Re: Migrating to a second subnet, multi-homed for a short time
On Thu, Apr 30, 2015 at 2:14 PM, francis picabia <fpicabia@gmail.com> wrote:
> We have a subnet running our legacy and primary DNS server
> where it remains as the last system on that subnet.
> Many things are pointing to that IP in resolv.conf, etc.,
>
> We want to put the DNS server on the new subnet with all
> the other systems, to simply configuration on the
> new Fortinet firewall.
>
> It seemed a good solution would be to add a new IP to the
> legacy system so it would be on two subnets, and adjust
> named.conf so it was listening on both IPs. Later we could
> update the multiple external settings pointing to the legacy subnet
> and eventually take it down.
>
> After the second IP is brought up, we have something like this:
>
> Kernel IP routing table
> Destination Gateway Genmask Flags Metric Ref Use Iface
> XXX.YYY.2.0 * 255.255.255.0 U 0 0 0 eth0
> XXX.YYY.200.0 * 255.255.248.0 U 0 0 0 eth1
> link-local * 255.255.0.0 U 1002 0 0 eth0
> link-local * 255.255.0.0 U 1003 0 0 eth1
> default dyna2-1.mydom 0.0.0.0 UG 0 0 0 eth0
>
> named is able to answer on either it's subnet 2 or subnet 200 address.
> However, systems on subnet 200 which have the XXX.YYY.2.ZZ address
> in the resolv.conf are timing out.
>
> If I try to ping the 2.ZZ address from a subnet 200 system, it fails.
> I think it is because the packet arrives at the 2.ZZ address, and
> it routes it through eth1 to get back to the subnet 200 destination.
> The remote subnet 200 system thinks the packet is spoofed since
> the answer came back from a IP different than it was sent to.
>
> Problems were restored simply by taking down eth1 for the time being.
>
> Is there an easy way to "make TCP/IP answer back on the same
> interface it was listening on when a packet is received"?
>
> I've seen articles mentioning multi-homing and balancing, but that
> doesn't seem to fit what we are looking for.
I think I found a blog that explains a solution with ip route commands
in simple terms.
https://kindlund.wordpress.com/2007/11/19/configuring-multiple-default-routes-in-linux/
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