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TIL: /proc/sys/net/ipv6/conf/$IFACE/accept_ra needs to be set to 2 (that kind of boolean...) to actually accept RAs when /proc/sys/net/ipv6/conf/all/forwarding is 1, but maybe not when /proc/sys/net/ipv6/conf/$IFACE/forwarding is 0.
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A lot more detail is documented e.g. at strugglers.net/~andy/blog/2011/09/04/linux-ipv6-router-advertisements-and-forwarding/