PC Networking

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

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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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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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.

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.

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.

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:

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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.

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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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.

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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.

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.

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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When you connect to a network, you usually have an IP address assigned automatically by the router. This process is necessary for devices to be able to communicate with devices on other networks. The switch service that assigns IP addresses is called Dynamic Host Configuration Protocol or DHCP.

DHCP works in a client-server model where a client broadcasts an IP address request to the local network and the DHCP server responds by assigning an IP address. Typically this happens as soon as the device connects to a network so it can start communicating as soon as possible.

When IP addresses are given, they are leased only for a certain period of time, then the DHCP server retrieves the assigned IP address. To avoid this, hosts will start renewing their current IP address allocation halfway through the lease term. Generally, if a lease expires and a new IP address is assigned to a host, a new one is randomly assigned, not the same as the previous one.

In-home and other small networks, the DHCP server is typically integrated into the router because this reduces the number of devices and reduces setup complexity. For large corporate networks, typically a single DHCP server will be used for multiple networks such as the entire building. In this case, DHCP relay agents are used in each interconnected network to forward DHCP traffic from the hosts to the DHCP server and forward the response back to the original host.

Typically, a DHCP server offers connected hosts a random IP address from a configured IP address pool, but it is also possible to manually assign an IP address to specific devices if required.

This is useful if there is some type of server on a network such as a NAS drive or a local webserver. By manually assigning an IP address to these servers, they are always in the same place and you don't have to worry about the IP addresses changing and causing problems.

DHCP Address Lease Stages:

The lease transaction between client and server consists of 4 steps;

If the client dynamically expects an IP when added to the environment, it does Discover for a DHCP server that can assign IP to it. The broadcast package that the client sends to the environment is called the DHCP Discover package. This process continues four times and if it cannot reach the server that will assign IP, it defines an address from the APIPA class.

If there is a DHCP server in the environment, it sends an Offer to the client. During this process, the server has reserved an IP for the client.

The client accepts the first offer returned to him and sends a Request package as feedback against this offer. At this stage, other servers sending offers to the client withdraw their offers.

In the last stage, the server confirms the previously reserved IP address for the client and sends the Acknowledgment package. Communication is always broadcast because the client does not have an approved IP address.

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All devices connected to the internet network have a specific address for data exchange. Internet Protocol Address, ie IP address, is the virtual address assigned to all computers connected to the Internet. IP addresses that provide internet network traffic are the most important part of the protocol that defines the working rules of the internet.

Internet protocol is the set of rules that determine the working process of the internet. This protocol controls the exchange of data between devices. In a sense, this system is similar to the legal system. Just as communication and relations in society are regulated by certain laws, communication on the internet is regulated by internet protocol. 

In order for the laws to be implemented in real life, people must have their identity information and addresses. In the virtual world, the IP address of every computer connected to the internet is defined. The internet address consists of unique numbers and is determined according to the computer coding system.

Computer systems and internet networks are based on numbers and coding. Each port on the Internet has a unique code. This code, the IP address, is a series of numbers divided into points. You must have a unique IP address for internet searchers to point to your site. Unique identification numbers are a unique language that ports use to find and understand each other. Traffic on the Internet depends on these IP addresses.

You can query to find out the IP address of your computer and other devices. You can see which internet service provider you are connected to and which network you are using by searching the IP address. You can use web tools designed for IP address query.

When you connect to the Internet, a private IP address is assigned to your computer or device you use. This address determines your connection location on the internet network. The last number of your IP address assigned to you by the service provider changes with each connection. Because these codes are dynamic codes. In the internet network system, the codes work continuously.

Dynamic coding is required for secure data exchange. Every device that connects to the Internet will have a completely unique IP address. You also have an IP address assigned to you by your service provider. You can find this address. You can change the name of the address as you wish. You can name your own link address. In the Internet access section, it is possible to change the IP address definition consisting of numbers and letters provided by your service provider. IP addresses are divided into public and private IP addresses. Even though the hidden addresses are in traffic on the internet, they cannot be detected by everyone.

Situations, where computers connected to the same network are identified with the same IP address, are called IP conflicts. This type of situation may occur when the IP address is changed. When there is an IP conflict, the computer cannot properly connect to the internet. Network connections will fail. It is not possible for different computers to connect to the computer network with the same IP addresses. 

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The word port, which computer users have heard in different areas as well as in daily use, is defined as the leading actor of a great stream. The port acts as a bridge for computer networks. This port, which has two protocols, TCP and UDP, is a core member that provides data flow between computers.

This port, which is frequently heard by users who are researched by going a little further than computer and internet use and focusing on solving the problems they encounter, is just one of the hundreds of technical computer terms.

All computers have a variety of IP addresses so that they can communicate with each other. IP addresses are divided into ports to create virtual buses for communication. This port, which is literally a port, is known as a bridge in computer networks. Port is one of the important members of data exchange between computers.

The port, which exchanges data over numbers, is divided into values starting from 0 up to 65535 in order to perform many operations at the same time. In this context, the port is divided into 2 types, TCP and UDP.

Port, the main function of which is to exchange data, is based on a number system. Each computer has an IP address and each IP address is divided into ports separately. This helps each IP address to exchange data for multiple purposes at the same time. Each program on computers works according to a port on a certain number.

An example of physical ports is all physical hardware inputs on the computer. Port relationships with IP are like the relationship between point A and point B to the route. Port is a gateway between IPs.

Port Forwarding is to stop or convert data to different tasks on the IP address of any computer or device. Port Forwarding is an extremely secure way to protect the IP address.

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UDP (User Datagram Protocol) is basically known as an alternative to TCP. UDP, which is a much faster protocol than TCP, is actively used for audio and video transmission.

The reason it works faster than TCP is that TCP is responsible for data integrity and authentication. UDP, unlike TCP protocol, is not responsible for data integrity and security.

It ensures fast transfer by enabling data transfer before an agreement is made with the receiving party. As a result, UDP is useful in time-sensitive communications such as voice over Internet Protocol (VoIP), domain name system (DNS) lookup, and video or audio playback. UDP is an alternative to the Transmission Control Protocol system.

The UDP protocol works similarly to TCP but discards all error checking items. All back and forth communication causes lag, which slows things down.

When an application uses UDP, packets are sent only to the recipient. The sender does not wait to make sure that the recipient has received the package, only continues to send subsequent packets. If the recipient occasionally misses several UDP packets, they are lost - the sender does not resend them. Losing all this overhead means devices can communicate faster.

UDP structure is used when speed is desired and error correction is not required. For example, UDP is often used for live streaming and online gaming.

Suppose you are watching a live video stream that is usually broadcast using UDP instead of TCP. The server just sends a constant stream of UDP packets to the watching computers. If you lose your connection for a few seconds, the video may freeze or jitter for a moment and then jump to the current bit of the stream. If you experience a minor packet loss, the video or audio may be distorted momentarily as the video continues to play without missing data.

This works similarly in online games. If you miss some UDP packets, player characters may appear to be teleported across the map while you receive new UDP packets. Since the game continues without you, there is no point in asking if you missed the old packs. The only thing that matters is what's happening on the game server right now, not a few seconds ago. Eliminating TCP's error correction helps speed up game connectivity and reduce latency.

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