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I have a headless mini computer with an Ethernet port, a Mac with a USB (not C) port, and a USB to Ethernet adaptor.

The mini computer is running standard CentOS Linux, and the eth interface is set to DHCP. The moment I connect the mini computer to a router via an Ethernet cable, it has access to the internet without any configuration.

The USB to Ethernet adaptor for my mac is also functional. I can connect it directly to the router and access the internet without configuration.

I would like to SSH into my mini computer over Ethernet, without needing the screen and keyboard, nor the intermediate router. However, I'm not getting any connectivity.

Do I need a cross-over Ethernet cable?

I know this is a very broad and ambigous question, but any pointers to what I could learn or investigate more about would be helpful.

closed as too broad by DavidPostill Jun 3 '18 at 11:21

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  • You should provide the make and models of the Ethernet cards used. With that information an answer might be possible. In any case you could just buy a crossover cable and test it yourself. – DavidPostill Jun 3 '18 at 11:22
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You are very unlikely to need a cross-over cable these days. Most ethernet ports are 'smart' & can auto-switch if cross-over is needed.

At the moment, both machines are expecting to find a DHCP server to tell them 'who to be'. As neither machine is serving DHCP, this will lead to both machines setting up a private, essentially non-routable address in the range 169.254.x.x
This should, in theory, work - but as it doesn't appear to be, then...

The first step is to remove DHCP & set them manually.

Set one machine to 192.168.0.1 & the other to 192.168.0.2
Gateway is strictly unimportant, as there's nowhere for them to go, but gateway 192.168.0.1 & a subnet default of 255.255.255.0 should work.

They are then on the same, routable, subnet & ought to be able to find each other.

On the Mac you can do this via the Location: drop menu, in Prefs > Network > so you can in future switch on the fly without needing to reconfigure. On CentOS, idk how this would be achieved.

  • To add, 1) Gigabit Ethernet ports are practically guaranteed to have auto-crossover; 2) Gateway can be left empty; 3) Subnet mask is still somewhat important (I don't think Windows has a special case for direct connections), but any value is fine as long as it covers both hosts' addresses. – grawity Jun 3 '18 at 8:06
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    And honestly, 169.254.x.x addresses would work just fine for this purpose. Non-routable doesn't mean non-usable. – grawity Jun 3 '18 at 8:07
  • In theory, absolutely; just that as it's not working using self-assigned addresses in practise, it seemed simpler to manually set an address. I was unsure about Gateway being empty; potential for the setup page to refuse 'no data', but picking one manually I didn't think would hurt anything. – Tetsujin Jun 3 '18 at 8:14
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    If you're only communicating between two, directly connected computers, APIPA/Local-Link addresses will work fine and gateway need not be set. It seems more likely to me that they're using older hardware that doesn't 'auto-switch' to cross-over mode. – 3D1T0R Jun 3 '18 at 8:17

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