Technical Tip: DHCP relay over WWAN

The built-in modem can be set up to enable DHCP relay via the AT commands:

1. To enter the AT command interface:

diagnose system modem com /dev/ttyusb2

2. To unlock the custom modem AT commands:

#at!entercnd="A710"

3. Check if the DHCPRELAYENABLE is enabled (in the output below, the DHCPRELAYENABLE is disabled):

#at!custom?
!CUSTOM:
GPSENABLE 0x01
GPSLPM 0x01
IPV6ENABLE 0x01
UIM2ENABLE 0x01
SIMLPM 0x01
USBSERIALENABLE 0x01
QXDMLOGENABLE 0x01

4. Enable the DHCPRELAYENABLE with the AT command:

#at!custom="DHCPRELAYENABLE",1

5. Check if DHCPRELAYENABLE is enabled:

at!custom?
!CUSTOM:
GPSENABLE 0x01
GPSLPM 0x01
IPV6ENABLE 0x01
UIM2ENABLE 0x01
SIMLPM 0x01
USBSERIALENABLE 0x01
DHCPRELAYENABLE 0x01 <----- Now enabled.
QXDMLOGENABLE 0x01

See the AT Command Reference.

To troubleshoot, collect the output of the following and upload it with a ticket:

diagnose system lte-modem traffic-status
fnsysctl ifconfig wwan
diagnose npu np6xlite dce 0
diagnose npu np6 dce 0
diagnose sniffer packet any "port 67 or port 68" 6 0 l

As of FortiOS 7.4.0, a new CLI setting was added to enable/disable DHCP relay packets flow through the modem interface (WWAN). Enable the dhcp-delay in the lte-modem config as shown below:

config system lte-modem
(lte-modem) # set dhcp-relay enable
(lte-modem) # end

The status can be verified via the following process:

diagnose sys lte-modem traffic-status
LTE Modem traffic status:
DHCP relay packets: blocked <-----