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Network Fundamentals

# Resource Records
The DNS distributed database consists of entities called **RR**s, or **Resource Records**. 

## Format
RRs contain some or all of the following values:
- **Name** of the domain.
- **Resource data (RDATA)** provides information appropriate for the type of resource record.
- **Type** of the resource record. We will discuss these shortly.
- **Time-to-live (TTL)** is how long the record should be cached by the client in seconds.
- **DNS Class**. There a many types of classes but we're mainly concerned with `IN` which implies the 'Internet' class. That's what all of our upcoming examples use so we won't be discussing it again. Another common value for the DNS Class is `CH` for 'CHAOS'. The CH class is mostly used for things like querying DNS server versions.

## Types of resource records

- **Address** type or `A` addresses contain IPv4 address to hostname mappings. They consist of:
  - The `name` is the hostname in question.
  - The `TTL` in seconds.
  - The `type` which is `A` in this case.
  - The `RDATA` which in this case is the IP address of the domain.
  -  **Example:** `educative.io. 299 IN A 104.20.7.183` where `educative.io` is the name, 299 is the TTL in seconds, `IN` is the class, `A` is the type of the RR, and `104.20.7.183` is the `RDATA`.
- **Canonical name** or `CNAME` records are records of alias hostnames against actual hostnames. For example if, `ibm.com` is really `servereast.backup2.com`, then the latter is the canonical name of `ibm.com`. 
  - The `name` is the alias name for the real or 'canonical' name of the server. 
  - The `RDATA` is the canonical name of the server.
  - **Example:** `bar.example.com.        CNAME  foo.example.com.`
- **Mail Exchanger** or `MX` records are records of the server that accepts email on behalf of a certain domain. We have seen this one before! 
  -  The `name` is the name of the host.
  - The `RDATA` is the name of the mail server associated with the host.
  - **Example:** `educative.io	IN MX 10 aspmx2.googlemail.com.`

These resource records are stored in text form in special files called **zone files**.

Let's now get into what DNS records and messages look like.

DNS: Records and Messages

Resource Records

Format