recvfrom on IPv6 sockets (GNU/Linux) gives unusual addresses for the client/from address of the format a00::<two_random_bytes>:</64_network_prefix>.
What are these addresses called, or by what search terms can more information be found?
Finally found the problem staring right at me; I used &addr instead of &addr.sin6_addr for function inet_ntop.
Sorry for bugging everyone with this silly problem.