Servers

The following set of notes corresponds to the Servers module of the Internet Design course, providing students an outline of the information they are expected to understand. The corresponding reading for this module is chapter two of i-Net+ Study Guide by David Groth, et. al., pages 128 - 163.

HTTP Servers

• HTTP servers are responsible for serving text and graphical content using Hypertext Transfer Protocol.
• An HTTP or web server runs an HTTP daemon, which is a program responsible for responding to requests from web browsers.The server may also be configured to process different types of scripts.
• An Internet web server is designed and configured to provide web content to the general public.
• An intranet server is configured to provide web content to authorized users only. Intranet servers are usually located behind a firewall within a local LAN and intended for use by only the members of the organization or company.
• An extranet server is an intranet server that also permits some limited access by the public or other authorized users outside the local area network. These servers are popular for business partners who wish to share information.
• A web server designed to provide e-commerce services may be called an e-commerce server. These services, which are supported through special programs or scripts, may include inventory searches, a shopping cart program, credit card verification, and an automatic email confirming your order.
• There are three main HTTP daemons in use today. These are HTTPd, Microsoft IIS, and Apache. Currently the most popular is Apache, which is on over 60% of all servers.
• The web server software HTTPd (HTTP daemon), was the original HTTP server software. HTTPd was developed by the National Center for Supercomputing Applications for the Unix operating system and is free, open source software.
• Microsoft's IIS (Internet Information Server) is packaged with Windows NT and Windows 2000. It provides a user-friendly graphical interface but works only on Windows NT or 2000 servers.
• Apache is a free, open source HTTP daemon built on HTTPd. Although originally designed for use on Linux / Unix systems, it is also available for use on Windows NT and OS/2. Apache is currently the most widely used HTTP daemon on the Internet.
• If the traffic on a web server becomes too heavy, the site may be mirrored. Mirroring a site means placing a copy of the site on another web server.

FTP Servers

• An FTP server runs an FTP daemon in order provide file services (download, upload, delete)using the FTP protocol.
• An FTP server requires a name and password from the user before allowing file transfers. For public downloads, "anonymous" transfers are permitted with the user name "anonymous" or "ftp" and any text for the password.
• Two popular FTP server programs are FTPd (FTP daemon) and Microsoft's IIS. FTPd is a free Unix-based program in wide use, while IIS is designed for specifically for use on Windows NT or 2000 server systems.

SMTP Servers

• SMTP (Simple Mail Transfer Protocol) servers allow users to send and receive email through the Internet.
• An SMTP server runs a daemon responsible for locating the IP address of the recipient email account, verifying that mail has been delivered, displaying an appropriate error message if mail is undeliverable, retrying to send mail if the recipient server is busy, and generally getting the mail through.
• Mail servers may also provide SMTP relaying services, which forward email coming from one SMTP to a different SMTP.
• Mail servers may also be used as list servers, allowing email messages to be sent to all the recipients who have signed up on a list, and allowing people on that list to in turn reply to the list. The email address for list servers are often majordomo@domain.com.
• The most commonly used SMTP daemon is Sendmail, another product of open source software.

Proxy Servers

• Proxy servers provide a means for workstations on a secure intranet or private LAN to access the Internet while preventing Internet traffic from accessing the private LAN.
• A proxy server sits between the Internet server and the client workstation, processing the requests made by the client and determining whether to pass those requests on to the Internet server or deny the request. Similarly, the proxy server can manage some of the requests sent from the Internet server. Proxy servers can be configured as to what types of services they permit.
• Since proxy servers can provide packet filtering, they can be used to restrict the types of files that network users access over the Internet. For example, a proxy server may be set to prohibit any MP3 files from being downloaded.
• Proxy servers may also provide caching services. With active caching, the server uses low activity periods to download and store documents that may be requested by network users, while passive caching determines whether or not to store documents as they are requested by users on the network.

NNTP Servers and Newsgroups

• NNTP (Network News Transfer Protocol) servers are used to provide access to newsgroups.
• Newsgroup servers store a variety of newsgroups, and can be configured to communicate with other NNTP servers so that each server has a copy of all the messages available within the newsgroup.
• If a newsgroup is moderated, any new messages posted to the newgroup are first reviewed by a moderator, who determines whether to publicly post the message or discard it.
• Newsgroups may be configured with an expiration date, thereby automatically erasing messages after they reach a designated age.

Other Servers

• A server running a telnet daemon is a telnet server. The telnet daemon, used mostly on Linux / Unix systems, allows a remote user to log onto the server and type in commands as if he were sitting directly at that computer.
• A directory server provides access to directories of information, such as phone books, email address lists, and various other directory-type information. Directory servers are also known as LDAP servers, since most conform to the Lightweight Directory Access Protocol.
• Mirror servers are backup servers that duplicate everything that takes place on the main server so that they can immediately replace the main server if it fails.
• A single computer may run a variety of daemons and thus be more than one type of server. For example, an HTTP server may also be a telnet and FTP server.