188.8.131.52. Q: Cannot ping6 to link-local addresses
Error message: "connect: Invalid argument"
Kernel doesn't know, which physical or virtual link you want to use to send such ICMPv6 packets. Therefore it displays this error message.
Solution: Specify interface like: "ping6 -I eth0 fe80::2e0:18ff:fe90:9205", see also program ping6 usage.
184.108.40.206. Q: Cannot ping6 or traceroute6 as normal user
Error message: "icmp socket: Operation not permitted"
These utilities create special ICMPv6 packets and send them out. This is done by using raw sockets in the kernel. But raw sockets can only be used by the "root" user. Therefore normal users get such error message.
Solution: If it's really needed that all users should be able to use these utilities, you can add the "suid" bit using "chmod u+s /path/to/program", see also program ping6 usage. If not all users should be able to, you can change the group of the program to e.g. "wheel", add these power users to this group and remove the execution bit for other users using "chmod o-rwx /path/to/program". Or configure "sudo" to enable your security policy.