Networking › Module 3 › Lesson 3
FTP, SSH, SMTP, IMAP
Common application protocols, ports, and security notes
Opening
Beyond the web browser
Not all network traffic is HTTP. File transfers, remote shells, and email each use dedicated protocols—and defenders spot anomalies faster when they know the expected port and whether traffic should be encrypted.
1. FTP — File Transfer (Port 21)
FTP moves files between client and server. Control channel: port 21. Data often uses separate ports (active/passive modes). Security note: Classic FTP sends credentials in plaintext. Prefer SFTP (SSH-based, port 22) or FTPS (FTP over TLS) for anything sensitive. Unexpected FTP listeners on a workstation are worth investigating.
2. SSH — Secure Shell (Port 22)
SSH encrypts remote login, command execution, and file copy (scp/sftp). It is the standard for administering Linux servers. Security note: Brute-force and credential-stuffing attacks target port 22 constantly. Use key-based auth, disable root login, and restrict source IPs where possible. Only connect to systems you are authorized to access.
3. SMTP — Sending Mail (Port 25)
SMTP (Simple Mail Transfer Protocol) relays email between mail servers. Port 25 is the traditional server-to-server path. Clients often submit mail on port 587 (submission, usually with STARTTLS) or 465 (SMTPS). Open relays and spoofed headers make SMTP a top channel for phishing—SPF, DKIM, and DMARC help receivers verify senders.
4. IMAP — Reading Mail (Port 143 / 993)
IMAP keeps messages on the server and syncs folders across devices. Port 143 is plain IMAP; port 993 is IMAPS (IMAP over TLS). POP3 (110/995) downloads mail to one device instead. For security, prefer encrypted ports (993, 995, 587) and modern auth.
Quick port reference:
FTP21 (control)—use SFTP/FTPS instead when possible
21 (control)—use SFTP/FTPS instead when possible
SSH22—encrypted remote access
22—encrypted remote access
SMTP25 relay; 587 submission; 465 SMTPS
25 relay; 587 submission; 465 SMTPS
IMAP143 plain; 993 IMAPS
143 plain; 993 IMAPS
Knowledge Check
Which protocol and port provide encrypted remote shell access?
Multiple choice
Knowledge Check
Why is plain FTP considered weak for credentials?
Multiple choice
Knowledge Check
IMAPS typically uses which port?
Multiple choice