MacDevCenter.com: Unix on Panther: Accessing the Internet

Authors' note: A network lets computers communicate with each other, sharing files, email, and much more. Unix systems have been networked for more than 25 years, and Macintosh systems have always had networking as an integral part of the system design from the very first system released in 1984.

This chapter introduces Unix networking: remotely accessing your Mac from other computers and copying files between computers. It also shows you how the Connect to Server capability of Terminal can make common connections a breeze once you've set them up the first time.

Remote Logins

There may be times when you need to access your Mac, but you can't get to the desk it's sitting on. If you're working on a different computer, you may not have the time or inclination to stop what you're doing, walk over to your Mac, and log in (laziness may not be the only reason for this: perhaps someone else is using your Mac when you need to get on it or perhaps your Mac is miles away). Mac OS X's file sharing (System Preferences → Sharing) can let you access your files, but there may be times you want to use the computer interactively, perhaps to move files around, search for a particular file, or perform a system maintenance task.

If you enable Remote Login under System Preferences -> Sharing, you can access your Mac's Unix shell from any networked computer that can run SSH (http://www.ssh.com/), OpenSSH (http://www.openssh.org/), or a compatible application such as PuTTY (a Windows implementation of SSH available at http://www.chiark.greenend.org.uk/~sgtatham/putty/). SSH and OpenSSH can be installed on many Unix systems, and OpenSSH is included with many Linux distributions, including Mac OS X.

Figure 8-1 shows how remote login programs such as ssh work. In a local login, you interact directly with the shell program running on your local system. In a remote login, you run a remote-access program on your local system; that program lets you interact with a shell program on the remote system.

When you enable Remote Login, the Sharing panel will display instructions for logging into your Mac from another computer. This message is shown in Figure 8-2.

To log into your Mac from a remote Unix system, use the command displayed in the Sharing panel, as shown in the following sample session where a user on a Red Hat Linux system is connecting to a Mac OS X computer (the first time you connect, you'll be asked to vouch for your Mac's authenticity):

To log in to your Mac from a Windows machine using PuTTY, launch the PuTTY application, specify SSH (the default is to use the Telnet protocol described later), and type in your Mac OS X system's IP address as shown in the Mac's Sharing panel. PuTTY will prompt you for your Mac OS X username and password. Figure 8-3 shows a sample PuTTY session.

Remote Access to Other Unix Systems

You can also connect to other systems from Mac OS X. To do so, launch the Terminal application. Then start a program that connects to the remote computer. In addition to ssh, some typical programs for connecting over a computer network are telnet, rsh (remote shell), or rlogin (remote login). All of these are supported and included with Mac OS X. In any case, when you log off the remote computer, the remote login program quits and you get another shell prompt from your Mac.

The syntax for most remote login programs is:

program-name remote-hostname

For example, when Dr. Nelson wants to connect to the remote computer named biolab.medu.edu, she'd first make a local login to her Mac named fuzzy by launching Terminal. Next, she'd use the telnet program to reach the remote computer. Her session would look something like this:

Welcome to Darwin!
~ 452 $ telnet biolab.medu.edu
  
Medical University Biology Laboratory
  
biolab.medu.edu login: jdnelson
Password:
  
biolab$
.
.
.
biolab$ exit
Connection closed by foreign host.
~ 453 $

Her accounts have shell prompts that include the hostname. This reminds her when she's logged in remotely. If you use more than one system but don't have the hostname in your prompt, see Section 1.3.1 in Chapter 1 or Section 10.1 in Chapter 10 to find out how to add it.

WARNING: Actually, Dr. Nelson would be unwise to use telnet to connect to the remote system, because ssh is a much more secure alternative and is highly preferred. However, some remote sites still stick with telnet, and while it's important to encourage them to switch to ssh-only access, you will still sometimes find yourself using telnet, as shown here.

Also, when you're logged on to a remote system, keep in mind that the commands you type will take effect on the remote system, not your local one! For instance, if you use lpr to print a file, the printer it comes out of may be very far away.

