Overview

The Domain Name System is a distributed database located on different servers that stores mapping information between domain names and IP addresses. DNS performs the translation of a domain name to an IP address and vice versa.

Some concepts in DNS

Domain name space

Is a tree structure containing the entire domain space as the following image.

Domain

Is a branch of the domain name space. The following is an example of the domain purdue.edu.

Domain name

Each node in the tree has a label. A domain name of any node in the tree is the order of labels in the path from that node to the root, each node's name is separated by a dot .

Resource record

Information about domain names is stored in resource records. Each resource record specifies information about a particular object. The DNS server uses these records to answer DNS queries. Here are a few common resource records:

• SOA - Start of authority: Record stores zone administration information and each zone has only one SOA.
• NS - Name server: Record stores the mapping of the domain name to a list of name servers.
• A: Record stores the mapping of a domain name to an IP address.
• PTR - Reverse-lookup pointer: Record stores the mapping of IP addresses to a domain name.
• MX - Mail exchange: Record stores a domain name mapping to a list of mail servers. For example, when you send mail to [email protected], the mail will be forwarded to the mail server specified in the MX Record.
• CNAME: Record stores the mapping of one domain name to another domain name.

Top level domain

The domain at the top level just below the root domain in the DNS hierarchy. Top level domains are divided into two main categories:

• Generic TLD (gTLD)
• Country Code TLD (ccTLD)

Some examples of gTLDs:

• .com: For commercial websites
• .org: For Non-Profits
• .edu: Limited to schools and institutions
• .net: Initially limited to network infrastructure organizations but now unrestricted

Some examples of ccTLD:

• .us: United States
• .in: India
• .uk: United Kingdom
• .vn: Vietnam

Image of the DNS tree structure:

Delegation và Authority

One of the main goals of DNS system design is to decentralize governance. It is achieved through delegation.

The authority that administers the domain is called the Authority. The authority governing root domains and gTLDs belongs to ICANN - Internet Corporation for Assigned Names and Numbers. ccTLD domains are governed by countries.

A domain management organization can divide it into sub-domains. Each sub-domain can delegate administration to other organizations. For example, the domain "standford.edu" is administratively delegated to Standard University.

Name server and zone

Name server: A server that stores resource records about the domain and name server information of its sub-domains. Name servers usually only contain complete information about a part of the domain name space called the zone.

Note: The root name server is the name server of the root domain or the root name server of the entire DNS system.

The following is an example of an edu domain divided into zones such as zone berkeley.edu, zone purdue.edu and zone nwu.edu zone. And edu itself is also a zone.

DNS Resolver

The client side of DNS is called DNS Resolver. DNS Resolver is responsible for performing DNS queries.

DNS Query

There are 3 types of DNS queries:

• Recursive query: Recursive query
• Iterative query: Iterative query
• Inverse query: Query domain name for specified IP address

Following are the steps when performing domain name resolution such as rutgers.edu.

• The resolver will send a query to the local DNS
• DNS local will look up rutgers.edu in its database. If yes return the IP address to the Resolver. If not proceed to the next step.
• Resolver will send query to root name server
• The root name server will delegate the query response to the name server of the .edu domain
• The name server of the domain .edu continues to delegate the query response to the name server of the domain rutgers.edu
• Returns the IP information retrieved from the A record to the Resolver.

A few tips when using DNS

Configure local DNS using hosts file

I have a situation where the customer will migrate to a new server without stopping the service on the old server and due to some security rules, the customer configures 2 servers with the following information:

• The domain name example.com is associated with the IP of server A.
• Both server A and server B have the same config server_name = example.com on the web server.

So how to access the website example.com of server B?

I added the following config to the hosts file with the path /etc/hosts on linux or mac machines.

SERVER-B_IP_ADDRESS example.com

Note: The host file is used as a resource record to resolve domain names to IP addresses.

Then if you type example.com into the browser, it will run to server B's example.com website instead of server A. To confirm if the config is working properly, turn on the browser's Developer Tools and then check the request's Headers information. has the following form:

Request URL: http://example.com/
Request Method: GET
Status Code: 200 OK
Remote Address: SERVER_ADDRESS
Referrer Policy: no-referrer-when-downgrade

Configure DNS for domain name

For a website to work, we must perform the operation of pointing the domain name to that website. And there are two ways to do this:

• Point the domain name directly to the IP address of the server hosting that website.
  or
• Add the server provider's name server information to the domain name.

After the configuration is complete, you can use the following tool to check the domain name configuration information:

https://ipinfo.info/html/ip_checker.php/

Using CNAME records

Suppose you have 2 websites example.com and example.vn on the same server. Then you can configure 2 domain names pointing to the same IP address, but if you intend to change the server's IP address, you must reconfigure the IP for both domain names.

However, if you use a CNAME record, for example, point domain name example.vn to domain name example.com as in the example below, then you need to configure IP only at domain main example.com.

NAME               TYPE    VALUE
---------------------------------------
example.vn.        CNAME   example.com.
example.com.        A      192.0.2.23

References

http://web.deu.edu.tr/doc/oreily/networking/dnsbind
https://www.thegeekstuff.com/2013/12/dns-basics