Ufw

Disable IPv6

/etc/default/ufw

# Set to yes to apply rules to support IPv6 (no means only IPv6 on loopback
# accepted). You will need to 'disable' and then 'enable' the firewall for
# the changes to take affect.
IPV6=no

/etc/ufw/sysctl.conf

# Uncomment this to turn off ipv6 autoconfiguration
net/ipv6/conf/default/autoconf=0
net/ipv6/conf/all/autoconf=0

Setup Rules

Adding a rule

# ufw allow log-all from 192.168.2.0/24 to any app OpenSSH 
Rule added
# ufw status verbose
Status: active
Logging: on (low)
Default: reject (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22/tcp (OpenSSH)           ALLOW IN    192.168.2.0/24 (log-all)

Inserting before

# ufw insert 1 allow log-all from 192.168.1.0/24 to any app OpenSSH 
Rule inserted
# ufw status verbose
Status: active
Logging: on (low)
Default: reject (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22/tcp (OpenSSH)           ALLOW IN    192.168.1.0/24 (log-all)
22/tcp (OpenSSH)           ALLOW IN    192.168.2.0/24 (log-all)

# ufw status numbered
Status: active

     To                         Action      From
     --                         ------      ----
[ 1] OpenSSH                    ALLOW IN    192.168.1.0/24 (log-all)
[ 2] OpenSSH                    ALLOW IN    192.168.2.0/24 (log-all)

Own applications

nrpe

/etc/ufw/applications.d/nrpe

[NRPE]
title=Nagios NRPE
description=Nagios Remote Plugin Executor
ports=5666/tcp

MySQL

/etc/ufw/applications.d/mysql

[MySQL]
title=MySQL Server (MySQL, MYSQL)
description=Old and rusty SQL server
ports=3306/tcp

Exim

/etc/ufw/applications.d/exim

[Exim SMTP]
title=Mail Server (Exim, SMTP)
description=Small, but very powerful and efficient mail server
ports=25/tcp

[Exim SMTP Virusscanned]
title=Mail Server (Exim, SMTP Virusscanned)
description=Small, but very powerful and efficient mail server
ports=26/tcp

[Exim SMTPS]
title=Mail Server (Exim, SMTPS)
description=Small, but very powerful and efficient mail server
ports=465/tcp

[Exim SMTP Message Submission]
title=Mail Server (Exim, Message Submission)
description=Small, but very powerful and efficient mail server
ports=587/tcp

Inspect your application profile

# ufw app info MySQL
Profile: MySQL
Title: MySQL Server (MySQL, MYSQL)
Description: Old and rusty SQL server

Port:
  3306/tcp