Comment # 5 on bug 1139983 from Stanislav Brabec

I spent two days in learning of the network code.

As far as I understand, the current agetty code (see comment 4) is not capable
to discriminate between global, link and ephemeral IPv6 address as well as it
is not capable to discriminate between primary and secondary IPv4 address.


If I read the documentation correctly, there are at least three completely
different implementations of IP addresses interface:

- POSIX getifaddrs() used by agetty.

- sockios ioctl() and its SIOC* commands, used by the old net-tools, e. g.
ifconfig.

- NETLINK_ROUTE netlink messages, used by iproute2, e. g. ip.

Only the first one is portable.

And probably only the last one is capable to provide ifaddrmsg, the structure
that reports type of the IP address as well as lifetime for ephemeral
addresses.

(For the first one I am pretty sure that it is not possible, as the documented
part does not contain it, and ifa_data is unused by kernel and set to zero in
glibc. I am not sure for the second interface.)

The third interface is used by ip, and so it seems to be possible to get the
requested information in that way.


Could you confirm that I have to rewrite the agetty to NETLINK_ROUTE to fix
this bug?

Are you aware of any sample code that is more readable than iproute2?

In case of rewrite to NETLINK_ROUTE, it could make sense to parse network
change messages, but it would require even larger rewrite of agetty.


Note that it is a bit confusing that linux/if_addr.h refer to ifa_flags, which
are part of struct ifaddrs from getifaddrs(). It is probably recycled in more
interfaces.

So, I definitely see that ifa_flags contains items starting with IFF_.

And that starting with IFA_F_ is surely not part of ifa_flags from
getifaddrs(), but probably returned in netlink messages.