You may receive the following error message while trying to enable ufw (ufw enable):
ERROR: problem running ufw-init
modprobe: ERROR: ../libkmod/libkmod.c:514 lookup_builtin_file() could not open builtin file '/lib/modules/4.4.0/modules.builtin.bin'
modprobe: FATAL: Module nf_conntrack_ftp not found in directory /lib/modules/4.4.0
modprobe: ERROR: ../libkmod/libkmod.c:514 lookup_builtin_file() could not open builtin file '/lib/modules/4.4.0/modules.builtin.bin'
modprobe: FATAL: Module nf_nat_ftp not found in directory /lib/modules/4.4.0
modprobe: ERROR: ../libkmod/libkmod.c:514 lookup_builtin_file() could not open builtin file '/lib/modules/4.4.0/modules.builtin.bin'modprobe: FATAL: Module nf_conntrack_netbios_ns not found in directory /lib/modules/4.4.0
The reason is that the modules are already enabled on your VPS but ufw is trying to enable them again.
To resolve the problem, simply edit your /etc/default/ufw file with a text editor like nano and comment the following line out:
IPT_MODULES="nf_conntrack_ftp nf_nat_ftp nf_conntrack_netbios_ns"
To comment it out, simply put a # in front of the line, like this:
#IPT_MODULES="nf_conntrack_ftp nf_nat_ftp nf_conntrack_netbios_ns"
Then save and exit and try enabling it again by command:
ufw enable