Skip to main content
sjoshi
Staff
sjoshi
Staff
June 25, 2026

Troubleshooting Tip: DHCP relay not working when passing through LTE interface on FortiWifi 50G-5G

  • June 25, 2026
  • 0 replies
  • 167 views

Description

This article describes an issue where DHCP relay is not working over a LTE interface on a FortiWifi 50G-5G.

Scope

FortiWifi.

Solution

Topology:

A private WAN APN is in use, and DHCP requests are being forwarded by the firewall to a DHCP server that is reachable through the LTE interface.

Phone -> Wi-Fi -> FortiGate 50G -> LTE (WWAN/SIM interface) -> DHCP server.


LAN interface:

edit "lan_network"
    set vdom "root"
    set dhcp-relay-service enable
    set ip 192.168.40.1 255.255.255.0
    set allowaccess ping
    set type switch
    set device-identification enable
    set lldp-transmission enable
    set role lan
    set snmp-index 115
    set dhcp-relay-ip "192.168.110.2"
next


DHCP server IP: 192.168.110.2.

get router info routing-table all


Example output:

B 192.168.110.0/24 [20/0] via 202.x.x.x (recursive is directly connected, wwan), 00:19:49, [1/0]


The route towards the DHCP server 192.168.110.2 is via the WWAN interface through BGP (Private APN).

Pinging connectivity to the DHCP server (192.168.110.2) is successful, but the issue is limited to DHCP relay traffic only.

DHCP discover packets are received on the FortiGate, and the FortiGate forwards them as unicast to the DHCP server, but no response is being received. It has been confirmed that the next hop is not receiving any DHCP Discover packets from the FortiGate.

DHCP debug output:

(xid:320ed962) received request message from 0.0.0.0:68 to 255.255.255.255 at lan_network
(xid:320ed962) got a DHCPDISCOVER
(xid:320ed962) Warning! can't get server id from client message
Insert option(82), len(13)
found route to 192.168.110.2 via 192.168.40.1 iif=32 oif=42/wwan, mode=auto, ifname=
(xid:320ed962) forwarding dhcp request from 192.168.40.1:67 to 192.168.110.2:67
(xid:320ed962) received request message from 0.0.0.0:68 to 255.255.255.255 at lan_network
(xid:320ed962) got a DHCPDISCOVER
(xid:320ed962) Warning! can't get server id from client message
Insert option(82), len(13)
found route to 192.168.110.2 via 192.168.40.1 iif=32 oif=42/wwan, mode=auto, ifname=
(xid:320ed962) forwarding dhcp request from 192.168.40.1:67 to 192.168.110.2:67


This is a limitation on FortiWiFi-50G-5G as it is an issue on the Telit modem where DHCP relay cannot be transmitted over WWAN. This is a new feature and it will be supported from v8.0.1.

If a solution is required on v7.4 and v7.6, raise a Fortinet Support ticket for the special build: Customer Service Tip: How to create a ticket for Fortinet TAC.

      Terms & ConditionsAccessibility statement