Enabling Secure Remote Access with SSH on Debian 10

SSH (Secure Shell) has become the standard for secure remote access, shell login, and file transfers between systems. Its encrypted connections, strong authentication mechanisms, and ubiquity have made SSH an indispensable tool for system administrators, developers, and power users alike.

In this comprehensive 2600+ word guide, we will cover everything you need to know to install, configure, connect to, and utilize key capabilities of an OpenSSH server on Debian 10.

An Overview of SSH

SSH stands for "Secure Shell", and most commonly refers to OpenSSH – an open source implementation of the SSH protocol that aims to be standards-compliant and interoperable with proprietary versions. It provides a secure channel for logging into remote systems, executing commands and transferring files.

The key capabilities enabled by SSH include:

• Encrypted remote shell access – SSH connections are fully encrypted using cryptographic ciphers like AES, Blowfish etc. to prevent eavesdropping. Modern ciphers with 128-bit and higher keys are considered unbreakable with current computing capacity.

• File transfers – The inbuilt SCP and SFTP protocols allow easy transfer of files over an SSH session. Using SCP, files can be directly copied from one system to another. Meanwhile, SFTP provides a simple interface much like typical FTP clients.

• Application tunneling – SSH port forwarding allows other TCP/IP applications and protocols to be tunneled through an SSH session. This allows services to communicate securely over untrusted networks.

• Automated connections – Using SSH public key authentication, connections can be established without manually entering passwords. Keys identify trusted systems.

Compared to unencrypted protocols like FTP and Telnet, SSH is much more secure thanks to its strong encryption, tamper-proof key exchange mechanisms, and integrity checking capabilities. These make it very difficult for eavesdroppers to intercept data or launch man-in-the-middle attacks. The fact that SSH is built into practically every modern operating system makes it universally accessible as well.

Installing OpenSSH Server on Debian 10

The OpenSSH server package comes preinstalled on some Debian editions, but can easily be added if missing. We will use the APT package manager:

sudo apt update
sudo apt install openssh-server openssh-client

This installs the sshd daemon that will handle SSH connections as a background service, as well as the client programs like ssh and scp.

With the OpenSSH server installed, we can check if sshd is running using systemctl:

sudo systemctl status sshd

If SSH has not started automatically, enable it so it runs on every boot:

sudo systemctl enable sshd

Or to start it immediately in the current session without a reboot:

sudo systemctl start sshd

Now our SSH server is up and running, listening for client connections on TCP port 22.

Understanding OpenSSH Encryption

A key advantage of SSH over plain terminal protocols is its support for strong symmetric encryption algorithms. The server and client negotiate an algorithm to use on connection:

Algorithm	Key Length	Relative Speed
AES	128-256 bit	Very fast
Blowfish	128-448 bit	Fast
3DES	168 bit	Slow

AES with a >128 bit key is recommended as it provides the best balance between security and speed. brute-force attacks are not feasible against such keys with current computing power.

Debian‘s default SSH cipher selection includes AES-CTR and AES-GCM stream ciphers which are very efficient in software. As extra protection against attacks, OpenSSH rekeys frequently to limit decryptable segments.

Connecting to the Server with SSH

To connect to our SSH server from a remote client, we need:

• Server IP address or hostname
• SSH client program
• Valid user credentials

When connecting for the first time, SSH displays the target host‘s key fingerprint and asks for confirmation before connecting:

ssh user@server-ip

The authenticity of host ‘server-ip (10.10.0.5)‘ can‘t be established.
ED25519 key fingerprint is SHA256:9gtYX+Wn4fcnpAFeLaMJZGEMs5jk199qHLYS/D0SBX0.
Are you sure you want to continue connecting (yes/no)?

This ensures attackers cannot spoof a server without being detected – providing protection against man-in-the-middle attacks.

After accepting the key, provide the user password when prompted, and you will be logged in. The connection is fully encrypted and secured against eavesdropping.

Key-based Authentication

Having to enter passwords repeatedly can be cumbersome. SSH keys allow authentication without passwords:

On the client, generate an RSA key pair:

ssh-keygen -t rsa -b 4096

Then copy the public key to any servers you want passwordless access to:

ssh-copy-id user@server-ip 

This adds your public key to .ssh/authorized_keys. Now logins can authenticate against the local key instead of using passwords.

Key pairs eliminate having to enter credentials manually while maintaining security. For best protection, secure the client‘s private key against access on untrusted systems.

Transferring Files Securely with SCP

The OpenSSH client provides the scp program that copies files between hosts similar to cp. Because connections are tunneled over SSH, transfers are encrypted end-to-end.

Basic SCP usage resembles regular shell copying:

scp file.txt user@server:/remote/path  

This copies file.txt into the /remote/path directory on server as user. Transfers can also work recursively on directories.

Advanced capabilities include retaining original permissions and timestamps, resuming interrupted transfers, limiting bandwidth throttle and more. This makes scp suitable for automated system migration processes.

Compared to unencrypted FTP or NFS, SCP provides reliable security for data transfers at reasonable speeds. Embedded SSH functionality delivers strong integrity checking and protection against tampering as well.

