how to install the program Apache and Allow in Firewall

 Step 1: Install Apache and Allow in Firewall

The Apache web server is among the most popular web servers in the world. It's well-documented, and has been in wide use for much of the history of the web, which makes it a great default choice for hosting a website.
We can install Apache easily using Ubuntu's package manager, apt. A package manager allows us to install most software pain-free from a repository maintained by Ubuntu. You can learn more about how to use apt here.
For our purposes, we can get started by typing these commands:

sudo apt-get update

sudo apt-get install apache2

Since we are using a sudo command, these operations get executed with root privileges. It will ask you for your regular user's password to verify your intentions.
Once you've entered your password, apt will tell you which packages it plans to install and how much extra disk space they'll take up. Press Y and hit Enter to continue, and the installation will proceed.

Set Global ServerName to Suppress Syntax Warnings

Next, we will add a single line to the /etc/apache2/apache2.conf file to suppress a warning message. While harmless, if you do not set ServerName globally, you will receive the following warning when checking your Apache configuration for syntax errors:

sudo apache2ctl configtest

Output
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message
Syntax OK

Open up the main configuration file with your text edit:

sudo nano /etc/apache2/apache2.conf

Inside, at the bottom of the file, add a ServerName directive, pointing to your primary domain name. If you do not have a domain name associated with your server, you can use your server's public IP address:

Note

If you don't know your server's IP address, skip down to the section on how to find your server's public IP address to find it.

/etc/apache2/apache2.conf

. . .
ServerName server_domain_or_IP

Save and close the file when you are finished.
Next, check for syntax errors by typing:

sudo apache2ctl configtest

Since we added the global ServerName directive, all you should see is:

Output
Syntax OK

Restart Apache to implement your changes:

sudo systemctl restart apache2

You can now begin adjusting the firewall.

Adjust the Firewall to Allow Web Traffic

Next, assuming that you have followed the initial server setup instructions to enable the UFW firewall, make sure that your firewall allows HTTP and HTTPS traffic. You can make sure that UFW has an application profile for Apache like so:

sudo ufw app list

Output
Available applications:
  Apache
  Apache Full
  Apache Secure
  OpenSSH

If you look at the Apache Full profile, it should show that it enables traffic to ports 80 and 443:

sudo ufw app info "Apache Full"

Output
Profile: Apache Full
Title: Web Server (HTTP,HTTPS)
Description: Apache v2 is the next generation of the omnipresent Apache web
server.

Ports:
  80,443/tcp

Allow incoming traffic for this profile:

sudo ufw allow in "Apache Full"

You can do a spot check right away to verify that everything went as planned by visiting your server's public IP address in your web browser (see the note under the next heading to find out what your public IP address is if you do not have this information already):

http://your_server_IP_address

You will see the default Ubuntu 16.04 Apache web page, which is there for informational and testing purposes. It should look something like this:

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