The programs rsh (also called rlogin) and ssh generally don't give you a login: prompt. These programs assume that your remote username is the same as your local username. If they're different, give your remote username on the command line of the remote login program, as shown in the next example.

You may be able to log in without typing your remote password or passphrase.[1] Otherwise, you'll be prompted after entering the command line.

Following are four sample ssh and rsh command lines. The first pair shows how to log in to the remote system, biolab.medu.edu, when your username is the same on both the local and remote systems. The second pair shows how to log in if your remote username is different (in this case, jdnelson); note that the Mac OS X versions of ssh and rsh may support both syntaxes shown depending on how the remote host is configured:

$ ssh biolab.medu.edu
$ rsh biolab.medu.edu
$ ssh jdnelson@biolab.medu.edu
$ rsh -l jdnelson biolab.medu.edu

About Security

Today's Internet and other public networks have users who try to break into computers and snoop on other network users. While the popular media calls these people hackers, most hackers are self-respecting programmers who enjoy pushing the envelope of technology. The evildoers are better known as crackers. Most remote login programs (and file transfer programs, which we cover later in this chapter) were designed 20 years ago or more, when networks were friendly places with cooperative users. Those programs (many versions of telnet and rsh, for instance) make a cracker's job easy. They transmit your data, including your password, across the network in a way that allows even the most inexperienced crackers to read it. Worse, some of these utilities can be configured to allow access without passwords.

SSH is different; it was designed with security in mind. It sends your password (and everything else transmitted or received during your SSH session) in a secure way. A good place to get more details on SSH is the book SSH: The Secure Shell, by Daniel J. Barrett and Richard Silverman (O'Reilly).

Transferring Files

You may need to copy files between computers. For instance, you can put a backup copy of an important file you're editing onto an account at a computer in another building or another city. Or, Dr. Nelson could put a copy of a data file from her local computer onto a central computer, where her colleagues can access it. Or you might want to download 20 files from an FTP server, but not want to go through the tedious process of clicking on them one by one in a web browser window. If you need to do this sort of thing often, you may be able to set up a networked filesystem connection; then you'll be able to use the Finder or local programs such as cp and mv. But Unix systems also have command-line tools for transferring files between computers. These often work more quickly than graphical tools. We explore them later in this section.

scp and rcp

Mac OS X includes both scp (secure copy) and rcp (remote copy) programs for copying files between two computers. In general, you must have accounts on both computers to use these. The syntax of scp and rcp are similar to cp, but also let you add the remote hostname to the start of a file or directory pathname. The syntax of each argument is:

hostname:pathname

hostname: is needed only for remote files. You can copy from a remote computer to the local computer, from the local computer to a remote computer, or between two remote computers.

The scp program is much more secure than rcp, so we suggest using scp to transfer private files over insecure networks such as the Internet. For privacy, scp encrypts the file and your passphrase.

For example, let's copy the files report.may and report.june from your home directory on the computer named giraffe.intuitive.com and put the copies into your working directory (.) on the machine you're presently logged in to. If you haven't set up the SSH agent that lets you use scp without typing your passphrase, scp will ask you:

$ scp giraffe.intuitive.com:report.may giraffe.intuitive.com:report.june .
Enter passphrase for RSA key 'taylor@mac':

To use wildcards in the remote filenames, put quotation marks ("name") around each remote name.[2] You can use absolute or relative pathnames; if you use relative pathnames, they start from your home directory on the remote system. For example, to copy all files from your food/lunch subdirectory on your giraffe account into your working directory (.) on the local account, enter:

$ scp "giraffe.intuitive.com:food/lunch/*" .

Unlike cp, the Mac OS X versions of scp and rcp don't have an -i safety option. If the files you're copying already exist on the destination system (in the previous example, that's your local machine), those files are overwritten.

If your system has rcp, your system administrator may not want you to use it for system security reasons. Another program, ftp, is more flexible and secure than rcp (but much less secure than scp).

