Re: Why does dhcpd have a routers (plural) option for a subnet?

On Thu, Sep 26, 2024 at 03:29:39PM +0100, Frank Leonhardt wrote:
> This would require the host to rotate on failed gateways. I've always
> thought this was a sensible and simple idea but networking geeks said it was
> a really bad one and router standby protocols were the way to go.
> 
> So the next interesting question would be which host stacks would accept
> multiple gateways and what would they do with them?


You have to think back to when this would have been developed.

The model of NAT didn't exist. IP addresses were unique and globally routable.

Routers were large devices that weren't entirely stable, mainly to route between multiple networks.

For a large campus, you probably had a router servicing every building, if not every floor.
You may have put multiple routers in to talk to different types of networks. Its entirely possible 
that another backup router would eventually lead you back to where you needed to go, perhaps over a
secondary (ie. slower) link. 

Any host stack should be able to handle routing tables dealing with the original scenario.
Network Engineers at ISPs do this all the time.

Now, with NAT being prevelent everywhere, you're going to have to go
through the device that holds your session table to have NAT work
back. Most firewalls won't let traffic coming in on the "wrong"
interface from passing through. Thus, we've collapsed everything down to requiring
the one gateway router/firewall device. 

This is one of the problems with NAT that old network people complain about.
NAT solved the Internet IP address limit problem, but with much reduced functionality and resiliency.