lsg30.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

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X.25



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X.25 address</FONT>



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20



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ISDN



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ISDN address</FONT>



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21



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RT



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Route through</FONT>



</TABLE><P>Some of the resource record types are now obsolete (numbers 3 and 4), and others are considered experimental at this time (numbers 13 and 17 through 21). In most resource records, you find only types A, NS, SOA, and PTR.



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<P>The Class field in the resource record format contains a value for the class of record. If no value is specified, the class of the previous RR is substituted. Internet name servers usually have the code IN in this field.



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<P>The Time-to-Live (TTL) field specifies the amount of time in seconds that the resource record is valid in the cache. If a value of zero is used, the record should not be added to the cache. If the Time-to-Live field is omitted, a default value is used. Usually this field tells the name server how long the entry is valid before it has to ask for an update.



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<P>The data section of the resource record contains two parts: the length of the current record and the data itself. The Data Length field specifies the length of the data section. The data is a variable length field (hence the need for a length value) that describes the entry. The use of this field differs with the different types of resource records.



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<P>Some resource record types have a single piece of information in the data area, such as an address, or at most three pieces of information. The only exception is the Start of Authority (SOA) resource record. The contents of the resource record data areas (except SOA's) are:



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RR Type



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Field Name



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Description</FONT>



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A



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Address



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A network address</FONT>



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NS



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NSDNAME



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Domain name of host</FONT>



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MG



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MGNAME



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Domain name of mailbox</FONT>



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CNAME



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CNAME



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Alias for the machine</FONT>



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HINFO



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CPU 



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String identifying CPU type</FONT>



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OS



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String identifying operating system</FONT>



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MINFO



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RMAILBX



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Mailbox responsible for mailing lists</FONT>



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EMAILBOX



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Mailbox for error messages</FONT>



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MB



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MADNAME



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Now obsolete</FONT>



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MR



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NEWNAME



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Rename address of a specific mailbox</FONT>



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MX



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PREFERENCE



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The precedence for delivery</FONT>



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EXCHANGE



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Domain name of host that will act as mail exchange</FONT>



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<A NAME="I2"></A>NULL



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Anything can be placed in the data field</FONT>



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PTR



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PTRDNAME



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Domain name that acts as a pointer to a location</FONT>



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TXT



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TXTDATA



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Any kind of descriptive text</FONT>



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WKS



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Address Protocol Bitmap



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A network address Protocol used Ports and protocols</FONT>



</TABLE><P>The Start of Authority (SOA) resource record format is used to identify the machines that are within a zone. There is only one SOA record in each zone. The fields in the SOA resource record are mostly used for administration and maintenance of the name server. The order of fields in the SOA RR is:



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