From DD-WRT Wiki
It is extremely important to follow the guidelines for your specific router. Please see Installation for more information. Remember to follow the peacock thread first for background , use this page to identify your router model correctly, and use the dd-wrt download search page (DD-WRT Downloads page or secure version) to ensure that you are getting the correct file (or files, as often is required) for the job.
This describes how to flash your Siemens SE505 Router using tftp and Suse Linux. For flashing the Siemens Gigaset SE505, you have to use the dd-wrt.v23_mini_generic.bin. Since the Siemens default settings do not fit to the Linksys/dd-wrt ones you have additionally to check your configuration details after flashing.
DD-WRT v23 (starting from 27/10/05 version) does now officially support the Siemens Gigaset SE505 router. That means you can use the very latest features like WPA2 on this unit too. Due the limited RAM of this unit, you cannot use all features at the same time, but from the base it is much better than the original firmware for this router.
 Install the tftp package on your Linux Box
Trivial File Transfer Protocol (TFTP) is a very simple Filetransferprotocol. Using Suse Linux you can do this by yast or typing as root user:
#smart install tftp
 Create a virtual Network Interface on your Linux Box
Your router can only be flashed from the IP 192.168.2.100. The easiest way is to create a virtual network interface. In Linux, most network interfaces (e.g. eth0 or ppp0) are associated to a physical device. However, there are exceptions to this rule. This can be done by yast oder you simply type as root user:
#ifconfig -a eth0:1 192.168.2.100 #route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.2.100 dev eth0:1
ifconfig should now print somethinge like:
eth0:1 Link encap:Ethernet HWaddr 00:00:00:00:00:00 inet addr:192.168.2.100 Bcast:192.168.2.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:10 Base address:0xdc00
 Restore the Router-IP
If you changed your router's IP, you have to restore the default IP to 192.168.2.1. This can be done by accessing your router via webinterface.
 Flashing the router
 Download the new Firmware
Download the new latest DD-WRT Firmware (micro) dd-wrt and save it to your Linux Box. Remember the directory.
 Reboot the Router
This can be done by using the router's webinterface (http://192.168.2.1). Klick the reboot button in the Management Menu
 Immediately start the tftp transfer
After klicking the reboot button you have to start the transfer immediately on your Linux Box. Remember that you have to do this in the directory where the .bin file was saved.
#tftp -v 192.168.2.1 -m binary -c put dd-wrt.v23_mini_generic.bin
After 5 to 10 minutes you should be able to access your router's new webinterface on http://192.168.1.1. The preset username is root, password ist admin.