By default the ISC DHCP Server logs to the common-all-garden /var/log/messages which is fine, but I've always preferred a little log apartheid on my systems. This snippet shows how to make dhcpd log to its own, exclusive log file.
Firstly create the log file itself..
The daemon which maintains the system logs is called syslog. Before modifying its config file, make a backup 'cos that's a good thing to do..
cp /etc/rsyslog.d/50-default.conf /etc/rsyslog.d/50-default.conf.bak
..then edit the file using your favourite text editor..
Add the following directive to the end of the config file..
# dhcp log
Finally restart the syslog service..
sudo restart rsyslog
Now when a new IP is given out, or the dhcpd daemon is restarted, the service will write to its own log file. Neat..
Logging the DHCP Service (FreeBSD)