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I have a MacBook Pro connected to an iMac 27" Mid-2011 with a Thunderbolt cable. I use this for Target Display Mode.

My iMac is connected to the Internet via an Ethernet cable, and I would like to share that connection to my MacBook Pro, using the Thunderbolt cable. Is it possible to use Internet Sharing to do this?

I've tried using the settings below, but they don't work. While the "Thunderbolt Bridge" has a self-assigned IP, the "Thunderbolt Internet" connection is dead. In the image Wi-Fi on the MacBook is On, but if I switch it Off, there is no Internet connection at all.

I'm using the Thunderbolt-to-Thunderbolt cable. enter image description here

iMac ("Sender"):enter image description hereenter image description hereenter image description hereenter image description hereenter image description here

MBP ("Receiver"):

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Update: Seems to work OK

Thunderbolt Bridge Internet Sharing now appears to work. On the receiving MacBook, the Thunderbolt Bridge service is placed on top and has a "green" marker. I get Internet access even if I disconnect the Wi-Fi, so it does indeed work.

However, I'm not sure what changed, except that I might've restarted the computers once or twice.

enter image description here

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    Just to clarify: Are you trying to share an Internet connection from your iMac to your MacBook Pro at the same time as the iMac is being used in target display mode?
    – Monomeeth ♦
    May 2, 2017 at 23:09
  • 1
    @Monomeeth: Good question, the answer is yes – I would like to share Internet from the iMac to MBP, with Target Display Mode being active at the same time. When I took the screenshots and Internet Sharing didn't work, TDM was not active however. So I'm not sure which effect it will have (if any).
    – P A N
    May 2, 2017 at 23:32
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    Your Sharing preferences on the iMac look fine. If possible it'd be good to also see the Network Preferences on your iMac for the Thunderbolt service (not just the main screen, but also the TCP/IP, DNS and Hardware tabs within Advanced).
    – Monomeeth ♦
    May 6, 2017 at 9:25
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    @Monomeeth: I added the images you requested
    – P A N
    May 7, 2017 at 17:04

1 Answer 1

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It is possible to share Internet from Ethernet to Thunderbolt!

  • Your gateway (iMac) Internet Sharing settings are ok:

    Share your connection from: Ethernet > to computers using: Thunderbolt bridge

  • Choose the Thunderbolt Bridge device in the network preferences of the client and set Configure IPv4 to Using DHCP.

Open Terminal on both Macs and enter ifconfig respectively.

The gateway (your iMac) should show a bridge device bridge0 crowded with the Thunderbolt devices (probably en2 & en3) with the IP-address 192.168.2.1:

...
bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=63<RXCSUM,TXCSUM,TSO4,TSO6>
    ether 6a:aa:ca:11:23:00 
    inet6 fe80::68aa:caff:fe11:2300%bridge0 prefixlen 64 scopeid 0x8 
    inet 192.168.2.1 netmask 0xffffff00 broadcast 192.168.2.255
    Configuration:
        id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
        maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
        root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
        ipfilter disabled flags 0x2
    member: en2 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 6 priority 0 path cost 0
    member: en3 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 7 priority 0 path cost 0
    nd6 options=1<PERFORMNUD>
    media: autoselect
    status: active
...

The client (your MacBook Pro) should also show a bridge device bridge0 probably containing en1 (a MBP without Ethernet) or en2 (a MBP with Ethernet) and maybe another en device. Usually its IP-address is 192.168.2.2.

...
bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=63<RXCSUM,TXCSUM,TSO4,TSO6>
    ether 6a:aa:ca:22:11:00 
    inet 192.168.2.2 netmask 0xffffff00 broadcast 192.168.2.255
    inet6 fe80::68aa:ca22:fe11:e000%bridge0 prefixlen 64 scopeid 0x8 
    Configuration:
        id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
        maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
        root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
        ipfilter disabled flags 0x2
    member: en1 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 6 priority 0 path cost 0
    nd6 options=1<PERFORMNUD>
    media: autoselect
    status: active
...

If the network 192.168.2.0/24 is already occupied by the Ethernet network, the next network 192.168.3.0/24 will be used.

If Ethernet to Thunderbolt Internet Sharing is enabled, the Target Display Mode doesn't work reliably here!

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  • Internet Sharing via Thunderbolt bridge now appears to work (see screenshot in updated post). I didn't do anything to make it happen, other than restart the computers once or twice. So maybe there was something that was making the Thunderbolt connection unreliable, in this regard. It sometimes happens that TDM can't be invoked either, without restarting one or both of the computers.
    – P A N
    May 7, 2017 at 17:06

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