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Our home network consists of a Fritz!Box 7390 as ADSL modem/gateway and an old Linksys WRT54G running Tomato as additional access point. The latter is dumbed down to be nothing but an access point, and it is hooked up to the Fritz!Box via LAN port. Both operate on the same WLAN SSID. WPA+WPA-2 Personal is used to secure the connections (with a shared key between the two APs). There's an additional switch on the LAN between the two.

Generally this setup seems to work ok, but I can see issues with WLAN devices not getting IP addresses assigned. In Windows 7 this shows up with a little yellow star on the network icon in the tray. It looks like the WLAN connection is there, but DHCP didn't go through. Changing to static IP addresses doesn't help either.

It seems to affect only WLAN devices, and in particular Windows. My wife's laptop hasn't managed to connect using Windows for three weeks now, despite reboots on all devices involved. It works fine when booted into Linux.

I have also seen the issue arise with Android phones occasionally, but they seem to recover on their own. I'm not sure if any Linux ever showed the problem -- most Linux boxes in the house are wired. It definitely mostly works: my wife uses Linux as default and I have tried to reproduce the issue in Linux to get some more output. It's not noticeable there.

I have been playing with various options on both routers/APs. So far nothing seems to help. By now I'm running out of ideas -- short of turning the second AP off, that is. Is there anything I can do to debug this? Is there any common reason for this kind of problem? Normally I'm happy to pull out the Wireshark to debug a TCP protocol, but in this case I don't even know were to start.

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Enable telnet on the Fritzbox and log in.

Use general Linux commands to debug.
To show interfaces: ifconfig -a
To show routing: netstat -rn

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I presume you are running a multiple access points to give better coverage, also running a DHCP server on each access point.
Simplify your network and try replacing the Lynksys with a wireless repeater.
My thinking here is perhaps the two DHCP servers are conflicting with each other.

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