[FS#151] RST packet as response to SYN is not forwarded to sender?

[LEDE Bugs]

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FS#151 - RST packet as response to SYN is not forwarded to sender?
User who did this: Ted Hess (thess)
Task details edited:
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Github issue #139 reported by: https://github.com/xnoreq

Following setup:
A host on the LAN sends a SYN packet through the router to a remote server. The server rejects it with tcp-reset.
tcpdump on the router shows the SYN packet being sent and the RST,ACK packet being received over eth0.2 (WAN interface), but the packet never makes it to the host on the LAN.

In iptables the reset packet shows up in -t mangle PREROUTING and then INPUT instead of FORWARD. Why?

I've tried disabling the firewall (all iptables flushed) and just added a single MASQUERADE rule ... same result.
This is on 15.05.1 with Linux 3.18.23.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=151