DNS Overview An overview of DNS, MX records and A records. The Internet is a global network of computers connected using the TCP/IP protocol. Each computer has a unique numerical TCP/IP address that other computers use to communicate with it. But people find it easier to refer to computers by name. Domain Name Service (DNS) takes care of this dilemma. Its purpose is to translate (or resolve, in technical terminology) TCP/IP addresses to computer names (and vice-versa). DNS services run on many computers on the Internet. DNS is designed so that if a computer is asked to resolve an address and does not know the answer, it will pass the request on to a higher level authority. Your computer, as a DNS client, will submit resolution requests to DNS servers when you try to connect to Internet sites such as www.rockliffe.com. The primary and secondary DNS servers for your computer are entered in numerical format in the Network TCP/IP Settings. The DNS database on the server contains a number of different mappings of names to addresses, called record types. The most common, A, is the one that most people think of as DNS. A records are the ones used for resolving WWW and FTP addresses. In general you will find one numeric address for a given host name. For example, the A record for www.rockliffe.com is: 165.87.194.133 www.rockliffe.com There is a second record type is called Mail eXchange (MX) records. These records are devoted to the delivery of mail. A MX record for a particular name will have several addresses and priorities. For example, to send e-mail to DEC, you need to use the name dec.com. DEC has the following MX records, each with a different priority number: 16.1.0.22 inet-gw-2.pa.dec.com 70 204.123.2.50 mail1.digital.com 80 16.1.0.33 inet-gw-3.pa.dec.com 90 16.1.0.23 inet-gw-2.pa.dec.com 100 These records tell mail software where to send the mail and what to do if the first machine is not available. If the mail server inet-gw-2.pa.dec.com is being rebooted, a mail server will send mail to mail1.digital.com or inet-gw-3.pa.dec.com (since these are the entries with the next higher priority in the MX record). Many destinations do not have MX records. In this case, mail software will attempt to use the A record instead. Mail will be held until it is possible to forward if none of these machines are available. The following points should be noted: · It is wise to ask your provider to act as a secondary mail server for you in case your machine becomes unavailable. For example, your disks could become full, your Internet connection could break, etc. · The MX records and A records for the same name could be different. In other words, one machine receives mail and another machine receives all other traffic. · If MX records exist, they MUST be correct, otherwise your domain will not receive mail.