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I don't see how to get Linux to automatically configure a link-local IPv6 address for a GRE tunnel endpoint, even though it seems to have that ability since 2011. Here is the transcript. The 10.10.10.10 address here is for the sake of demonstration, but the same thing happens with actual tunnels that work otherwise (including with IPv6). Enabling MULTICAST is necessary for other reasons, but doesn’t seem to affect the outcome in any case.

# uname -srvo
Linux 4.16.12-1-ARCH #1 SMP PREEMPT Fri May 25 23:30:31 UTC 2018 GNU/Linux
# ip -V
ip utility, iproute2-ss180402
# ip tun add test mode gre remote 10.10.10.10
# ip link set test multicast on
# ip link set test up
# ip link show test
11: test@NONE: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/gre 0.0.0.0 peer 10.10.10.10
# ip addr show test
11: test@NONE: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN group default qlen 1000
    link/gre 0.0.0.0 peer 10.10.10.10
# sysctl net.ipv6.conf.test.disable_ipv6
net.ipv6.conf.test.disable_ipv6 = 0
# sysctl net.ipv6.conf.test.autoconf
net.ipv6.conf.test.autoconf = 1

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