Merge pull request #29767 from poettering/host-to-host

automatic support for host-to-host networking that only does link-local addressing

TODO

@@ -133,16 +133,11 @@ Deprecations and removals:
 
 Features:
 
-* hwdb: add a new field that can be used to mark network devices where only
-  ipv4ll/ipv6ll makes sense, i.e. which should not expect to be connected to
-  the internet, i.e. for direct host-to-host connection. Set it for thunderbolt
-  networking, and for "laplink"-style network cables. Then make netword match
-  against that prop in a .network file we ship by default. Would be
-  particularly useful in storage target mode (i.e. NVME-TCP) modes. i.e. do
-  what https://christian.kellner.me/2018/05/24/thunderbolt-networking-on-linux/
-  and
-  https://github.com/NetworkManager/NetworkManager/blob/main/data/90-nm-thunderbolt.rules#L11
-  do for NM, but generically.
+* networkd: controlled by build-time options install some of the
+  *.network.example files we install currently as regular files *.network. For
+  many cases it's what you want: just configure the damn network. Hopefully
+  this can be enabled on Fedora even, though into a separate sub-RPM, so that
+  we don't conflict with NM.
 
 * add support for activating nvme-oF devices at boot automatically via kernel
   cmdline, and maybe even support a syntax such as

hwdb.d/82-net-auto-link-local.hwdb

@@ -0,0 +1,12 @@
+# This file is part of systemd.
+
+# Network interfaces for which only Link-Local communication (i.e. IPv4LL, …)
+# makes sense, because they almost certainy will point to another host, not an
+# internet router.
+
+# (Note: matches against drivers go into 82-net-auto-link-local.rules instead)
+
+# Prolific USB-to-USB links (https://www.prolific.com.tw/US/ShowProduct.aspx?pcid=43)
+usb:v067Bp25A1*
+usb:v067Bp27A1*
+ ID_NET_AUTO_LINK_LOCAL_ONLY=1

network/80-auto-link-local.network.example

@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: MIT-0
+#
+# This example config file is installed as part of systemd.
+# It may be freely copied and edited (following the MIT No Attribution license).
+#
+# To use the file, one of the following methods may be used:
+# 1. add a symlink from /etc/systemd/network to the current location of this file,
+# 2. copy the file into /etc/systemd/network or one of the other paths checked
+#    by systemd-networkd and edit it there.
+# This file should not be edited in place, because it'll be overwritten on upgrades.
+
+# Enable IPv4LL + IPv6LL (but no DHCP/IPv6RA) for local links
+[Match]
+Type=ether
+Property=ID_NET_AUTO_LINK_LOCAL_ONLY=1
+
+[Network]
+LinkLocalAddressing=yes
+DHCP=no
+IPv6AcceptRA=no
+MulticastDNS=yes

rules.d/82-net-auto-link-local.rules

@@ -0,0 +1,15 @@
+# do not edit this file, it will be overwritten on update
+
+ACTION=="remove", GOTO="net_link_local_end"
+SUBSYSTEM!="net", GOTO="net_link_local_end"
+
+# Network interfaces for which only Link-Local communication (i.e. IPv4LL, …)
+# makes sense, because they almost certainy will point to another host, not an
+# internet router.
+
+# (Note: matches against VID/PID go into 82-net-auto-link-local.hwdb instead)
+
+# Thunderbolt host-to-host connections
+DRIVERS=="thunderbolt-net", ENV{ID_NET_AUTO_LINK_LOCAL_ONLY}="1"
+
+LABEL="net_link_local_end"