Debugging the DHCP client is only possible after it has booted. If DHCP problems are experienced, you must boot with DHCP disabled. The steps here can be performed only once after the host has booted, preferably in single-user mode.
The DHCP agent can be configured to log details of the packets exchanged with the server. To do this, the agent must be started with debug mode turned on:
/sbin/dhcpagent -n -d3 &
The -d3 flag turns on debug at leve1 3, and the -n flag says "don't configure the interface even if DHCP is successful".
Level 3 and levels below that return appropriate information for the user. Levels above level 3 are appropriate only for developers or those with even more expertise, since they return raw packets of information.
Only one instance of dhcpagent can run at a time, so before the agent can be started in this manner, any previously invoked agent must be stopped. To do this, find the agent's process ID and send it the terminate signal:
kill -TERM process_id_of_dhcpagent
After an agent has been started in debug mode, try to configure an interface manually by typing:
ifconfig interface_name auto_dhcp
The packets that are sent, and any that are received, will be displayed.
During the time that DHCP tries to configure an interface, the interface is unable to send or receive packets. Other network services, such as NIS or NFS, may be adversely affected while the interface is down.