Protocols and Servers

Telnet

• Purpose: Remote terminal access.
• Protocol: Telnet.
• Port: 23 (TCP).
• Insecure: Sends data in cleartext (unencrypted), including usernames and passwords.
• Vulnerable to eavesdropping: Attackers can easily capture credentials from network traffic.
• Example: telnet <machine_ip>

Hypertext Transfer Protocol (HTTP)

• Purpose: Transfer web pages and other web resources.

• Cleartext Protocol: Sends data unencrypted.

• Can be interacted with using Telnet: Manually send GET requests to retrieve web pages.

• Example:

  telnet <machine_ip> 80
  GET /index.html HTTP/1.1
  Host: <any_value>
  

• Web Servers:

  • Apache: https://www.apache.org/
  • IIS: https://www.iis.net/
  • nginx: https://nginx.org/

  • Web Browsers:

  • Chrome, Edge, Firefox, Safari, etc.

File Transfer Protocol (FTP)

• Purpose: Transfer files between computers.

• Cleartext Protocol: Sends data unencrypted, including login credentials.

• Can be partially interacted with using Telnet: Can log in and execute some commands, but not transfer files.

• Two Modes:

  • Active: Data transfer originates from server (port 20).
  • Passive: Data transfer originates from client (port > 1023).
  • Example:

  telnet <machine_ip> 21
  USER <username>
  PASS <password>
  

• FTP Servers:

  • vsftpd: https://security.appspot.com/vsftpd.html
  • ProFTPD: http://www.proftpd.org/
  • uFTP: https://www.uftpserver.com/

  • FTP Clients:

  • Console FTP client (Linux)

  • FileZilla: https://filezilla-project.org/
  • Web browsers (limited support)

Simple Mail Transfer Protocol (SMTP)

• Purpose: Send email.
• Mail User Agent (MUA): Email client (e.g., Thunderbird, Outlook).
• Mail Transport Agent (MTA): Server that sends email.

Post Office Protocol 3 (POP3)

• Purpose: Receive email.
• Downloads emails to local device: Emails are deleted from the server after download.

Internet Message Access Protocol (IMAP)

• Purpose: Receive email.
• Synchronizes email across devices: Emails are stored on the server and can be accessed from multiple devices.