I already have router which is Ubuntu PC. I need to connect my DD-WRT asus router WAN to Ubuntu routing PC in the way so DD-WRT will just bridge connection between wireless and WAN interfaces. Is it possible?
Looking at your other post along with this, my first questions back to you is why do you have your ubuntu PC running as your router? DD-WRT has really good protocol for firewall protection. Bridging a connection from your Ubuntu Desktop PC to a DD-WRT router will not be impossible, but I feel that you will have more success with having the router directly connected to you ISP.
With that said, DD-WRT has guides on how to place the router into a "repeater" mode here. Follow those instructions and you'll be able to forward the connections of the router and attached devices to the ubuntu machine. This is not a "bridged" connection, but rather using the router as a simple "repeater" that sends the packets both ways. You will then need to share the internet connection of the ubunut pc by following these instructions.