Bridged network issue iMac 2019

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MRL
Posts: 4
Joined: 25. Aug 2019, 15:59

Bridged network issue iMac 2019

Post by MRL »

I have a 5 year old iMac using Apple AirPort Extreme (Broadcom) WiFi adapter connection to internet. No problem with Windows and Ubuntu VMs using bridged network with iMac WiFi adapter. In fact running Ubuntu server. Just purchased 2019 iMac and am also using Apple AirPort Extreme WiFi to connect to internet. Both iMacs are too far to have wired connection to router. Created Windows and Ubuntu VMs on new iMac with same configuration as old iMac. No internet connection in either VM. Tried every possible configuration. Obviously the newest Apple AirPort Extreme WiFi adapter on the 2019 iMacs (adapter does not list Broadcom in description) does not work with Virtual Box. Can anyone replicate this problem or offer suggestions? I suspect the newest Apple WiFi adapter needs to be addressed in a Virtual Box update. Thanks. BTW, both iMacs are running Mojave 10.14.6. BTW2 Parallels has no problem with the new iMac WiFi for Windows but I cannot create a custom Ubuntu server with Parallels - thus why I need Virtual Box to work with new Apple WiFi adapter.
socratis
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Posts: 27330
Joined: 22. Oct 2010, 11:03
Primary OS: Mac OS X other
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Guest OSses: Win(*>98), Linux*, OSX>10.5
Location: Greece

Re: Bridged network issue iMac 2019

Post by socratis »

Bridged over wireless don't always play nice. Bridged networking is outside the WLAN specification. Bridging to wireless is not really bridging. The guest shares the MAC of the host and the host does a sort of MAC-NAT translation based on IP addresses. Promiscuous mode doesn't exist in the official WLAN specifications. It may or may not work. Some combinations of Routers/Access Points, WLAN cards and drivers work, some don't. See: Bridging & Wifi - Supported hardware and add your experience. For example, it works fine in my home, but not in my office. Same laptop, same VM. Try to see if it works either with wired bridged or with NAT.

And here's a more technical explanation, pay special attention to the last paragraph:
vushakov in ticket [url=https://www.virtualbox.org/ticket/10019#comment:18]10019:18[/url] wrote: Many wifi routers now try to use unicast link-level destination for broadcast/multicast IP destination. The reasons are explained in http://tools.ietf.org/html/draft-vyncke ... ficient-01 - that is in context of IPv6, but the same logic applies to IPv4 (IPv6 is hit harder since it relies more on multicast). Behavior varies between wifi routers, so you may get bridged setup working with some and not working with others.

If the wifi router that is not working for you just uses unicast delivery for multicast, then 4.3.16 should help (a typical packet capture can be seen in #12207). In this case the host was receiving DHCP replies intended for the guest (broadcast IP, but unicast to host MAC), but was not rewriting MAC address correctly, so the guest was not receiving the packet. If you plug another computer into the wired port of the router to capture DHCP exchange as seen on the wired side, you would see the same DHCP replies sent to ethernet broadcast on the wired connection. So this is just an optimization for wifi that some routers do.

Unfortunately - and this is orthogonal to multicast/unicast issue above - some routers will send DHCP replies to broadcast IP, but to the unicast client MAC address (i.e. guest MAC in this case) fetched from the DHCP request. These packets will never be even seen by the host. I'm afraid the packet captures in comment:14 is an example of that. In the ethernet capture you can see DHCP replies unicast to guest and in the wireless capture you don't see any replies at all. I have one router like this (though it at least uses ethernet broadcast for its DHCP NAKs, so you can see something in the wireless capture :).

This latter kind of routers has problems with DHCP, but usually you can work around it by not using DHCP and using static IP instead. E.g. I cannot connect to that router of mine with DHCP, but if I use static IP in the guest then I get normal connectivity. Yes, this is suboptimal :(, but better than no connectivity if you must use bridged for some reason.
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