non-legacy iptables backend change breaks net scripts

[sarnold]

At some point after bionic the default iptables (command) backend changed from legacy to nf_tables that results in the current net scripts failing to setup the correct masq ports.

Log errors:

2020-06-02 19:13:00 UTC node_tools.network_funcs.do_net_cmd +308: ERROR [2912] net cmd fpn0-setup.sh msg: iptables v1.8.4 (nf_tables): unknown option "--dport"
Try `iptables -h' or 'iptables --help' for more information.
iptables v1.8.4 (nf_tables): unknown option "--dport"
Try `iptables -h' or 'iptables --help' for more information.
iptables v1.8.4 (nf_tables): unknown option "--dport"
Try `iptables -h' or 'iptables --help' for more information.
iptables v1.8.4 (nf_tables): unknown option "--dport"
Try `iptables -h' or 'iptables --help' for more information.
4 warnings/errors

Current manual workaround:

As documented here (https://github.com/kubernetes/website/commit/9cdaf4ead1f4c514a6a2582ab00e01efad331a0d/) the affected user node can be "fixed" by setting the default back to legacy using the following command:

update-alternatives --set iptables /usr/sbin/iptables-legacy

followed by a restart of the fpnd service.

Found in:

# cat /etc/os-release 
PRETTY_NAME="Kali GNU/Linux Rolling"
NAME="Kali GNU/Linux"
ID=kali
VERSION="2020.2"
VERSION_ID="2020.2"
VERSION_CODENAME="kali-rolling"
ID_LIKE=debian
ANSI_COLOR="1;31"
HOME_URL="https://www.kali.org/"
SUPPORT_URL="https://forums.kali.org/"
BUG_REPORT_URL="https://bugs.kali.org/"

[sarnold]

The above fixes startup but kali/rolling 64bit on rpi3 is still not shutting down correctly; the log finishes with repeated fpn0 shutdown msgs and is missing output for the cmds that follow iface shutdown.