How to Enable SSH on Linux Mint

SSH, or Secure Shell, is an encrypted network protocol that allows secure remote login from one computer to another. It is commonly used to securely administer remote servers, transfer files, and tunnel other applications in a secure way.

Enabling SSH on your Linux Mint desktop or server provides a secure way to remotely manage the system via encrypted connections. In this comprehensive 2600+ word guide, we‘ll cover everything from SSH encryption and OpenSSH configuration to troubleshooting from an expert developer‘s perspective.

An Overview of SSH Encryption

SSH utilizes both asymmetric and symmetric encryption to secure connections between clients and servers:

• Asymmetric Encryption – Uses public-private key pairs to establish the initial connection. The key exchange algorithm generates temporary session keys encrypted by the public key the server has for the user trying to log in. Popular asymmetric algorithms used include RSA and ED25519.

• Symmetric Encryption – Once an encrypted tunnel is established using the asymmetric keys, a faster symmetric cipher encrypts the actual data in transit. Common symmetric ciphers include AES, Blowfish, 3DES, and RC4.

So on initial connection, slow but secure asymmetric encryption is used just for negotiating session keys. But bulk data transfer uses rapid symmetric encryption through the encrypted tunnel for performance.

SSH also provides strong integrity checking via cryptographic hashing functions like SHA-1 and SHA-2 to prevent against data tampering. Overall this mix of encryption, hashing, and public-private key pairs allows SSH to provide secure remote shell access.

Now that we understand the encryption behind SSH at a lower level, let‘s move on to actually installing and configuring the SSH server components.

Installing & Configuring OpenSSH Server

The OpenSSH suite contains the tools needed for both the SSH server and client side. This includes:

• sshd – This SSH daemon listens for connections from clients and handles encryption, authentication, and session management.

• ssh – An SSH client for remotely logging into other servers and tunneling connections.

• ssh-keygen – Generates public-private key pairs for SSH authentication.

On Linux Mint, installing the OpenSSH meta-package provides all these tools:

sudo apt install openssh-server

The configuration file for sshd is located at /etc/ssh/sshd_config. This controls the listening port, protocol versions, encryption algorithms, user access restrictions, and authentication mechanisms allowed.

Some key options to pay attention to:

• Port – The port number for sshd to listen on. Change this to hide SSH on a non-standard port.

• LoginGraceTime – Maximum time allowed for failed login attempts before disconnecting. Lowers brute force risk.

• MaxAuthTries – Maximum authentication attempts permitted per connection. Another way to limit brute force guessing.

• PermitRootLogin – Allows direct root user SSH logins. It‘s better to disable this and use sudo privileges instead.

There are many more options that can tighten down control and access at a very granular level. The Debian/Ubuntu OpenSSH packages provide very robust configuration through this file.

Comparing SSH Key Pairs vs Password Authentication

For proving a user‘s identity to SSH, two main options exist – plain password authentication or SSH key pairs.

SSH key pairs are the more secure option because passwords have risks:

• Passwords can be guessed through brute force attacks
• Users create weak passwords that are easy to crack
• Shoulder surfing exposes passwords being entered
• Keyloggers record passwords during entry

Whereas SSH key pairs provide multi-factor security validating "something you have" (the private key file) and "something you know" (the passphrase). The private keys are also long cryptographic data mathematically computed, not simple passwords chosen by a human that tend to be weak.

So from a security standpoint, SSH key pairs are far superior to simple password authentication, preventing many avenues of attack. Plus SSH keys facilitate automation workflows by not requiring interactive password prompts. The only downside is slightly more complex initial setup.

With SSH keys added to user accounts, we can configure our SSH server for key-based auth:

Enable key auth:

PubkeyAuthentication yes

Disable password auth:

PasswordAuthentication no 

This forces keys to be used rather than fall back on simple passwords. Now let‘s take a look at another use case for SSH – tunneling connections for proxies or VPN access into private networks from remote locations.

SSH Port Forwarding & Tunneling

In addition to providing secure remote shell terminal access, SSH also facilitates tunneling other types of TCP connections.

This works by using the encrypted SSH connection as a secure tunnel to route other application protocols through like a proxy or VPN. Traffic is forwarded from the SSH client to the SSH server then on to the final destination protected within the encrypted tunnel the whole way.

SSH port forwarding can be used for:

• Accessing websites & apps on remote private networks
• Sharing access to internal databases/APIs with app servers
• Making connections not normally possible due to firewall rules
• Adding an extra layer of encryption to non-encrypted protocols

Common examples include tunneling RDP to access remote desktops, running MySQL queries securely via an SSH tunnel, or configuring local programs to use SSH as a SOCKS proxy for all connections.

Port forwarding mappings are defined in the SSH client config file at ~/.ssh/config like:

Host tunnel-server
  HostName 1.2.3.4
  User myuser

  LocalForward 9000 127.0.0.1:80
  RemoteForward 9001 127.0.0.1:3306

This tunnels:

• Local port 9000 to the remote server‘s port 80
• Remote port 9001 to the local server‘s port 3306

Allowing for creative possibilities in accessing resources across private remote networks.

Now that we understand SSH concepts more fully, let‘s circle back to the server setup steps.

Configuring Linux Mint‘s Firewall for SSH

By default Linux Mint includes a firewall called UFW that blocks all incoming connections except essential system services.

So we need to create firewall rules allowing SSH traffic on the TCP ports the SSH daemon is listening on (22 by default):

sudo ufw allow ssh

We can verify the new firewall rule is active:

sudo ufw status

And ensure UFW itself is enabled, activating our firewall policies:

sudo ufw enable

With this complete, external hosts can now connect via SSH for secure remote access shell sessions.

Troubleshooting SSH Issues from a Developer Lens

As a developer relying on SSH daily for remote server access, one becomes very accustomed to occasional SSH issues.

Here are some developer-focused troubleshooting techniques for debugging SSH connection problems:

Check connectivity to server

Typically layer 1-3 networking issues. Can we ping? Telnet to ports? traceroute to trace routing issues? DNS problems?

Verify user account validity

Does SSH user have a valid shell set? Or is it a system user without shell access for remote SSH connections?

Inspect server-side logs

/var/log/auth.log for SSH daemon logs. Failed connections or errors logged here from server perspective.

Analyze traffic signatures

Use tcpdump or Wireshark during attempted SSH connections for clues. Seeing initial TCP handshake? Encrypted packets after? Where is it failing?

Local client SSH verbosity

Use -vvv flag on ssh client for extremely verbose debug output. Exposes negotiation issues in detail as they happen.

Strace application calls

Advanced technique! Use strace on a process to log all system calls in extreme detail. Applied against the sshd process reveals minute app execution flows.

So through a mix of general networking knowledge plus Linux server and application troubleshooting techniques, we can tackle almost any SSH irregularity at a very deep level as developers.

Final Thoughts

Accessing remote infrastructure securely with SSH is an indispensable tool for Linux developers and administrators alike.

We‘ve explored both theoretical and practical facets around leveraging OpenSSH – from encryption mechanisms to extensive server configurations through a developer lens.

Follow along to harden remote access authentication via SSH keys rather than simplistic passwords prone to cracking. Utilize port forwarding for creative access across restrictive network policies. Analyze extensive logs and traffic captures to isolate stubborn SSH issues that may occur.

Once familiar with OpenSSH installs, key generation, and troubleshooting, you can securely manage infrastructure confidently using this powerful encrypted protocol.

So whether directly administering remote servers or tunneling access to restrictive internal development resources, SSH skills put power in the hands of developers for flexibility in how they access computing environments.