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NETLINK upcalls and wireless events

To: hostap@xxxxxxxxx, netdev@xxxxxxxxxxx
Subject: NETLINK upcalls and wireless events
From: Pavlin Radoslavov <pavlin@xxxxxxxx>
Date: Mon, 22 Mar 2004 18:12:40 -0800
Cc: pavlin@xxxxxxxx
Sender: netdev-bounce@xxxxxxxxxxx
[Apology for the cross-post, but I am not sure which is the right
 mailing list for this question]


BACKGROUND:

I am using Linux NETLINK sockets to monitor network interface events
in the kernel such as "interface added/deleted", "address
added/deleted", etc.

The RTM_NEWLINK message type upcall from the kernel to user-space is
used to signal that a new network interface has been created (e.g.,
a tunnel interface, a PC-Card has been inserted, etc).
One of the attributes of that message (see rtnetlink(7)) is
IFLA_IFNAME and is used to store the device name (e.g., "tun0",
"eth0", etc).


PROBLEM:

So far everything worked fine on a desktop: whenever I add a tunnel,
the kernel sends to user-space RTM_NEWLINK, I check the IFLA_IFNAME
attribute to get the name of the new interface, etc.

However, a fellow tried same software on a laptop in wireless
environment, and then RTM_NEWLINK messages started to appear for no
apparent reason. Those messages did not contain the IFLA_IFNAME
attribute which was strange and I started to suspect that it could
be a kernel bug. After some investigation I found that the
RTM_NEWLINK message can be generated not only when there is a new
interface, but in some other cases as well. Inside file
linux/wireless.h (I am looking into version 15 from 12.7.02) I found
the following comment: 

/* ----------------------- WIRELESS EVENTS ----------------------- */
/*
 * Wireless events are carried through the rtnetlink socket to user
 * space. They are encapsulated in the IFLA_WIRELESS field of
 * a RTM_NEWLINK message.
 */


In other words, some wireless events may also generate RTM_NEWLINK
message as well. I don't have a laptop with Linux to verify that
myself, but I am quite sure that some wireless events do generate
RTM_NEWLINK with no IFLA_IFNAME.


QUESTION:

If I receive an RTM_NEWLINK message, is there a reliable way to
find-out whether the message is generated because of a wireless
event or because indeed there is a new network interface installed
in the kernel.

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?

Thanks,
Pavlin

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