Your PC does have a local DNS cache. Assuming you are using Windows, you can display the cache using the command ipconfig /displaydns. If for some oddball reason you have a problem with your local DNS cache you can flush the cache using the command ipconfig /flushdns.
Your ISP’s DNS server will usually cache DNS entries as well.
When your browser sends out a DNS request, if that machine doesn’t have a listing for it then the request gets automatically forwarded up to the next machine in the DNS hierarchy.
Now suppose that a website moves. They need to re-register the new IP address with the old domain name. This information goes to the TLD’s DNS server immediately (that’s what registering is), but it won’t go to all of the lower ones right away. So for a little while, you’ll have low-level DNS servers going “Oh, yeah, I know where that site is!”, and still giving out the old IP address. Ideally, whoever owns the website that moved will keep the old address live for a little while, to forward people to the new one, but sometimes this isn’t possible, which can lead to a period as long as days where the site really is up and working, but some segment of the site’s users can’t find it, until the information about the new address propagates through the system.
As a general statement, this is not true. Each TLD has its own rules. Some require a physical presence, others don’t care. I don’t see anything in the current nic.ly rules requiring servers in Libya.
This rule by Libya gains some revenue/jobs for their country, in addition to the fees for the name – they have to rent space for the server, hire someone to maintain it, etc.
Governments often tries to use their spending to encourage local jobs & economic development – for example, many state government construction contracts will require that x% of the workers on the project live in the state, or they will give bonus points on the bid if they employ x% of workers living in the state. (Just like many of us try to spend our money at companies that treat their workers right, or care for the environment, or donate to my favorite charity, or whatever else I consider important.)
But what is that heirarchy ?
Your workstation queries the home/SOHO router, the router is a DNS relay (so that the workstations can always rely on it)… The router has learn the DNS servers to use from the ISP (Via PPP or DHCP …)., which are typically the ISP’s DNS server
The ISP may then run a set of redundant servers, but I see no reason they set up a heirarchy of their own… they may allocate usage to be internal only, external only, or both, but never really a chain, but they could, I just don’t see why they’d chain them along.
A very small ISP may cheat and set their DNS server to RELAY to their upstream ISP’s DNS servers…
But then the big ISP’s DNS server will not have an upstream. It will only have the root servers.
So lets use the example www.youtu.be (the ip address of youtu.be is obtained the same way, as ip address of youtu.be is the same as if the hostname is “null”,
you ask for the name server for youtu.be is what the ip address of the HOST youtu.be is., while you ask the nameserver for .be what the nameserver for youtu.be is ! See the difference ? I use the redundant www. only to make it easier to write clearly. clear as mud at this point )
a. If the DNS server has www.youtu.be in cache, it goes to f.
b. If it does not know the AUTHORATIVE DNS server for youtu.be , it will ask the authorative DNS server for .be , the TLD’s (TLD=top level domain) authorative server list ; if it does know go to e.
c. If it does not know the AUTHORATIVE DNS server for .be , it will ask the root servers. Therefore the DNS server must have its list of root servers maintained. (This is the bootstrap, if the DNS server doesn’t know any DNS server IP address, it will never be able to learn of any !)
d. Ask the TLD server for the ip addresses of authoritive servers for youtu.be
e. Ask the authoritive server for youtu.be what the ip addresses of www.youtu.be
f. Respond back with the answer. (and it provides some other info, eg a recommendation from youtu.be’s config on how long to remember the answer. !)
So no the root servers do not know everything. They know where all the TLD’s can be found only.
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