Linux+: Operating System Intro 06 – Linux Networking Networking is a very important part of businesses and even in homes. Being able to share resources among computer systems can be imperative. Even if a local network is not needed some rely on the International Network (Internet). For systems in a local environment, which are connected by some type of network medium, files can be shared between systems. Files are shared by using the Network File System (NFS) protocol. NFS allows a non-local directory and file structure to be mounted as if it were local. The NFS system is usually referred to as a File Server. The resources are the files, or more generically, the Hard Disk Drive (HDD). In this case, the client systems can contain smaller HDDs while the File Server has a large HDD capacity. NOTE: By centralizing the files for the users on a File Server only one system needs to be backed up. When only one system is backed up in the case of an HDD failure or a major catastrophe, time and money is saved. It would require more time and money to equip each system with a backup device and the time to perform the routine file backup. Other resources can be shared among a local network. These resources are not limited to CD/DVD drives, printers, modem banks, gateways to the Internet, network scanners, etc. For the Internet other types of resources can be shared. It is possible to share these resources on a local network as well, but they do tend to be used more on the Internet. NOTE: Keep in mind that the other types of servers do work in a local networked environment. For example, a web server can be placed on a local network of a business for employees to view current company news which may not be wanted on the Internet for everyone to access. There are six basic server types as follows: Domain Name Service (DNS) Server Electronic Mail (E-Mail) Server File Server Firewall Server Web Proxy Server Web Server Domain Name Service (DNS) Server Services within a company may require the use of a Domain Name. A Domain Name is something like Linux.Org. Within a company, a Web Server or E-Mail server can be set up to allow in-house employees access specific information. When the client opens a web browser and types the in-house company web address they will be directed to the Web Server and the page will load. For example, if the internal Domain Name structure was mycompany.com you could access the Web Server if the server name was WebServer at WebServer.mycompany.com. Some applications, such as Internet Browsers, rely on Domain Names to operate just as they do on the Internet. These applications connect to a server on the local network using the server’s Domain Name. The functionality needed requires a Domain Name Server to provide the server’s IP Address for the given Domain Name. For a DNS Server application, go to isc.org/products/bind. NOTE: For more information on DNS, see TCP/IP Service: Domain Name System (DNS). Electronic Mail (E-Mail) Server E-Mail is a very crucial service for most companies. It allows mail to be sent in-house as well as to clients on the Internet. The best part of E-Mail is that it is free to send and receive without having to spend a lot of money on stamps as things used to be done before the Internet. E-Mail is made up of three different protocols: Post Office Protocol (POP) Internet Message Access Protocol (IMAP) Simple Mail Transfer Protocol (SMTP) POP and IMAP are used to access an E-Mail Server from a client application. A good application to support this feature is Dovecot. It can be found at dovecot.org. SMTP is the protocol used to transfer E-Mails between servers. A good application to support SMTP is Postfix. The application can be downloaded at postfix.org. NOTE: For more information on POP, IMAP and SMTP, see TCP/IP Protocol: Electronic Mail (E-Mail) Protocols. File Server File Servers are basically any system, usually a Server version of a Linux distro, which is set to share certain folders. The great thing about File Servers is that security can be implemented to provide some type limitations to certain files. Access to folders and files can be allowed to anyone on a network, but usually there are limits placed on them. Set limits on files and folders as needed. A good server version of a Linux distro, such as Ubuntu, can be found at www.ubuntu.com/download/server. NOTE: For file sharing on the Internet, use a File Transfer Protocol (FTP) Server. Firewall Server It is imperative for networks to remain separate from the Internet. No one wants people on the Internet to have access to company servers. Firewalls provide a barrier to block unwanted traffic to and from the Internet. Data transfers over the Transmission Control Protocol/ Internet Protocol (TCP/IP) use Ports. Applications can be used on a Firewall Server to limit or stop traffic on these Ports. For example, a Firewall can be set to only allow Web browsing by enabling access through Port 80. By blocking all other Ports E-Mail and File Transfers through the File Transfer Protocol (FTP) would not be possible. A good firewall application is iptables found at www.netfilter.org/projects/iptables/index.html. NOTE: A basic overview of TCP/IP can be found here: Transmission Control Protocol/Internet Protocol (TCP/IP). Basics on TCP/IP version 6 are at Internet Protocol version 6 (IPv6) Basics. Web Proxy A Web Proxy is useful for companies that have limited bandwidth for the employees to go to the Internet. A Proxy Server requests web pages and stores them for a specified time before deleting the page from the local cache. When a network client requests a web page the Proxy Server checks its cache for the existence of the page. If it is there it sends the page from cache. Because of this, the bandwidth is preserved for other requests which are not in the cache. Network clients receive cached web pages faster than when it has to be retrieved from the Internet. It is possible for the Proxy Server to also cache downloaded files. The Proxy Server can also log all visited web sites and from which system the request was sent. In this way, it is possible to track users who may be going to inappropriate web sites during work. A good Web Proxy application is Squid found at www.squid-cache.org. Web Server Web Servers are widely known on the Internet. However, it is not widely known that a Web Server can be placed on a company network and allowed to be accessed only by the systems on the internal network. Companies can have private web sites accessible only to employees. Some companies have their business databases accessible through web pages by the employees which can include taking orders over the phone or for billing purposes. A Web Server contains web pages accessible by web browsers. All sites on the Internet accessed by a web browser to view pages are a Web Server. A good Web Server is the Apache HTTP Server found at httpd.apache.org.