AVM Fritz!box DHCP Problem

Problem: I wanted to provide a CWMP (TR-069) ACS URL to an AVM modem using DHCP on the WAN Interface, but the DHCP server ignores this vendor-option.

Discussion: Following the TR-069 specs, the CWMP-ACS Server can be included in an dhcp response using vendor-option (SubOption 1). Usually vendor options should work with isc-dhcp-servers like that:

option space vo;
vo.acsurl code 1 = text;vo.provcode code 2 = text;
vendor-option-space vo;

option vo.acsurl “http://acs.server/”;
option vo.provcode “modemtype”;

But in this case the isc-dhcp (4.1.1) ignored this option.

After some cross-checks with working setups, I recognized, that theĀ  AVM modems (7360 and 7390) are missing an option in the dhcp request.
Option “Parameter-Request Option 55” doen’t include “Vendor-Option 43”, which means isc-dhcpd doesn’t send the vendor-option, because the AVM tells it wouldn’t accept it.

Solution: You can force the isc-dhcpd to send vendor-option even though the AVM doesn’t request it. You have to add the following line to your dhcpd.conf:

option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,2b);