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Secure Shell

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Summary

PDF p.150

Secure Shell (SSH) provides secure remote access to command line terminals, primarily for remote administration and secure file transfer (SFTP). SSH servers use public/private key pairs (host keys) for identification, and various client authentication methods are supported. Managing SSH keys is critical for security.

In plain words

Supplementary — not from your PDF

SSH gives an encrypted command-line session for remote administration, and SFTP/SCP for file transfer. The server proves its identity with a host key. Users can authenticate with a password, a public key (listed in the server's authorized keys), or Kerberos. Managing keys matters: remove old or compromised keys and regenerate them.

Detailed explanation

PDF p.150
  • SSH Overview
    • Purpose: Secure remote access to command line terminals.
    • Uses: Remote administration, secure file transfer (SFTP).
    • Products: Numerous commercial and open-source options, with OpenSSH being the most widely used.
  • Host Key
    • Identification: SSH servers identified by a public/private key pair (host key).
    • Mapping: Host names mapped to host keys manually or through enterprise software.
    • Compromise: Host key must be changed if compromised to prevent spoofing attacks.
  • SSH Client Authentication
    • Methods
      • Username/Password: Credentials verified by SSH server against a local database or RADIUS server.
      • Public Key Authentication: Remote user's public key added to a list of authorized keys on the SSH server.
      • Kerberos: Client submits Kerberos credentials (Ticket Granting Ticket) to the server, validated by the Ticket Granting Service.
    • Key Management: Critical to manage valid client public keys to prevent attacks. Compromised keys should be deleted and regenerated.
  • SSH Commands
    • Connecting to SSH Server
      • Command: ssh bobby@10.1.0.10
    • Creating and Copying Key Pair
      • Commands
        • ssh-keygen -t rsa
        • ssh-copy-id bobby@10.1.0.10
    • Using Standard Linux Shell Commands: At SSH prompt, use standard commands and exit to close the connection.
    • Copying Files with SCP
      • From Remote to Local: scp bobby@10.1.0.10:/logs/audit.log audit.log
      • From Local to Remote: Reverse the arguments.
      • Copying Directories: Use -r option for recursive copy.

Important terms

taken from the text above
Products
Numerous commercial and open-source options, with OpenSSH being the most widely used.
Identification
SSH servers identified by a public/private key pair (host key).
Mapping
Host names mapped to host keys manually or through enterprise software.
Compromise
Host key must be changed if compromised to prevent spoofing attacks.
Username/Password
Credentials verified by SSH server against a local database or RADIUS server.
Public Key Authentication
Remote user's public key added to a list of authorized keys on the SSH server.
Kerberos
Client submits Kerberos credentials (Ticket Granting Ticket) to the server, validated by the Ticket Granting Service.
Key Management
Critical to manage valid client public keys to prevent attacks. Compromised keys should be deleted and regenerated.
Command
ssh bobby@10.1.0.10
Using Standard Linux Shell Commands
At SSH prompt, use standard commands and exit to close the connection.
From Remote to Local
scp bobby@10.1.0.10:/logs/audit.log audit.log
From Local to Remote
Reverse the arguments.
Copying Directories
Use -r option for recursive copy.
SSH Secure Shell

Examples & real-world scenarios

Supplementary — not from your PDF
  • ssh admin@10.1.0.10 to reach a Linux server.
  • ssh-keygen to make a key pair and ssh-copy-id to install the public key.
  • scp to copy a log file from a server for review.

Scenario

An administrator leaves the company. Their public key must be removed from every server's authorized keys, or it still grants access. Tracking SSH keys is part of account deprovisioning.

Common mistakes

Supplementary — not from your PDF
  • Forgetting that SSH keys are credentials that need lifecycle management.
  • Ignoring a changed host key warning. It could mean the server is being impersonated.

Practical skills

Supplementary — not from your PDF
  • Set up key-based SSH authentication in your own lab.
  • Explain why host keys matter.

What I should remember

Key Points PDF p.150
  • SSH Overview
    • Purpose: Secure remote access.
    • Uses: Administration, file transfer.
    • Products: OpenSSH, others.
  • Host Key
    • Identification: Public/private key pair.
    • Mapping: Manual or enterprise software.
    • Compromise: Change if compromised.
  • SSH Client Authentication
    • Methods: Username/password, public key, Kerberos.
    • Key Management: Critical for security.
  • SSH Commands
    • Connecting: ssh bobby@10.1.0.10
    • Key Pair: ssh-keygen -t rsa, ssh-copy-id bobby@10.1.0.10
    • Shell Commands: Standard Linux commands, exit.
    • SCP: Copy files, directories with -r.