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We're using the Ethernet connection as a point-to-point connection to a lighting console and then the WiFi to connect to the main network. The issue is whenever we turn the WiFi on/off the Ethernet interface, en0, loses its IP and will not work again until its IP is changed.

The Ethernet connection has a fixed IP, whereas the WiFi uses DHCP.

The issue is the software we're using with the lighting desk loses connection to the Mac, and it's not practical to keep on changing the IP of en0 to reset each time. Is there anyway of stopping this from occurring? Even if it means throwing together a script that can switch the WiFi independently instead.

One further question: provided the two networks have different prefixes (10.x.x.x and 192.168.1.x), should the traffic remain on the separate networks, and the applications shouldn't get 'confused', right? (I've found the route command, using that to setup all connections to a particular IP to use a particular interface should solve any issues right?)

Thanks in advance, and if you need any more information let me know and I'll get what I can.

2 Answers 2

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First, in…

System Preferences > Network > [Gear-icon button menu at bottom of interface list] > Set Service Order

…make sure your Wi-Fi interface is prioritized above your Ethernet interface (you can drag them around to reorder them).

If that doesn't solve the problem, make sure you're setting the Ethernet's IP address through the GUI, not via ifconfig. OS X's network interface management code relies upon what's called the "System Configuration" database to know how to configure your various network interfaces, and setting an IP via ifconfig doesn't actually create an entry in the System Configuration database. So the next time the System Configuration database gets updated or reapplied (like if you enable/connect another network interface or get a new DHCP lease), other interfaces may get reset to their last known setting in the System Configuration database, meaning you'd lose any IP address you'd configured through ifconfig or similar Unix-level tools.

If that's not what's going on for you, you'll need to describe exactly how you're going about setting the static IP address (GUI? ifconfig? networksetup? something else?), and what you mean when you say en0 "loses" it (It disappears from the Network pref pane GUI? It stops showing up under ifconfig en0? Something else?)

And yes, if the interfaces are on separate subnets (what you called "prefixes") the routing code in the network stack should not get confused.

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  • Hey Spiff, thanks for answering. In answers to your questions: the service order is correct (Ethernet over Wifi), it's being configured via the GUI, and when I say it 'loses its IP' the interface remains visible everywhere (in the GUI and in ifconfig) but the IP in ifconfig is no longer there (although it still is in the GUI). I'm not at the Mac at the moment so will have to check how it compares to a disabled/unplugged interface but if I remember rightly it doesn't have any network info (subnet mask or IP). I've learnt something about how OSX deals with networks, thanks for that.
    – Skyrail
    Apr 4, 2014 at 9:37
  • @Skyrail I said Wi-Fi over Ethernet. If Wi-Fi is your main Internet connection and Ethernet just connects to a single device stub network, then you want Wi-Fi ranked above Ethernet.
    – Spiff
    Apr 4, 2014 at 14:33
  • right, sorry, the ethernet connection is our priority though, or would that cause trouble as everything else will want to get to the internet (via WiFi) and not to the lighting desk? We ordered it Ethernet over WiFi as the software was losing connection to the lighting desk so I hoped the WiFi wouldn't affect the ethernet if en0 was a higher priority, did I think wrongly?
    – Skyrail
    Apr 4, 2014 at 15:59
  • The ranking is just about which interface is meant to be your main connection to the rest of the Internet, or at least the rest of a multi-hop network. If one interface connects to a multi-hop network, it should be ranked above any interfaces that only connect to isolated single LANs.
    – Spiff
    Apr 4, 2014 at 16:06
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Having internet sharing enabled may do this on Mac OS.

Check in System Preferences, Sharing, then if Internet Sharing is enabled over the ethernet interface. Disabling it may solve your problem, although you will of course lose the shared connection.

[was having the same problem, this was the reason]

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