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Unformatted text preview: s%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (12 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System Host information: two arbitrary character strings specifying the CPU and operating system. Not all sites
provide HINFO records for all their systems, and the information provided may not be up to date.
sun % host -t hinfo sun
sun.tuc.noao.edu HINFO Sun-4/25 Sun4.1.3
Mail exchange records, which are used in the following scenarios: (1) A site that is not connected to the
Internet can get an Internet-connected site to be its mail exchanger. The two sites then work out an
alternati ve way to exchange any mail that arrives, often using the UUCP protocol. (2) MX records
provide a way to deliver mail to an alternative host when the destination host is not available. (3) MX
records allow organizations to provide virtual hosts that one can send mail to, such as
cs.university.edu, even if a host with that name doesn't exist. (4) Organizations with firewall
gateways can use MX records to limit connectivity to internal systems.
Many sites that are not connected to the Internet have a UUCP link with an Internet connected site such as
UUNET. MX records are then provided so that electronic mail can be sent to the site using the standard
user@host notation. For example, a fictitious domain foo.com might have the following MX records:
sun % host -t mx foo.com
foo.com MX relayl.UU.NET
foo.com MX relay2.UH.NET
MX records are used by mailers on hosts connected to the Internet. In this example the other mailers are
told "if you have mail to send to firstname.lastname@example.org, send the mail to relay1.uu.net or
relay2.uu.net." MX MX records have 16-bit integers assigned to them, called preference values. If multiple MX records exist
for a destination, they're used in order, starting with the smallest preference value.
Another example of MX records handles the case when a host is down or unavailable. In that case the
mailer uses the MX r...
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