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Appendix C


Domain Name System (DNS)

Domain Name System (DNS) is the standard protocol used by the Internet to map host names to IP addresses and vice versa. The DNS protocol is defined in RFCs 1034, 1035.

The DNS protocol was developed to allow organizations to maintain their own database of host name to IP address mappings, while allowing remote organizations to access their names database to resolve queries. The biggest difficulty the DNS protocol has to overcome is 'who has the correct database to answer this query?' This is achieved by organizing the name space into a hierarchical structure with a name server being authoritative for a particular domain. The name server for each domain only has to know of its connected subdomains.

A name server has a dual purpose, either answering the query (if it has the answer in its database) or informing the host of a more suitable name server. Thus a host generating a query does not have to know of the whole tree, just the IP address of a name server at the root of the tree. This name server will inform the querying host to try one of the name servers authoritative for the relevant sub domain. The querying host will continue querying relevant name servers, getting closer to the desired domain

each time, until finally it queries the one authoritative for the full query.

The DNS record structure is not only employed to hold address to name mappings, it is also used to alias machines, store details of available services on a given machine, inform which machines can operate as mail

relays and as name servers to other domains.

DNS is important when sending Internet mail from company to company, as it would be unrealistic to define every Internet address for all the companies you may wish to send to.

When sending mail, SMTPDS has to be able to determine the IP address of a mail server capable of receiving mail for that domain. For example, to send mail to user@example.com the SMTPDS will have to be able to contact the SMTP mail server responsible for example.com.

The required information will be stored in the DNS database for example.com, as an MX (Mail Exchanger) record. SMTPDS sends a DNS query to resolve the host name and IP address of the mail server at example.com. On receiving the information it will contact the mail server, via the SMTP protocol and negotiate the delivery of the mail for example.com.

MX records

An MX record has the following format:

Domain  Class  MX  Preference_value  mail_server
 	 	 	 

Mail forms an important part of your companies operation, therefore it is advisable to have more than one mail server set up to receive mail, for resilience. Multiple MX records allow you to specify any number of mail servers and also to define a preference value for each one. The lower the preference value the more likely that the host is to receive mail.

MAILsweeper will have to be referenced in your MX record as the most preferred mail server for receiving Internet SMTP mail.

For an example of resilience, you could configure the following:

example.com IN MX 5 MAILsweep1.example.com
               IN MX 10 MAIL2.example.com

In this example, all Internet mail is delivered to MAILsweep1. If MAILsweep1 is off-line mail is sent instead to MAIL2. MAIL2 must be configured to deliver its mail to MAILsweep1, for validation, when it comes back on-line.



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