DNS Record Types

DNS Record Types

• A record: represents an IPv4 address, a 32-bit address

  • Example: cisco.com has an A record with IP address 23.33.250.163

• AAAA record: represents an IPv6 address, a 128-bit address

  • Example: cisco.com has a AAAA record with IP address (IPv6 address not shown)

• NS record: represents name servers for a domain

  • Example: cisco.com has three name servers: ns1.cisco.com, ns2.cisco.com, and ns3.cisco.com

• CNAME record: represents an alias or canonical name

  • Example: www.cisco.com has a CNAME record that maps to another name, which then maps to an A record or AAAA record

• SOA record: represents the start of authority for a domain

  Example: cisco.com has an SOA record that provides information about the domain

  An SOA (Start of Authority) record is like the boss of a domain. It keeps important information about the domain such as its name, who controls it, when it was last updated, and how often it should be checked for changes. Just like a company has a CEO who is in charge, the SOA record is in charge of the domain.

  To look up information in an SOA (Start of Authority) record for a domain, you would typically need to use a DNS lookup tool or access the domain's DNS settings through your domain registrar or hosting provider. Here's a general guide on how you can perform an SOA record lookup using a command-line tool like nslookup:

  1. Open a command prompt or terminal window.

  2. Type nslookup followed by a space.

  3. Type -type=soa followed by a space and then the domain name you want to look up the SOA record for.

  4. Press Enter to execute the command.

  For example, if you want to look up the SOA record for the domain "example.com", you would type:

  ```

  nslookup -type=soa example.com

  ```

  The output will provide you with information such as the primary name server, responsible party email address, serial number, refresh interval, retry interval, expiry interval, and time to live (TTL) values defined in the SOA record for the specified domain.

  - Serial number: The serial number is a unique identifier that helps track changes to the DNS zone file

  A DNS zone file is a text file that contains entries mapping domain names to corresponding IP addresses. It is used by DNS servers to resolve domain names to IP addresses.

  DNS zone files can be accessed and viewed by those with administrative access to the DNS server. The specific method for accessing and looking up a DNS zone file may vary depending on the DNS server software being used. Typically, administrators can access the zone file through the DNS server's configuration settings or by directly accessing the server's file system.

  If you have access to the DNS server, you should consult the documentation for the specific server software you are using to find detailed instructions on how to access and view the DNS zone file.. When a change is made to the zone file, the serial number should be incremented to reflect the update.

  - Refresh interval: The refresh interval defines how often a secondary DNS server should check with the primary DNS server

  No, the primary DNS server and authoritative name server are not the same.

  The primary DNS server refers to the DNS server responsible for maintaining the original master copy of a zone file containing DNS records for a domain.

  On the other hand, an authoritative name server is a DNS server that has the definitive information about a specific domain name or resource record, meaning it is deemed authoritative for that domain. An authoritative name server can be the primary DNS server for a domain, but it can also be a secondary server that holds a copy of the zone data from the primary server.

  In essence, while the primary DNS server refers to a specific role within DNS management, the authoritative name server pertains to a server that has the ultimate authority for a particular domain's DNS records.to see if the zone file has been updated. It specifies the frequency at which the secondary server should attempt to synchronize its data with the primary server.

  - Retry interval: The retry interval specifies how long a secondary DNS server should wait before retrying a failed zone transfer after the refresh interval has passed. It defines the period of time the secondary server should wait before attempting to contact the primary server again if a previous attempt failed.

  - Expiry interval: The expiry interval defines the maximum amount of time that a secondary DNS server can continue to provide authoritative responses for a zone without successfully completing a zone transfer from the primary server. After this time period elapses, the secondary server will stop responding to queries for that zone.

  - Time to Live (TTL): The Time to Live value in the SOA record specifies the duration for which a DNS record can be cached by other DNS servers or clients. It indicates how long a DNS resolver should consider the record valid before it needs to be refreshed from the authoritative DNS server.

• PTR record: represents a pointer record for reverse lookup

  • Example: doing a reverse lookup on an IP address (23.33.250.163) returns the name associated with it (cisco.com)

• MX record: represents mail exchange servers for a domain

  • Example: cisco.com has three mail exchange servers with MX records, each with A records and AAAA records for their IP addresses

DNS Resolution Process

• Client makes a request to a DNS server
• DNS server may not have the information, so it queries other DNS servers
• Information is cached by the DNS server and returned to the client
• Clients and servers have caches to improve performance

Using nslookup

• nslookup is a tool for querying DNS servers
• Can be used to query specific record types, such as A records or MX records
• Can be used to perform reverse lookups using PTR records
• Can be used to query authoritative DNS servers for a domain

Learn about the different types of information stored in DNS, including IP addresses, email servers, and authoritative DNS servers. This video explores various DNS record types and demonstrates their usage.