DNS - Domain Name System, Domain Name Server
System set up specifically to make it easy for people to find computers on the network (computers do not need easy to remember names).
BIND - Berkeley Internet Name Daemon
BIND is the distributed database program that is queried by other programs on the internet to resolve a domain name. BIND was written for Unix, but has now been ported to Windows NT as well. BIND is a free product and is maintained by the
Internet Software Consortium.
Additionally, Active Directory in Windows uses it's own version of DNS to
maintain it's Active Directory Tree.
HOSTS
Prior to the invention of DNS/BIND each host on the network had to have a HOSTS file to allow resolution of host names to machine addresses. This system did not scale well at all, in fact as the network began to grow the file was outdated before you could download it. The inability keep a single file up to date was the reason that it bacame necessary to have domains and a distributed method of providing name resolution.
Record
There are several types of records in BIND. Records are the listings for the individual hosts that BIND uses to resolve names to IP addresses, or to find other DNS servers.
Authoritative
A DNS server is authoritative only for the zone that it holds the records for. A DNS server may non-authoritatively answer queries to resolvers if it has the data in cache as the result of a previous query.
The domain name space has a tree structure much like a file system. A domain is a branch of the domain name space. for instance there is a domain parkland.cc.il.us or the domain uiuc.edu. Domains can have sub domains such as cso.uiuc.edu. A host such as surt.cso.uiuc.edu (my unix desktop workstation) is in multiple domains, uiuc.edu and cso.uiuc.edu. There could be a host called surt.uiuc.edu which could be a different machine with a different IP address.
Top-Level Domains
com, edu, gov, mil, net, org, int(international organizations), us (countries)
There are many more top level domains. What is .jp ?
Name Resolution - How a resolver queries a DNS Server
Resolvers
Resolvers are client programs running on hosts that access name servers. Resolvers do the following:
Query a name server
Interpret the response which may be a resource record or an error
Return the information to the program that requested it
The resolver is a set of library routines that are integral with the hosts TCP/IP stack and client programs are hooked into the routines for internet services. An example would be a browser querying a DNS server to find the IP address of www.google.com.
The majority of the 'work' done in the resolution is performed by the DNS server that the resolver originally asked. The resolver is just waiting for an answer, but if the DNS server does not know the IP address of a host, it queryies another name server. The name server that was queried by the resolver continues to query other name servers on behalf of the host resolver until the name is resolved or an error is returned by the root name server for the zone.
To find a host IP or name on the internet we can query a DNS server directly with the nslookup command. Remember, nslookup does not tell you if the host is running and connected to the network, it just tells you if a host is registered. The man page for nslookup conatins a lot of information about DNS.
A host can have many different types of records listed for it (the same IP address) in the DNS tables. Here are three common types of records, if you were to be operating a DNS server there would be many other types of records that you would have to maintain.
A Record
An Alias Record is a record in a file that BIND uses to look up host name to IP pairs.
alpo.dogfood.com IN A 192.17.243.31
Another possibility would to have what would be called a multi homed host.
kibblesnbits.dogfood.com IN A 192.17.243.32
kibblesnbits.dogfood.com IN A 192.17.243.102
C-NAME Record
This is known as a Cannonical Name record, or commonly a C-NAME record. A host can have many of these. A host with multiple C-NAMES can host many different web sites.
www.dogfood.com IN CNAME 192.17.243.32
www.kibblesnbits.com IN CNAME 192.17.243.32
MX Record
This is known as a Mail Exchange Record.
dogfood.com IN MX 1 mailserver1.dogfood.com
dogfood.com IN MX 2 backupmailserver.dogfood.com
DNS and BIND, P. Albitz and C. Liu, O'Reilly Publishing 4th Edition. This book (The Grasshopper book) covers version 9 of BIND, the latest release.
The manual page for nslookup, dig, or host is a good source of information. To view this page, issue the command "man nslookup" from the command prompt on a unix system.