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Mutihoming with one Ethernet interface Network
Like many small businesses we have taken advantage of the 'free' DSL services offered as part of an existing mobile phone contract. In this case we use Orange for mobile service (we're based in the UK) and they offer 2 Mbit internet with a wireless router for no extra if you have at least one pay monthly account. I thought that it would be a useful addition to our network for testing and backup, especially as it is from a different ISP and delivered over a physically separate landline from our business internet.

The wireless router arrived yesterday and the DSL activation happened on time so that the line came up first time when I plugged the line into the router. I'd heard a few horror stories about Orange's support for Macs so I was pleasantly surprised when I popped the install CD into my MacBook and it came with full OSX installation instructions. 5 minutes later I was surfing on the internet via my shiny new Orange broadband.

Now that we had two internet accesses functioning, how to use them both?

I originally thought that I would keep our 8 Mbit business broadband link and the new line on separate networks so that I would not have to change any server or client configurations, just point my test machines at the new default gateway.

However, with a little thinking about how much work it would be to switch all clients over in the event of a line failure (it does happen), especially if no IT savvy people were available at the time of failure, I started Googling for suggestions on how to link all computers to both gateway routers.

I investigated using a spare Linksys router with new firmware to act as a load balancer, but that would introduce a new single point of failure. After thinking a bit more about OSX's multihoming feature I decided to do the following:
  • At each Mac, clients and servers, add a new interface (System Preferences » Network » Show Network Port Configurations » New.) I called ours orange internet, using port 'Built-in ethernet.'
  • Set up the new port with 'Configure IPv4: Manually,' giving it a 192.168.1.x address. (The original LAN address range is, mask, and default gateway is
  • On each machine, arrange the order of ports so that the default ethernet for each is first in the list.
  • On the server, setup a new DNS zone on 192.168.1.x with the same config as the original.
  • Optional: Use dyDNS or similar to assign a domain to the new external IP
Now, if the first interface is not available then it will fail-over to the other one and I can chose which gateway to use at each machine by re-arranging the port order.

[robg adds: My apologies to pglock; this hint was submitted a longgg time ago. For some reason, I marked it as a draft, then forgot about it. Yikes. There's a much older variation on this hint in the system, but this one contains updated information.]
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Mutihoming with one Ethernet interface
Authored by: tempel on Jun 08, '09 08:21:32AM
One thing is not clear to me: It says to setup DNS on the server. Does this mean this multihoming approach won't work with an extra server that provides DNS for the network? What if I have no dedicated server available?

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Mutihoming with one Ethernet interface
Authored by: theilgaard on Jun 08, '09 11:54:00AM

Usually when you want a failover option, the right way to do it is with a firewall/router that supports failover. In this way nothing needs to be changed on the clients or servers, as the failover is handled automatically at the gateway.

It's annoying for mobile users to have to select between locations in the network preference.

If you have 3g mobile coverage in your area you can with the right firewall, use that as fail over, it works well here in Denmark.

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Mutihoming with one Ethernet interface
Authored by: ckizer on Jun 08, '09 06:08:59PM

Actually with this method there is no settings to change, this is a "true" failover, in the sense it's 100% automatic. No switching of anything is required. This is one of the best, fastest, and better than ANY switch or firewall I've used, even more expensive $500 cisco routers.

You can also modify this method to bind to connections together, and router different traffic over internet connects. AKA bittorrent out one, and regular traffic over another. Or all upstream traffic out one, so it doesn't slow down the network.

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Mutihoming with one Ethernet interface
Authored by: jdmuys on Jun 10, '09 07:02:26AM

I have a similar need, but I don't need failover, I need protocol-based traffic splitting (or possibly IP-based).

In details: I want some protocols to be handled through one network interface (eg http), and other ones through another (eg ssh, irc, ftp).

I found one possible way, but it's not protocol based, it's IP-based: using the route terminal command.

So is it possible to route some protocols through an interface, some others through a different one (and the rest through the default one)?

If not, using IP-based routing is a possibly working scenario for us. Is there a nicer to configure it than through the terminal? I found IP Net Router which seems to be able to do so, but it's very complex.

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Mutihoming with one Ethernet interface
Authored by: tc_nyc on Jun 10, '09 04:58:08PM

Did you experiment and test this with a simulated failure?

It seems to me that this might not work because when the primary ISP connection goes down, the cable/dsl modem will still provide a link and DHCP addresses even though traffic will not flow. I don't think OS X will treat a virtual ethernet port as 'dead' or 'unplugged' unless the physical link actually goes down, so it will never actually switch to the other connection.

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Mutihoming with one Ethernet interface
Authored by: pglock on Jun 11, '09 08:41:51AM

The idea of multihoming was a good one, but...'s been a while since I wrote the original hint. It was a good idea when the cost of multi-WAN routers were in the high-end enterprise budget range. I've since invested in a Netgear FVS 336G which gives me load balancing, failover, and a SSL VPN gateway in one box.

Just as an added complication, we are about to get a third DSL line at home (it's actually cheaper than a basic BT line) so I'll separate my children's traffic from our business by putting them on a different wireless network.

How many DSL/cable connections do you have?

Too lazy to think of an original signoff...

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Mutihoming with one Ethernet interface
Authored by: mezis on Jun 21, '09 02:44:49AM
I also run the network at a SMB where we chose to subscribe with two ISPs (Free and Orange).
Not wanting to advertise, but...
We invested in a Peplink multi-wan router that takes care of connection fail-over and load-balancing for both incoming and outgoing solution. AFAIK this is fairly unique and scalable, and has worked very well for us for over 6 months now (not a single router reboot, gave us 100% service uptime with 99.4% uptime for each connection).

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