3.7.3 SSH security basics

Why SSH Security Matters

SSH (Secure Shell) is usually the primary way to remotely administer Linux systems. If SSH is compromised, an attacker effectively gets a shell on your machine with your privileges (or root, if they can escalate). This chapter focuses on practical, baseline measures to harden SSH on typical Linux servers.

You’ll use:

The SSH server config: /etc/ssh/sshd_config
The SSH client config: /etc/ssh/ssh_config and ~/.ssh/config
The ssh, sshd, and sshd-keygen tools
Systemd commands like systemctl restart sshd (name may be ssh on some distros)

Assume SSH is already installed and working; here we focus on making it safer.

Understanding SSH Keys vs Passwords

SSH supports two common authentication methods:

Password authentication

You type a username and password.
Vulnerable to brute-force guessing if SSH is exposed to the internet.
Reuse of passwords across services is especially dangerous.

Public key authentication

You generate a key pair: private key (kept secret) and public key (shared).
Server stores your public key in ~/.ssh/authorized_keys.
When you connect, the server challenges your client to prove it has the private key.
More resistant to brute-force attacks, especially if keys are protected with a passphrase.

For most servers, public key authentication should be the default, and password logins should be disabled once keys are in place.

Generating and Using SSH Keys Securely

Creating a key pair

Run this on the client machine (your laptop/workstation):

ssh-keygen -t ed25519 -C "you@example.com"

-t ed25519: uses a modern, strong key type.
-C is an arbitrary comment (e.g., email or purpose).
Default file is ~/.ssh/id_ed25519 (private) and ~/.ssh/id_ed25519.pub (public).

When prompted:

Choose a strong passphrase. This encrypts your private key at rest.
Don’t share the private key file with anyone.

To see your public key:

cat ~/.ssh/id_ed25519.pub

Never share the private key (id_ed25519); you may share the .pub file.

Installing your public key on the server

On the client, you can copy your key to a server:

ssh-copy-id user@server.example.com

This:

Appends your public key to ~user/.ssh/authorized_keys on the server.
Sets basic permissions correctly.

If ssh-copy-id is unavailable, do it manually:

On client:

   cat ~/.ssh/id_ed25519.pub

On server (logged in as user or via sudo su - user):

   mkdir -p ~/.ssh
   chmod 700 ~/.ssh
   echo "ssh-ed25519 AAAA... your-comment" >> ~/.ssh/authorized_keys
   chmod 600 ~/.ssh/authorized_keys

Now test login from client:

ssh user@server.example.com

If it logs in without asking for the account’s password (but maybe asking for key passphrase), key auth is working.

Hardening `sshd_config`

The SSH daemon configuration file is usually /etc/ssh/sshd_config.

Before editing, backup it:

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

After changes, test config for syntax errors:

sudo sshd -t

If there’s no output, configuration syntax is OK. Then restart:

sudo systemctl restart sshd
# or, on some systems:
sudo systemctl restart ssh

Disable root login

Logging in directly as root over SSH is risky. Use a regular user and sudo instead.

In /etc/ssh/sshd_config:

PermitRootLogin no

Or, more permissive but still safer than full root logins:

PermitRootLogin prohibit-password

This allows only key-based root logins (not recommended for beginners; prefer no).

Enforce key-based authentication

Once keys are set up and tested, disable password auth:

In /etc/ssh/sshd_config:

PasswordAuthentication no
KbdInteractiveAuthentication no
UsePAM yes

Notes:

Only do this after confirming that your keys work, or you can lock yourself out.
Keep console/physical access or out-of-band access (e.g., cloud provider console) for recovery.

Restrict SSH protocol and algorithms

Modern OpenSSH only supports protocol 2; ensure it:

Protocol 2

Algorithms:

Old/weak ciphers and MACs should be disabled, but default modern OpenSSH configs are generally sane.
For most beginners, rely on the distro defaults; aggressive tuning belongs in more advanced chapters.

Limiting Who and How People Can Log In

Restrict which users can SSH

You can explicitly allow only certain users or groups:

In /etc/ssh/sshd_config:

AllowUsers alice backupuser
# or
AllowGroups sshusers

If you use AllowUsers or AllowGroups, only those listed will be allowed.

Alternatively, you can deny specific accounts:

DenyUsers test temp
DenyGroups nogroup

Don’t use both Allow and Deny in complex ways unless you understand the precedence rules; for simple setups, stick to one approach.

Use non-default port (with care)

Changing the SSH port from 22 to another (e.g., 2222) does not add real cryptographic security, but it:

Reduces noise from automated bots scanning the internet.
Makes logs less cluttered with generic brute-force attempts.

In /etc/ssh/sshd_config:

Port 2222

Then update firewall rules and connect with:

ssh -p 2222 user@server.example.com

This is “security through obscurity” and should supplement, not replace, proper hardening.

