With complex DHCP configurations, it is often difficult to tell which parameter applies to any given host, but remember two things: First, host or group declarations can specifically override the global definition, and host declarations can override the group declarations. Second, definitions are not necessarily applied in the order in which they appear in the dhcpd.conf file. The values are applied starting from the specific and moving to the more general. That is, the server first checks for a specific host configuration, then for a group configuration, then the subnet configuration, followed by a shared-network configuration, followed by the global declarations. Configuration options are only added to and not overwritten. Therefore, the configuration for the smaller, more specific units (like hosts) overrides those of the more general units (like global para-meters). When you are troubleshooting DHCP, always start at the bottom and work your way up.
Perhaps the most basic troubleshooting technique is to look at the leases the server has assigned. This is done by looking at the leases file (/var/state/dhcp/dhcp.leases), which maintains the current state of all active leases. One thing to note is that this file is rewritten from time to time to keep the file from growing too large. First a temporary copy is made and the old file is renamed dhcpd.leases~. Although it is rare, sometimes the server dies at this point. When this happens, there is no dhcpd. leases file and the server cannot restart. Rather than simply creating…
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