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DNS ( Domain Name System )

Basically, If we have used the internet, it means we have used the DNS. DNS is used to convert human readable names(i.e https://google.com) to Internet Protocol(IP) (such as http://172.217.166.110).

Ip Addresses are used by computers to identify each other on the network. And there are two types of IP addressess

A Domain Name System is also a Distributed Database, Partitioned into zones which are managed in a distributed, decenteralized way which are available globally, accross the internet.

Database typically have indexes,DNS indexes are those funny dotted names, which are properly called domain names. Domain Names are actually paths in a inverted tree.

DNS Tree

(Source: Google images)

NameServers,Zones and Authority

DNS is a client-Server System.

  • The Servers are called name servers or DNS servers.
  • The clients are called resolvers, and they query name servers.

DNS Resolver to NS

(Source: Google images)

Name Servers Load data in zones, the administrative units in DNS.

  • Name servers may load thousands of zones, but no name server loads the entire namespace.
  • A name server that loads an entire zone is authoritative for that zone.

Name Servers are reponsible for answering the queries sent by the resolvers. But the Name servers also query other Name Servers.

Authoritative Name Server

  • A name server authoritative for one or more zones.
  • In common Usage, a name server whose primary function is to answer non-recursive queries for data in its autoritative zones. Some even have recursive disabled.
  • Also meaningful when a specific context is specified.

Recursive Name Server

  • Name server willing to handle recursive queries.
  • Virtually all recursive name servers also cache,so caching name servers is roughly equivalent.
  • Some recursive name servers are basically dedicated to recursion

Primary and Secondary

A primary Nameserver is the one where it gets the zone detail from file. Now it transfers it to the other name server is called Secondary NameServer by Zone Transfering.

  • The secondary name server sends the AXFR query to primary nameserver to initailly get the all DNS details. And periodically it checks the primary name server by sending a SOA query with serial number for the particular refresh time. If the Referesh time is over again the secondary server sends SOA query and primary server sends the serial number if it is increased, Then the secondary server asks the updated DNS details.

Resolvers

All the OS have the DNS resolvers,Such as nslookup,dig. It is a software application. Resolvers takes applications requests for data in DNS and translates them into DNS queries.

Resolvers then awaits a response, * It may retransmit the query after a timeout. * It may fall back to querying a different name servers, or a differnt naming server entirely.

when it receives a response, the resolver unmarshals it into data structure that passes it back to application.

DNS Resolutions

The below image shows whenever a client hits the request, How DNS resolves the query.

DNSResolution

(Source: google images)

DNS Recursion

Queries are either recursive or non-recursive, also called as iterative.

  • Accepting a recursive query obliges a name server to do all the work necessary to find an answer

    • which may inculde following serveral referrals
    • the name servers may not send a referral in response

  • A nameserver reponds to a non-recursive query with the best answer it already knows, often a refferal.

DNS Caching

All the DNS enteries are cached. Meaning if we hit the google.com

 * Client -> NS -> (Root Domain) (1 Referel)
 * (Root domain) -> NS -> (Top level domain) ( 2 Referal)
 * (Top level domain) -> NS -> google.com

When we first hit the google.com it takes two referals to finally reach the required results. Next time when we hit the same google.com. It will take zero referrals. Now when we hit he "facebook.com

* Client -> NS -> (top level domain) ( 1 referal).
* (TLD) -> NS -> facebook.com

Time To Live

Cached data must eventually time out in order to force name servers to re-query autoritative name servers and see changes. Each resource record has a value associated with it called time to live, or TTL, which governs how long another name server may cache that record

  • TTL range from seconds to hours to days
  • The administrator of the zone that contains the record decides on the TTL.
  • A name server that has a record cached decrements its TTL over time. when the TTL reaches zero, the name server deletes the record.
  • TTL has no effect on a name server authoritative for the zone that contains the resource record.

Server selection and Retransmissions

  • Most zones have more than one name server

    • Recursive name servers need to choose which one(s) to query.

  • Queries and responses are generally sent over UDP

    • So they can get lost
    • Both resolvers and name server must handle retansmissions.

  • Retransmission algorithms also depends on implementation

  • However, all name servers send queries and await responses

    • if they don't receive a response within a timeout, they retransmit the query - to another name server, if possible.

Resource Records

A resource record, commonly referred to as an RR, is the unit of information entry in DNS zone files; RRs are the basic building blocks of host-name and IP information and are used to resolve all DNS queries. Resource records come in a fairly wide variety of types in order to provide extended name-resolution services

Resource Record Types:

  • A

    The record A specifies IP address (IPv4) for given host. A records are used for conversion of domain names to corresponding IP addresses.

  • AAAA

    The record AAAA (also quad-A record) specifies IPv6 address for given host. So it works the same way as the A record and the difference is the type of IP address.

  • PTR

    As opposed to forward DNS resolution (A and AAAA DNS records), the PTR record is used to look up domain names based on an IP address.

  • CNAME

    The CNAME record specifies a domain name that has to be queried in order to resolve the original DNS query. Therefore CNAME records are used for creating aliases of domain names. CNAME records are truly useful when we want to alias our domain to an external domain. In other cases we can remove CNAME records and replace them with A records and even decrease performance overhead.

  • MX

    The MX resource record specifies a mail exchange server for a DNS domain name. The information is used by Simple Mail Transfer Protocol (SMTP) to route emails to proper hosts. Typically, there are more than one mail exchange server for a DNS domain and each of them have set priority.

  • SRV

    The SRV record is a Domain Name System (DNS) resource record that is used to identify computers that host specific services.

  • TXT

    The text record can hold arbitrary non-formatted text string. Typically, the record is used by Sender Policy Framework (SPF) to prevent fake emails to appear to be sent by you.

  • Wildcards

  • RRset