The firewall on a Raspberry Pi will, by default, block incoming connections to certain ports. This is a security feature, but may cause problems if you try to configure an SSH server on your Raspberry Pi, host a website, or otherwise host some service that needs to accept incoming connections. The usual answer to this problem is to configure the firewall to accept the connections on the particular ports that you need to open. Another alternative is to disable the firewall completely.
The default Ubuntu firewall is ufw, which is an acronym for “uncomplicated firewall.” It comes automatically installed on all editions of Ubuntu Desktop and Ubuntu Server. Ufw is a frontend for the typical Linux iptables commands, but it is developed in such a way that basic firewall tasks can be performed without the knowledge of iptables. You may find you need to disable ufw if it is blocking some necessary connections, or you can enable it for increased security.
Checking for open ports on Ubuntu Linux is an essential part of security administration. Some Linux software works by listening for incoming connections. A simple example would be a web server, which handles user requests whenever someone navigates to a website. As a Linux administrator or user, it’s important to always know which ports of your system are open to the internet. Otherwise, you could be unaware of outside connections being made to your computer, which consumes bandwidth and resources, along with being a potential security hole.
hosts.deny file can be used on a Linux system to deny connection attempts from one or more IP addresses, hostnames, or domains. It can work with any TCP wrapped service on your system. The
hosts.deny file is used in conjunction with
hosts.allow to determine whether a connection attempt gets accepted or denied.
A Linux system can utilize the
hosts.allow file to specify which IP addresses, hostnames, or domains are permitted to connect to it. This works specifically for TCP wrapped services. The
hosts.allow file is used in conjunction with
hosts.deny to determine whether a connection attempt gets accepted or denied.
A firewall is a line of defense on your network, primarily used to filter incoming traffic, but also used for outbound rules and other network related security. All major Linux distros come with a software firewall built into them, since it is part of the Linux kernel itself. Any user can configure their system firewall to get started with securing network traffic, but there are many alternatives to the default which will extend or simplify the functionality.
Firewalld is the default high-level firewall manager on the Red Hat family of distributions. One of its peculiarities is that it defines a series of so called firewall zones: each zone can be considered like a different level of trust and can be configured to allow traffic through a specific set of ports. While Firewalld comes with some predefined zones which can be easily examined and modified, sometimes we may want to create our custom zones from scratch.
SSH stands for secure shell and is the primary method of remote access and administration on Linux systems. SSH is a client-server service providing secure, encrypted connections over a network connection. After downloading Ubuntu 22.04 Jammy Jellyfish or upgrading to Ubuntu 22.04, it will probably be one of the first things you want to configure.
The SSH protocol operates on port 22 by default. In order to accept incoming connections on your SSH server, you will need to ensure that port 22 is allowed through the firewall.