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James L
Asked: 2011-03-02 03:48:13 +0800 CST

Netinstalling CentOS if the gateway is in a different subnet

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I have a KVM host (A) running a virtual machine (B). They each have their own external IP address and the networking is setup using bridging between eth0 and br0 on A. B uses eth0, with A being the gateway.

The problem is that the two external IP addresses are on different subnets (different /8s in fact) so by default, B claims it cannot reach A (Network Unreachable).

I can resolve this by adding a static route on B:
echo "any host gateway_ip dev eth0" > /etc/sysconfig/static-routes

Modifying /etc/init.d/networking to reload the gateway after applying static routes (I only added the final line before fi): 

if [ -f /etc/sysconfig/static-routes ]; then  
    grep "^any" /etc/sysconfig/static-routes | while read ignore args ; do  
        /sbin/route add -$args  
    done  
    route add default gw "${GATEWAY}"  
fi

If I then restart networking, it comes online. How can I do this (or work around it some other way) prior to the system being installed, ideally inside an Anaconda kickstart file?

virtualization centos redhat networking kvm-virtualization

2 Answers

  1. The /etc/sysconfig/static-routes file is deprecated, and it uses the route command syntax, which is also deprecated.

    You can drop rules using the ip syntax for the eth0 interface in the file:

    /etc/sysconfig/network-scripts/route-eth0

    There's no hook to do this directly in a kickstart file, so you would have to do it in the %post section.

    • 2

  2. since you're running VMs, why not use templates and snapshots instead of kickstarting? Customizations can be put in the /etc/rc.d/sysinit file, under

    # Configure machine if /.unconfigured exists:

    then run sys-unconfig to prepare the VM to become a template.

    After this is done, you can clone the images with qemu-img convert or take snapshots from the original image, to save disk space, and attach the cloned/snapshotted images to new VMs. They will all have the same configuration and files, minus the typical settings removed by sys-unconfig. And whatever else you add to rc.sysinit as well of course.

    • 1