The result of a blogging accident

Disabling the DHCP server on a Motorola SURFboard SBG900 (or, how to turn it into an ethernet access point)

Unfortunately, there’s no proper way to disable DHCP server, but you can at least limit it’s damage. This is with a Telstra BigPond firmware.

In the “Gateway” tab, select “LAN” → “DHCP Server Config”. Set the following:

* IP Address: The static IP address the modem should use. * Starting IP Address: The static IP address of a system permanently connected to the network. * Number of DHCP Users: 1 * DHCP Server Lease Time: 0 (this actually sets it to 20 seconds, though.) * Time To Live: 0 hops * Maximum Transmission Unit: 0 bytes

Click “Apply”.

Additionally, disable NAT on the device. In the “Gateway” tab, select “LAN” → “NAT Config”. Untick “Enable NAT”. Click “Apply”.

This should break DHCP functionality enough that it won’t operate. Even if it does operate, only one computer will be effected. Make sure your real DHCP server is set as “authoritative”. You may occasionally have issues, however the renewal time is very low, so the damage should be limited and the DHCP clients should get an IP from the correct location instead.

All in all, turning this cable modem into a standalone AP is very dodgey. It is very limited in it’s configuration options, and it only has HTTP-based configuration (no Telnet or SNMP). It also doesn’t seem to bridge DHCP connections between wired and wireless. Time for a soldering iron to find me a serial console for this modem.