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