PC Networking PC Networking

What is SSH (Secure Shell), and What are its Features?
Secure Shell (SSH) is a network protocol that provides users, especially system administrators, a secure way to access a computer over an unsecured network.

SSH is a network protocol that provides users, especially system administrators, a secure way to access a computer over an unsecured network.

SSH provides strong authentication and encrypted data communication between two computers connected over an open network such as the internet. SSH is widely used by network administrators to remotely manage systems and applications. Thus, this protocol allows users to log in to another computer over the network, execute commands, and move files from one computer to another.

SSH refers to both the cryptographic network protocol and the suite of utilities implementing this protocol. SSH uses a client-server model that connects an SSH server where the session ends with a secure shell client application that displays the end of the session.

The SSH protocol implementations often include support for application protocols used for terminal emulation or file transfers. SSH can also be used to create secure tunnels for other application protocols or to securely run Window System graphics sessions remotely. By default, an SSH server listens to standard Transmission Control Protocol (TCP) port 22.

Secure Shell Features


It allows users to remotely access SSH-enabled network systems or devices, as well as automatic operations.
Initiates a secure and interactive file transfer session.
Automatic and secure file transfer.
It realizes the secure transmission of commands to remote devices or systems.
Provides secure management of network infrastructure components.

SSH Client

SSH client usually refers to your own computer with which you want to connect with the server. To establish a connection to the remote server, you must install or use software that creates an SSH connection. These programs are often called SSH clients.

SSH Server

Most software for clients runs on servers. There is also software specially designed for SSH servers. It is common practice to initially launch SSH on servers. This ensures that the server can be accessed from outside via SSH at any time.

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What is Telnet Protocol in Networking?
Telnet is a TCP/IP protocol developed to connect to a multi-user machine on the Internet network from another remote machine and is the general name given to programs that do this work.

In order to enter the connected machine, you must have a user name there and a telnet access program for the connection to be made. However, some libraries and public telnet-based web services may not ask for a user name during the connection, or what you need to type a username and password will appear automatically when you connect.

Telnet is widely used today to access BBS (Bulletin Board Systems) systems over the Internet. Telnet access programs come with the operating system in most of today's operating systems. Multi-user operating systems usually offer users a text-based interface and all operations in these systems are performed from the command prompt via the keyboard.

When you connect to your virtual server with the Telnet program, you are connected to the remote UNIX operating system. This means you can type UNIX commands, run programs, edit your website as if you were sitting in front of the machine.

In Telnet programs, the address to be telnet can be entered from the command line, similar to the above, as well as through the menu system of the program. Some Telnet programs, which were very popular in the 1980s such as ZMODEM and KERMIT, also support computer-to-computer file transfer protocols.

Telnet is an insecure protocol. Telnet protocol sends your user name and password in PLAIN TEXT format, which is a format that can be easily seen on the network you are connected to. This means that your username and password can be easily seen by anyone listening to the network. If you trust everyone on your network, it is okay to use Telnet. But if you have even the slightest concern about security, it would be beneficial not to use telnet to connect to your account.

As mentioned before, the HTTP, FTP or Telnet protocol used in the Internet environment basically enables a user to connect to another computer remotely through a computer and to benefit from the resources as permitted by the protocols and the connected computer allows.

While there was no such progress in computer technology and the Internet environment, computers were working character-based. However, over time, character-based and cool screens left their places to graphics-based screens that emphasize visuality.

Web-based applications have spread to almost the entire Internet environment, and the servers providing only character-based services are very few. However, there are still servers that can only receive character-based service using Telnet.

However, the reason for using Telnet is not to be able to connect to a number of servers whose number has decreased significantly. The reason that distinguishes it from other protocols and makes it useful is that it is a protocol capable of Terminal Emulation.

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What is HTTPS (Secure Hypertext Transfer Protocol)?
What is HTTP?
HTTP (Hypertext Transfer Protocol) is the processing of information shared between client and server without any encryption. In short, there is http:// at the beginning of a website address, and the information you will share and your computer, in general, are not protected by security protocols and become vulnerable to external threats.

What is HTTPS?
HTTPS (Secure Hypertext Transfer Protocol) adds a security measure that allows encrypted transmission of any information between client and server. This is configured with a prevented SSL certificate.

This SSL Certificate, which should be included in banks, e-commerce sites that offer online shopping services, and similar websites, use different encryption methods for sending and storing personal information written on forms on payment pages and card information.

Thus, it provides a safe online shopping opportunity. In other words, e-commerce sites with https:// at the beginning have SSL certificates and guarantee card security in online shopping.

What are the Features of HTTPS?
On websites that do not have a security certificate and specified as HTTP, all data shared between the client and the server is transmitted unencrypted. These websites, which are not protected by any security protocol, remain open to external threats and problems may occur at any time due to lack of security.

HTTPS encrypts all data between the client and the server and provides a safe environment for both itself and its users with the security measures it takes. With the security protocol called SSL certificate, your website is protected against fiber attacks in the best way. Today, many e-commerce sites guarantee the security of their users' personal data by obtaining SSL certificates.

What are its Advantages?
It writes HTTPS in the URL of websites that secure its data with a security certificate, thus protecting against external threats. The advantages obtained with an SSL certificate can be briefly listed as follows:


Information is encrypted during transmission. In this way, the data of the users cannot be obtained by third parties. This allows users to shop on the website with peace of mind.
Since payment transactions are made on e-commerce sites, extra security measures must be taken. Otherwise, credit card information may be passed on to third parties. Therefore, thanks to the SSL certificate, all information provided during the payment is protected by an encryption method. Therefore, it is safe to shop from places specified as HTTPS.
The data upload speed of sites with a security certificate is much higher. Sites that are meticulous about SEO have the opportunity to rank much higher in the search engine with their SSL certificate.

