Tham khảo tài liệu 'learning publishing dns in action ebook_3', công nghệ thông tin, hệ điều hành phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 2 Type Name Description of the RDATA field NXT Next domain Name of another domain. Authenticating a nonexistent domain name and type. A6 A6 host address Can contain up to three fields prefix length part of an IP version 6 address and prefix name. Table The most common RR DNS Protocol The DNS protocol works with several types of operations. The most commonly used operation is a DNS QUERY. It is a query that enables the obtaining of one or more records from the DNS database. The DNS QUERY operation was for a long time the only operation possible in the DNS system. New modifications to the DNS protocol have brought new kinds of operations as DNS NOTIFY or DNS UPDATE. These will be dealt with in the next chapter. The DNS protocol operates on a query answer basis. A client sends a query to a server and the server answers it. DNS protocol uses name compression in order to make DNS packets as compact as possible. The DNS protocol is an application-layer protocol and as such it does not carry out packet transfer on its own. The packet transfer is delegated to a transport protocol. Unlike the overwhelming majority of other application protocols DNS protocol uses both UDP and TCP. Each query and the answer to it are transferred by the same transport protocol. With translation queries asking RR UDP is preferred. Where a DNS answer is longer than 512 B the answer includes only a 512 B part of the information and the truncation TC bit is set in the header to mark that the answer is incomplete. The complete answer can be requested by the client via TCP. For zone transfer between a primary and a secondary name server TCP is used. Name servers wait for queries both on the 53 UDP port and the 53 TCP port. Some UDP implementations do not fill in the checksum field in the UDP packet header and take advantage of this option. This feature can be useful for example for NFS but it is precarious with DNS. A network failure can result in a meaningless answer especially .
