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DNS Resolution.

Discussion in 'Training & Development' started by beaumontdvd, Apr 13, 2010.

  1. beaumontdvd

    beaumontdvd Kilobyte Poster

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    Hi everyone thats reading this, just to clear things up. Dns serves as like a phone book for the internet by translating hostnames to iP. Does it do this by sending off a request to the dns server and then searching through the cache on which dns server has previously resolved that request then returning the correct address to the user.

    Also for a ISP would there domain name servers that resolve it to IP be the same as another user from a differnt ISP. So basically does everyone use the same dns server as I have seen some ones that can be used as a temporary measure.

    Thanks,
    Dave
     
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  2. SimonD

    SimonD Terabyte Poster Moderator

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    Internal DNS basically resolve around the use of Root Hint servers or Forwarders.

    Essentially a machine sends a request out to a DNS server asking if they know the address of xxx.yyy.com, if they don't they will direct them to an upstream server, eventually getting to a root hints server that will know who handles the TLDN (in this case .com).

    The DNS server handling .com will then query his db to find out if he knows xxx.yyy.com, if he doesn't he will know where yyy.com resides and forward the request to him, yyy.com will then be asked if he knows xxx.yyy.com, because he is authoritive for yyy.com he will know the answer and provide the address for xxx.yyy.com

    Each ISP will have their own DNS servers for which they would be either be caching or holding primary zones for the zones it's authoritive for (eg virginmedia.com would be the zone that virginmedia.com DNS servers would be authoritive for, anything beyond the scope of it's zones would be resolved by someone else).

    As I mentioned earlier, a DNS server can either rely on the Root Hint servers (there are 13 of them) or they can rely on forwarders, if you don't manually enter an address for a forwarder the DNS server will generally use Root Hints, the only occasion that it wouldn't would be if the DNS server considers itself to be a Root DNS server (ie it has a (.) entry in the DNS zone).

    As far as cached records go, those records will only stay valid for the period of the TTL part of the DNS record, that TTL is configured by the authoritive keeper of those records.

    It's completely up to the individual on who they use as a forwarding DNS server, whether it's your ISP or the likes of OpenDNS, as long as they can do either forwarding or root hint lookups themselves you should pretty much be able to get any address on the internet (of course that's assuming that it is reachable).
     
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  3. beaumontdvd

    beaumontdvd Kilobyte Poster

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    Thanks mate I appreciate it, I understand now. So basically the root hints server handling the .com is highest in the hierachy and can resolve it to ip. But untill it reaches that it will search portions of the address untill it can determine where the ip is in its database file? Is that right mate?

    +1 for the great explanation thanks again,

    Dave
     
    Certifications: 070-271, 070-272, (MCDST)Level 1,2,3 NVQ
    WIP: 070-270, A+, N+, S+,MCDST 7 Upgrade
  4. SimonD

    SimonD Terabyte Poster Moderator

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    No, the root hints will point you to the authoritive dns server for that dns zone, the root hints are the root of DNS hierarchy, they service the top level domains, which service the next level etc, at some point in the chain a DNS server somewhere should be pointing to the TLDN (.com, .org, .gov, .mil etc), they inturn look for the next step and so on until the final address is found.

    Have a look here for a clearer picture
     
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