[FS#993] gre tunnel dropping all TX packets

[LEDE Bugs]

The following task has a new comment added:

FS#993 - gre tunnel dropping all TX packets
User who did this - chandlerding (chandlerding)

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Issue root caused:
 firewall default rule zone_wan_dest_ACCEPT is droping CSTATE INVALID packets.and gre packets are not trackable by default.
Install kmod-nf-nathelper-extra or add iptables -A output_rule -p gre -j ACCEPT

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=993#comment3660