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  • Multi Homed server & default gateway settings

    Hi all,

    I have a 2008 r2 server with two NICS. Only the first one has a Default Gateway - second does not (if I try to put one in - I get a warning from Windows).

    Plan - I would like to install Virtualbox and set it up so that the VM only uses the second NIC. I tested it, and it will not reach the internet (obviously).

    So - is it OK to setup the second NIC with the same gateway - in spite of the warnings?

    Thanks,
    Rob

  • #2
    Re: Multi Homed server & default gateway settings

    In the past I have ignored the warnings with no visible side effects, but I have a vague memory of a very good reason why the warning is there (but unfortunately not what that reason was )

    EDIT: this seems to cover it: http://hardforum.com/showthread.php?t=1699524
    Tom Jones
    MCT, MCSE (2000:Security & 2003), MCSA:Security & Messaging, MCDBA, MCDST, MCITP(EA, EMA, SA, EDA, ES, CS), MCTS, MCP, Sec+
    PhD, MSc, FIAP, MIITT
    IT Trainer / Consultant
    Ossian Ltd
    Scotland

    ** Remember to give credit where credit is due and leave reputation points where appropriate **

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    • #3
      Re: Multi Homed server & default gateway settings

      Thanks for the fast reply.

      Just as I finished the original post, I was thinking that my linux boxes (I have one with 8 NICs) all have DG set for each NIC - with no problems.

      Rob

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      • #4
        Re: Multi Homed server & default gateway settings

        Originally posted by Sternfan2012 View Post
        Thanks for the fast reply.

        Just as I finished the original post, I was thinking that my linux boxes (I have one with 8 NICs) all have DG set for each NIC - with no problems.

        Rob
        You don't actually have gateways per NIC, you have them per host. Each "default gateway" setting generates a default route (0.0.0.0/0) in the routing table. Having a gateway setting for each NIC is actually quite misleading.

        Most OSes will treat identical route entries with the same metric as redundant routes and attempt to load balance across them. If one or more of the gateways represent a NATed path, or if they don't all actually represent a path to the 0.0.0.0/0 network, this has the potential to get ugly really fast.

        Multiple default gateways makes no sense in most scenarios.

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        • #5
          Re: Multi Homed server & default gateway settings

          Admittedly I'm not up to speed on networking in Type 2 hypervisors (bridged, NAT'ed, etc.) but the VM only needs to use the physical connectivity of the host NIC. It shouldn't need to use the layer 3 components of the host NIC, so the host NIC doesn't need a DG and doesn't even need an ip address unless you want to access the host itself via that NIC.

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