Binding to fe80::%<scope> - c

Question

Trying to get daemon to bind to fe80::%en0.
I'm not 100% sure this is possible, but it really seems like for the link-local address to be useful it should be.
If you want to accept packets from your peer on the other side of the link, it seems stupid to have to specify the full link-local address.
The code looks like this:
if (DEBUG_ENABLED4) {
if (salocal.ss_family == AF_INET) {
char buffer[INET_ADDRSTRLEN];
struct sockaddr_in *addr = (struct sockaddr_in *)&salocal;
inet_ntop(addr->sin_family, &addr->sin_addr, buffer, sizeof(buffer));
DEBUG4("[FD %i] Binding to address %s port %u -- bind(%i, %p, %u)",
this->fd, buffer, ntohs(addr->sin_port), this->fd, &salocal, salen);
} else if (salocal.ss_family == AF_INET6) {
char buffer[INET6_ADDRSTRLEN];
struct sockaddr_in6 *addr = (struct sockaddr_in6 *)&salocal;
inet_ntop(addr->sin6_family, &addr->sin6_addr, buffer, sizeof(buffer));
DEBUG4("[FD %i] Binding to address %s port %u scope ID %i -- bind(%i, %p, %u)",
this->fd, buffer, ntohs(addr->sin6_port), addr->sin6_scope_id,
this->fd, &salocal, salen);
}
}
rad_suid_up();
rcode = bind(this->fd, (struct sockaddr *)&salocal, salen);
rad_suid_down();
The debug output looks like this:
Sat Nov 7 19:58:28 2015 : Debug: [FD 12] Binding to address fe80:: port 1812 scope ID 4 -- bind(12, 0x7fff599dc4d0, 28)
Sat Nov 7 19:58:28 2015 : Error: Failed binding to auth address fe80:: port 1812 bound to server default: EADDRNOTAVAIL: Can't assign requested address
Sat Nov 7 19:58:28 2015 : Error: /usr/local/freeradius/etc/raddb/sites-enabled/default[215]: Error binding to port for fe80:: port 1812
Ifconfig output looks like this:
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=10b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV>
ether a8:20:66:xx:xx:xx
inet6 fe80::aa20:66ff:fe1a:8bcd%en0 prefixlen 64 scopeid 0x4
Values all seem to match up, but bind returns EADDRNOTAVAIL. Is there some magic socket option I need to enable to make this work? Or do I need to check for that link local IP prefix specifically and then resolve it to the full address for the interface described by the scope_id/zone_id?

Answer 1

In short: if you want to bind to a specific address you'll have to specify that address. A host can have multiple link-local addresses. If you want to bind to all link-local addresses automatically you'll have to auto-discover them and create a socket for each of them.