IPv6 link-local addr && %interfacename

[Matthias Apitz]

Hi,

I have a small question re/ the IPv6 link-local address; I configured
IPv6 in my 11-CURRENT with:

/etc/rc.conf:

ifconfig_em0_ipv6="inet6 accept_rtadv"
rtsold_enable="YES"

The em0 interface now looks like this:

# ifconfig em0
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=9b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>
        ether 00:0c:29:47:a3:8d
        inet6 fe80::20c:29ff:fe47:a38d%em0 prefixlen 64 scopeid 0x1 
        inet 192.168.56.131 netmask 0xffffff00 broadcast 192.168.56.255 
        nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>
        media: Ethernet autoselect (1000baseT <full-duplex>)
        status: active

I have a small C-written test client which connects to port 22 to the addr given
in argv1, reads one line (in our case the GM message of the sshd) and
prints it:

$ ./ipv6-client ::1
host: ::1
read: SSH-2.0-OpenSSH_6.6.1_hpn13v11 FreeBSD-20140420

it does not work with the link-local addr:

$ ./ipv6-client fe80::20c:29ff:fe47:a38d    
host: fe80::20c:29ff:fe47:a38d
ssh: connect: Network is unreachable

but with appending %em0 it does work:

$ ./ipv6-client fe80::20c:29ff:fe47:a38d%em0
host: fe80::20c:29ff:fe47:a38d%em0
read: SSH-2.0-OpenSSH_6.6.1_hpn13v11 FreeBSD-20140420

My question is: What does the %em0 mean in the IPv6 addr and why it is
not working without it?

The failing sys call is the connect(2):

...
socket(PF_INET6,SOCK_STREAM,6)                   = 3 (0x3)
connect(3,{ AF_INET6 [fe80::20c:29ff:fe47:a38d]:22 },28) ERR#51 'Network is unreachable'

I consulted the handbook and the RFC:

https://www.freebsd.org/doc/handbook/network-ipv6.html
http://www.ietf.org/rfc/rfc3513.txt

Both do not give any explanation re/ the %interface value.

Thanks

	matthias

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