ethernet-shared internet gives self-assigned ip address

I just thought of another thing to check. IPv6. Are either your Mac Pro or your Macbook Air using IPv6 vs IPv4?

I generally put my Macs into for "Link-Local Only" for IPv6

System Preferences -> Network -> <interface (WiFi or Ethernet)> -> Advanced -> TCP/IP -> Configure IPv6

This is how I set my Macs.

While out on a bicycle ride this morning, I was listening the MacGeekGab podcast, while they were not talking about Internet Sharing, they did mention IPv6, and I realized that maybe I should ask if your Mac Pro and/or Macbook Air are using IPv6, and make the above suggestion.

NOTE: I still think an Ethernet switch is the best long term approach, but that does not preclude solving your Internet Sharing issue.

Read some articles : macOS Sierra: Share the Internet connection on your Mac

: macOS Sierra: Share the Internet connection on your Mac

: If a device sharing the Internet connection of your Mac isn't connecting to the Internet - Apple Support

If you plug the Macbook Air into ethernet cable you currently have your Mac Pro, does it work?

That is to say, if you eliminate the middle-man and internet sharing, does it work? That would isolate if it was something related to the Ethernet adapter you are using on the Macbook Air, a problem with the Macbook Air using Ethernet, or a problem with the Mac Pro internet sharing or the Mac Pro's Ethernet 2 port.

If directly connecting the Macbook Air to the Ethernet cable you normally plug into the Mac Pro works, then consider an inexpensive Ethernet switch, which will allow you to share that Ethernet line with both the Mac Pro and the Macbook Air.

Anyway, once you know if it is the Macbook Air, or Macbook Air Ethernet adapter vs the Mac Pro or Internet Sharing running on the Mac Pro, additional experiments and suggestions can be made.

Passive FTP should have zero effect on any of this. Passive FTP ONLY applies when you enable proxies, as in your are going to send all your networking stuff through a proxy server. If you have enable proxies, that in itself could be a problem, as generally only users inside a corporation trying to access the Internet through the company firewall need to setup proxies.

...I'm sure my setup is correct as I was advised by Apple's Linc Davis.

Unless Linc has taking a job recently with Apple, all his posts in this forum have been as an Apple customer helping other Apple customers. The bulk of the help here is from volunteers that just like to help fellow Mac users.

As for your ISP being involved. It is unlikely. As generally what you plug into your home network behind the router is totally up to you. Sometimes a router does get flaky, but not all that often these days.

As an experiment, on The Macbook Air, you could create a new System Preferences -> Network -> Location -> Edit Locations... -> [+], and disable all the network interfaces for that location except the one you want to use, so that when that Location is specified as the location to use, then there is no question which interface the Macbook Air can be using (Apple Menu -> Locations is how you quickly switch locations). NOTE: Do NOT touch your normal location (typically called Automatic, unless you have been creating locations before I suggested it)

Another trick is to go to the Macbook Air -> System Preferences -> Network -> network interface you are using -> Advanced -> TCP/IP -> DHCP Client ID, and put in a simple unique name, with no spaces or special characters. Such as FunksterMBAir. Hopefully you get the idea. Reasonable short and simple. Sometimes routers are more friendly when the device identifies itself. Sometimes this is just an experiment that changes nothing.

So let us know how the experiments went.

Many thanks for your quick response. I should have mentioned that plugging the ethernet cable normally going into the Mac Pro directly into the Macbook Air (via the ethernet adapter) does work fine, and I get full speed internet. So that seems to indicate the problem is not the adapter.

and

Mac Pro is connected by ethernet cable in Ethernet 1 port to an Airport Extreme router (in Bridge mode),

Is the Airport Extreme in the same room? It should have several extra Ethernet ports you should be able to use on the back. So why not just plug the Macbook Air into one of the other Airport Extreme ports?

