Understanding Linux Server And Clients In Security
What Is Covered
An Introduction to Client-Server Communications
Servers are computers with the software installed that enables them to provide services to clients across the network. There are many types of services. Some provide external resources such as files, email messages, or web pages to clients upon request. Other services run maintenance tasks such as log management, memory management, disk scanning, and more. Each service requires separate server software. For example, the server in the figure uses file server software to provide clients with the ability to retrieve and submit files.
Client-server communications is discussed in more detail later in the course.
The image is a diagram that shows a server sending files to a client. Resources are stored on the server. A client is a hardware/software combination that people use directly. Files are downloaded from the server to the client.
Servers, Services, and Their Ports
In order that a computer can be the server for multiple services, ports are used. A port is a reserved network resource used by a service. A server is said to be “listening” on a port when it has associated itself with that port.
While the administrator can decide which port to use with any given service, many clients are configured to use a specific port by default. It is common practice to leave the service running in its default port. The table lists a few commonly used ports and their services. These are also called “well-known ports”.
| Port | Description |
|---|---|
| 20/21 | File Transfer Protocol (FTP) |
| 22 | Secure Shell (SSH) |
| 23 | Telnet remote login service |
| 25 | Simple Mail Transfer Protocol (SMTP) |
| 53 | Domain Name System (DNS) |
| 67/68 | Dynamic Host Configuration Protocol (DHCP) |
| 69 | Trivial File Transfer Protocol (TFTP) |
| 80 | Hypertext Transfer Protocol (HTTP) |
| 110 | Post Office Protocol version 3 (POP3) |
| 123 | Network Time Protocol (NTP) |
| 143 | Internet Message Access Protocol (IMAP) |
| 161/162 | Simple Network Management Protocol (SNMP) |
| 443 | HTTP Secure (HTTPS) |
Ports and their uses in network communications are discussed in more detail later in the course.
Clients
Clients are programs or applications designed to communicate with a specific type of server. Also known as client applications, clients use a well-defined protocol to communicate with the server. Web browsers are web clients that are used to communicate with web servers through the Hyper Text Transfer Protocol (HTTP) on port 80. The File Transfer Protocol (FTP) client is software used to communicate with an FTP server. The figure shows a client uploading files to a server.
The image is a diagram that shows a client uploading files to a server. A client is a hardware/software combination that people use directly. Files are uploaded from the client to the server for storage. Resources are stored on the server.
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