The Unix ftp program does FTP transfers from the command line. There are also a number of easy-to-use graphical FTP tools available from the Apple web site (go to "Get Mac OS X Software . . . " from the Apple menu and click on Internet Utilities). But we cover the standard ftp program here. The computers on either end of the FTP connection must be connected by a network (such as the Internet).

To start FTP, identify yourself to the remote computer by giving the username and password for your account on that remote system. Unfortunately, sending your username and password over a public network means that snoopers might see them—and use them to log into your account on that system.

A special kind of FTP, anonymous FTP, happens if you log into the remote server with the username anonymous. The password is your email address, such as alex@foo.co.uk. (The password isn't usually required; it's a courtesy to the remote server.) Anonymous FTP lets anyone log into a remote system and download publicly accessible files to their local systems. Here's how that might look:

Command	Description
`put` filename	Copies the file filename from your local computer to the remote computer. If you give a second argument, the remote copy will have that name.
`mput` filenames	Copies the named files (you can use wildcards) from the local computer to the remote computer.
`get` filename	Copies the file filename from the remote computer to your local computer. If you give a second argument, the local copy will have that name.
`mget` filenames	Copies the named files (you can use wildcards) from the remote computer to the local computer.
`prompt`	A "toggle" command that turns prompting on or off during transfers with the `mget` and `mput` commands. By default, `mget` and `mput` will prompt you "`mget` filename?" or "`mput` filename?" before transferring each file; you answer `y` or `n` each time. Typing `prompt` once, from an `ftp>` prompt, stops the prompting; all files will be transferred without question until the end of the `ftp` session. Or, if prompting is off, typing `prompt` at an `ftp>` prompt resumes prompting.
`hash`	Displays progress marks on file uploads and downloads so you can gauge progress. Particularly helpful with large transfers.
`cd` pathname	Changes the working directory on the remote machine to pathname (`ftp` typically starts at your home directory on the remote machine).
`lcd` pathname	Changes `ftp`'s working directory on the local machine to pathname. (`ftp`'s first local working directory is the same working directory from which you started the program.) Note that the `ftp lcd` command changes only `ftp`'s working directory. After you quit `ftp`, your shell's working directory will not have changed.
`dir`	Lists the remote directory (like `ls -l`).
`binary`	Tells `ftp` to copy the file(s) that follow it without translation. This preserves pictures, sound, or other data.
`ascii`	Transfers plain-text files, translating data if needed. For instance, during transfers between a Microsoft Windows system (which adds Control-M to the end of each line of text) and a Unix system (which doesn't), an `ascii`-mode transfer removes or adds those characters as needed.
`passive`	Toggles the setting of passive mode. This may help ftp to run correctly if you are behind a firewall. If you put the command `setenv FTP_PASSIVE 1` in your .tcshrc, all your ftp sessions will use passive mode.
`quit`	Ends the `ftp` session and takes you back to a shell prompt.

Dave Taylor is a popular writer, teacher and speaker of business and technology issues. The founder of The Internet Mall and iTrack.com, he's been involved with UNIX and the Internet since 1980. He's also been a Mac fan since the year it was released.

Brian Jepson is an O'Reilly editor, programmer, and co-author of Mac OS X Panther for Unix Geeks and Learning Unix for Mac OS X Panther. He's also a volunteer system administrator and all-around geek for AS220, a non-profit arts center in Providence, Rhode Island. AS220 gives Rhode Island artists uncensored and unjuried forums for their work. These forums include galleries, performance space, and publications. Brian sees to it that technology, especially free software, supports that mission. You can follow Brian's blog here.

Unix on Panther: Accessing the Internet

Remote Logins

Web and FTP Access

Remote Access and the Outside World

Remote Access to Other Unix Systems

About Security

Transferring Files

scp and rcp

FTP

Command-line ftp

Table 8-1. Some ftp commands

SFTP: FTP to secure sites

FTP with a web browser

Other FTP solutions

Easy Shortcuts with Connect to Server

Practice

Footnotes