正解: B
The correct answer is B. 22.
Port 22 is the default port for SSH (Secure Shell), a protocol that allows secure and encrypted remote login and command execution over a network. SSH uses public key cryptography to authenticate the client and the server, and to encrypt the data exchanged between them. SSH can be accessed through different ports, depending on the configuration of the server and the client. The default port for SSH is 22, but it can be changed to any other port for security reasons1.
Port 20 is the default port for FTP (File Transfer Protocol) data transfer, a protocol that allows file transfer between a client and a server over a network. FTP uses two ports: one for data transfer (port 20) and one for control commands (port 21). FTP does not provide encryption or authentication, and it is vulnerable to various attacks, such as packet sniffing, spoofing, or brute force. To secure FTP connections, protocols such as FTPS (FTP over SSL) or SFTP (SSH File Transfer Protocol) can be used2.
Port 23 is the default port for Telnet, a protocol that allows remote login and command execution over a network. Telnet does not provide encryption or authentication, and it sends data in plain text, which can be easily intercepted or modified by attackers. Telnet is considered obsolete and insecure, and it should be replaced by SSH or other secure protocols3.
Port 25 is the default port for SMTP (Simple Mail Transfer Protocol), a protocol that allows sending and receiving email messages over a network. SMTP does not provide encryption or authentication, and it can be exploited by spammers or hackers to send malicious or unwanted emails. To secure SMTP connections, protocols such as SMTPS (SMTP over SSL) or STARTTLS (Transport Layer Security) can be used.
Therefore, port 22 is the only port that allows for secure communications by default, as it uses SSH protocol.
For more information about ports and their security, you can refer to the following web search results: How to secure your TCP/IP ports, What is an SSL port? A technical guide for HTTPS, HTTPS Port: What It Is, How to Use It, and More (2023).
