How to troubleshoot why ath0 can't connect to a passwordless wireless network?

[Yuri]

Quoting "Sam Leffler" <sam at freebsd.org>:

> You seem to say your network is open (no security).  If not you need  
> to show your network configuration.

That's right, network is open (no security).

> tcpdump can be used to tap traffic at 3 layers: 802.3, 802.11, and  
> driver.  It can be used to identify where packets are lost in the  
> hierarchy (if at all).

'tcpdump -vv -i ath0' shows nothing when I run 'dhclient ath0' or
'dhcping -s 255.255.255.255'. dhclient always fails, dhcping
says "no answer".
But tcpdump shows traffic between other hosts on the same network.

> Assuming packets are going out but not coming back you can sniff  
> from a 3rd station to look for traffic in the air but not received.

No, now packets don't seem to go out at all. I said before that packets
went out when interface in fact was in 'monitor' mode.

> Given how little info you posted it's virtually impossible to advise  
> you what is wrong.  When in doubt c&p real output; describing a  
> problem often causes useful info to be left out.

There is no "real output" from tcpdump, this is a problem.

How to find out for example why 'sendto' call from dhcping doesn't  
result in any packets on ath0 interface which is 'UP' and 'associated'?

I used this same system to connect to other open WiFi networks without  
the problem. So it seems to be network-specific. And it fails only for  
certain networks.

Yuri