Web hosting account - NS NS records are used to specify a
NS NS records are used to specify a zone’s primary server and all its secondary servers. An NS record points to a master name server of the given zone, with the resource data field containing the hostname of the name server. You will meet NS records in two situations: The first situation is when you delegate authority to a subordinate zone; the second is within the master zone database of the subordinate zone itself. The sets of servers specified in both the parent and delegated zones should match. The NS record specifies the name of the primary and secondary name servers for a zone. These names must be resolved to an address so they can be used. Sometimes the servers belong to the domain they are serving, which causes a “chicken and egg” problem; we can’t resolve the address until the name server is reachable, but we can’t reach the name server until we resolve its address. To solve this dilemma, we can configure special A records directly into the name server of the parent zone. The A records allow the name servers of the parent domain to resolve the IP address of the delegated zone name servers. These records are commonly called glue records because they provide the “glue” that binds a delegated zone to its parent. CNAME This record associates an alias with a host’s canonical hostname. It provides an alternate name by which users can refer to the host whose canonical name is supplied as a parameter. The canonical hostname is the one the master file provides an A record for; aliases are simply linked to that name by a CNAME record, but don’t have any other records of their own. PTR This type of record is used to associate names in the in-addr.arpa domain with hostnames. It is used for reverse mapping of IP addresses to hostnames. The hostname given must be the canonical hostname. MX This RR announces a mail exchanger for a domain. Mail exchangers are discussed in “Mail Routing on the Internet”. The syntax of an MX record is: [domain][ttl][class]MX preference host host names the mail exchanger for domain. Every mail exchanger has an integer preference associated with it. A mail transport agent that wants to deliver mail to domain tries all hosts who have an MX record for this domain until it succeeds. The one with the lowest preference value is tried first, then the others, in order of increasing preference value. HINFO This record provides information on the system’s hardware and software. Its syntax is: [domain][ttl][class] HINFO hardware software The hardware field identifies the hardware used by this host. Special conventions are used to specify this. A list of valid “machine names” is given in the Assigned Numbers RFC (RFC-1700). If the field contains any blanks, it must be enclosed in double quotes. The software field names the operating system software used by the system. Again, a valid name from the Assigned Numbers RFC should be chosen. An HINFO record to describe an Intel-based Linux machine should look something like: tao 36500 IN HINFO IBM-PC LINUX2.2 and HINFO records for Linux running on Motorola 68000-based machines might look like: cevad 36500 IN HINFO ATARI-104ST LINUX2.0 jedd 36500 IN HINFO AMIGA-3000 LINUX2.0 Caching-only named Configuration There is a special type of named configuration that we’ll talk about before we explain how to build a full name server configuration. It is called a caching-only configuration. It doesn’t really serve a domain, but acts as a relay for all DNS queries produced on your host. The advantage of this scheme is that it builds up a cache so only the first query for a particular host is actually sent to the name servers on the Internet. Any repeated request will be answered directly from the cache in your local name server. This may not seem useful yet, but it will when you