I have a few networks that all my systems are connected to. Two of them have internet connections and the third is strictly internal and has my file servers, NAS and other devices I don't want to allow outside access to.

I can find which internet connection is being used by default by theach system but when it comes to connecting to each machine within the network, I don't know how it is being done and which network connection is being used (I guess I coud look at adapter statistics but often all machines are busy transfering so it gets messy.

I'm wondering what is the best way to handle the hosts file when there are at least 3 IP addresses, each on a different network, for each machine.

Is there any standard way of handling this type of setup and which network should be used for which type of traffic. Also the hosts file issue is something I'd love some suggestions on.

Anyone have any thoughts on this? Thanks.

You're asking three different questions here:

1. How does routing work when a computer is connected to more than one network?
2. How does name resolution work in a scenario where multiple network connections are involved?
3. Are there any guidelines/best practices one should follow when setting up a network like this?

As for the first question, the critical part of the setup has to do with the choice of "default gateway". In most networks, IP addresses are handed out automatically via DHCP, and along with then IP address, each client gets told which gateway to use. That's problematic when several networks are involved, because multiple "default" gateways simply doesn't work in most scenarios. (And it doesn't make sense anyway; how can there be more than one "default" of anything?)

When a host is told to use two "default" gateway, the TCP/IP standard dictates that traffic should be load balanced across both/all gateways. If one or more of those gateways perform Network Address Translation (which most do these days if they're connected to the Internet), the result will be slow/unstable/no Internet access. There is no way to configure the DHCP servers/scopes in such a way that one gateway will take precedence over the other. In general, multiple network connections should be avoided for all non-routers, unless all but one network has a gateway.

About name resolution: You mention the "hosts" file, which is an ancient way of providing name resolution in networks. Sure, it works, but you really should consider setting up a local DNS service, if at all possible.

But again, if you have multiple network connections that each provide different DNS server settings via DHCP, it's all going to be a big mess. You can indeed specify multiple DNS servers on a host, but the host will interpret that at "always use the first server unless it's completely unavailable, and only then proceed down the list". Unless you can redesign your network, the "hosts" file may actually be your best bet.

"Best practices" would be to separate the networks using a firewall/router instead of connecting each client to multiple networks. That would be the quickest and most efficient way to solve all your issues. In a corporate environment one would usually configure a VLAN for each network, and since switches supporting VLANs/802.1q are really cheap these days, that's what I'd recommend.

hosts files are great for some situations. In most default installs the hosts file is used at each look up for a name to ip. It can be set to be a lower priority if needed. For example dns doesn't resolve so then go look at hosts or other for resolution. I use them all the time but for other reasons.

To select a route based on the application would require usually a layer 7 plus firewall or appliance. It may be possible in some apps to tell the gateway or other feature to determine path. Some of the older apps in linux may have that feature. Many newer ones simply default to the host computer setting.

I'd agree that 2 or 3 real network (or vlan) would suite you but at some point you have to realize they may end up being connected if you break any rules to keep them apart.