NETLINK upcalls and wireless events

[Gargi D]

Hi,

I was wondering if you could help me in understanding the usage of
netlink sockets.  Wireless events such as a signal that a new interface
has been created, can anyway be read from 

/proc/net and var/lib/pcmcia/stab

Why would then want to use netlink sockets to get an indication of the
same.


Regards, 


Miss. Gargi Dadhich 
Member Technical Staff 
Networking and Internet Software Group (NISG) 
C-DAC, Pune 

Tel:    020-25694000/1/2/3      Extn: 484 
Fax:    020-25694059 



-----Original Message-----
From: hostap-bounces+gargid=cdacindia.com at shmoo.com
[mailto:hostap-bounces+gargid=cdacindia.com at shmoo.com] On Behalf Of
Pavlin Radoslavov
Sent: Tuesday, March 23, 2004 7:43 AM
To: hostap at shmoo.com; netdev at oss.sgi.com
Cc: pavlin at icir.org
Subject: NETLINK upcalls and wireless events


[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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