DNS Transactions

DNS transactions are a regular occurrence over the Internet. The two most common types of transactions are discussed in the following list:

• DNS zone transfers. In this type of transaction, synchronization of new/updated domains in the backup or secondary DNS server takes place. The secondary DNS server compares its serial number to the serial number of the primary server at regular intervals. If the serial number on the primary server is more recent, the secondary server downloads the new copy of the zone. This is called the zone transfer. You will learn more about zones later in this tutorial.
• DNS queries/responses. In this type of transaction, the client sends a DNS query, and the server replies through a DNS response. Clients usually choose three domain name servers to send their queries. These servers are the default servers defined on the client computer that are provided by the ISP. These servers are checked one by one until you obtain the desired result. The query keeps passing through the name servers until a valid response is generated. The name server can also forward the query to another name server if it cannot resolve the query. In this case, the query keeps on passing through the servers and when an answer is found, it is sent back to the original name server. The answer is first cached by the original server and then sent back to the client.

The DNS query and responses that take place between the DNS server and client have five sections. These sections are discussed in the following list:

• Header section. This section contains information about the type of message and other sections that are present in the message.
• Question section. This section contains information concerning the object of the query.
• Answer section. This section contains RRs regarding the answer.
• Authority section. This section contains either a Statement of Authority (SOA) or name server records. These name server records belong to the zone of authority for the owner name of the RR(s) in the Answer section.
• Additional section. This section contains additional information for the receiver.

Cached answer can be used by that DNS to resolve similar subsequent queries for the same DNS information. Thus, caching increases the efficiency but can lead to security loopholes.