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When switching FortiGates, certain ISP providers require the FortiGate to release the DHCP address to assign the same IP to the new FortiGate.
How to configure the FortiGate to release an IP assigned by a DHCP server:
Switch the IP address in the Interface configuration settings from DHCP to Manual.
For example, the FortiGate below was assigned the IP address 10.4.9.23.
In the CLI, set the mode to static and set the IP addresses as below:
F4 (port1) # set mode ?
static Static setting. <----- Manual IP assignment.
dhcp External DHCP client mode.
pppoe External PPPoE mode.
F4 (port1) # set mode static
F4 (port1) # set ip 10.4.9.1/24
F4 (port1) # show
config system interface
edit "port1"
set vdom "root"
set ip 10.4.9.1 255.255.255.0
set allowaccess ping https ssh http telnet
set type physical
set snmp-index 1
next
end
After changing the IP address from DHCP to Manual, the following can be seen on a Sniffer trace.
Unicast packet from the FortiGate to DHCP server releasing the IP using option 53:
Frame 5: 590 bytes on wire (4720 bits), 590 bytes captured (4720 bits)
Ethernet II, Src: 00:41:6c:6f:10:01 (00:41:6c:6f:10:01), Dst: 00:41:6c:6f:0f:02 (00:41:6c:6f:0f:02)
Internet Protocol Version 4, Src: 10.4.9.23, Dst: 10.4.9.2
User Datagram Protocol, Src Port: 68, Dst Port: 67
Dynamic Host Configuration Protocol (Release)
Message type: Boot Request (1)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0xba6ad46e
Seconds elapsed: 0
Bootp flags: 0x0000 (Unicast)
Client IP address: 10.4.9.23
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 0.0.0.0
Client MAC address: 00:41:6c:6f:10:01 (00:41:6c:6f:10:01)
Client hardware address padding: 00000000000000000000
Server hostname not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Release)
Length: 1
DHCP: Release (7)
Option: (61) Client identifier
Option: (54) DHCP Server Identifier (10.4.9.2)
Option: (255) End
Padding […]:
Or run the following commands in CLI to debug DHCP client traffic:
diagnose debug reset
diagnose debug application dhcpc -1
diagnose debug console timestamp enable
diagnose debug enable
To stop debugging:
diagnose debug reset
diagnose debug disable
Related article:
Technical Tip: Diagnosing DHCP on a FortiGate
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