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1.21. Unix Networking and Communications

Generally speaking, a network lets two or more computers communicate and work together. Partly because of the open design of Unix, a lot of networking development has been done in this operating system. Just as there are different versions of Unix, there are different ways and programs to use networks from Unix.

There's an entire chapter devoted to Connectivity (Chapter 46), but for now, here's a quick review of the major networking components.

The Internet
The Internet is a worldwide network of computers. Internet users can transfer files, log into other computers, and use a wide range of programs and services.

The World Wide Web is a set of information servers on the Internet. The servers are linked into a hypertext web of documents, graphics, sound, and more. Point-and-click browser programs turn that hypertext into an easy-to-use Internet interface. (For many people, the Web is the Internet. But Unix lets you do much more.)

A Unix facility that's been around for years, long before networking was common, is electronic mail. Users can send electronic memos, usually called email messages, between themselves. When you send email, your message waits for the other user to start his own mail program. System programs can send you mail to tell you about problems or give you information. You can send mail to programs, asking them for information. Worldwide mailing lists connect users into discussion groups.

The ftp program is one way to transfer files between your computer and another computer with TCP/IP, often over the Internet network, using the File Transfer Protocol (FTP).

Unix-to-Unix Copy is a family of programs (uucp, uux, uulog, and others) for transferring files and email between computers. UUCP is usually used with modems over telephone lines and has been mostly superceded by Internet-type connections.

Usenet isn't exactly a network. It's a collection of hundreds of thousands (millions?) of computers worldwide that exchange files called news articles. This "net news" system has thousands of interactive discussion groups -- electronic bulletin boards -- for discussing everything from technical topics to erotic art.

This utility logs you into a remote computer over a network (such as the Internet) using TCP/IP. You can work on the remote computer as if it were your local computer. The telnet program is available on many operating systems; telnet can log you into other operating systems from your Unix host and vice versa.

This starts a "remote shell" to run a command on a remote system without needing to log in interactively. If you don't give a command, rsh acts like rlogin. This is often used to start remote X Window System (Section 1.22) programs whose display opens on your local system. Section 6.10 has examples -- as well as details on problems you can have running rsh for any application.

ssh acts like rsh (and rlogin), but it makes a secure encrypted connection to the remote computer. It also can encrypt X Window System (Section 1.22) connections, as well as other types of connections, between hosts. The utility ssh-agent allows remote logins without typing a passphrase. We've included an entire chapter on ssh (Chapter 51).

This is a "remote cp" program for copying files between computers. It has the same command-line syntax as cp except that hostnames are added to the remote pathnames.

This is a secure version of rcp that uses the ssh protocol. ssh-agent works here, too.

NFS isn't a user utility. The Network FileSystem and related packages like NIS (the Network Information Service) let your system administrator mount remote computers' filesystems onto your local computer. You can use the remote filesystem as easily as if it were on your local computer.

This sends messsages to another user's screen. Two users can have a discussion with write.

A more sophisticated program than write, talk splits the screen into two pieces and lets users type at the same time if they wish. talk can be used over networks, though not all versions of talk can talk to one another.

Internet Relay Chat allows multiple users to carry on multiple discussions across the Internet and other networks. One popular IRC client is irc.

-- JP

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