A Direct Ethernet Link between a MacBook Air and an Arch Linux ThinkPad

In this post, we will describe how to connect a MacBook Air M1 to a ThinkPad P15 Gen 1 running Arch Linux with a single Ethernet cable, without using a router or a switch. We will give each machine a fixed address and set up passwordless SSH from the Mac to the ThinkPad. This Ethernet link will connect the two machines (as described in this setup). We will use the Mac to launch Julia workers and Mathematica kernels on the ThinkPad and use both machines concurrently to run our embarrassingly parallel computations. For remote access over the internet, see the notes on remote login, where we have used Tailscale.

1. Overview

The Mac will use the address 192.168.0.1 and the ThinkPad has the IP address 192.168.0.2, both being on the network 192.168.0.0/16. All traffic between the two machines goes over the cable. Since neither machine has a gateway on this network, internet traffic will continue to go through Wi-Fi, and the link works even without an internet connection.

Note that the setup has the following limitations:

1. The cable can connect only two machines. Adding a third machine will need an Ethernet switch.
2. Many home routers assign addresses of the form 192.168.0.x. If the Mac’s Wi-Fi network does so too, the Mac might route traffic for 192.168.0.2 through Wi-Fi. In that case, we use a range that no other network uses, for example 10.10.0.1 and 10.10.0.2 with the subnet mask 255.255.255.0.
3. macOS names a USB network adapter when it first detects it, so the interface may not be called en7 on another Mac or another USB-C port.

2. The cable

The MacBook Air has no Ethernet port, so we use a Cat6 cable with a USB-C plug on one end and an RJ45 plug on the other. The ThinkPad has an old-fashioned RJ45 port. The USB-C plug contains a Gigabit Ethernet adapter:

MacBook Air M1 (USB-C)
      │
USB-C to RJ45 Cat6 cable
      │
ThinkPad P15 Gen 1 (RJ45)

3. Checking the link

Once the cable is plugged in, both machines should detect the link, even before we assign any addresses. On the Mac, run:

ifconfig en7

The output should include:

status: active
media: 1000baseT <full-duplex>

Here, status: active means that the Mac detects a live port at the other end of the cable, and 1000baseT <full-duplex> means that the link runs at 1 Gb/s in both directions simultaneously. If en7 does not exist, networksetup -listallhardwareports lists all network ports with their device names, and the cable appears as a USB LAN port.

On the ThinkPad:

ip addr

The Ethernet interface is enp0s31f6, and its flags should include UP and LOWER_UP:

enp0s31f6: <BROADCAST,MULTICAST,UP,LOWER_UP>

UP means that the interface is enabled, and LOWER_UP means that it detects the cable.

4. Fixed addresses

Without a router, there is no DHCP server to assign addresses, so we assign them manually. On the Mac, we open System Settings $\to$ Network, select the Ethernet interface, open its details, and set:

Configure IPv4: Manually
IP address:     192.168.0.1
Subnet mask:    255.255.0.0
Router:         (blank)

We leave the router field blank so that the Mac continues to use Wi-Fi for internet access. After applying the changes, ifconfig en7 should show:

inet 192.168.0.1

On the ThinkPad, NetworkManager manages the Ethernet interface. In its settings, we edit the connection Wired connection 1, which NetworkManager creates for the Ethernet port, and set:

IPv4 method: Manual
Address:     192.168.0.2
Netmask:     255.255.0.0
Gateway:     (blank)

We save the connection and reconnect it. To verify the address, run:

ip addr show enp0s31f6

The output should include inet 192.168.0.2/16.

5. Connectivity

5.1 Ping

We can now test the link in both directions. From the Mac, run:

ping -c 4 192.168.0.2

and from the ThinkPad:

ping -c 4 192.168.0.1

The option -c 4 sends four packets and then stops. If we do not specify the flag, ping runs until we press Ctrl-C. Both commands should report 0% packet loss.

5.2 ARP

A successful ping does not tell us which interface the Mac used. For that, we check the Mac’s ARP table, which maps IP addresses to the hardware (MAC) addresses of network cards:

arp -a | grep 192.168.0.2

The output should look like:

isengard (192.168.0.2) at 84:a9:38:5b:3d:ff on en7 ifscope [ethernet]

The entry shows the ThinkPad’s name and the MAC address of its network card, and on en7 confirms that the Mac reached the ThinkPad through the cable rather than through Wi-Fi. If there is no entry yet, we send a single ping first:

ping -c 1 192.168.0.2
arp -a | grep 192.168.0.2

6. SSH server on the ThinkPad

Unlike the setup in the notes on remote login, which uses Tailscale SSH, this link uses the OpenSSH server. We can install it from the official Arch repositories and enable it:

sudo pacman -S openssh
sudo systemctl enable --now sshd

enable starts the server at every boot, and --now also starts it immediately. To check that it is running:

systemctl status sshd

The output should show active (running).

7. First SSH login

From Terminal on the Mac, we run a single command on the ThinkPad:

ssh subhodeep@192.168.0.2 hostname

SSH asks for the ThinkPad’s password, runs hostname on the ThinkPad, and returns to the Mac. The output should be the ThinkPad’s hostname:

isengard

This command tests the cable, the addresses, the SSH server and the login at once.

8. Passwordless login

Julia and Mathematica can launch their remote workers over SSH but cannot enter a password, so the Mac must log in with a key. If the Mac does not have a key pair yet, we can create one as follows:

ssh-keygen -t ed25519

We then copy the public key to the ThinkPad. ssh-copy-id adds it to ~/.ssh/authorized_keys on the ThinkPad and asks for the password one last time:

ssh-copy-id subhodeep@192.168.0.2

Repeating the test from Section 7 should now print isengard without asking for a password.

To avoid typing the address each time, we can define a host alias in ~/.ssh/config on the Mac:

Host isengard
    HostName 192.168.0.2
    User subhodeep

After saving the file, ssh isengard opens a shell on the ThinkPad. Julia and Mathematica can use the alias too.

9. Julia and Mathematica

Julia’s addprocs and Mathematica’s LaunchKernels start their workers through non-interactive SSH sessions, which may not read the same start-up files as an interactive login. We therefore check that such a session can find both programs:

ssh isengard hostname
ssh isengard julia -e 'println(VERSION)'
ssh isengard wolframscript -code '1+1'

The first command should print isengard, the second the Julia version installed on the ThinkPad, and the third 2, which also confirms that Mathematica starts and finds its licence. If all three succeed without a password prompt, the Mac can launch Julia workers and Mathematica kernels on the ThinkPad over the cable, as shown in the benchmark notes.

10. Performance

The link runs at 1 Gb/s. After protocol overhead, large file transfers reach about 100–115 MB/s, and the latency is below a millisecond. This is sufficient for embarrassingly parallel jobs. The link also works well for remote development, for example with VS Code’s Remote SSH, and for copying large files with rsync or scp, as described in the rsync guide.

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We used the Portronics EtherC, a USB-C to RJ45 Cat6 cable, for our setup.