netplug: T5476: rewrite dhclient helper from Perl -> Python

There are two hooks called for bridge, ethernet and bond interfaces if the
link-state changes up -> down or down -> up.

The helpers are:
* /etc/netplug/linkdown.d/dhclient
* /etc/netplug/linkup.d/dhclient

As those helpers use Linux actions to start/restart the dhclient process in
Perl it's time to rewrite it. First goal is to get rid of all Perl code and the
second is that we now have a Proper Python library. Instead of checking if the
process is running the then restarting it without even systemd noticing
(yeah we might get two processes beeing alive) we should:

* Add a Python helper that can be used for both up and down (see man 8 netplugd
  FILES section)
* Query the VyOS CLI config if the interface in question has DHCP(v6)
  configured and is not disabled
* Add IPv6 DHCPv6 support

MAN page: https://linux.die.net/man/8/netplugd

1 files changed, 41 insertions, 0 deletions

diff --git a/src/etc/netplug/netplug b/src/etc/netplug/netplug
new file mode 100755
index 000000000..60b65e8c9
--- /dev/null
+++ b/src/etc/netplug/netplug
@@ -0,0 +1,41 @@
+#!/bin/sh
+#
+# Copyright 2023 VyOS maintainers and contributors <maintainers@vyos.io>
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+
+dev="$1"
+action="$2"
+
+case "$action" in
+in)
+   run-parts --arg $dev --arg in /etc/netplug/linkup.d
+    ;;
+out)
+   run-parts --arg $dev --arg out /etc/netplug/linkdown.d
+    ;;
+
+# probe loads and initialises the driver for the interface and brings the
+# interface into the "up" state, so that it can generate netlink(7) events.
+# This interferes with "admin down" for an interface. Thus, commented out. An
+# "admin up" is treated as a "link up" and thus, "link up" action is executed.
+# To execute "link down" action on "admin down", run appropriate script in
+# /etc/netplug/linkdown.d
+#probe)
+#    ;;
+
+*)
+    exit 1
+    ;;
+esac