DNS Recap¶

What?¶

The DNS (Domain Name System) translates human readable domain names like www.bfh.ch to IP addresses like 127.0.0.1. The IP addresses can then be routed by routing protocols and finally be resolved to physical MAC address using ARP.

How?¶

The DNS has a strong hierarchy starting at the root domain '.' , followed by so called TLD (Top Level Domains, like 'ch' or 'com'). The hierarchy is a tree and each node forms its one namespace. All 'ch' domains can be found within the '.ch' namespace. For example the domain of the BFH is located at '.ch.bfh' and forms the namespace for all subdomains of 'bfh.ch'. Any node is addressed using his FQDN (Fully Qualified Domain Name). The namespace of the BFH can therefore be resolved by the FQDN 'bfh.ch.'. Each namespace forms also a zone. The zones are the domain of the namespace and all its sub-domains. This means that for example the subdomains 'ti.bfh.ch' or ahb.bfh.ch are part of the zone 'bfh.ch'.

Resource Records (RR)¶

DNS entries follow a specific form specified in the RFC-1035 (which is part of Internet Standard STD 13). They are called resource records and specify following fields per entry

NAME: The human readable domain www.bfh.ch
TYPE: The type of the RR (RR can be of various types like A for IPV4-addresses, MX for mail exchanges, SOA for zone of authority, AAAA for IPV6-addresses and many more \(\rightarrow\) see section 3.2.2 of RFC-1035)
CLASS: Network class of the RR (mostly IN for Internet)
TTL: How long shall the RR live in caches.
RDLENGTH: Length of the RDATA
RDATA: detailed information defining the RR

Reverse lookup¶

DNS also supports reverse lookups. This means, that for a given address a human readable domain can be looked up. For this purpose also RR must be created in the DNS config which map addresses to domains. Then you can do a normal lookup.

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