[CentOS] Configuring source-specific routing

[Les Mikesell]

On Thu, May 2, 2013 at 12:31 PM, Michael Mol <mikemol at gmail.com> wrote:
>>> with its default gateway pointing toward the ISP handling it.   DNS
>> service is simple enough to have standalone servers for each instance
>> you need.
>
> This would also require either resources or underlying authorizations I
> don't have.

CentOS VMs are really, really cheap....

>> Web browsers are actually very good at handling multiple IPs in DNS
>> responses and doing their own failover if some of the IPs don't
>> respond.
>
> It varies greatly by client software. And given the explosion of
> unreliable network connections (wifi, mobile), some of that failover
> logic's margin is already lost in dropped packets between the client and
> their local network gateway.

Yes, but typically they can deal with receiving multple IPs from the
initial DNS lookup even if some are broken better/faster than getting
one IP which subsequently breaks and then having to do another DNS
lookup to get a working target.   At least the few broswers I tested a
while back did...

>> For other services you might need to actively change DNS to drop IPs
>> if you know they have become unreachable, though.
>
> Yup. That's what I was planning on doing, more or less. Start with
> ordering IPs by route preference, drop IPs by link state. I just wish I
> could drive it by snooping OSPF...

I don't think you can count on your ordering reaching the clients or
meaning anything to them if it does.  And some applications won't ever
do a lookup again.

--
   Les Mikesell
     lesmikesell at gmail.com