MAC address pass through

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Level 1

27 points

MAC address pass through

I've started using a Plugable docking station which allows your USB-C MBP to use a wired connection on the dock. This works great except that your MBP gets assigned the IP address for the docking station's ethernet port's MAC address, which makes it impossible to switch between several different laptops. Each will get assigned the same IP address from a DHCP server that assigns IP addresses to MAC addresses, because the DHCP server only sees the docking station's MAC address.

It might be possible to configure the DHCP server to assign IP addresses based on host names (not sure), but the other option would be to pass through a MAC address from the laptop to the docking station. It doesn't seem like that's supported by macOS 10.12 though. Anybody have any ideas for how to work around this limitation?

MacBook Pro (13-inch, 2017, 4 TBT3), macOS Sierra (10.12.5)

Posted on Jul 11, 2017 4:54 PM

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Answer 1

BobHarris

Level 9

56,945 points

Jul 11, 2017 6:41 PM in response to pumpichank

Some router DHCP implementations can use the System Preferences -> Network -> Advanced -> TCP/IP -> DHCP Client ID. You could set that per Mac, even per network interface if you want.

I know that the Apple Airport Extreme WiFi base stations support using the DHCP Client ID to assign the IP address to a Mac. If your router supports using the DHCP Client ID, you could make sure each Mac gets its own unique IP address.

If your router does NOT support DHCP Client ID, then you could change each Mac to "Using DHCP with manual address", or "Manually" (System Preferences -> Network -> Advanced -> TCP/IP)

Because you most likely do not want your Macs to always use that fixed IP address, you could use a utility such as ControlPlane to switch the Network Location based on whether you are at home, or somewhere else. Each System Preferences -> Network -> Location can have its own TCP/IP setup.

<http://www.controlplaneapp.com>

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Answer 2

Kappy

Level 10

362,007 points

Jul 11, 2017 5:40 PM in response to pumpichank

That isn't supported in any way because each network device must have its own hardware MAC address. There is no reason for it to pass through nor does it matter whether other devices receive IP addresses from the dock or another Ethernet port. If for some odd reason you want different computers to receive the same IP address then you will have double NAT problems to deal with or errors trying to use multiple devices with the same IP address.

Maybe you aren't explaining things clearly, but I don't understand why you see a problem. Your dock most likely has only one Ethernet port. It sees each connected computer as a separate MAC device. There is no need to pass anything through to it.

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Answer 3

pumpichank Author

Level 1

27 points

Jul 11, 2017 7:05 PM in response to Kappy

Thanks for the reply. Yep, I'm sure I didn't describe the problem very clearly. 🙂

The docking station has its own MAC address hardwired to ethernet. That's the MAC that the DHCP server assigns an IP address to. When I plug MBP #1 into the dock, I'll see the dock show up in the list of available network interfaces. If I enable that, my MBP will get assigned that IP address on that interface.

Now I unplug MBP #1 and plug in MBP #2. As soon as I enable the dock's network interface, the MBP will get assigned the same IP address (i.e. the one assigned to the dock's MAC address). This causes a conflict.

For example, if that IP address is mapped to hostname 'foo', then things like ssh get into trouble because with MBP #1 connected, the IP address will have one host key, but with MBP #2 connected, that same IP address will have a different host key.

What I would like is for the DHCP server to see a different MAC address through the dock depending on which MBP is connected to it. I get that this may not at all be possible, either because of limitations in Network preferences or in the hardware/software configuration options of the dock. The dock's manufacturer suggests that a "pass through" of a unique MAC address per machine to the dock is possible on Windows, so I'm kind of just throwing out a question as to whether the same is possible on macOS.

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Answer 4

pumpichank Author

Level 1

27 points

Jul 11, 2017 7:07 PM in response to BobHarris

Thanks! The Client ID is definitely something to investigate, and may indeed solve the problem. I'm using isc-dhcp-server on Ubuntu to assign addresses, currently only MAC based assignments. I'll look into whether it supports Client ID.

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Answer 5

Kappy

Level 10

362,007 points

Jul 11, 2017 7:11 PM in response to pumpichank

Now I unplug MBP #1 and plug in MBP #2. As soon as I enable the dock's network interface, the MBP will get assigned the same IP address (i.e. the one assigned to the dock's MAC address). This causes a conflict.

How so? The first Mac is not longer using the dock or the IP address. I've seen this many times and there were no conflicts. What router do you use for the dock? The dock should be connected to an Ethernet port on the router. The computer then connects to the dock.

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Answer 6

BobHarris

Level 9

56,945 points

Jul 12, 2017 8:23 AM in response to pumpichank

Another thing you could try, although it might be a pain to implement, but maybe see if you can change the MAC address of the interface: 

http://osxdaily.com/2012/03/01/change-mac-address-os-x

NOTE: You will have to figure out what ifconfig interface name is assigned to this Ethernet port.

Then use a utility such as ControlPlane to run a script that applies the ifconfig command when you connect to the docking station

<http://www.controlplaneapp.com>

Then again, there may be automated implementation issues, as you need to be 'root' to change the MAC address, and I'm not sure ControlPlane will be able to do that. Then again, I do not know that it cannot.

BUT, if you can use the DHCP Client ID, that would be so much cleaner.

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Answer 7

pumpichank Author

Level 1

27 points

Jul 12, 2017 8:23 AM in response to BobHarris

Yep, thanks! I was thinking about MAC spoofing.

Thank you all for your great suggestions.

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