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This question can be treated as a sister question of previous one posted by myself. It is very tedious that when you want to bind a link local address to an IPv6 socket, you need to set the sin6_scope_id field of the sockaddr_in6 struct. I'm wondering if someone can provide a solution following good practice.
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IPv6 link-local addresses are defined by RFC 4291 (IPv6 Addressing Architecture) and are covered by the prefix fe80::/10. In practice, only fe80::/64 is used.
Consequently, IPv6 hosts usually have more than one IPv6 address assigned to each of their IPv6-enabled network interfaces. The link-local address is required for IPv6 sublayer operations of the Neighbor Discovery Protocol, as well as for some other IPv6-based protocols, such as DHCPv6.
The IPv6 link-local address is not unique on the node it's only unique for the NIC which is why you have to specify the scope-id. In other words it is perfectly valid to have multiple adapters with exactly the same IPv6 address.
This means you should take in as input the scope-id or suitable text form (%eth0, %1) that you can pass to getaddrinfo().
One method is to take in a IPv6 link-local address, enumerate the interfaces and if only one matches use that, if more than one match then bail out with a list of interfaces and get the user to specify which one in full form.
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