DNS 101

Route 53 - Amazons DNS service

History: Route 53 (Route 66 is the first roud across america, DNS is on port 53)

What is a DNS?

• its a phone book basically, (look up name, get phone number)
• DNS is used to convert human friendly domain names into an internet protocoal (IP) address (IPV4 or IPV6)

IPv4 vs IPv6

• The IPv4 space is a 32 bit field and has over 4 billion addresses
• IPv6 was created to solve this depletion issue and has an address space of 128 bits

Top Level Domains

• If we look at common domain name such as google.com, bbc.co.uk. you will notice a string of characters separated by periods.
• The last word in a domain represents the "top level domain".
• The second word in a domain name is known as a second level domain name (this is optional though and depends on the domain name)

Example

• .com
• .edu
• .gov
• .co.uk (uk is top level, co is second level)
• .gov.uk
• .com.au

These top level domain names are controlled by Internet Assigned Numbers Authority (IANA) in a root zone database which is essentially a database of all available top level domainds

• You can visit this database by visiting http://iana.org/domains/root/db

Domain Registrars

• Since all of the names in a given domain name have to be unique there needs to be a way to organize all this to avoid duplication.
• This is where domaind registrars come in.
• A registrar is an authority that can assign domain names directly under one or more top level domains.
• These domains are registeed with InterNIC, a service of ICANN, which enforces uniquesness of domain names across the internet.
• each domain name becomes registed in a central database known as the WholS database

Sample domain Registrars (where you go and buy domain name)

• Amazon (new one actually)
• GoDaddy.com
• 123-reg.co.uk

Start of Authority Records (SOA)

The SOA records store information about

• The name of the server that supplied the data for the zone
• The administrator of the zone
• The current version of the data file
• The default number of seoncds for the time-to-live file on resoruce records

NS Records

NS stands for Name Service Records

• They are used by Top Level Domain servers to direct traffic to the Content DNS service which continas the authoritative DNS records.

Sample Flow of Execution

• Browser goes TDL
• TDL has the NS, return NS records
• Then we go to the NS records, returns SOA
• SOA will have all the DNS records,

Whats an "A" records

• An "A" records is the fundamental type of DNS record.
• The "A" is A records stands for Address.
• The "A" records is used by a computer to translate the name of the domain to an IP address
• Example http://www.acloud.guru might point to http://123.10.10.80.

TTL

What's an TTL?

• The length that a DNS records is cached on either the Resolving Server or the users own local PC is equal to the value of the "Time to Live" (TTL) in seconds.
• The lower the time to live, the faster changes to DNS records take to propagate through the internet.
• Default TTS is 48 hours (If you make a dns change it could take 48 hours to propagate through the internet)

CName

What's a CName?
A Canonical Name (CName) can be used to resolve one domain name to another.

• For example, you may have a mobile website with the domain name http://m.acloud.guru that is used for when users browse to your domain on their mobile devices. You may also want the name http://mobile.acloud.guru to resolve to this same address
• So instead of having 2 IP addresses you can have one for both CNames. (Another reference that we can look up for the IPv4 Address)
• Examples
  • Batman -> See West Adam
  • Adam West -> 123 1234 3456

Alias Records

• Alias records are used to map resource records sets in your hosted zone to Elastic Load Balanced, CloudFront distributions, or S3 buckets that are configured as websites.

• Alias records work like a CNAME record in that you can map one DNS name (www.exaple.com) to another "target" DNS name (elb1234.elb.amazonaws.com)

• Key difference - A CName can't be used for named domain names (zone apex record.) You can't have a CNAME for http://acloud.guru, it must be either an "A" records on an Alias.

Route53 Exam Tips

• ELBs do not have pre-defined IPv4 addresses; you resolve to them using a DNS name
• Understand the difference between an Alias Records and a CNAME
• Given the choice, always choose an Alias Records over a CName

Common DNS Types

• SOA Records
• NS Records
• A Records
• CName
• MX Records
  • Used for mail
• PTR Records
  • Reverse of a A records, look up a name against a IP address