Share internet from Ethernet 2 to Ethernet 1

Question

Hello,
My Mac Pro is connected to a router by its Ethernet 2 port; I want to connect another computer (Linux) to Ethernet 1 (so the Linux computer can connect to the Internet passing by the Mac Pro). I can easily share the connection from Ethernet 2 to AirPort (e.g. to synchronise my iPhone) using the Sharing pane of System Preferences, but sharing from Ethernet 2 to Ethernet 1 does not work.
I tried several configurations; sometimes I can just ping from the Linux computer to the Mac Pro (Ethernet 1 address only), sometimes I can ping up to my home's router and other times I can't ping at all. With all the configurations I tried, Internet can't be reached, even when I can ping the router.
Searched on various forums, found no help.
How should I go for this?

Answer 1

Grant Bennet-Alder wrote:
Your use of Ethernet 1 to be shared, and Ethernet 2 to connect your Mac to the Internet makes me crazy.
But they SHOULD be interchangeable, so you should be able to use either one for either function.
This evening, I got an “amusing” surprise, which tends to mean it makes me crazy as well. Another computer on my network, which is pluged into the main router, could not connect to the Internet. I then tried to go to 192.168.2.1 (my router's IP), to look at its state, and instead Safari showed the HTML page of my web sharing of my Mac Pro. My Mac Pro had been temporarily my DHCP server instead of the usual one because I confused Ethernet 1 and Ethernet 2… Tired of trying too much, I guess.
So, well, I'll definitively use Ethernet 1 to connect to the Internet and Ethernet 2 to share it, so it's clearer.
When I think about WHY it might make me crazy, it reminds me of a fundamnetal rule: Your Mac will send packets bound for the Internet to the TOPMOST working interface listed in System preferences > Network. If you have not already demoted the port you intend to Share to the end of that list, you should do so now. Use the gear Icon.
In that list, Ethernet 2 seems to be the first one (unless it starts at end?). I have this, as it appears in the list of services:
Bluetooth PAN
DUN Bluetooth
AirPort
Ethernet 2
Ethernet 1
Firewire
I don't know what these Bluetooth have as differences, because I never used them. But it looks like Ethernet 2 is before Ethernet 1. Do you think I should put Ethernet 1 at the very top of the list (I don't recall why I put it in this order in the first place)?
If Ethernet 1 is the port being shared to the Linux machine, you do not configure it AT ALL. If you have changed its settings, set it back to DHCP.
Anything you change Manually will mess up Sharing. You may even want to delete that port and Restart your Mac to get all pre-conceived notions removed.
Ok, very nice to know, thank you!
So, now Ethernet 1 is connected to my home's Router and Ethernet 2 to the Linux computer. I did first set Ethernet 2 to DHCP, following your suggestion, but it assigned the 169 range of IP address; then I configured it as “DHCP with a manual address” (remember, I translate to English) and put 192.168.100.1 as the IP adress. From Linux (still using DHCP), I couldn't even ping 192.168.100.1.
Then, I chose to set 192.168.2.28 as Ethernet 2's IP (trying to put it in the same subnet as Ethernet 1). At that point, from the Linux box, I could connect to 192.168.2.28, 192.168.2.222 and even to my router (192.168.2.1), but pinging beyond the router (on the Internet) still fails and, of course, browsing doesn't work. But it's better.
I'd like to avoid restarting my Mac for now (I've a lot of applications open, in use). Is there anything else I should do now?
Thank you for your replies so far!