[FS#342] iptables v1.4.21: can't initialize iptables table raw

[LEDE Bugs]

The following task has a new comment added:

FS#342 - iptables v1.4.21: can't initialize iptables table raw
User who did this - Jo-Philipp Wich (jow-)

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Ideally we should check `/proc/net/ip_tables_names` and `/proc/net/ip6_tables_names`
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=342#comment1062