DNS Servers

DNS Records are stored distributed across global, a DNS lookup will involve multiple Servers of different types.

From the aspect of DNS client there are two types of query, recursive and iterative, which is primarily controlled by the DNS client. Client can set a Recursion Desired (RD) bit in request header to indicate that.

• Recursive query means the client only sending one request, the DNS Resolver giving the resource back by querying recursively.
• Iterative query means the DNS Resolver return the location of a nameserver or even root server, then client send query(s) to the location responded iteratively until the resource is resolved.

DNS Resolver

DNS Resolvers are services standing between DNS client and Nameservers. Any query made by DNS client will hit a DNS Resolver first. The resolver will try its best to resolve the requested resource record, and there are several possible actions that an resolver can take:

• If the record is already part of its cache, it return the result right away.
• In a recursive query, it send a request to the Root Nameserver, then to a TLD Nameserver, finally to a Authoritative Nameserver. If everything went well, with the final IP address in hand, it return it to the client.
• In a iterative query, it return the location of a nameserver closest to the requested resources per its knowledge.

Root Nameserver

There are 13 DNS root nameservers known to every DNS Resolver. Each nameserver has one IP address, but distributed globally, using anycast routing to distribute traffic.

A root server accepts a recursive resolver’s query which includes a domain name, and the root nameserver responds by directing the recursive resolver to a TLD nameserver, based on the extension of that domain (.com, .net, .org, etc.).

TLD Nameserver

TLD(Top-Level Domain) Nameserver maintains information for all the domain under the common domain extension, such as .com, .net.

Authoritative Nameserver

The authoritative nameserver maintains information specific to the domain name it serves, such as google.com. When DNS resolver reaches the authoritative nameserver for resolution query, it will give back the IP address if an A or AAAA record is found. If an CName record is found, the alias domain will be returned to resolver, and resolver will have to initialize a new round of look up.

Record Types

DNS records (aka zone files) are instructions that live in authoritative DNS servers and provide information about a domain. Records includes:

• A (Address record): The record that holds the IP address of a domain.
• AAAA (IP Version 6 Address record): The record that contains the IPv6 address for a domain (as opposed to A records, which stores the IPv4 address).
• CNAME (Canonical Name record): Forwards one domain or subdomain to another domain, does NOT provide an IP address.
• MX (Mail exchanger record): Directs mail to an email server.
• TXT (Text Record): This record lets an admin store text notes in the record. These records are often used for email security.
• NS (Name Server records): Stores the name server for a DNS entry.
• SOA (Start of Authority): Stores admin information about a domain.
• SRV (Service Location record): Specifies a port for specific services.
• PTR (Reverse-lookup Pointer record): Provides a domain name in reverse lookups.
• CERT (Certificate record): Stores public key certificates.

Reverse DNS Lookup

While DNS lookup refers to resolving IP address(es) from domain name, Reverse DNS Lookup refers to resolving domain name from IP address(es). There is no way for the nameserver to know this unless the there are corresponding PTR records.

Subdomains

To save some money, I put all my website under one domain. — Zane Chen

Subdomain is commonly used to logically split the domain, so different websites can be hosted on child domains under one main domain.

To create Subdomain, A, AAAA or CNAME record is needed. For example, to host my portfolio at portfolio.my-domain.com, a CNAME record with host as portfolio and value point the server is required.