Protecting SSH with Firewalls

Basic idea: only allow SSH from networks or IPs that should reach it.

Examples:

Using UFW (commonly on Ubuntu)

Allow SSH on the default port:

sudo ufw allow ssh

If you changed the SSH port (e.g., to 2222):

sudo ufw allow 2222/tcp

Restrict to a specific IP:

sudo ufw allow from 203.0.113.10 to any port 22 proto tcp

Check rules:

sudo ufw status verbose

Using firewalld (Fedora/RHEL family)

Allow SSH in the active zone:

sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --reload

If using a custom port:

sudo firewall-cmd --permanent --add-port=2222/tcp
sudo firewall-cmd --reload

For tighter restrictions (e.g., only from a specific subnet), use --add-rich-rule or rich rules as appropriate for your environment.

Using Fail2ban (Brute-Force Protection)

Fail2ban monitors logs and bans IP addresses with repeated failed login attempts.

Install (example on Debian/Ubuntu):

sudo apt install fail2ban

Basic SSH protection is often enabled by default. To enable or tune it:

Copy default config:

   sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local

Edit /etc/fail2ban/jail.local, look for [sshd]:

   [sshd]
   enabled  = true
   port     = ssh
   filter   = sshd
   logpath  = /var/log/auth.log
   maxretry = 5
   bantime  = 600

maxretry: number of failed attempts before ban.
bantime: ban duration in seconds.

Restart Fail2ban:

   sudo systemctl restart fail2ban

Check banned IPs:

sudo fail2ban-client status sshd

Fail2ban configuration can be more advanced, but this baseline cuts down on brute-force noise.

Securing SSH Client-Side

Use `~/.ssh/config` for safer defaults

Create or edit ~/.ssh/config on your client:

Host server-example
    HostName server.example.com
    User alice
    Port 2222
    IdentityFile ~/.ssh/id_ed25519
    IdentitiesOnly yes

Now connect with:

ssh server-example

This helps avoid mistakes like specifying the wrong port or user, and enforces use of your key.

Protect your private keys

Set proper permissions:

  chmod 700 ~/.ssh
  chmod 600 ~/.ssh/id_ed25519

Never email or upload private keys.
Use an SSH agent so you don’t type your passphrase every connection:

  eval "$(ssh-agent -s)"
  ssh-add ~/.ssh/id_ed25519

Many desktop environments start an agent automatically.

Verifying Host Keys and Avoiding MITM

When you first connect to a host, SSH shows a message like:

The authenticity of host 'server.example.com (203.0.113.5)' can't be established.
ED25519 key fingerprint is SHA256:....
Are you sure you want to continue connecting (yes/no/[fingerprint])?

SSH stores the server’s host key fingerprint in ~/.ssh/known_hosts.
On later connections, it checks if the key changed; if it did, you’ll get a warning (possible man-in-the-middle attack or server reinstall).

Basic practices:

For important servers, verify the fingerprint out-of-band (e.g., from your provider’s web console or documentation) before accepting.
If you see a warning about a changed host key, don’t blindly accept it. Confirm why it changed (e.g., server rebuilt) before proceeding.

You can inspect a known host entry:

ssh-keygen -lf ~/.ssh/known_hosts

Or from a host key file on the server:

sudo ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub

Practical Hardening Checklist

For a typical internet-facing server:

Generate an ed25519 SSH key on your client; protect it with a passphrase.
Install your public key on the server (ssh-copy-id).
Confirm you can log in using the key.
On the server, edit /etc/ssh/sshd_config:

Protocol 2
PermitRootLogin no
PasswordAuthentication no (after confirming key works)
Optionally change Port from 22 to a non-standard value.
Optionally add AllowUsers yourusername.

Test SSH config: sudo sshd -t.
Restart SSH service.
Configure firewall (UFW or firewalld) to:

Allow SSH only from networks that need it.
Use the correct port.

Install and enable Fail2ban (or equivalent) to limit brute-force attempts.
Keep SSH server and system packages up to date with your distro’s package manager.
Periodically review /var/log/auth.log or equivalent for unusual SSH activity.

These steps provide a solid baseline of SSH security for most beginner-to-intermediate Linux administrators.

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3.7.3 SSH security basics

Why SSH Security Matters

Understanding SSH Keys vs Passwords

Generating and Using SSH Keys Securely

Creating a key pair

Installing your public key on the server

Hardening `sshd_config`

Disable root login

Enforce key-based authentication

Restrict SSH protocol and algorithms

Limiting Who and How People Can Log In

Restrict which users can SSH

Use non-default port (with care)

Protecting SSH with Firewalls

Using UFW (commonly on Ubuntu)

Using firewalld (Fedora/RHEL family)

Using Fail2ban (Brute-Force Protection)

Securing SSH Client-Side

Use `~/.ssh/config` for safer defaults

Protect your private keys

Verifying Host Keys and Avoiding MITM

Practical Hardening Checklist

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