Category Archives: Networking

OpenWRT: Easy and secure guest WLAN access

I use OpenWRT on my TP-Link TL-WDR3500, and I have a guest WLAN defined on each of the two radios (2.4 and 5 GHz). The guest WLANs are isolated from my LAN, i. e. guest WLAN stations can’t talk to any of my own hosts (either on the WLAN, or in the LAN, i. e. hosts connected via Ethernet). Guest WLAN stations also can’t talk to each other. The actual OpenWRT configuration (apart from passwords, of course ;-)) is not a secret, I will publish an article about that soon.

For security reasons I didn’t want a static guest WLAN password, but one that changes daily, so that I don’t have to manually revoke the right to use my WLAN by changing the password all the time. So I created two tiny scripts, one that actually changes the active WLAN password every day, and one CGI script that displays the password so that I can give it to my guests.

Here’s the first one that sets the password. I run it from cron at 00:01 every day:

 DATE=`date -I`
 PWD=`echo -n "${SALT}${DATE}" | md5sum | cut -c1-16`

 if [ `uci get wireless.@wifi-iface[2].network`x = guestlanx ]; then
   uci set wireless.@wifi-iface[2].key=$PWD
 if [ `uci get wireless.@wifi-iface[3].network`x = guestlan2x ]; then
   uci set wireless.@wifi-iface[3].key=$PWD
 if [ $CHANGE -eq 1 ]; then
   uci commit wireless

And here’s the CGI script that needs to go to /www/cgi-bin to show the current password:

DATE=`date -I`
PWD=`echo -n "${SALT}${DATE}" | md5sum | cut -c1-16`

echo "Content-Type: text/plain"
echo ""
echo "Today's Guest Password for $SSID is $PWD"

Don’t forget to make the scripts executable by running “chmod +x <script>“.

If you find this helpful I would appreciate your feedback.

Windows 7 PPPoE-Protokoll schlecht implementiert?

Anläßlich eines Problems mit meinem Vodafone 16 MBit/s-DSL-Anschluss — Geschwindigkeit ging plötzlich dramatisch in die Knie, ca. 1-2 MBit/s nur noch! — habe ich testweise die PPPoE-Verbindung direkt vom Laptop unter Windows 7 über das Arcor-DSL Speed-Modem 200 zum Konzentrator bei Vodafone aufgebaut. Auf diese Weise wurde das Modem als “Schuldiger” ausgemacht: Ein baugleiches Ersatzmodem lieferte sofort über 14 MBit/s!.

Nachdem ich dann wieder das DSL-Modem mit dem TP-Link TL-WDR3500-Router (mit OpenWRT als Firmware) verkabelt hatte, stellte ich plötzlich erstaunt Folgendes fest: Die Ping-Round-Trip-Zeiten gingen von 31-32 ms (unter Windows 7 als PPPoE-Client) deutlich herunter auf 21 ms (mit OpenWRT Barrier Breaker r39582 als PPPoE-Client). Das ist insofern sehr erstaunlich, da ja nun eine 802.11an-WLAN-Strecke und der Router als zusätzliche Latenz erzeugende “Komponenten” hinzu kamen!

Ich interpretiere das so, dass die PPPoE-Implementierung unter OpenWRT der von Windows 7 deutlich überlegen ist, da sie offensichtlich “schneller” bzw. “effizienter” ist. Bevor jetzt jemand sagt “Vielleicht hast Du einen krötenlangsamen Laptop verwendet?” — nein, das ist nicht der Fall, es war ein Lenovo X220 mit einem Core i5-Prozessor mit 2.5 GHz…. Und der Laptop war dauernd “idle”… 🙂

Eure Meinung zu dieser Interpretation würde mich sehr interessieren, daher würde ich mich über Kommentare freuen.

OpenWRT on the TP-Link TL-WDR3500

I got myself a TP-Link TL-WDR3500 since it boasts great hardware (see below for detailed info), and at the same time is supported by OpenWRT which I easily found out by searching in the OpenWRT forums.

Here’s the direct link to the firmware image (current “unstable” or “bleeding edge” OpenWRT release “Barrier Breaker” — i. e. not current stable one, which is Attitude Adjustment — build r36486) which you can use to upgrade a device with the factory firmware still installed. (Update: The link refers to the “trunk”, i. e. the development branch, where daily builds are available.)

Installing OpenWRT using the stock firmware’s “Firmware Upgrade” function worked smoothly. Less than 5 mins. after I started the upgrade I had OpenWRT running (thanks, folks!).

Continue reading OpenWRT on the TP-Link TL-WDR3500


I’m hosting my domain myself on a dedicated root server, and I wanted my Internet router to automatically update a hostname in my own domain (in a designated dynamic zone) with my current public IP. With OpenWRT this was easily accomplished. I used these instructions as a starting point.

When trying to check whether everything was set up correctly I always got some strange error from the following command:

# ACTION=update INTERFACE=wan /sbin/hotplug-call iface

It turned out that the following statement

config_get ipaddr wan ipaddr

did not return the currently assigned IP address in my case, but just an empty response, so I got the following error message:

could not read rdata
syntax error

(For testing I hooked a spare router with a fresh OpenWRT install with the WAN port into my LAN, and configured the WAN interface to receive its IP address via DHCP from out of the LAN. In “production” the WAN interface receives its IP via PPPoE.)

Some friendly guy in the OpenWRT forum suggested I try the following instead:

. /lib/functions/
network_get_ipaddr ipaddr wan

And indeed this worked well.

Cisco VPN install nightmare on Vista

Here’s another Cisco VPN client nightmare for you:

The old 4.9.x.x Cisco VPN client does’t run under Vista anymore. So I downloaded the most current version our organization has available, I started the installer and pretty quickly received an error message that simply said: “Internal Error 2738″.

I thought maybe the install file was corrupt, so I redownloaded it — same error.

Now I read the readme file (which I normally don’t do ;-)) It said you need a Microsoft hotfix in order to be able to install the VPN client. So I downloaded that one as well and retried the installation after rebooting the machine — same error message agin.


So I googled for this problem and quickly came across this website — which indeed fixed the problem for me.

Thanks, Microsoft, for making such a lousy job of not registering said DLL. And thanks, Cisco, for not pointing your customers to this problem.

Gaaaawd, how I hate monopolies…