How can I get isc-dhcp-server/dhcpd to listen on one interface only?

Question

I would like my isc-dhcp-server instance to listen on my LAN interface only because I'm investigating an issue with my "WiFi router" and configured INTERFACES='p18p1' in /etc/default/isc-dhcp-server where p18p1 is my LAN interface. However, after restart with sudo systemctl restart isc-dhcp-server.service if see

udp        0      0 0.0.0.0:67              0.0.0.0:*                           23088/dhcpd         

in the output of sudo netstat -tupln | grep 67 which leads me to believe that the interfaces configuration value is ignored. Specifying interfaces p18p1; in /etc/dhcpd/dhcpd.conf causes the systemd unit to fail to start - it seems to enforce specification of interfaces in /etc/default/isc-dhcp-server.

I'm using Ubuntu 17.04.

Solution

Try seting the INTERFACE variable in /etc/default/isc-dhcp-server to the desired interface. For example:

INTERFACES="eth2"

Comments

[heemayl]

Whats the output of systemctl cat isc-dhcp-server.service? Also if you do sudo /etc/init.d/isc-dhcp-server restart, does it work?

[Mehmet Fide]

didn't work on Ubuntu 20.04