NETLINK upcalls and wireless events

jamal hadi
Tue Mar 23 20:53:02 PST 2004

On Tue, 2004-03-23 at 12:57, jt at wrote:

> > Checking whether the message contains the IFLA_WIRELESS field is
> > probably not an option, because I may be compiling the code on a
> > slightly older Linux box that doesn't have IFLA_WIRELESS defined.
> > The only solution I have so far is to check whether the message
> > contains the IFLA_IFNAME field, but this is more like a work-around
> > rather than a solution. Any suggestions?
> 	The method I personally use is that I keep a little database
> of interfaces (I need it for other reasons). Therefore, I know if the
> interface is new (not in my database) or already exist (present in my
> database).

This is the best way to do it.

> 	Note that in theory, RtNetlink messages may be lost (due to
> memory pressure). Also, you won't only get NEWLINK when interface
> register, but also when interface goes up and down, or when multicast
> or promiscuous flags are changed. Which mean that in practice you want
> to be much more clever than what you do now.

If you maintain a database in userspace the real good check would be
the ifindex since this would be unique for the device and persistent. 
The name can be changed. 
If you already have the ifindex in your database then clearly thats just
an event announcement.
Next thing to check would be the state of the flags/changemask to see if
the admin status changed (eg ifconfig down ).
And of course whatever the IFLA_* attribs (which are optionaly sent).
Typically you can catch lost messages in netlink since the socket gets
its error flag set. You can use that as an event to check for changes.

> 	Both methods you suggest would also work. If IFLA_WIRELESS
> doesn't exist, just define it :
> -----------------------------
> #endif /* IFLA_WIRELESS */
> -----------------------------
> 	The latest method is to look at both ifi_flags and
> ifi_change. If ifi_change is 0, it is usually a wireless event (but
> not always).

mask should really indicate that something new is in the flags so
hopefuly it is not being arbitrarily set to 0 to indicate events.
Not too familiar with IFLA_WIRELESS - but is this a link event or is 
something conviniently using the link message? What kind of things does
it announce/configure?

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