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Learning about DNS

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  3. AWS Certified SysOps Administrator – Associate (SOA-C01)
  5. Learning about DNS
  • service to translate domain names into IP addresses
  • Example, domain name www.example.com might translate to 198.105.232.4.
  • works in an complex and hierarchical manner.
  • Working
    • if one DNS server doesn’t know IP address, it asks another one, and so on, till IP address is returned.
    • Domain names are organized as a tree or hierarchical in manner
    • Each node of the tree, called a domain, is given a label.
    • The domain name of the node is the concatenation of all the labels on the path from the node to the root node like network.support.vskills.in
  • 2 Types of DNS systems
    • Recursive DNS
      • It knows where to get corresponding IP address for a domain.
      • It connects to an Authoritative DNS to get this information,
      • then cache this information for a period of time.
      • It does not itself have target IP addresses.
    • Authoritative DNS
      • The final authority on DNS information.
      • keep the IP addresses for domain names
      • provide an update mechanism for changing those values
  • 2 types of IP addresses:
  • IPv4
    • 32 bit address
    • 4 billion different addresses (4,294,967,296)
  • IPv6
    • Created to solve depletion issue of IPv4 address space
    • 128 bit address
    • 340 undecillion addresses (340,282,366,920,938,463,463,374,607,431,768,211,456)
  • Top Level Domains: Signified by the last word in a domain name – .com, .edu
  • Controlled by the Internet Assigned Numbers Authority (IANA)
  • Stored in a root zone database which is a database of all available TLDs (Top Level Domains)
  • Database can be found at http://www.iana.org/domains/root/db
  • Domain Names:
    • All names in a given domain name have to be unique
    • DNS registrars are authority’s that can assign domain names directly under one or more TLD’s
    • Domains are registered with InterNIC, as service of ICANN, which enforces uniqueness of domain names across the internet
    • Each domain name becomes registered in a central database known as the WhoIS database
    • Popular domain registrars include godaddy.com, namecheap.com, Route53 etc..
  • SOA (Start of Authority) Records store information about:
    • Name of the server that supplied the data for the zone
    • Administrator of the zone
    • Current version of the data file
    • Number of seconds a secondary name server should wait before checking for updates
    • Number of seconds a secondary name server should wait before retrying a failed domain transfer
    • Maximum number of seconds that a secondary name server can use data before it must either be refreshed or expired
    • Default number of seconds for the TTL (Time to Live) on resource records
  • DNS Record Types:
    • NS or Name Server Records are used by TLD’s to direct traffic to the content DNS server which contains the authoritative DNS records
    • A or Address records are used by a computer to translate the name of the domain to an IP address
    • CNAMES or Canonical Names can be used to resolve one domain name to another
      • CNAME’s can’t be used for naked domain names (zone apex). As such awsdocs.com must be either an A record or an Alias record
    • Alias records are used to map resource record sets in your hosted zone to ELBs, CloudFront Distributions, or S3 Buckets that are configured as websites
      • Alias records work like CNAME records in that you can map one DNS name to another target DNS name
      • Alias records can save time because Route53 automatically recognizes changes in the record set that the alias resource record set refers to
      • You are NOT charged for requests to Alias records, you ARE charged for requests to CNAMES, so using Alias records is cheaper
    • TTL or Time to Live is the length that a DNS record is cached on either the resolving server or the users local PC. The lower the TTL, the faster changes to DNS records take to propagate throughout the internet
  • ELBs do not have a pre-defined IPv4 address, DNS names are used for ELB resolution
  • Always use an Alias record over a CNAME where possible, as it’s cheaper and faster

DNS Routing

The Name Resolution process is done as

  • Upon receiving query from client, the local nameserver will check if it has the authority for the required domain name. If it has, the local nameserver returns the IP address sought. Otherwise, go to step 2.
  • Query one of the root nameservers to find the server authoritative for the next level down.
  • Querying this second nameserver for the address of a DNS server with detailed knowledge of the second-level domain.
  • Repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.

 

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