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Unformatted text preview: ecords only if it can't connect to the destination using TCP. In the case of the author's
primary system, which is connected to the In ternet by a SLIP connection, which is down most of the time,
sun % host -tv mx sun
Query about sun for record types MX
Trying sun within tuc.noao.edu ...
Query done, 2 answers, authoritative status: no error
sun.tuc.noao.edu 86400 IN MX 0 sun.tuc.noao.edu
sun.tuc.noao.edu 86400 IN MX 10 noao.edu
We also specified the -v option, to see the preference values. (This option also causes other fields to be
output.) The second field, 86400, is the time-to-live value in seconds. This TTL is 24 hours (24 x 60 x 60).
The third column, IN, is the class (Internet). We see that direct delivery to the host itself, the first MX
record, has the lowest preference value of 0. If that doesn't work (i.e., the SLIP link is down), the next
higher preference is used (10) and delivery is attempted to the host noao.edu. If that doesn't work, the
sender will time out and retry at a later time.
In Section 28.3 we show examples of SMTP mail delivery using MX records. NS Name server record. These specify the authoritative name server for a domain. They are represented as
domain names (a sequence of labels). We'll see examples of these records in the next section. These are the common types of RRs. We'll encounter many of them in later examples. 14.7 Caching file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (13 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System To reduce the DNS traffic on the Internet, all name servers employ a cache. With the standard Unix implementation,
the cache is maintained in the server, not the resolver. Since the resolver is part of each application, and applications
come and go, putting the cache into the program that lives the entire time the system is up (the name server) makes
sense. This makes the cache available to any applications that use the server. Any other hosts at the site that use this
name server also share the server's cache.
In the scenario that we've used f...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12