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What is FTP (File Transfer Protocol)?
FTP is a file transfer protocol and is the most commonly used protocol for modifying, updating, and uploading files, folders, images, or text files on your website. FTP is the simplest way to send files and folders online from your computer to the server. For example, if you want to upload WordPress on your website, you can send your files to the server via FTP.

With the data transfer between two different computers, it becomes easier to reach the desired content. Moreover, this process, which is provided extremely quickly, creates the opportunity to use the time in the best way. The FPT server, which provides data exchange and connection between computers at the same time, thus obtains a wide range of use.

In order to use FTP, you must have the information to connect to your FTP server. This is the name or IP address of the FTP server is your username, password, and the port number of the server to connect to. You also need a program to send your files via FTP to the server.

There are certain FTP server programs prepared with care. These programs make it much more practical to access data via FTP server addresses. The desired file can be downloaded or uploaded within seconds. In addition, the target source can be scanned with the file explorer feature. Thus, it becomes easier to see under which file the desired data is located.

FileZilla, WinSCP, and CyberDuck are among the most popular FTP server programs. After entering the FTP address, it is necessary to fill in the user name and password information. After the requested information is completed, the main source can be accessed and the desired file can be imported from there. Programs that display the download process offer a user-friendly experience.

What is Anonymous FTP?
Some FTP services accept connections over the internet with an anonymous user who is active by default. In this way, any user can download the files within his/her authority to his/her computer or send any file on his/her computer to the other computer via FTP service in line with the competence of his/her authority.

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What is the OSI Reference Model?
The OSI model (Open System Interconnection) defines a computer network framework for implementing protocols at seven layers. A protocol in networking terms is a form of negotiation and rule between two networking organizations. The purpose is to define how communication between the two computers will be.

The OSI is a conceptual model that characterizes and standardizes the communication functions of a telecommunications or information processing system, regardless of its underlying internal structure and technology, and its purpose is the interoperability of various communication systems with standard protocols.

In simpler words, the OSI model is a tool used by IT professionals to actually model or monitor the actual flow of data transfer across networks. So basically the OSI model is a logical representation of how network systems should send data to each other.

The OSI Model divides this data communication procedure into different components. Its purpose is to guide vendors and developers so that the digital communication products and software programs they create can work together and facilitate clear comparisons between communication tools. Most vendors involved in telecommunications attempt to describe their products and services in relation to the OSI standard.

Moreover, it is very important for an IT professional to have a clear idea of the OSI Model. This is because they can narrow down and find out what part of the problem is using OSI Layers in case of certain network issues. That's why it's very useful to have an OSI Layered approach to troubleshooting networking issues.

Advantages of the OSI Model

It creates a common platform for software developers and hardware manufacturers that encourages the creation of network products that can communicate with each other over the network.
It helps network administrators by dividing the big data exchange process into smaller segments.
Because the layers are independent, it prevents changes in one layer from affecting other layers.
Standardization of network components allows for multiple vendor development.
Very well configures functions specific to each layer.
It reduces complexity and speeds up evolution.

Layers of the OSI Model

Physical Layer
Data Link
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer

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What is DNS (Domain Name System)?
DNS is the system that provides the connection between domain names and IP addresses of internet sites. In short, DNS converts domain names to IP addresses so you can access any website you want without entering complex IP addresses into your internet browser.

For example, to enter Google, one of the most visited websites in the world, you need to enter an IP address of 172.217.23.14. However, thanks to the DNS system, typing www.google.com into your internet browser will be sufficient. DNS servers connect the written domain name and IP address and open the Google website.

How Does DNS Work?
The DNS system works through servers located in many different parts of the world, called DNS servers. This system establishes a connection between the domain name and the IP address within seconds, without you realizing it. The DNS system basically consists of 4 sub-servers.

1. DNS Cursor

This sub-server is the first server you encounter in the DNS system. When you type a domain name in your internet browser, the domain name is greeted by the DNS cursor and subjected to a deep query. If the query is successful, further steps are taken.

2. Root Server

The domain name passing through the DNS cursor comes to the root server and begins to be resolved there. At this point, simple domain names, which we can see visually, reveal the complex IP addresses they carry. There are different types of DNS resolvers.

3.TLD Server

The TLD server, like the root server, is part of the parsing process. In this final resolution step, the domain name is converted to the IP address it represents. Every website is resolved by the .com TLD server at the end of the domain name.

4. Authorized Server

The place where everything happens is the authoritative server. The domain name was queried in the DNS cursor and resolved in the root and TLD servers reach the authorized server like the IP address. If the proxy server accesses the record of the IP address, the website opens. If there is no access, additional information is requested by the cursor.

DNS Resolvers
There are different types of DNS resolvers as we explained in the root server step. These; It is a recursive DNS resolver and authoritative DNS resolver. These two resolvers, which look similar, actually have quite different roles in the DNS system.

1. Recursive DNS Resolver

The recursive DNS resolver is the responsive resolver that can be repeatedly requested by the client. What this server does is to search the cache for the answers to the question marks in the resolution process until the domain name reaches the authorized server and reveals the necessary records.

2. Authorized DNS Resolver

The authoritative DNS resolver works with the authoritative server and is responsible for keeping all records. It is the last link in the long parsing chain. At the end of the process that resolves the domain name and reaches the IP address it represents, the authorized server opens the website without the need for another query thanks to the authorized DNS resolver.

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