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Mac OS X appears to have a new address convention that I can't find
documented. The link-local address fe80:4::X:X:X:X stands for
fe80::X:X:X:X%en0, if en0 is the interface with index 4. (I.e., it would
be fe80::X:X:X:X%4 on Windows.) The number 4 could be different numbers.
The interface addresses seem to be stored with these pseudo-zone IDs at
a low level, because they appear that way when they come from libdnet.
This is what "nmap --iflist" shows:
lo0 (lo0) fe80:1::1/64 loopback up 16384
en0 (en0) fe80:4::xxxx:xxxx:xxxx:xxxx/64 ethernet up 1500 XX:XX:XX:XX:XX:XX
The OS X network tools seem to hide this. This is what "ifconfig" and
"netstat -rn -f inet6" show:
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
en0: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::xxxx:xxxx:xxxx:xxxx%en0 prefixlen 64 scopeid 0x4
fe80::1%lo0 link#1 UHLI lo0
fe80::xxxx:xxxx:xxxx:xxxx%en0 XX:XX:XX:XX:XX:XX UHLI lo0
(Not the "scopeid 0x4".) Also, if you use one of these addresses, it
magically transforms into one without the pseudo-ID (getaddrinfo does
this):
$ ping6 fe80:4::1234
PING6(56=40+8+8 bytes) fe80::xxxx:xxxx:xxxx:xxxx%en0 --> fe80::1234%en0
$ ping6 fe80:1::1234
PING6(56=40+8+8 bytes) fe80::1%lo0 --> fe80::1234%lo0
This translation is messing up our netmask comparisons for the purpose
of routing. If you use a normal link-local address starting with fe80:0,
then it doesn't compare equal with the fe80:4/64 interface address. If
you try to use the 4, then getaddrinfo turns it into fe80:0 anyway, and
the comparison still fails.
So for this reason I added a canonicalize_address function, which calls
getnameinfo on an address, then calls getaddrinfo on the returned
string, to mangle an address the same way the OS would do it. One would
hope this is a no-op in cases other than the one I have described.
Now "nmap --iflist" shows:
lo0 (lo0) fe80::1/64 loopback up 16384
en0 (en0) fe80::xxxx:xxxx:xxxx:xxxx/64 ethernet up 1500 XX:XX:XX:XX:XX:XX