Introduction to HTTP: DNS Servers


Overview

Throughout this series, we will be tackling the basics such as:-
(Part 1) How does DNS work? [You are here!]
(Part 2) Network Stack, OSI Model
(Part 3) HTTP Methods and Formats
(Part 4) Client Identification
(Part 5) Basic/Digest Authentication
(Part 6) HTTPS working with SSL/TLS


What is HTTP?

HyperText Transfer Protocol (HTTP) is a protocol originally devised by Sir Tim Berners Lee in 1989, and it forms the basis for how web pages are communicated from the web server to the client’s browser.

Sir Tim Berners Lee, the Father of World Wide Web. Photo courtesy of CNET

DNS Servers

Is the connection to the webpage established immediately after typing in the Domain Name, such as medium.com? Definitely not!

Machines, unlike us, recognize the location of webpages by IP Addresses. These string of numbers, such as 104.16.121.127, are more machine friendly especially since there are millions of domain names on the Web.

Domain Name System (DNS) plays a crucial role in the whole HTTP request process, as it allows us to call a webpage by typing a simple domain name, http://www.medium.com instead of 104.16.121.127 every time you want to access the site.

Without DNS, your brain would be filled with numbers just trying to remember the IP Addresses for every single website you use!

Flow of DNS Resolution, maintained by a distributed database system

Now that we know that an IP Address is requested every time we type in the domain name, let’s find out where this request searches for the correct IP Address.

Local Cache

A cache is a block of memory for temporary storage of data that has a high probability of being used again. The first thing that happens is the DNS Resolver (residing in your computer) will check the browser’s cache, followed by the computer’s DNS cache. If the website had been accessed recently, it would have the IP address cached in the system.

In that case, the browser can immediately call the IP Address to retrieve the webpage!

One thing to note here is that every cache has an expiry date, called the “Time to Live” setting, which determines how long the cache may be stored when the website is accessed. We will address how that works later on.

DNS Recursor

If the IP Address can’t be found in the local cache, it will then request from the DNS Recursor, which is often the DNS Server of your Internet Service Provider (ISP).

These Internal DNS Servers have caches from websites that their clients have visited recently. Again, if the IP Address can’t be found here, it will be passed on to the next Domain Server.

Root-Level Domain Server

The Root-Level Domain Server (RLDS), or sometimes called the ‘ . ’ Name Server, is simply a gatekeeper for requests. It reads the request and locates the appropriate domain server to redirect to.

As such, it plays an important role in redirection to the next layer of Domain Servers, and are dispersed all around the world to prevent malicious attacks from bringing down the World Wide Web by targeting the RLDS.

Top-Level Domain Server

The Top-Level Domain Server (TLDS), is the name server for domains that end with their specific domain suffixes such as .com.org or .io. After being passed down by RLDS, this layer works in the same way as the second gatekeeper. It takes the requests and runs through its DNS Server to redirect the request to the last and final stop, the Second-Level Domain Server.

Since the number of domain names are increasing exponentially, it is impossible for the RLDS to be able to store or redirect such a sheer amount of IP Addresses. As such, it is redirected to the TLDS to diversify the processing power and memory required.

Second-Level Domain Server

This layer is where all the information is stored about the domain is accessible. This DNS Server is usually owned by the institute that is responsible for hosting your website.

As such, a request for the record of the domain is sent to this DNS Server, and greeted with the IP Address, along with other important information such as the server it is on, and the alias it has.

Success!

The browser now receives the IP Address, which will be used to establish a connection with the host server using TCP/IP and retrieve the webpage via HTTP. This will be discussed in Part 2.


“Time to Live” Setting

DNS Records have a Time to Live (TTL) Setting, which determines the amount of time that any of the domain servers are allowed to cache the record.

Caching is important as it reduces the loading time for the page, since the DNS information will have to be reacquired every time the domain name is requested. Hence, a high TTL would allow the DNS records to stay alive for a longer period of time, and allow webpages to load faster.

Why don’t all DNS Records have a high TTL then?

By having a high TTL, it would mean that changes to the DNS would not be reflected to visitors immediately, but only after the DNS Record has expired.

For example, if we were to change the host for this webpage, and have a high TTL, the changes would not appear on the visitors browser immediately. This might result in broken links and users not being able to access the webpage.


Hostname  —  IP Address Relationship

So a single domain name is attached to one IP Address?

The answer is yes… and no. It can be, but doesn’t have to be a one-to-one relationship.

Single Hostname, Multiple IP Addresses

A single hostname such as http://www.google.com can correspond to multiple IP Addresses, to balance the load on the server since there is a significant amount of users calling on the same webpage at any one given point in time.

DNS Servers use a “Round Robin” method, such that all IP Addresses are equally utilized.

Multiple Hostname, Single IP Address

The purpose for this might be for referral links. For example, searching amazon.com/products/pc will show the product screen for PCs. Although amazon.com/products/pc?user=cherdon will also show the same webpage, any purchase would tell Amazon that I was the referrer, allowing me to gain commission from it.

Companies often buy multiple domains that link to the same webpage as well. For example, google.com and google.net would link you to the same search engine webpage.


Conclusion

The DNS Server is very important as it stores a database for machine-friendly IP Addresses under user-friendly Domain Names. Now that we have learnt how DNS Servers work together in a distributed database, let us explore how the connection with the host server is established with the IP Address in Part 2!

**Update: Part 2 will be released 27th August, 2018.

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