Create DNS-records-overview.md

DNS-records-overview.md

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+# Under constrution!!
+This document lists the basic usage of commonly used DNS records. It can be used to track commonly made mistakes.
+
+# A
+* Points to an IPv4 address.
+* Does not point to anyhting else.
+* Record does not start (left side) with _ or -.
+
+# AAAA
+* Points to an IPv6 address.
+* Does not point to anyhting else.
+* Record does not start (left side) with _ or -.
+
+# MX
+* Points to an A and/or AAAA record.
+* Preferrably does not point to other record types, but the use of CNAME records is seen in practice. RFC's are inconsistent on this.
+* Has a priority value. 
+
+# CNAME
+* Points to other records (A, AAAA, CNAME).
+* Be carefull with CNAME chaining; don't use too many CNAMEs in a row.
+* The end of a CNAME chain is always an A an/or AAAA record.
+* **Can only be combined with NS / SOA records if left side is equal.**
+
+# SRV
+* Records starts (left side) with _.
+* Points to an A and/or AAAA record. 
+* Has a priority value. 
+
+# NS
+* Points to an A and/or AAAA record.
+* **Used to point to subzones.** 
+* **Used to indicate what is inside the parent zone.** 
+* **Each zone needs this to indicate what is inside the parent zone as a reference to this zone.**
+
+# SOA
+* Mandatory for every DNS zone.
+* Contains the following information (seperated by a single white space):
+   * FQDN of the primairy name server followed by a trailing dot.
+   * e-mail address of the DNS administrator (followed by a trailing dot, the @ replaced with a dot).
+   * an opening round bracket "(".
+   * serial number that is changed (increased) on every zone change.
+   * refresh time (in seconds) for a secondary name server to check the primairy name server for changes in the zone.
+   * retry time (in seconds) for a secondary name server for requesting the serial number when the primary name server did not respond on the previous request. 
+   * expire time (in seconds) after which a secondary name server should stop answering requests if the primary name server is not responding. 
+   * Negative caching time to live (in seconds) which is the time a caching name server should cache a negative result (indicating that a name does not exist) coming from the authorative name server before trying again.
+   * a closing round bracket ")".
+