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What is the Secure Shell (SSH) protocol?

In the World of cybersecurity and secure network management, the ‘Secure Shell (SSH) protocol’ plays a pivotal role. This article aims to demystify SSH, exploring its significance in ‘secure data transmission,’ ‘remote server management,’ and ‘firewall bypass.’ Whether you’re an IT professional or simply curious about ‘internet security,’ this in-depth look at SSH will provide valuable insights into its uses and vulnerabilities.

What is the Secure Shell (SSH) protocol?

The Secure Shell (SSH) protocol is a secure method for transmitting commands to a computer over an insecure network. It utilizes cryptography to both authenticate and encrypt connections between devices. SSH also enables tunneling, allowing data packets to traverse networks that would otherwise be inaccessible. Primarily, SSH is used for remote server control, infrastructure management, and file transfer.

Secure Shell (SSH) protocol

Similar to how a store owner might instruct employees from a distance while traveling to ensure the store’s smooth operation, SSH allows administrators to manage servers and devices remotely. Unlike older remote management protocols like Telnet, which exposed commands to potential interception (akin to customers overhearing private instructions if the store owner were on speakerphone), SSH prioritizes security, as implied by its name—Secure Shell.


What does SSH do?

Establishing remote encrypted connections

SSH facilitates a link between a user’s device and a distant machine, typically a server, employing encryption to garble the data transmitted through this connection. Any third party attempting to intercept this data would only encounter what appears as random, meaningless static unless they manage to decrypt it. (SSH employs encryption methods that significantly hinder decryption by unauthorized individuals.)

The concept of tunneling

In the realm of networking, tunneling serves as a mechanism for routing packets across a network by utilizing a protocol or route that would otherwise be inaccessible to them. This process involves encapsulating data packets* with supplementary information—referred to as headers—to modify their intended destination. SSH tunnels utilize port forwarding, a method detailed further below, to direct packets from one machine to another.

How does SSH work?


SSH functions through the TCP/IP protocol suite, an essential framework that supports a significant portion of the Internet’s operations. TCP, or Transmission Control Protocol, and IP, the Internet Protocol, work in tandem to format, route, and deliver data packets. IP specifies essential information such as the packet’s destination IP address (similar to an address for delivery), while TCP determines the specific port at that address where the packet should arrive (akin to a designated gate or entrance).

TCP operates as a transport layer protocol primarily concerned with managing the movement and delivery of data packets. Usually, additional protocols are layered over TCP/IP to structure transmitted data into a usable format for applications. SSH represents one of these protocols, just like other examples including VPN, SSL, or DNS.

Public key cryptography

SSH is labeled as “secure” because it integrates encryption and authentication through a method called public key cryptography. This cryptographic approach involves using two distinct keys to either encrypt or sign data. One of these keys, the public key, is accessible for use by anyone, while the other key, the private key, remains confidential and is held by the owner. Verification of the key owner’s identity relies on having possession of the private key that corresponds to the public key.

These “asymmetric” keys, named so due to their different values, also enable both sides of the connection to establish identical, shared symmetric keys for further encryption over the channel. Once this negotiation is completed, both parties use these symmetric keys to encrypt the data they exchange.

In an SSH connection, each side possesses a public/private key pair, and they mutually authenticate each other using these keys. This distinguishes SSH from HTTPS, where, in most cases, only the web server’s identity is verified in a client-server connection. Other differences include that HTTPS typically does not grant the client access to the server’s command line, and firewalls may sometimes block SSH while seldom interfering with HTTPS.


Although public key cryptography is involved in authenticating the connected devices in SSH, a well-secured computer will still prompt the individual using SSH for authentication. This commonly involves entering a username and password.

Once the authentication process is finalized, the user gains the ability to execute commands on the remote machine as if they were operating on their own local machine.

SSH tunneling, or ‘port forwarding’

Port forwarding operates in a manner similar to rerouting a letter through intermediaries. For instance, if Tom sends a letter to Jessica, who then forwards it to Alex, it follows a similar pattern to how port forwarding functions, redirecting data packets initially aimed at a specific IP address and port on one machine to an IP address and port on a different machine.

Consider an office scenario: A manager needs to access a specific device within a highly secured internal network, which only accepts data from devices within the same network. To facilitate this access, the manager can connect to an intermediary server within the network, which permits external internet traffic. By utilizing SSH port forwarding, the manager can reach the intended device. From the device’s perspective, it appears that the data packets originate from within the internal network.

