Domain Name System (DNS)

Terms

Domain Name System

The domain name system, more commonly known as “DNS” is the networking system in place that allows us to resolve human-friendly names to unique IP addresses.

Top-Level Domain

A top-level domain, or TLD, is the most general part of the domain. The top-level domain is the furthest portion to the right (as separated by a dot). Common top-level domains are “com”, “net”, “org”, “gov”, “edu”, and “io”. Top-level domains are at the top of the hierarchy in terms of domain names. Certain parties are given management control over top-level domains by ICANN (Internet Corporation for Assigned Names and Numbers). These parties can then distribute domain names under the TLD, usually through a domain registrar.

Domain Name

A domain name is the human-friendly name that we are used to associating with an internet resource. For instance, “google.com” is a domain name. Some people will say that the “google” portion is the domain, but we can generally refer to the combined form as the domain name.

The URL “google.com” is associated with the servers owned by Google Inc. The domain name system allows us to reach the Google servers when we type “google.com” into our browsers.

SubDomain

DNS works in a hierarchy. TLDs can have many domains under them. For instance, the “com” TLD has both “google.com” and “ubuntu.com” underneath it. A “subdomain” refers to any domain that is part of a larger domain. In this case, “ubuntu.com” can be said to be a subdomain of “com”. This is typically just called the domain or the “ubuntu” portion is called a SLD, which means second level domain.

Likewise, each domain can control “subdomains” that are located under it. This is usually what we mean by subdomains. For instance you could have a subdomain for the history department of your school at “www.history.school.edu”. The “history” portion is a subdomain.

Name Server

A name server is a computer designated to translate domain names into IP addresses. These servers do most of the work in the DNS system. Since the total number of domain translations is too much for any one server, each server may redirect request to other name servers or delegate responsibility for a subset of subdomains they are responsible for.

Name servers can be “authoritative”, meaning that they give answers to queries about domains under their control. Otherwise, they may point to other servers, or serve cached copies of other name servers’ data.

Zone File

A zone file is a simple text file that contains the mappings between domain names and IP addresses. This is how the DNS system finally finds out which IP address should be contacted when a user requests a certain domain name.

Zone files reside in name servers and generally define the resources available under a specific domain, or the place that one can go to get that information.

Records

Within a zone file, records are kept. In its simplest form, a record is basically a single mapping between a resource and a name. These can map a domain name to an IP address, define the name servers for the domain, define the mail servers for the domain, etc.

Root Servers

As we said above, DNS is, at its core, a hierarchical system. At the top of this system is what are known as “root servers”. These servers are controlled by various organizations and are delegated authority by ICANN (Internet Corporation for Assigned Names and Numbers).

There are currently 13 root servers in operation. However, as there are an incredible number of names to resolve every minute, each of these servers is actually mirrored. The interesting thing about this set up is that each of the mirrors for a single root server share the same IP address. When requests are made for a certain root server, the request will be routed to the nearest mirror of that root server.

What do these root servers do? Root servers handle requests for information about Top-level domains. So if a request comes in for something a lower-level name server cannot resolve, a query is made to the root server for the domain.

The root servers won’t actually know where the domain is hosted. They will, however, be able to direct the requester to the name servers that handle the specifically requested top-level domain.

So if a request for “www.wikipedia.org” is made to the root server, the root server will not find the result in its records. It will check its zone files for a listing that matches “www.wikipedia.org”. It will not find one.

It will instead find a record for the “org” TLD and give the requesting entity the address of the name server responsible for “org” addresses.

TLD Servers

The requester then sends a new request to the IP address (given to it by the root server) that is responsible for the top-level domain of the request.

So, to continue our example, it would send a request to the name server responsible for knowing about “org” domains to see if it knows where “www.wikipedia.org” is located.

Once again, the requester will look for “www.wikipdia.org” in its zone files. It will not find this record in its files.

However, it will find a record listing the IP address of the name server responsible for “wikipedia.org”. This is getting much closer to the answer we want.

Domain-Level Name Servers

At this point, the requester has the IP address of the name server that is responsible for knowing the actual IP address of the resource. It sends a new request to the name server asking, once again, if it can resolve “www.wikipedia.org”.

The name server checks its zone files and it finds that it has a zone file associated with “wikipedia.org”. Inside of this file, there is a record for the “www” host. This record tells the IP address where this host is located. The name server returns the final answer to the requester.

DNS Records

Types d’enregistrements de ressources DNS

CNAME: enregistrement de ressource Alias.Redirige un domaine vers un autre dans le système DNS. Quand un serveur de noms recherche un domaine et trouve un enregistrement CNAME, il remplace le premier nom de domaine par le CNAME, puis recherche le nouveau nom.

SRV: enregistrements de ressources de localisateur de service. Utilisé par Skype Entreprise Online et Exchange Online pour coordonner le flux d’informations entre les services Microsoft. Par exemple, les enregistrements SRV sont requis pour voir les informations de présence dans Outlook Web App et pour utiliser Skype Entreprise Online, Skype ou d'autres outils de messagerie instantanée avec des utilisateurs dans d'autres sociétés

Enregistrement SPF (Sender Policy Framework): Enregistrement TXT qui empêche l’usurpation d’adresse de courrier et le hameçonnage.

MX Record

SOA Records

PTR Records

The PTR records are used define a name associated with an IP address. PTR records are the inverse of an A or AAAA record. PTR records are unique in that they begin at the .arpa root and are delegated to the owners of the IP addresses. The Regional Internet Registries (RIRs) manage the IP address delegation to organization and service providers. The Regional Internet Registries include APNIC, ARIN, RIPE NCC, LACNIC, and AFRINIC.