1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

DNS/DHCP Outages

Discussion in 'Networks' started by Fergal1982, Feb 13, 2007.

  1. Fergal1982

    Fergal1982 Petabyte Poster

    4,196
    171
    211
    I had my server freeze this evening, taking my DNS and DHCP offline for a while. In resolving it, it led me to a question. I know i've asked questions about this stuff before (i never can seem to get my head around it).

    Whilst the system was down i noticed some things happening. I was unable to navigate to any websites, but online resources already in place operated without issue (ie msn, yahoo, WoW). I seemed to be able to navigate to another machine by IP, but renewing the IP killed the system.

    Now im pretty sure i understand by DHCP operated in the manner it did, but correct me if im wrong:

    When you obtain your Lease, it lasts for a certain period of time (say 7 days). once you have your lease, the system just periodically checks for a network connection to the dhcp server (such as when you reconnect to the network, bootup, etc), so its only really when the lease expires that you notice major issues if an outage occurs whilst you are working.

    DNS im a bit confused about. it seems that your programs only need the DNS server functioning in order to establish the connection. once its running, it doesnt appear to require DNS. This seems contrary to what i would believe would happen. Surely whenever you try to communicate with the network resource. no?

    Im confused.
     
    Certifications: ITIL Foundation; MCTS: Visual Studio Team Foundation Server 2010, Administration
    WIP: None at present
  2. AJ

    AJ Administrator Administrator

    6,771
    102
    221
    Fergal, I in know way consider myself an expert in DNS or DHCP but here we go .................

    When your pc gets an IP addy, if set up, your DHCP server will register this address and name with thew dns server. Now if server are down then the clients will refer to their DNS cache to resolve name to IP addy if the DNS server is not on-line. If this can not be resolved then you wil get a DNS error. Internally, a broadcast for name to IP can happen and this will resolve internally but will not allow external resolution of name to IP.

    Right I'll happily be corrected by others.
     
    Certifications: MCSE, MCSA (messaging), ITIL Foundation v3
    WIP: Looking at doing ..................
  3. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

    8,871
    167
    256
    Yup and don't forget that if the client can't contact a DHCP server it will not keep the lease even if it hasn't expired - W2K and XP will assign themselves an APIPA address 169.254.x.x and keep trying to contact a DHCP server every five minutes. This will break connectivity with just about everything on the subnet except other APIPA clients.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  4. Sparky
    Highly Decorated Member Award

    Sparky Zettabyte Poster Moderator

    10,190
    296
    319
    Remember there is DNS cache of the PC, run ipconfig /displaydns at the command line to see what is there.
     
    Certifications: MSc MCSE MCSA:M MCSA:S MCITP:EA MCTS(x5) Security+ Network+ A+
    WIP: Exchange 2007\2010
  5. Fergal1982

    Fergal1982 Petabyte Poster

    4,196
    171
    211
    I tried that, all i got was

    Could not display the DNS Resolver Cache.
     
    Certifications: ITIL Foundation; MCTS: Visual Studio Team Foundation Server 2010, Administration
    WIP: None at present
  6. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

    8,871
    167
    256
    Ferg, make sure your "DNS client" service is running.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  7. Fergal1982

    Fergal1982 Petabyte Poster

    4,196
    171
    211
    that sorted it. should i have it running as standard? what benefit/effect will it have on the system?
     
    Certifications: ITIL Foundation; MCTS: Visual Studio Team Foundation Server 2010, Administration
    WIP: None at present
  8. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

    8,871
    167
    256
    Yes it should start automatically I believe.

    The benefits are that it should in theory speed up DNS resolution as the client will check it's cache before going out on the wire to contact a DNS server.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)

Share This Page

Loading...