Upgrading and Repairing Networks (5th Edition)

Several files are generally associated with the user logon process for Unix systems. These files can be located in different directories, and the fields within some of these files can vary from one implementation to another. However, the following two files are generally used:

• /etc/passwd This is the password file. It is used to store the username, the password (in encrypted format), and other information specific to the user account. This file has its file protection value set to be world-readable so that anyone can access the file when logging in to the system. It also means that when someone gets into your system, he or she can usually copy this file and then begin to crack the passwords it contains.

• /etc/groups This file contains a list of user groups and a numerical value associated with each group. A field in the /etc/passwd file references a group in this file using this value.

The /etc/passwd File

The Unix operating system usually authenticates users by comparing their credentials with those stored in one or more files on the server. This is similar to the method used for the older Novell-based bindery, in which users must authenticate to each server they want to access. The typical username/password exchange is used, and the /etc/passwd file is the standard file used to store most user information. It is a simple text file that stores data using ASCII characters, and it's world-readable because access to the file is required during the logon process.

The fields in this file store information such as the username, the home directory, the default shell, and an encrypted password, among other things. This file is one of the most vulnerable and soughtafter files by hackers. You might think that it's a safe file because the password field in this file is encrypted. Not true! After a hacker has access to this file, a large number of utilities can be downloaded from the Internet to run against a password file to decrypt the password. Many hackers just use a dictionary and known encryption techniques and then compare the result with the value found in your /etc/password file. When a match is found, the hacker knows your password for that account.

Keep in mind that this file is world-readable. That means after someone has broken into even the most restricted account, if they can get to a shell command prompt, they can most likely copy this file and use it to further compromise accounts that have been granted much greater access rights to the system.

Note

This chapter covers files used to secure individual or groups of Unix servers and workstations in a network. It should be obvious that managing a large number of workstations, even using things such as NIS, can be a difficult task from a security standpoint. For this reason, every network that connects to another outside network, or the Internet, needs a good firewall. The authentication files discussed in this chapter help protect an individual Unix system. A firewall can help protect the entire network from outsiders. Chapter 45, "Firewalls," contains more information on this important topic.

After the root password is discovered, or the password to any account that has administrator-equivalent privileges is discovered, your system is wide open to attack. This is just one of many reasons why it is very important to use a long, meaningless, and complex password consisting of a mix of letters, numbers, and symbols. Password complexity makes it more difficult to use either dictionary or brute-force attacks to break a password. However, don't make your password so difficult to remember that you have to write it down.

This is the format for the /etc/passwd file, on most systems:

username:password:uid:gid:GECOS:homedir:shell

Note that the colon character (:) is used to separate fields. If a field is to be left blank, you'll see two colons in a row. The fields in this file are detailed here:

• username The account name used to log in to the account.

• password The encrypted password for the user account. An asterisk character (*) in this field means that the account is disabled. If this field is left blank, no password is required for the account. Unless you have a very good reason, you should not have any account with a blank password on a networked computer. Any access can usually lead to further penetration by a clever user. An x character in this field generally means that a shadow password file, discussed later, is in use.

• UID A numerical value that the system gives to the account to identify the user when running processes or evaluating access to files and other system resources. A value of zero for this field is used to indicate the superuser, or a user who has the same privileges as root. On some systems values from 1 to 99 are reserved for use for system processes, such as background daemons.

• GID A numerical value that identifies a user group to which the account belongs. The file /etc/group contains a listing of user groups and the numbers associated with them. Group membership can be used to make managing access to system resources, such as files and directories, an easier task. Access to a resource can be granted to the group. The alternative method is to grant access individually, which is a time-consuming process when you have a large number of users who access similar resources.

• GECOS Yet another computer acronym! GECOS stands for General Electric Comprehensive Operating System. It is used to hold comments about the user, such as office and telephone number. This field can also be used to hold text that is used by certain applications, such as the finger utility. If more than one item is included in this field, commas should separate the items. It is common to store the user's full name in this field.

• homedir This text field specifies the user's home directory. When the user logs in to the system, he or she is initially placed in this location in the file system. As with most operating systems, a separate home directory is maintained for each user for storing his or her own files. Home directories also can contain subdirectories to make organizing one's files a simple task.

• shell The user on a Unix system interacts with the system using one of the many shell applications available for Unix today. This field in the /etc/passwd file is used to specify the shell program that will be invoked when the user logs in to the system.

A typical entry in the /etc/passwd file looks like this:

jdoe:Gfjhjo9Uia$jpo2dYtaGGdsh:223:100:John Doe:/home/jdoe:bash rsmith:HuiTytsm$ld34tTbd9Saa2:119:110:Rob Smith:/home/rsmith:bash

Note that the second field, the password field, appears to have nonsense characters. This is the encrypted password that is highly prized by network intruders.

