DHCP VMkernel renewal behavior
Article ID: 309593
Updated On:
Products
VMware vSphere ESXi
Issue/Introduction
Symptoms:
When a VMkernel interface has acquired its IP lease from a DHCP server in another subnet, you may see messages in the
When a VMkernel interface has acquired its IP lease from a DHCP server in another subnet, you may see messages in the
/var/log/dhclient.log file similar to:Where2012-08-29T21:36:24Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:36:35Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:36:49Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:37:08Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:37:24Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:37:39Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:37:52Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:38:01Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:38:19Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:38:29Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:38:41Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:38:53Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:39:09Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 672012-08-29T21:39:24Z dhclient-uw[4884]: DHCPREQUEST on vmk1 to 192.168.2.210 port 67
192.168.2.210 is the IP of the DHCP server.Environment
VMware ESXi 4.1.x Installable
VMware vSphere ESXi 5.1
VMware vSphere ESXi 5.0
VMware ESXi 4.1.x Embedded
VMware vSphere ESXi 5.1
VMware vSphere ESXi 5.0
VMware ESXi 4.1.x Embedded
Cause
Initially, the VMkernel client interface sends a DHCPDiscover to discover available DHCP servers and request an IP address. This is a Broadcast message.
A DHCPOffer is sent from the DHCP server to the client offering an IP, which is Unicast message.
At time T1 (defaults to 0.5 * duration_of_lease), the client moves to the RENEWING state and sends a DHCPREQUEST via Unicast to the DHCP server to extend its lease.
This request is logged in /var/log/dhclient.log as seen in the Symptoms section.
VMkernel interfaces other than vmk0 only have a direct path to their connected network and they cannot route through the default gateway.
Because the VMkernel does not have a route to the DHCP server, this Unicast request is never sent.
When a DHCP server is on another subnet, the VMkernel cannot renew the lease by Unicast as it cannot reach the DHCP server.
A DHCPOffer is sent from the DHCP server to the client offering an IP, which is Unicast message.
At time T1 (defaults to 0.5 * duration_of_lease), the client moves to the RENEWING state and sends a DHCPREQUEST via Unicast to the DHCP server to extend its lease.
This request is logged in /var/log/dhclient.log as seen in the Symptoms section.
VMkernel interfaces other than vmk0 only have a direct path to their connected network and they cannot route through the default gateway.
Because the VMkernel does not have a route to the DHCP server, this Unicast request is never sent.
When a DHCP server is on another subnet, the VMkernel cannot renew the lease by Unicast as it cannot reach the DHCP server.
Resolution
To resolve this issue, give the DHCP server an interface on the same subnet as the VMkernel port to allow DHCP renewal via Unicast.
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