[FS#1083] ar71xx Kernel 4.9 PPTP Passthrough not working

[LEDE Bugs]

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FS#1083 - ar71xx Kernel 4.9  PPTP Passthrough not working
User who did this - Martin Bouska (maabo)

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yes, me. 
Way to reproduce: 
1. flash SNAPSHOT image lede-ar71xx-generic-tl-mr3420-v2-squashfs-sysupgrade.bin
2. opkg install kmod-nf-nathelper-extra
3. try the PPTP connection from LAN to external host without succes

I have tried to compile the image from trunk sources, then tried the image generator, but it is always the same:

the module **nf_nat_pptp is loaded, but not working with 4.9.65-1 kernel**

I found that in kernel logs this message appeared:
//"nf_conntrack: default automatic helper assignment has been turned off for security reasons and CT-based firewall rule not found. Use the iptables CT target to attach helpers instead."//
Im not sure, if it is relevant to this issue - maybe not. 

I attached some more detailed infos to this post. If You need some other traces to investigate, dont hasitate to contact me.
thanks

Martin

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1083#comment3932