Using a Shadow Password File

To plug the password security hole presented by the /etc/passwd file, a technique called a shadow password file is usually used. This file contains the actual passwords, also in encrypted format. However, the shadow password file is not world-readable, and permissions are set so that only the root account can access this file.

On many systems, the name of the shadow password file is /etc/shadow. Check your documentation to determine the exact path on your system. Also note that on some older systems (and a few current ones), you'll have to load an extra component to install the shadow password file capabilities. Keep up-to-date by consulting your vendor's Web site.

The contents of this file also can vary from one system to another. However, the following format contains the fields used in most implementations:

username:password:last:may:must:warn:expire:disable:reserved

The following list describes these fields:

• username This field is used for the same purpose as it is in the /etc/passwd file. It is the login name for this user's account.

• password The user account password stored in encrypted format.

• last The number of days, since January 1, 1970, that the password for this account was changed.

• may The number of days that must pass before the password for this account can be changed.

• must The number of days after which the password for this account must be changed.

• warn The number of days before the password expires to warn the user about the upcoming password expiration.

• expire The number of days that must pass before the account password expires and the account becomes disabled.

• disable The number of days, since January 1, 1970, that the account has been disabled.

• reserved This field is reserved for future use.

As you can see, using a shadow password file gives you additional control over accounts, such as setting password expiration values. It also gives you informational fields (such as last and disable that can be used when managing user accounts.

The /etc/groups File

This file contains a list of user groups and a numerical value used to identify the group. The syntax for entries in this file is as follows:

groupname:grouppassword:groupID:username1,username2 ...

• groupname A name associated with the group. Using a meaningful name can help you simplify user administration.

• grouppassword Yes, you can place a password on a group, but this is generally not done. Instead, this field is usually left blank.

• groupID This is the group's ID number, which is used by the operating system to identify the group. It is often referred to as the GID. This number can range from 0 to 32,767, and the numbers 0 to 10 are generally reserved for system groups. For example, in most Unix implementations, the root user has a GID of zero.

• username1,username2 ... This is a comma-delimited list of members of the group. Separate each using a comma.

Users can be members of more than one group. If this is the case, the first group is the user's primary group, and the GID of this group is used when files are created or saved. Users can use the groups command to view the group(s) of which they are a member. The command chgrp can be used to change the current default group.

Adding or Removing User Accounts

To add or remove a user from a Unix/Linux box, you can edit the password and group files. However, whenever you make an edit to such an important file, there is always the chance that something will go wrong and you'll end up rendering an account, or possibly a system, unusable. It's advisable to always make a backup of an important file before making any edits.

After making entries in the /etc/passwd and /etc/groups files, you'll have to create the user's home directory and install any files that are part of your standard distribution, such as shell files.

However, on most systems you won't have to go through all this trouble. Instead, most versions of Unix or Linux provide a program that can be used to perform all the functions needed to add or remove a user. On FreeBSD Unix, for example, the adduser command can be used. The syntax for this command is as follows:

adduser [-dDv] [-c changetime] [-C class] [-e expiretime] [-g primarygroup] [-G gecos] [-h homedirectorybasedir] [-H homedirectory] [-m homedirectorymode] [-p passwd] [-P encryptedpasswd] [-s shell] [-S skeletondir] [-u uid] [username...]

• -v This is the typical Unix/Linux "verbose" option. It causes the adduser command to output more information about its operations. Good to use if you're new at this.

• -d This is equivalent to the rmuser command (remove user) described later in this section.

• -D If you use the uppercase "D" character, the adduser command does not actually add a user. Instead, it sets defaults to use the next time adduser is invoked.

• -C class This specifies a login class for the user. FreeBSD Unix provides a file called /etc/login.conf that stores class definitions for users. Classes can be used to customize authentication methods and the user environment.

• -e expiretime This field is not generally used. It is intended to set an expiration time for the password.

• -g primarygroup This sets the user's default group value. If you do not include this on the command line, adduser will prompt you for it.

• -G gecos This is the GECOS comment field. You'll be prompted for this value if it is not included on the command line. Substitute text after the G command that you want to appear in the GECOS field.

• -h homedirectorybasedirectory Use this to specify the home directory for the user. The value of homedirectory is the base directory under which the user's directory is created using the username.

• -H homedirectory This version of the home directory option lets you specify the complete path of the user's home directory.

• -m homedirectorymode Use this to specify the mode for the user's home directory. The default is 0775.

