DHCP
Fix Duplicate UID Lease Error
Error...
dhcpd: uid lease 192.168.0.88 for client 40:83:de:b9:7e:3f is duplicate on 192.168.0.0/24
Fix...
Check to make sure that you do not have matching MAC address in your configuration file AND the leases files...
grep -c '40:83:de:b9:7e:3f' /etc/dhcp/dhcpd.conf /var/lib/dhcp/dhcpd.leases*
Stop the DHCP server...
service isc-dhcp-server stop
Clear the leases files...
cd /var/lib/dhcp rm dhcpd.leases*
Start the DHCP server...
service isc-dhcp-server start
(Optional - fix the directory and file permissions and ownership for isc-dhcpd-4.2.4)
chmod g+w /var/lib/dhcp && chown -R dhcpd:dhcpd /var/lib/dhcp
Fix Big Leases File
https://www.reddit.com/r/networking/comments/5h1bej/isc_dhcp_lease_file_too_big/
https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1186662
https://help.ubuntu.com/community/isc-dhcp-server
Find DHCP Server On Network
IPV4
sudo nmap --script broadcast-dhcp-discover -e <network-interface>
IPV6
sudo nmap --script broadcast-dhcp6-discover -e <network-interface>
Thanks to Superuser.
sudo nmap -sU -p 67 --open 192.168.1.0/24
https://www.unix.com/ip-networking/171195-dhcp-server-discover.html
Monitor DHCP Traffic From Command Line
sudo tcpdump -i <network-interface> port 67 or port 68 -e -n
http://ask.xmodulo.com/monitor-dhcp-traffic-command-line-linux.html
Monitor Logs
sudo journalctl -u dhcpd.service -f
https://www.shellhacks.com/journalctl-tail-service-logs-systemd-journal/
HOWTO: VLANS
https://www.ossramblings.com/using-multiple-vlan-with-dhcp-ubuntu
https://community.spiceworks.com/topic/1331562-isc-dhcp-server-for-multiple-vlans
HOWTO: FIX: DNSMasq DHCP Server With UFW
sudo ufw allow from any port 68 to any port 67 proto udp
HOWTO: List Leases On The Command Line
egrep "lease|hostname|hardware|\}" /var/lib/dhcp/dhcpd.leases
or
egrep "lease|hostname|hardware|\}" /var/lib/dhcpd/dhcpd.leases
Thanks - https://unix.stackexchange.com/questions/91799/command-to-list-assigned-dhcp-addresses/
LTSP and Vendor Classes
set vendor-string = option vendor-class-identifier;
http://chrisjrob.com/2009/04/14/determine-vendor-class-identifier/#comment-3962986253
