[FS#396] odhcpd: IPv6 relay mode is unreliable

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Stijn Tintel (stintel) 

Attached to Project - LEDE Project
Summary - odhcpd: IPv6 relay mode is unreliable
Task Type - Bug Report
Category - Base system
Status - Researching
Assigned To - Hans Dedecker
Operating System - All
Severity - Medium
Priority - Medium
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - This seems to be a longstanding bug, and has been reported several times in OpenWrt and odhcpd bugtrackers:

https://dev.openwrt.org/ticket/16835
https://dev.openwrt.org/ticket/16923
https://dev.openwrt.org/ticket/18056
https://github.com/openwrt/odhcpd/issues/37
https://github.com/openwrt/odhcpd/issues/92

As I am currently forced to use relay mode, I'm also bitten by this issue. What I noticed so far:
* After reboot, IPv6 simply doesn't work for the clients.
* Sometimes restarting odhcpd solves the problem, but not always and/or not for all clients

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=396