DNS

7.1 DNS

A global list of sites

Each site could keep a list of all sites on the network (or have access to such a list).
BITNET kept such a list, of thousands of international sites.
ARPANET did this at first, too.

A decentralised list - DNS

DNS - Domain Name System

The Internet has long since grown too big to keep a list of all sites anywhere, let alone have one at each site. This means sites do not know a priori if an email address or web site name is valid. They must go out on the net and find out.

The upside is that each subzone of the net can develop rapidly, without having to tell everyone else what it is doing. Say the University of MIT is responsible for all addresses:

  *.mit.edu

It is assigned a segment of the numerical address space:

  106.132.*.*

and can assign and reassign names to this as it likes. i.e. It doesn't need permission to add new machines to the Internet, nor does it need to publicise those machines. Other sites find out about these machines if and when they need to (which may be never). When given the name of a machine:

  jimmy6.chemistry.mit.edu

they talk to the DNS server for .edu, which gives them the name of the DNS server for .mit.edu, which gives them the real address of the site (or says "invalid site").

You don't clutter up your site with info about other sites, 90 percent of which you will never actually need to know about. You don't have 90 % of Internet traffic being the announcements of new sites. Also the Internet can expand very fast, as each subzone can build and rebuild its own topology as it pleases. Don't even need to report how much of that address space is used.

If a DNS server does not know the answer, it returns the address of a better DNS server to ask.
From here.

DNS so important / done so often that there are duplicate DNS servers, caches of remote information, etc. (without going as far as the complete cache of all sites on the net).

DNS uses UDP (not TCP) for address lookups.

The 123 root nameservers of the world, as at 2006.
There is a single root nameserver in Ireland, run by INEX in Dublin. It is a nameserver for .com and .net.
From here.
Latest and more detailed map here.

DNS Lookup tools

DNS Lookup

Network tools
DNS
- Explanation of DNS record types
- Zone file format

In DCU Windows:
ipconfig /all
You may get something like this:

  CA DNS server: 136.206.11.247
 DCU DNS server: 136.206.1.3

At home on ISP will have different DNS server.

7.1.2 Aliases

Many-to-1:
Can set up multiple aliases for same machine ("site.com" = "www.site.com" = "ns.site.com" = "ftp.site.com"), etc.
Lookup our web server:
www.computing.dcu.ie and:
www.compapp.dcu.ie
and you find:
```
www.computing.dcu.ie = 
www.compapp.dcu.ie = 
elbrus.computing.dcu.ie = 
elbrus.compapp.dcu.ie = 
136.206.11.240
```
These are all the same machine.
See Different forms of URL.
1-to-many:
Can set up "virtual machines" distributed over many real machines (e.g. "www.yahoo.com" is not 1 machine, but a "farm" of machines).
Both:
Lookup of mailhost.computing.dcu.ie shows a hybrid of both the above:
mailhost alternates among different machines for load balancing and some of those machines double up as DNS servers.