• -p passwd This option enables you to enter, in clear text, a password for the user account.

• -P encryptedpasswd This option enables you to enter, in encrypted format, a password for the user account. You must use either -p or -P. If neither is used, you will be prompted for a value.

• -S skeletondir You can set up a "skeleton" directory to serve as a template to use when creating user home directories. This option enables you to specify the name of the template to use for this user's home directory and possible subdirectories.

• -u uid Enter the user's ID (UID) with this option. FreeBSD starts ordinary users' UID at 100 and increments the value. Other flavors of Unix/Linux may use different starting values.

• username ... You can enter one or more user account names separated by spaces.

If you enter the adduser command with no command-line arguments, the command will prompt you for the information it needs (such as a username) to create the new account. You can use the large number of options available with the command to construct your own command to add users. If you need to add a large number of users at a time, or if you use the same command-line options frequently, create script files that can be used to invoke the adduser command for your customized requirements.

By using this command, you not only make the necessary entries into the /etc/passwd and /etc/groups files, but also create the user's home directory.

The rmuser command can be used to remove a user. Simply follow the command with the username that is to be removed. The -v option is the only other command-line option available with this command.

The addgroup command works similar to the adduser command and helps automate the process of managing entries in the /etc/groups file. The syntax for this command is much simpler because there are fewer fields in the groups file, and no directories or other data structures need to be created. The syntax for this command is as follows:

addgroup [-vd] [-g gid] [-m members] [groupname...]

• -v Again, this causes more output to be displayed during the execution of the command. Useful when you are first learning to use the system.

• -d Similar to the adduser -d command, this command is the equivalent of using the rmgroup command that is used to remove a group. For example, addgroup d groupname will remove that groupname from the /etc/groups file.

• -g gid This is used to specify the group ID (GID) for the group you are adding. If you omit this, the next available (unused) group number will be used.

• -m members Use this to specify the initial members of the group, separating each by a space when more than one member is entered.

• groupname This is the name you want to give to the group.

Other versions of Unix/Linux have similar programs. For example, Compaq's True64Unix uses the useradd command. The syntax is similar to that of the adduser command, but a few other options are available. For example, the -x option enables you to further specify options relating to NIS, password expiration times, and so on. Red Hat's Linux distribution enables you to use a similar useradd command. However, it also offers a more complex tool called linuxconf, which can be used in text mode, and also in a GUI mode using an X Window Systems interface or a Web browser. This tool goes far beyond simple user configuration tasks, including options for managing groups, file systems, system services, and many other objects.

Check the documentation of your Unix or Linux version to determine the commands and exact syntax for any script files or other utilities that can be used to automate user management.

Tip

In addition to any printed documentation you get with the Unix or Linux product you purchase, you can get online help. The command MAN followed by a topic will display text for the command you specify. The term MAN comes from Manual Pages. Consider this as a first resort when you need help with command syntax, and so on.

Using a Linux GUI Utility to Manage Users

Both Unix and Linux systems come with a graphical interface that can be used to perform many of the functions you can do at the command line. Some of these utilities provide all the functionality of the command-line version, whereas others offer a subset. For example, in Figure 38.1 you can see an example from Linuxthe Red Hat User Manager.

To bring up this utility, click on the Red Hat (the equivalent of the Windows Start button), and then from the menu click on System Settings and finally Users and Groups.

The only account that exists on this computer now is the root account, which exists on the computer when it is first booted. It was set up during the operating-system installation. As the "superuser" account, this has the rights and privileges to do anything on the computer, depending on what mode the Linux box is booted into. You can also enter user accounts during the installation and use the user manager utility to add or modify accounts later.

To add users, use the Add User button at the top of the window. Similarly, to add a new group that you can use to categorize users, use the Add Group button. In Figure 38.2 you can see an example of the Create New User dialog box.

Here you can enter information that looks a lot like that used in Windows NT 4.0. There are just a few fields such as these:

• User Name This is the logon username.

• Full Name As you may have guessed, this is the user's full nameuseful in large networks.

• Password You'll have to guess this one.

• Confirm Password Okay, if you got the last one right, then you might just get this one right, too.

• Login Shell Unix/Linux operating systems have different "shells," which are basically command environments that wrap around the Unix kernel, and each shell has its own set of commands. Different users prefer different command environments. The default is the "bash" shell, located in /bin/bash, as the figure shows. However, if you click the down arrow on the Login Shell field, you'll see a lot of options you or your users can use instead of the default bash shell.

