve4 and dns.resolve6 methods with {ttl: true} param.
It caches addresses for that particular hostname for DNS TTL time and returns one address if you specified {all: false} (default value)
and array of addresses if {all: true}.

If you didn't specify family type ({family: 4} or {family: 6}) the method searches for addresses of {family: 4} and {family: 6} in parallel.
After that, if you specified {all: true} it returns an array in form [[...IPv4],[...IPv6]], in other case it returns IPv4 or IPv6 address.
(IPv4 has more priority).

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---


Examples

`javascript
lookup('hostname', {all: true}, (error, results) => {
// results is an array that contains both IPv4 and IPv6 addresses (Ipv4 first).
//
// error - null
// results - [
// { address: '1.2.3.4', family: 4 },
// { address: '5.6.7.8', family: 4 }
// ]
});
`

`javascript
lookup('hostname', {all: false}, (error, address, family) => {
// address and family of the first resolved IP (IPv4 or IPv6 if supported).
// error - null
// address - '1.2.3.4'
// family - 4
});
`

`javascript
lookup('hostname', {all: false, family: 4}, (error, address, family) => {
// address and family of the first resolved IP (IPv4 only).
});
`

`javascript
lookup('hostname', {all: false, family: 6}, (error, address, family) => {
// address and family of the first resolved IP (IPv6 only).
// will return an error if IPv6 is not supported. See NodeJS dns.lookup doc.
});
``

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---

Similar packages

Yahoo tried to solve this problem in own way https://github.com/yahoo/dnscache.

The big disadvantages if this package are:
- monkey patching dns module
- does not support DNS TTL
- cache just one IP address and use it for every request (no advantage of round-robin if you have dns resolver that returns several addresses)

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