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1. Obtain a certificate (Let's Encrypt)1.1 advance preparation①Enable mod_ssl# a2enmod ssl2. SSL/TLS (Let's Encrypt) configuration for Apache2①Edit Apache2 SSL-related configuration files3. SSL/TLS (Let's Encrypt) settings on the mail server3.1 Obtaining a certificate for the mail serverObtain a certificate for the mail server, but it cannot be obtained in the same way as above, so the following with the "--standalone" option fails.

1.Antivirus software Clamav installedInstall Clam AntiVirus, a free anti-virus software for Linux, as an anti-virus measure.By installing this anti-virus software, you can not only scan the entire server for viruses, but also scan incoming and outgoing mail for viruses if you build and configure a mail server.2. Mail server installationPostfix was developed as a Mail Transport Agent (MTA) to replace sendmail, and is a mail server that is highly compatible with sendmail, secure, easy to maintain, and fast.In addition, since Postfix only functions as an SMTP server for sending mail, the POP server Dovecot for receiving mail will be installed separately in the latter half.3 Applied ClamAV to mail server PostfixSet up Postfix and Clamav to work together to scan incoming and outgoing mail in real time.4 Apply spam checks to the mail server4.1 Spam checking using spamassassin and procmail①spamassassin and procmail installation

Web server Apache2 installationAllow http:80 and https:443 ports in UFW before installation3.  Apache2; Using Perl ScriptsEnable CGI to make Perl scripts available①Install Perl4 Apache2 : Using PHP Scripts①Install PHP5. Virtual Host SettingsCopy the default configuration file (file name is arbitrary, in this case vhost-yourdomain.conf as an example) and configure the virtual host6. Digest authentication with Apache2Since Basic Authentication, a well-known authentication authorization method for http, sends authentication information in plain text, there is a risk of ID and password leakage if the packet is intercepted.On the other hand, Digest Authentication encrypts and transmits authentication information, so there is almost no risk of information leakage.

1. Install a time synchronization service NTP server# apt -y install chrony# vi /etc/chrony/chrony.conf# Lines 20-23.：Comment the default settings and add the NTP server for your time zone.#pool ntp.ubuntu.com     iburst maxsources 4#pool 0.ubuntu.pool.ntp.org iburst maxsources 1#pool 1.ubuntu.pool.ntp.org iburst maxsources 1#pool 2.ubuntu.pool.ntp.org iburst maxsources 2

Create a key pairCreate a public/private key pair for a user connecting to a Linux server using OpenSSH.Use ssh-keygen to create the key pair.The creation of a public/private key pair must be done as a user with remote login privileges.If you do not specify the destination and file name, id_ed25519 and id_ed25519.pub will be created in /home/(user name)/.ssh/. On the way, enter the password for the key.

1. SSH Service Security SettingsThe SSH service allows the root user to log in by default, and since the root user already knows the user name and can log in to the server with administrative privileges once the password is known, we will deny this setting.1.1 Creating a General UserIf you have created a general user when installing Debian, this procedure is not necessary.If the only user created on the server is root, remote login via SSH will not be possible, so if a user has not been created during OS installation, a user must be created in advance.Users can be created with the "useradd" command. The "-m" option creates a home directory and the "-p" option specifies the password.For example, to set "debianuser" as the user account name and "123456" as the password, execute the following