How can I diagnose long DHCP response time?
I'm not really sure what's happened, but some of my PCs take a very long time to receive a DHCP response. In fact, one Windows 7 machine doesn't receive a response in time and several boot processes fail because they depend on the network connection.
The Windows 7 machine is a fresh install with basically nothing installed.
I'm 99% certain the problem is my Dell PowerConnect 2824 switch or the pfSense (RC2) box.
How can I start diagnosing this? The switch is using VLANs but has it's DHCP server disabled.
How long is "a very long time"?
I would start by taking a machine that does correctly receive an IP address and making sure it can do so on each of your LAN segments. If it can then it's perhaps a client hardware or OS problem if it can't then it's a switch/pfSense problem.
It's taking about 60 seconds maybe?
Windows 7 boots to the desktop and then I get the "No Network Available" icon in the taskbar and then applications begin to display errors because the network connection isn't available. About 30 seconds later it receives the DHCP response and it works.
Another example is PXE booting. On several machines it takes 60 seconds or longer to receive a DHCP response. (ie. it sits at the "DHCP …" prompt)
These problems seem to have started after implementing the Dell managed switch but nothing it's really only a very simple VLAN setup.
Can I somehow monitor pfSense to determine when exactly it gets the DHCP request so I can see where the problem is originating from?
Interesting question. You can see the DHCP logs under status > system logs > DHCP but there's no dynamic view so you can only look after the fact. If you have everything time synced that shouldn't be problem.
You can tail the dhcp log in the console for real time view.
Edit: Except that you can't use tail because of the circular log format :-[. Instead use:
[code]clog -f /var/log/dhcpd.log
You can do a packet capture on the interface the DHCP server is listening on and after that analyze it with wireshark or any other tool.