Home / Apple / Macs cannot see each other over an Ethernet cable. MacOS is unable to configure 169,254 routing. Why? How do I fix it? Infringes RFC 3927? Tip: the apartment?

Macs cannot see each other over an Ethernet cable. MacOS is unable to configure 169,254 routing. Why? How do I fix it? Infringes RFC 3927? Tip: the apartment?



(For wordy title: Macs connected to an Ethernet cable cannot see each other – MacOS is unable to configure 169.254 routing, AKA link local address configuration. Why? How do I fix it? Does it break RFC 3927? Tip : One Mac has a standard Ethernet connection (via Wi-Fi), which can host multiple homes.)

I’m trying to communicate between two Macs via an Ethernet cable. This should “just work”, I thought, but it does not seem that MacOS automatically sets things up properly. Failed to configure 169,254 routing, AKA link local addressing so Macs can communicate properly. In other words, it is unable to fully configure the en0 (TP Ethernet) interfaces so that link-local addressing works automatically. I thought since it apparently followed RFC 3927 on dynamic configuration of IPv4 link local addresses, would “just work”

;, but not so.

Especially that is automatically assign IPs to the devices, but that’s it NOT route packages / set up the route table accordingly.

From RFC:

Abstract

… This document describes how a host can automatically configure an interface with an IPv4 address within the prefix 169.254 / 16 that is valid for communication with other devices connected to the same physical (or logical) link.

Does MacOS break RFC 3927? Why shouldn’t two Macs connected over an Ethernet cable, each with automatically assigned 169,254 / 16 link local addresses, see each other?

This can be a solution; I have not figured it out yet: sudo route -n add -net 192.168/16 169.254.14.233. (This is the auto-assigned IP.)

It adds a few lines to the timetable (output from netstat -rn| grep -v ::):

169.254.14.233     ac:87:a3:20:db:5e  UHLSW           2        0     lo0        
192.168.0/16       169.254.14.233     UGSc            0        0     en0        


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