Linux Servers

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DevynCJohnson

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Servers are important in the industry and the field of computers. Servers are an important component in networks. Linux, like any system, supports networking and can be used as a server. There are numerous server types. A variety of server types exist due to the many kinds of needs clients and networks may require. Thankfully, Linux supports all of them.

Some of the most popular distros to use as a Linux server include Ubuntu, RedHat, CentOS, OpenSuse, Mandriva, Xandros, Debian, and others. No matter which distro is chosen, there are many server types that an admin may choose based on the network's needs. Also, a single Linux machine can be multiple server types at once.

NOTE: CentOS is RedHat without the paid support and any proprietary software.

Application Server - A server that has the sole purpose of running some type of software.

Cache Server – A cache server stores previously visited webpages. A network in a business may query the cache server for a webpage. If the webpage is not cached on the server, then the server will get the webpage and send it to the client. Cache servers can help increase perceived Internet speed since previously accessed pages are cached on the local network. Squid (http://www.squid-cache.org/) is an example of a cache server.

Database Server - A database server provides databases services to clients. The server has some type of database or multiple databases (SQL, JSON, etc.). Some databases server software includes Ingres, PostgreSQL, MySQL, Informix, and many others. Some database server software is cross-platform and others can only be installed on certain systems. Database server may be used to track and store information such as accounting, banking, products/merchandise data, sales, etc.

DHCP Server - DHCP servers assign IP addresses to clients on the network. This saves time since admins and users will not need to manually configure their IP addresses.

DNS Server - DNS servers allow Domain Names to be resolved to IP addresses. Examples of DNS server software includes BIND and djbdns. DNS servers can be public (like the ones used for the Internet) or private (like within a company network).

Fax Server - Offers fax services to clients. Fax servers are useful on networks with many fax requests. HylaFAX (http://www.hylafax.org) is a specific example of software used by Linux to become a fax server. HylaFax is capable of using Fax over IP (FoIP). To install HylaFAX on Debian-based distros, type "apt-get install hylafax-server". If it is not found, then a repo/PPA may need to be added.

File Server
- Provides files. Many servers match this description such as FTP, HTTP, Samba, and other servers. Some FTP server include ProFTPD, Pure-FTPd, and vsftpd.

Game Server
- Some games (like Minetest) can be hosted on a server. Such servers provide a way for many client (players) to join the same game.

Intrusion Detection Systems (IDSs) - These servers monitor networks for malicious activity and log such events.

POP3 and IMAP (incoming mail) - Servers that receive mail (mainly to be sent to the recipient client/user) may use the POP3 or IMAP protocol. Some examples of outgoing mail servers include qpopper, UW IMAP, Courier-IMAP, and others.

Print Server - A print server provides print services to clients. Such print servers may be real/physical such as a hardware printer or virtual like a PDF-printer. Samba servers are an example of a printer server that serves Linux and Windows clients.

LDAP Server - Provides LDAP services. LDAP servers offer information directories. This allows a user to login to a network once and then have access to all of the resources. (http://www.linux.org/threads/tcp-ip-protocol-lightweight-directory-access-protocol-ldap.5047/)

Monitoring Server - Some servers monitor a network for certain activity. For instance, a Multi Router Traffic Grapher (MRTG) server keeps statistics concerning network traffic. SNMP (Simple Network Management Protocol) servers watch networks for important events that may require an admin's attention.

NTP Server - NTP servers synchronize the time/clocks on the networks. This is important because some network services or processing may be dependent on precise time. Also, when viewing logs for issues, it may be important to know specifically where the error occurred first. If some of the computers have their time off by a few seconds, this can make it difficult to track the issue. For instance, if a client and a server crash a few seconds apart from each other, but the time is not synchronized, then how will the admin know which crash caused the other system to crash? On very large networks that span across a country or the world will need to be synchronized, especially if the data is to be synced with the newest data. "ntpd" is a popular NTP server daemon for Linux and other Unixoid systems. An alternative to the NTP protocol is SNTP (Simple Network Time Protocol) which is a more lightweight and simplified NTP.

Proxy Server – Proxy servers get data and send requests on behalf of the client. Proxies an be used to protect privacy, control content, log client activity, data caching, etc. Cache servers are a special form of proxy server. Some proxy server software includes Polipo, Varnish, Nginx, HAProxy, and others.

Router - Routers are specialized servers. They route and transfer data packets and traffic. Routers may be an embedded device, laptop, or a full-sized server. Routers may use a variety of protocols such as Routing Information Protocol (RIP), Virtual Router Redundancy Protocol (VRRP), Cache Array Routing Protocol (CARP), and others.

SMTP (Outgoing mail) - Servers that send/deliver emails to clients typically use SMTP. Examples of SMTP servers include exim, postfix, qmail, sendmail, and others.

Sound Server - These servers provide multimedia broadcasting and streaming. Many "Internet Radio Stations" are sound servers. Icecast and Jamendo are examples of sound servers. Also, "SoundCloud.com" is a sound server with a web server interface.

SSH - Secure Shell provides a security tunnel which may be used to transfer files, X11, remote login (like VNC), etc. An example of an SSH server daemon is OpenSSH.

Tripwire - Tripwire is a special network service that monitors files on a network and then reports malicious or invalid changes/differences. This helps to detect malicious code or intrusions.

VNC - VNC servers allow a client to access the software and data of a remote computer (the server). Clients using one operating system can use the software of a different operating system without virtualization. An admin could setup dummy clients that are meant to access a different operating system depending on the needed task. Assume a business has a piece of software that only works on MS-Windows and other software on a Linux system. The dummy clients can login to the needed operating system depending on the needed software.

Virtualization Server - This is a server that specializes in virtualization of multiple operating systems. VMware Server is a popular example. An alternative is Xen. Xen is a hypervisor that provides virtualization. Citrix XenServer (http://www.citrix.com/products/xenserver/overview.html), XenServer (http://www.xenserver.org/), and Amazon EC2 Cloud (http://aws.amazon.com/ec2/) are two examples of Xen servers.

Web Server - Web servers provide websites. A popular web server is Apache, which is open source and free.
 

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