FTP, NFS, SMB, TELNET, SSH, RDP

• uses TCP

• Data connection -

• Control Connection

  

NFS

• file sharing protocol, native unix/linux

  

• clients access files from server using RPC Calls (UDP or TCP)

SMB

• for windows

  

• unlike FTP, SMB is secure and have specific access rules for Users and also common public shares

Aspect	NFS	SMB
Native OS	Unix/Linux	Windows
Auth model	UID/GID	User accounts
Encryption	v4 only	SMB3
Firewall	v4 easy	Easy
ACLs	POSIX	NTFS
Performance	High (Linux)	High (Windows)
AD integration	Weak	Native

Telnet - not secure

telnet <hostname> <port>

This command checks if this specific port is open or not (accepting connections or not)

SSH

• similar to Telnet but Uses strong encryption

  

  

  

  

  

RDP

• also allows remote clients accessing physical resources like printers on server environement

How SSH Works

SSH connection

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1. Client connects to server

• Client opens TCP connection to server
• Default port: 22
• Server is listening for SSH connections

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2. Encryption protocol agreement

• Server sends:
  • Supported encryption protocols
  • Supported protocol versions
• Client compares with its supported list
• A common protocol + version is selected
• SSH session initialization begins

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3. Server identity verification

• Server presents its asymmetric public key
• Client verifies this key (known host check)
• Confirms server authenticity

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4. Diffie-Hellman key exchange (key creation)

4.1 Public parameters

• Client and server agree on:
  • A large prime number (shared, public)
  • A generator (public)

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4.2 Private key generation

• Client generates a private random number
• Server generates a private random number
• These private values are never shared

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4.3 Public key exchange

• Client computes a public value using:
  • Its private number
  • Shared prime & generator
• Server does the same
• Public values are exchanged

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4.4 Shared secret computation

• Client uses:
  • Its private number
  • Server’s public value
  • Shared prime
• Server uses:
  • Its private number
  • Client’s public value
  • Shared prime
• Both independently compute the same shared secret

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5. Symmetric session key creation

• Shared secret is used to derive:
  • Symmetric encryption key
• This key will encrypt and decrypt all traffic

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6. Encrypted channel established

• SSH switches to symmetric encryption
• All further communication is encrypted

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7. User authentication

• Client authenticates using:
  • Password or
  • SSH public key
• Authentication data travels inside encrypted tunnel
• Server verifies credentials

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8. Shell session opened

• If authentication succeeds:
  • Server opens requested shell environment
• Client can now:
  • Execute commands
  • Receive output

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9. Secure session active

• Inputs → encrypted → server
• Outputs → encrypted → client
• Session continues until terminated

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Final one-line summary

SSH establishes a TCP connection, negotiates encryption, verifies the server, securely creates a shared symmetric key using Diffie-Hellman, authenticates the user, and then opens an encrypted shell session.