Using SSH

SSH is a standard feature in Linux and Mac operating systems. However, Windows machines may need an additional SSH client application installed. Users operating Mac and Linux systems can directly utilize SSH commands by accessing the Terminal application.

What is SSH used for?

From a technical standpoint, SSH can transport a diverse array of data across a network, and SSH tunneling offers versatile configurations for various purposes. However, the most common SSH usage scenarios encompass:

  1. Remote management of servers, infrastructure, and employee computers.
  2. Secure file transfers (notably more secure than unencrypted protocols like FTP).
  3. Accessing cloud services without exposing local machine ports to the internet.
  4. Establishing remote connections to services within a private network.
  5. Bypassing firewall restrictions.

What port does SSH use?

The default port for SSH is Port 22. Occasionally, firewalls might restrict access to specific ports on servers located behind the firewall while allowing Port 22 to remain open. This makes SSH advantageous for reaching servers positioned on the opposite side of the firewall: packets aimed at Port 22 remain unblocked and can subsequently be directed to any other port.

Are there any security risks associated with SSH?

Malicious actors can exploit any protocol, and SSH’s encryption and tunneling capabilities make it an attractive target for attackers. SSH has been utilized in various documented attacks with the intent of extracting sensitive data, creating unauthorized entry points into secure networks, and even obtaining root access on servers.

Certain attack methods can also compromise SSH keys, enabling access to private computers and servers. In reality, managing SSH keys poses a significant security challenge for large organizations, given that their numerous servers may employ thousands or even millions of keys, making it exceedingly difficult to monitor and update them. Once an attacker gains access to a key, they may maintain persistent access for months or even years.

How does SSH contrast with other protocols for tunneling?

One primary distinction between SSH and other tunneling protocols lies in the OSI layer they function within. GRE, IP-in-IP, and IPsec are all network layer protocols and operate between IP addresses without consideration for ports, which are a concept of the transport layer.

Additionally, SSH employs TCP, a major transport layer protocol extensively utilized on the Internet. UDP, or the User Datagram Protocol, is another widely used transport layer protocol characterized by its “best-effort” approach, prioritizing speed but occasionally resulting in packet loss. While TCP is slower than UDP, it ensures the delivery of all packets in sequence, thus offering greater reliability.

Unlike SSH, IPsec uses UDP instead of TCP, facilitating the passage of IPsec packets through firewalls. Consequently, IPsec tunnels are generally faster than SSH tunnels but may encounter packet loss during transmission. GRE and IP-in-IP can operate using either TCP or UDP.

Furthermore, SSH encrypts only one application at a time, unlike IPsec, which encrypts all network traffic regardless of its application origin. This is why SSH isn’t typically used to establish VPNs.


What is SSH?

SSH (Secure Shell) is a secure protocol used to transmit commands and data over an insecure network, employing encryption for secure connections between devices.

How does SSH work?

SSH operates on the TCP/IP protocol suite, uses public key cryptography for authentication, and establishes encrypted connections for secure data transmission.

What is the default port for SSH?

The default port for SSH is 22, often left open by firewalls for secure access.

What are the primary uses of SSH?

SSH is commonly used for remote server management, secure file transfers, accessing cloud services, connecting to private networks, and bypassing firewall restrictions.

Are there security risks with SSH?

Yes, SSH can be targeted by attackers due to its encryption and tunneling capabilities. Compromised SSH keys and mismanagement are security concerns.

How does SSH differ from other tunneling protocols?

SSH operates at the transport layer, aware of ports. In contrast, network layer protocols like GRE, IP-in-IP, and IPsec do not consider ports. SSH uses TCP for reliability, while IPsec uses UDP for faster but potentially lossy transmissions.

How do I use SSH on my computer?

Linux and Mac systems come with SSH built-in, while Windows may require an SSH client installation. Users on Mac and Linux can access SSH commands via the Terminal application.

What distinguishes SSH from HTTPS?

SSH focuses on secure data transmission, server management, and infrastructure control, using public-private key pairs. HTTPS primarily verifies web server identities for secure web browsing.


Secure Shell (SSH) is an essential protocol for secure communication over insecure networks. With robust encryption and versatile functionalities, SSH facilitates remote server management, secure file transfers, and firewall bypassing. However, potential security risks, especially in key management, highlight the need for vigilance. Despite vulnerabilities, SSH remains a cornerstone in secure network communication, enabling safe data transfer and remote server control.


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