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  • Fallover DNS?

    I am trying to setup DNS so we have a 3 tier fail-over
    www points to ip1
    if ip1 unreachable, points to ip2, if ip2 unreachable points to ip3
    what is the best way to do it ?
    having 3 dns servers ? and each one of them pointing to a different IP ?, and assigning those dns servers to the domain ?
    ns1.xxxa = ip1
    ns2.xxxb = ip2
    ns3.xxxc = ip3
    will the first DNS be called first before any of the others, or can somebody get thru to the dns3 first even if dns1 ip is up ?
    Christopher Padfield
    Web Based Helpdesk
    DeskPRO v3.0.3 Released - Download Demo Now!

  • #2
    I dont think you want to use 3 nameservers, since that would give you more stable DNS. You also shouldn't keep 3 different zone records (or A records for redirecting WWW) in this case.

    I'm not using a failover DNS myself - but here is how you could use DNS to do failover:

    create round robin IP addresses:

    www.example.com => IP1
    www.example.com => IP2
    www.example.com => IP3


    Assuming you have 3 web servers all with a shared or syncronised mysql server. This would roughly balance the users hitting your site across 3 servers/IP addresses.

    DNS isn't great for failover, from what I know at least anyway.
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    • #3
      Yeah, that does round robin but I am more looking for fail over, ie if the site istn't accessable. It seems DNS istn't the way to do it, unless you "outsource" your DNS at which point they can do server monitoring and then update the DNS if your primary goes down. Of course cached DNS results won't help that much then, and it seems lots of ISPs won't take account of the cache time you can set.
      Christopher Padfield
      Web Based Helpdesk
      DeskPRO v3.0.3 Released - Download Demo Now!

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      • #4
        You could maybe do it using a squid proxy which would redirect to multiple backend webservers - and in the event of one being down fallover to the next one. Might be worth a look at anyway. Would also be inefficient 99% of the time too..
        HP DL-380 G6, 2x E5520, 28GB RAM, 4x300GB SAS, VMWare ESXi
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        • #5
          What I am really trying to do is get datacenter fall over, ie have a server in another datacenter, that is why I wanted to use DNS because that would be associated with the domain name itself, not with the datacenter but I see how that won't work now.

          The squid proxy won't help because that would have to go in datacenter1 so if datacenter1 is down, it dosen't help, of course it would help if the web server only is down.

          I think remote DNS is the only solution, but then that adds another level of failure.
          Christopher Padfield
          Web Based Helpdesk
          DeskPRO v3.0.3 Released - Download Demo Now!

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          • #6
            Well, I don't see how DNS will help you - so I would be interested to know what you do decide on, and how it'll work.
            HP DL-380 G6, 2x E5520, 28GB RAM, 4x300GB SAS, VMWare ESXi
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            • #7
              I am really interested to know what others do to mitigate against potential datacenter loss. With datacenters going down during 911, Lousiana, Florida storms, LA brownouts etc its going to happen again sometime.
              Christopher Padfield
              Web Based Helpdesk
              DeskPRO v3.0.3 Released - Download Demo Now!

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              • #8
                nothing short of using rsync to another server in another data centre and having a short TTL on a domain so you can change the DNS to the backup server in another data centre.

                You can't really get something thats automatic.
                Scott MacVicar

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                • #9
                  Yeah, so it seems.

                  I have seem some DNS services that automate the changing the DNS part for you, ie they will probe your server every minute to see if the server is down. If it is they will then change the DNS for you automatically to your other datacenter.

                  I guess that service can be distributed as well, ie they can have DNS servers in mutliple places all pointing to either your Datacenter A if its up, or Datacenter B if its down, so its not going to break.

                  So, perhaps you could replicate this yourself like this:

                  a) Datacenter A has 2 dns servers
                  b) Datacenter B has 2 dns servers
                  c) All 4 DNS servers point to Datacenter A for domain.com
                  --- Datacenter A goes down ---
                  d) Datacenter B realises Datacenter A is down
                  e) Datacenter B updates its DNS to point to itself
                  f) Datacenter A dns servers unreachable, datacenter b dns servers should now be used.
                  g) People should now be directed to Datacenter B.

                  Of course, again its going to depend upon the time it takes for ISPs to update their cache, and apparently lots don't check the TTL.
                  Christopher Padfield
                  Web Based Helpdesk
                  DeskPRO v3.0.3 Released - Download Demo Now!

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                  • #10
                    Does anybody know if major sites like amazon, paypal etc are all situtated in a single datacenter? Perhaps they rely on the datacenter not going down?
                    Christopher Padfield
                    Web Based Helpdesk
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                    • #11
                      I assume they would be using multiple load balanced connections, each with copies of the data required. It's the only way to get true availability.

                      Should scale easily too by adding more front end servers behind the load balancers?
                      Code:
                              |                  |
                              | IP1              | IP2
                      [Load balancing]   [Load balancing]  
                         |        |          |       |  
                      [Server][Server]   [Server][Server]
                           |     |          |        |
                        [SAN/Storage]--- [SAN/Storage mirror]
                      Bleh..
                      HP DL-380 G6, 2x E5520, 28GB RAM, 4x300GB SAS, VMWare ESXi
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                      Unreal Tournament : Assault forums - irc://irc.utassault.net:6667 -

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                      • #12
                        try http://www.tzoha.com/ it does it fairly well
                        Webmaster in charge of technical stuff and taking out the trash. www.disboards.com, www.wdwinfo.com
                        www.dreamsunlimitedtravel.com and a few others I am forgetting!

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                        • #13
                          Originally posted by Martz
                          I assume they would be using multiple load balanced connections, each with copies of the data required. It's the only way to get true availability.

                          Should scale easily too by adding more front end servers behind the load balancers?
                          Code:
                                  |                  |
                                  | IP1              | IP2
                          [Load balancing]   [Load balancing]  
                             |        |          |       |  
                          [Server][Server]   [Server][Server]
                               |     |          |        |
                            [SAN/Storage]--- [SAN/Storage mirror]
                          Bleh..
                          That sort of stuff in Datacenter #1 is done, the issue is really about handing Datacenter #1s death. Datacenter #2 would not be the actual sites, just a holding page with info etc.

                          Thanks alexi, looking into it. Got to say outsourcing DNS services scares me somewhat, but it might bee best solution.
                          Christopher Padfield
                          Web Based Helpdesk
                          DeskPRO v3.0.3 Released - Download Demo Now!

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                          • #14
                            If it makes you feel any better I have been using Tzo in all kinds of different ways for at least 5 years in my work as a network consultant. They really do a pretty good job.
                            Webmaster in charge of technical stuff and taking out the trash. www.disboards.com, www.wdwinfo.com
                            www.dreamsunlimitedtravel.com and a few others I am forgetting!

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                            • #15
                              I have been checking them out. One issue I have is the use of a *lot* of subdomains, which is something I am going to have to work out.
                              Christopher Padfield
                              Web Based Helpdesk
                              DeskPRO v3.0.3 Released - Download Demo Now!

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