Networking considerations

The MSL VM reaches the network through macOS using NAT, like WSL 2’s default networking mode. All distributions share one network namespace: they have the same IP address, and one localhost between them.

Identify IP addresses

Every distribution has the same address, on the VM’s network interface eth0:

$ ip -4 addr show eth0          # in any distribution

The Mac, seen from Linux, is the VM’s default gateway. MSL adds it to each distribution’s /etc/hosts as host.internal:

$ ip route show default         # in the distribution: "default via <Mac's address>"
$ getent hosts host.internal

You rarely need either address: use localhost from macOS, and host.internal from Linux.

localhost

A server that listens on localhost, or on all addresses, in any distribution answers on localhost on macOS, over IPv4 and IPv6:

$ python3 -m http.server 8000      # in a distribution
$ curl http://localhost:8000       # on macOS

MSL watches the TCP ports that Linux programs listen on and opens the same ports on macOS’s 127.0.0.1 and ::1. Forwarding stops when the program stops listening. UDP isn’t forwarded. The connections go through msl-portd, a process that runs only while a port is forwarded, as WSL’s wslrelay.exe does.

• Port already in use on macOS. If a macOS program already uses the port, MSL skips it and logs it in ~/Library/Application Support/msl/msld.log. Stop the macOS program, or use another port in Linux.
• Other users on the Mac. Forwarded ports are on the Mac’s loopback address, so any user logged in to the same Mac can connect to them, as with WSL’s localhost forwarding.
• Turning it off. Set localhostForwarding = false in ~/.mslconfig, then run msl --shutdown. See Advanced settings configuration.

Forwarded ports are for connections from the Mac itself. MSL doesn’t expose distribution ports to other machines on your network.

Reach a server on the Mac from Linux

Use host.internal:

$ curl http://host.internal:3000    # a server running on macOS, from a distribution

The macOS server must listen on an address the VM can reach, not only on 127.0.0.1, and the macOS firewall must allow the connection.

DNS

Name lookups go through macOS’s own resolver, so they give the same answers as on the Mac: VPNs with split DNS, custom resolvers in /etc/resolver, .local names and entries in the Mac’s /etc/hosts all work. Each distribution’s /etc/resolv.conf points at a small resolver inside the VM, 10.255.255.254, which forwards to macOS. This is WSL’s dnsTunneling, and it’s on by default.

• To use the VM network’s DNS server instead, set dnsTunneling = false in ~/.mslconfig.
• To manage /etc/resolv.conf yourself, set generateResolvConf = false under [network] in the distribution’s /etc/wsl.conf.

Hostname and /etc/hosts

A distribution’s hostname is the Mac’s host name, with any characters Linux doesn’t allow replaced. MSL writes /etc/hostname and /etc/hosts each time the distribution starts. /etc/hosts contains localhost, the hostname, host.internal, and the entries from the Mac’s own /etc/hosts, except its loopback ones.

In /etc/wsl.conf, [network] hostname sets another hostname, and generateHosts = false keeps your own /etc/hosts and /etc/hostname, as in WSL.

Not available

• Mirrored networking. WSL’s networkingMode = mirrored, which gives Linux the host’s own network interfaces, isn’t supported. MSL uses NAT only.
• Other networking modes and settings. networkingMode, firewall, autoProxy and the other WSL networking keys in .wslconfig are accepted and ignored.