  Note

  As mentioned in the main text, shells in Unix/Linux are basically command environments, similar to the commands you can enter in a DOS system (a DOS shell implemented by the program COMMAND.COM or CMD.EXE and other program files). Every operating system has a command interface of some sort. Another thing to consider when deciding which shell to use in your network is that each shell has its own commands, and thus, it's own syntax and command structure for creating script files. Script files are a valuable part of most operating systems because they group together commands to perform one or more functions. This saves the network administrator the chore of having to enter voluminous commands on many workstations. Script files are also used to perform setup duties for users when they log in to the system. So when choosing which shell to use, evaluate the commands and their flexibility and find out from seasoned Unix/Linux employees which is the one they prefer. If an administrator or programmer has to learn a new shell syntax from scratch, you might lose their years of experience using another shell. And you can let each user account use a different shell.

• Create Home Directory If you want to give this user his own home directory, select the Create Home Directory check box and use the default shown in the Home Directory field, or change it to suit the specifications in your LAN. This is the default directory in which a user's files will be stored. Unless this is a special account, such as one used only by an in-house written application, each user will most likely need a home directory. Note that due to the capability of the Unix/Linux operating systems to mount file systems from another computer, you can specify here a home directory on another computer by specifying the full pathname to the directory.

  Note

  You can learn more about mounting file systems in Unix/Linux using the Network File System protocols that were first developed by Sun Microsystems, and later ported to most modern operating systems, by reading Chapter 40, "Network Printing Protocols."

• Create a Private Group for the User Generally it is a good idea to plan your network based on some business organization and create user groups to gather together users who perform similar functions. However, sometimes you get a new user for which there is no match for your existing groups. Use this check box to later enable you to create a new group for the user.

• Specify User ID Manually In general, when you are adding a lot of users, selecting a user ID is not that important, except that each user should have a unique ID. The group ID and the user ID can uniquely identify a particular user on the system. If you don't want to use the default user ID displayed in this window, click on the Specify User ID Manually check box and enter a new value.

When you are finished entering information for a new user, click the Cancel button if you've changed your mind (or any time when you are entering information and are not sure of what data needs to be entered). If the information is correct, click on the OK button. The new user account then shows up in the window shown earlier in this chapter, in Figure 38.1.

As you enter information in this dialog box, you'll notice that other fields are filled in automatically with default information. For instance, when you enter the username, the Home Directory field expands from /home/ to /home/<username>, where username is the name you chose for the login username. If you like this method for naming home directories for clients, this utility will save you a lot of time.

When you want to review a user's account properties, you can click on the user in the main Red Hat Users Manager and use the Properties button. Figure 38.3 shows the basic information entered in the Create New User dialog box. Note that you can use this dialog box after the account has been created to change the user's password, something the Help Desk has to do often in many networks. You can also update additional information by using the tabs at the top of the dialog box.

These are the tabs you can use:

• User Data

• Account Info

• Password Info

• Groups

The first tab (User Data) enables you to change the basic information you entered when creating the account, such as changing the password.

You can use the Account Info tab to enforce an account expiration date or lock the account. The Password Info tab lets you find out the last date that the password was changed, and enable a password change policy. These are the items that make up this policy:

• Days Before Change Allowed Use this to restrict arbitrary changes in a very short length of time, which can indicate intruder attempts.

• Days Before Change Required All good things must come to an end, or be re-created. A good password must be changed to something else after this value expires. You should keep this to 3090 days, depending on the value you place on the date hosted by your network.

• Days Warning Before Change This is the number of days before the expiration of a password that users will begin to get notices that the password is about to expire.

• Days Before Account Inactive Those who don't change their password, or use their account, will no longer be able to access the account after this number of days.

• The Groups tab As shown in Figure 38.4, this allows you to use check boxes to select (or view) the groups that the user is a member of. The groups that the user is a member of may determine some of the capabilities the user has on the system.

This simple graphical interface allows you to perform a number of functions that would require many command-line interface commands. As the fast pace of computer and software development continues, it's possible, and very likely, that Linux may become a competitor for the desktop. The main driving forces for the desktop are cost, ease of use, and the applications. Windows operating systems can be more complex. In many cases Linux can be implemented more easily and at a much lower cost.

In the main window (shown in Figure 38.1), there is another tab right next to the Users tab: the Groups tab. Selecting this tab enables you to manage the groups of users on your network. If you simply click on the Groups tab, you will see a name for the group(s) on the computer, the group ID (GID), and the members of each group. You can use tabs at the top of this window to change this data. In Figure 38.5 you can see the Group Members window, which lets you see the members of each group.

You can also click on any group to select it and then use the Properties button at the top of the window to see a dialog box showing the group name and the members of the group, should the member accounts not fit on the main display.

Lastly, the Delete button (in Figure 38.1) can be used to remove a user account from the system.