System Administration Guide: IP Services

DHCP Client Shutdown

At shutdown, the client sends a Release message to the server that assigned addresses to the client, to indicate that the client will no longer use one or more of the assigned addresses. When the DHCPv4 client system shuts down normally, dhcpagent writes the current configuration information to the file /etc/dhcp/interface.dhc, or for DHCPv6, to /etc/dhcp/interface.dh6. By default, the lease is saved rather than released, so the DHCP server does not know that the IP address is not in active use, which enables the client to easily regain the address on next boot. This default action is the same as the ifconfig <interface> dhcp drop command.

If the lease in that file is still valid when the system reboots, dhcpagent sends an abbreviated request to use the same IP address and network configuration information. For DHCPv4, this is the Request message. For DHCPv6, the message is Confirm.

If the DHCP server permits this request, dhcpagent can use the information that it wrote to disk when the system shut down. If the server does not permit the client to use the information, dhcpagent initiates the DHCP protocol sequence described in How DHCP Works. As a result, the client obtains new network configuration information.