Make D-Link router copy all packets to a specific port

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http://unix.stackexchange.com – I want to use a D-Link ADSL2 router/modem ( DSL-2640B ) to sniff all network traffic (for a security audit. I don't have time to scan all machines there and I want to know if they are quiet or sending 10MB/s of sensitive data to some location ;) ). There will be a net connection on LAN port 1 and a PC with a packet sniffer on port 2. I want it to copy (-tee) all packets from wlan to port 2 while allowing them to pass to port 1 (where the gateway will be). Since this router is NOT supported by dd-wrt, my best option is openwrt. I also have a DIR-635, but I really don't want to flash it since (HowTos)