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Multiple ISP DNS Records for RPC over HTTP

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  • Multiple ISP DNS Records for RPC over HTTP


    I am having a problem figuring out how to ensure outlook connectivity for out-of-office users for my organization. Any help would be appreciated. I will use to help explain my situation.

    We currently have two internet connections from two different ISPs for redundancy. ISP A (, and ISP B (

    ISP A, being the Primary, hosts our primary A record:

    A -
    A -

    ISP B, the secondary, hosts:

    A -

    My understanding is that we cannot add an A record on ISP B for because there can only be one per domain.

    Our outlook clients are configured to connect to using RPC over HTTP. But when ISP A goes down (which it does frequently) our out-of-office clients can't connect to our email server through Outlook because they can't find the DNS record for They are forced to use OWA which isn't optimal.

    We need some way to ensure that even if ISP A goes down Outlook can still connect.

    I feel like there is a simple solution to this problem that we are missing.

    I greatly appreciate any help, thank you.

  • #2
    Re: Multiple ISP DNS Records for RPC over HTTP

    Two options that I know of:

    1) Get a slash /24 from ARIN ( or whoever your regional authority is. You need to setup an ORG, apply for an AS, and then apply for the IP block. (probably costs $1000 - $2000 /yr) Once you have that you need to then peer with your ISPs, advertising your IP block so that there is then multiple routes to your network (over both ISP circuits). Then change your DNS to one of the IP addresses you've been assigned. You then have fault tolerance if one of your ISPs go down.

    2) There are some DNS hosting services that say they provide fault tolerance by testing connectivity to the IPs you choose and then updating the DNS entry automatically when there's an outage.

    The first option is not easy because you have to justify the need for the IP addresses and it is more technical and requires you have a router capable of BGP. But the first option is more preferable since it is robust and is the accepted way of obtaining route fault tolerance on the Internet.

    The second option I'm not a huge fan of because the way they accomplish their fault tolerance is through extremely low TTL values on the DNS records and that makes for more traffic and potentially higher latency to your resources.

    Network Consultant/Engineer
    Baltimore - Washington area and beyond


    • #3
      Re: Multiple ISP DNS Records for RPC over HTTP

      The simple solution is a load balancer located in a data centre.
      All your DNS records point to the load balancer and then load balancer deals with the connection to the server over whichever ISP is up.

      You will need a dual WAN router so that Exchange only sees a single default gateway.

      Simon Butler
      Exchange MVP

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      • #4
        Re: Multiple ISP DNS Records for RPC over HTTP

        Thank you for the replies. I will investigate these options further. A DNS hosting service seems like a good option for us.