If there is a long Ethernet cable (going through walls and such) going from the Airport Extreme to the Mac Pro, then get an inexpensive Ethernet 5-port switch, and share that Ethernet cable with both the Mac Pro and the Macbook Air. You can pick them up at places like Staples, Walmart, Target, BestBuy, or on-line via Amazon.com, Newegg.com etc... I would expect the price to be in the $30-$40 range (maybe less).

NOTE: It would be interesting to figure out why your System Preferences -> Sharing -> Internet Sharing is not working on the Mac Pro. But it seems to me a simple hardware solution would be a better long term solution.

No, as mentioned, Airport Extreme is at far end of another building about 30 metres away, that is the issue. I do have a long cable going under floors, the back garden etc and that was hard enough to put in, I don't want to do a second cable! I have also tried home-plugs (internet through the electrical circuit, that's what they're called here in UK), but the performance in this out-building is relatively quite poor for some reason, although good in the house.

I fully understand NOT wanting to run a 2nd Ethernet cable, and I also understand the PowerLine Ethernet adapters not working well.

But you should be able to use an inexpensive 5-port Ethernet switch. You need at least 3 ports. 1 port for the 30 meter cable. 1 port for the Mac Pro and 1 port for the Macbook Air.

Generally the smallest switch is a 5-port. That gives you 2 spare ports for a network attached printer, an Apple TV, a TV that is network capable, etc...

NOTE: If you find a 3 or 4 port that is much less expensive than a 5 port, OK. But since 5-ports are the most common, they are more likely to have commodity pricing. You should be able to get something for under $30 US (not sure what that would be UK, but since many vendors make Ethernet switches, you should be able to find a vendor that offers something that costs less than a cheap dinner for 2 (maybe not burgers at McDonald's, but not much more either).

I have been thinking about getting the Ethernet switch and had a look on Amazon.co.uk for 2-port switches (I figured that was all I needed) but am a bit confused about "managed" or "unmanaged". I saw some with actual switch-buttons on them, but I don't really want to be doing switching manually, if that's what unmanaged means. Can you throw any light on that? I guess an Ethernet splitter is not going to do the job.

A simple "Unmanaged" switch is all you need.

It does not need to have any buttons. All it needs is a power brick to plug into the Mains (electrical outlet for those outside the UK). And enough RJ45 standard Ethernet ports for the 30 meter cable, the Mac Pro and the Macbook Air.

You DO NOT need PoE (power over Ethernet). You need special devices to use PoE, so do not be confused by that term either.

I've been using them for years. In one case it was a similar situation. I had an Ethernet cable running through the dinning room wall down to the basement, across the basement up through the 1st floor hall closet into the 2nd floor hall closet, through the wall into the back bedroom. I had a 5-port switch in the dinning room where my Wife's office was, and a 5-port switch in the upstairs bedroom where I had my office. Both of them were inexpensive unmanaged 5-port switches.

I'm thinking it will be simplest to go for the switch, as you suggest, or for that matter even a basic wireless router, to give my some mobility with the MB Air in that outside room. Just so long as it works!

A basic WiFi router, that you put into "Bridge" mode, would give you a similar result, plus WiFi support in the part of the house. It would just cost more than a switch, but if you want WiFi service in that area, like for you iPhone, iPad, or other WiFi devices, it might be the better option.

It can be fun figuring out how to put some WiFi routers into bridge mode. Apple routers are easy, and you have already done it. Other vendors often go into bridge mode if you turn of their DHCP server. Some need you to turn off the NAT server. Like I said it is different for each, but some Google searching should turn up instructions.

A WiFi router will have a built-in Ethernet switch, so you can run the 30 meter cable into the WiFi router's WAN port, and use one of the switched ports for your Mac Pro, and either us another port for your Macbook Air, or just connect to the WiFi.

Didn't get anywhere with the experiments, I'm afraid... still just "no IP address" or "self-assigned".

I'm sorry you cannot get Internet Sharing to work on your Mac Pro. It really should work and do exactly what you want.

Then again a dedicated Ethernet switch should do the job and not fight you about it.