Re: ARP dropped during WPA handshake

[James Cameron <quozl@laptop.org>]

On Fri, Mar 13, 2015 at 11:29:34AM -0500, Dan Williams wrote:
> On Fri, 2015-03-13 at 16:53 +0100, voncken wrote:
> > > > Below, a tcpdump capture from sta.
> > > > 17:43:12.964096 EAPOL key (3) v2, len 95
> > > > 17:43:12.998439 EAPOL key (3) v1, len 117
> > > > 17:43:13.062409 ARP, Request who-has 10.32.61.100 tell 10.32.0.1,
> > > > length 28
> > > > 17:43:13.079989 EAPOL key (3) v2, len 151
> > > > 17:43:13.082764 EAPOL key (3) v1, len 95
> > > > 17:43:14.062381 ARP, Request who-has 10.32.61.100 tell 10.32.0.1,
> > > > length 28
> > > > 17:43:14.127101 ARP, Reply 10.32.61.100 is-at b8:88:e3:45:1d:c6 (oui
> > > > Unknown), length 46
> > > > 17:43:14.127123 IP 10.69.1.201.41690 > 10.32.61.100.5001: UDP, length
> > > > 1470
> > > > 17:43:14.127136 IP 10.69.1.201.41690 > 10.32.61.100.5001: UDP, length
> > > > 1470
> > > >
> > > > You can see the ARP request during the WPA Handshake.
> > > 
> > > During the initial WPA handshake the connection is not fully set up, and so
> > > no general traffic can (nor should) pass between the STA and AP.
> > > That includes ARP and any L2/L3+ protocols, except for EAP and wifi
> > > management packets.
> > > 
> > > The interface itself must be IFF_UP before it can pass traffic, including the
> > > WPA handshake traffic.  IFF_UP only means that the interface can be
> > > configured at the L2 level and the hardware is active, it does *not* mean the
> > > interface can pass traffic.
> > > 
> > > Whatever is causing the ARPs shouldn't be doing that yet, and should be fixed
> > > to use the interface's "operstate" or IFF_LOWER_UP instead of IFF_UP.  Only
> > > when the supplicant changes the interface's operstate to IF_OPER_UP is the
> > > interface *actually* ready to pass traffic.  IFF_UP is not sufficient.
> > > 
> > 
> > Thanks for your reply. 
> > 
> > It seems wpa_supplicant set the operstate to IF_OPER_DORMANT when he received the ASSOCIATED Event from the driver (through netlink). And set the operstate to IF_OPER_UP in case of wpa handshake success.
> > 
> > Is it normal the local ip stack send arp when netdev it is on IF_OPER_DORMANT state?
> 
> I'm not sure the kernel stack cares much as long as the device is up.
> It is requesting the ARP because some application is attempting to
> communicate with that IP address.  That application should probably be
> waiting until the interface is actually ready to communicate, which
> means IF_OPER_UP.
> 
> But if this is the first WPA handshake with the AP during the initial
> connection, the wifi device shouldn't even have an IP address yet, so
> nothing should be doing ARP on the interface yet.

I thought that ARP was a means to get an IP address before an
interface had an IP address, so the interface spends some time without
an IP address yet generating ARP.

> Perhaps whatever is assigning the IP address to the interface is
> doing it too early, before the interface is IF_OPER_UP?
> 
> Dan
> 	
> > 
> > > 
> > > 
> > > > 	Any suggestion will be appreciate.
> > > >
> > > > Cedric.
> > > > >
> > > > > > Thanks for your help.
> > > > > >
> > > > > > Cedric Voncken
> > > > > >
> > > > > >
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> > 
> > 
> 
> 
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-- 
James Cameron
http://quozl.linux.org.au/