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When the FortiGate interface mode is configured as DHCP, the interface will be the DHCP client, and after reaching the DHCP lease time, FortiGate will send the DHCP request message to the server for renewal of the IP.
Run the following DHCP debugs when FortiGate acts as a client to viewthe DHCP request message:
diagnose debug reset
diagnose debug application dhcpc -1
diagnose debug enable
To stop the debug processes in the end, press Ctrl + C and enter 'diagnose debug disable'.
DHCP request to renew the IP address:
2025-05-26 04:07:28 timer 0xe6d3b40(bind_lease -> state_renewing) will expire in 1 secs
2025-05-26 04:07:29 timer 0xe6d3b40 expired, take action
2025-05-26 04:07:29 T1 has expired, state renewing.
2025-05-26 04:07:29 make request
2025-05-26 04:07:29 make dhcp message, code=3
2025-05-26 04:07:29 Insert option(255), len(0)
2025-05-26 04:07:29 Insert option(53), len(1)
2025-05-26 04:07:29 Insert max message len (1458)
2025-05-26 04:07:29 Insert option(57), len(2)
2025-05-26 04:07:29 Insert requested options
2025-05-26 04:07:29 Insert option(55), len(13)
2025-05-26 04:07:29 Insert customer options
2025-05-26 04:07:29 Insert class ID option
2025-05-26 04:07:29 Insert option(60), len(18)
2025-05-26 04:07:29 Insert client ID
2025-05-26 04:07:29 Insert option(61), len(7)
2025-05-26 04:07:29 Insert hostname
2025-05-26 04:07:29 Insert option(12), len(16)
2025-05-26 04:07:29 get_dhcp_msg_len, 306
2025-05-26 04:07:29 too small, extend to 548
2025-05-26 04:07:29 Sending request!
2025-05-26 04:07:29 Send a packet out.
After getting the DHCP acknowledgment, FortiGate will perform the gracious ARP and continue to use the same IP for the interface.
2025-05-26 04:07:29 ###############3Receive packet:
len=342
2025-05-26 04:07:29 del hw header
2025-05-26 04:07:29 ether_type:0800
2025-05-26 04:07:29 hw addr from: 00:50:56:E3:FD:5C
2025-05-26 04:07:29 del ip udp header
2025-05-26 04:07:29 final dhcp message len:300
2025-05-26 04:07:29 DHCP Message received.
2025-05-26 04:07:29 parse dhcp options
2025-05-26 04:07:29 parse dhcp option buffer (60 bytes)
2025-05-26 04:07:29 option[53], len:1
2025-05-26 04:07:29 option[54], len:4
2025-05-26 04:07:29 option[51], len:4
2025-05-26 04:07:29 option[1], len:4
2025-05-26 04:07:29 option[3], len:4
2025-05-26 04:07:29 option[6], len:4
2025-05-26 04:07:29 option[15], len:11
2025-05-26 04:07:29 option[28], len:4
2025-05-26 04:07:29 DHCPACK received
2025-05-26 04:07:29 handle received dhcp options!
2025-05-26 04:07:29 lease ip:88B5A8C0
lease time: 600, renew: 0, rebind: 0
2025-05-26 04:07:29 Ack: expiry 600 secs renew: 300 secs rebind: 450 secs
2025-05-26 04:07:29 binding lease
2025-05-26 04:07:29 make arp check
2025-05-26 04:07:29 Broadcasting ARPOP_REQUEST for 192.168.181.136
2025-05-26 04:07:29 Sending arpcheck!
2025-05-26 04:07:29 Send an arp packet out.
2025-05-26 04:07:29 add hw header
2025-05-26 04:07:29 set dst hw addr as: FF:FF:FF:FF:FF:FF
2025-05-26 04:07:29 src hw addr: 50:E5:C8:00:94:00
2025-05-26 04:07:29 result:60
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FortiGate will generate a log with the log description 'DHCP client lease granted', which provides information regarding the DHCP IP renewal.
date=2025-05-26 time=04:12:34 eventtime=1748257953738633912 tz="-0700" logid="0100026004" type="event" subtype="system" level="information" vd="root" logdesc="DHCP client lease granted" msg="interface port1 gets a DHCP lease, ip:192.168.181.136, mask:255.255.255.0, gateway:192.168.181.2, lease expires:Mon May 26 04:22:29 2025"
Related article:
Technical Tip: Diagnosing DHCP on a FortiGate
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