network: configure a tun host0 interface in a container

While containers often have their host0 network provided by veth when
the container runtime is privileged, unprivileged containers tend to
have their network provided via slirp4netns or pasta. These tools use a
tun interface rather than a veth interface and systemd should still set
configure such networks.

We should have different .network files for the veth and tun use cases
as there may arise a need to configure them differently.  We should not
rename 80-container-host0.network as that would cause existing drop-ins
to no longer apply.

Closes: #32095
Fixes: f139393dd2 ("network: use Kind= instead of Driver=")
Signed-off-by: Helmut Grohne <helmut@subdivi.de>

network/80-container-host0-tun.network

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: MIT-0
+#
+# This config file is installed as part of systemd.
+# It may be freely copied and edited (following the MIT No Attribution license).
+#
+# To make local modifications, use "networkctl edit". See networkctl(1) for details.
+# This file should not be edited in place, because it'll be overwritten on upgrades.
+
+# This network file is inside unprivileged containers that rely on network being
+# provided as a tun link via slirp4netns(1) or pasta(1).
+
+[Match]
+Kind=tun
+Name=host0
+Virtualization=container
+
+[Network]
+DHCP=yes
+LinkLocalAddressing=yes
+
+[DHCP]
+UseTimezone=yes

network/meson.build

@@ -5,6 +5,7 @@ if conf.get('ENABLE_NETWORKD') == 1
                 '80-6rd-tunnel.network',
                 '80-6rd-tunnel.link',
                 '80-container-host0.network',
+                '80-container-host0-tun.network',
                 '80-container-vb.network',
                 '80-container-vb.link',
                 '80-container-ve.network',