File Management with SFTP

SFTP (SSH File Transfer Protocol) serves another popular mechanism for file operations over an SSH session. It offers an interactive FTP-like interface rather than scp‘s single-command approach:

sftp user@server  

Upon connection this opens up an interactive prompt where you can execute familiar commands:

sftp> ls
sftp> pwd
sftp> get file.txt
sftp> exit

SFTP is generally slower than SCP but allows easier exploration and management of remote file systems. Complex tasks can be automated via sftp batch scripts rather than interactive usage.

Overall, SSH‘s underlying security protections make both SCP and SFTP hugely preferable over risky plain FTP for system administration.

Port Forwarding

A highly useful SSH capability is TCP/IP connection tunneling or port forwarding. This uses the encrypted SSH transport to provide connectivity to other services that may not have native encryption.

Local port forwarding maps a local listening port to connect to a specified remote host and port via the SSH tunnel:

# On local system
ssh -L 9999:web.host:80 user@ssh.host

This forwards local connections to localhost port 9999 over ssh to web.host:80. Software on the local system communicating with the forwarded port enjoys protection against eavesdropping and alteration of data.

Dynamic application-level port forwarding uses SOCKS and establishes tunneling dynamically based on name lookups:

ssh -D 9999 user@ssh.host

Now local programs can be configured to proxy traffic to remote destinations via SOCKS on port 9999 securely.

Port forwarding is invaluable for linking sensitive services across untrusted networks without compromising security.

Optimizing SSH Performance

While SSH is encrypted and provides integrity protection, these safeguards can have performance impacts depending on cipher choice, protocol options and environmental factors:

• Stronger ciphers like AES take more computation than weaker ones like DES/3DES.
• Using message authentication codes (MACs) adds an integrity layer but slows each packet.
• Network reliability impacts client-side latency as packets may need resending if dropped.
• Client hardware performance limits encryption/decryption throughput.

On reliable LAN links with modern CPUs, the difference between SSH and unencrypted traffic is small thanks to fast stream ciphers and session caching. But connection setup takes longer than plain Telnet/RSH so SSH can feel slower for very brief sessions with many short commands.

Where high throughput is needed, parallel SCP file transfers or SSH connection multiplexing help significantly. By opening multiple TCP streams in parallel or reusing established sessions, effective bandwidth is multiplied while avoiding new handshakes.

For example, with 3 streams 100 MB/s total can be reached using:

scp -3 server:large-file .

On WAN/Internet links with higher latency and packet loss, performance over plaintext protocols improves significantly. The ssh protocol minimized sensitive data transfer unlike Telnet. Creative forwarding can also avoid issues like chattiness.

With computational power ever increasing, we can employ strong ciphers today without noticeable SSH slowdowns in most settings.

Hardening SSH Security

Though encrypted, SSH backends still need hardening against misuse. Recommended practices include:

• Changing the default listening port away from predictable port 22 using sshd‘s Port directive.
• Limiting authentication methods offered in sshd_config. PubkeyAuthentication with locked down keys is best.
• Installing OS updates regularly to benefit from vulnerability fixes.
• Using firewall rules and/or TCP Wrappers to restrict source IP ranges that can access SSH.
• Protecting private keys from access on compromised systems.

Optional steps like obscuring the SSH banner help raise effort levels for attackers.

Following such guidelines minimizes the attack surface despite SSH‘s frontline defenses.

Troubleshooting SSH Issues

Sometimes SSH connections fail unexpectedly. Here are some common scenarios and solutions:

Problem: SSH instantly closes without prompting for a password.

Cause 1: Client cannot communicate on port 22 to the server. A local or network firewall is likely blocking traffic. TCP RST packets indicate active rejection.

Fix 1: Update firewall policies to open outbound port 22 access.

Cause 2: The server is not listening on port 22 at all. sshd is likely not running.

Fix 2: Check sshd status with systemctl and review logs. Start the sshd process if stopped.

Problem: Authentication prompt appears but password never accepts

Cause: Incorrect username, password or credential permissions/ownership.

Fix: Double check client credentials are accurate for target account. Reset password or ownership if permissions got altered.

Problem: Extreme latency before key prompt or commands lag after login

Cause: Network issues like packet loss or hostile traffic shaping are impacting SSH.

Fix: Trace route repeatedly from client to diagnose inconsistent ICMP timings or high packet loss on some hops. Contact your ISP if routing infrastructure needs investigation.

Addressing such problems ensures reliable SSH functionality.

Wrapping Up

We have explored how OpenSSH provides universal, secure remote access with features including:

• Encrypted connections preventing eavesdropping using ciphers like AES.
• SCP and SFTP options for easy yet secure data transfers
• Port forwarding to tunnel services over SSH dynamically
• Key-based logins for passwordless convenience
• Configuration tweaks to harden security against attacks

With ongoing development and support across every platform, OpenSSH continues proving itself as a robust, trusted protocol for connecting systems located anywhere through untrusted networks. Its strong 256-bit encryption and public key infrastructure integrate security capabilities rivaling expensive commercial alternatives.

So whether you are a home user maintaining remote servers or an enterprise managing global infrastructure, SSH remains the versatile solution for remote shell and file transfer duties. This guide covers the key theory and practical aspects for utilizing OpenSSH effectively.