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Behind the Commands: Configuration Files
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In the previous section, you learned about the commands to manage users from a console environment. All these commands put the user-related information into what are known as
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CHAPTER 5 CONFIGURING YOUR SERVER FOR SECURITY
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configuration files, and a configuration file is also used for default settings that are applied when managing the user environment. The aim of this section is to give you some insight into the following configuration files: /etc/passwd /etc/shadow /etc/login.defs
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/etc/passwd
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The first and probably most important of all user-related configuration files is /etc/passwd, which is the primary database for user information: everything except the user password is stored in this file. Listing 5-1 should give you an impression of what the fields in this file look like. Listing 5-1. Contents of the User Database file /etc/passwd root@RNA:~# cat /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh lp:x:7:7:lp:/var/spool/lpd:/bin/sh mail:x:8:8:mail:/var/mail:/bin/sh news:x:9:9:news:/var/spool/news:/bin/sh uucp:x:10:10:uucp:/var/spool/uucp:/bin/sh proxy:x:13:13:proxy:/bin:/bin/sh www-data:x:33:33:www-data:/var/www:/bin/sh backup:x:34:34:backup:/var/backups:/bin/sh list:x:38:38:Mailing List Manager:/var/list:/bin/sh irc:x:39:39:ircd:/var/run/ircd:/bin/sh gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/bin/sh nobody:x:65534:65534:nobody:/nonexistent:/bin/sh dhcp:x:100:101::/nonexistent:/bin/false syslog:x:101:102::/home/syslog:/bin/false klog:x:102:103::/home/klog:/bin/false mysql:x:103:106:MySQL Server,,,:/var/lib/mysql:/bin/false bind:x:104:109::/var/cache/bind:/bin/false sander:x:1000:1000:sander,,,:/home/sander:/bin/bash messagebus:x:105:112::/var/run/dbus:/bin/false haldaemon:x:106:113:Hardware abstraction layer,,,:/home/haldaemon:/bin/false gdm:x:107:115:Gnome Display Manager:/var/lib/gdm:/bin/false sshd:x:108:65534::/var/run/sshd:/usr/sbin/nologin linda:x:1001:1001::/home/linda:/bin/sh zeina:x:1002:1002::/home/zeina:/bin/sh
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CHAPTER 5 CONFIGURING YOUR SERVER FOR SECURITY
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You can see that /etc/passwd uses different fields, and they all are separated with a colon. Here s a short explanation of these fields: Loginname: This is the first field and it stores the user s login name. In older UNIX versions, the field was limited to eight characters. Fortunately, Ubuntu Server does not have this limitation. Password: In the old days of UNIX, this file stored the encrypted passwords. The only problem was that everyone including an intruder was allowed to read the /etc/ passwd file. This poses an obvious security risk, so passwords are now stored in the configuration file /etc/shadow, which is discussed in the next section. The x in the password field denotes the use of shadow passwords. UID: As you already learned, every user has a unique user ID. Ubuntu Server starts numbering local user IDs at 1000 and typically the highest number that should be used is 65535. GID: As discussed in the previous section, every user has a primary group, and its group ID (GID) is listed here. This is the numeric ID of the group that the user uses as his primary group. For ordinary users, the GID defaults to 100 (which belongs to the group users). GECOS: The General Electric Comprehensive Operating System (GECOS) field is used to include some comment to make it easier for the administrator to identify the user. However, the GECOS field is optional, and it s often not used at all. Home directory: This is a reference to the directory that serves as the user s home directory; it is typically the location in which a user stores files. Note that it is only a reference and has nothing to do with the real directory; just because you see a directory listed here doesn t mean that it actually exists. Shell: The last field in /etc/passwd refers to the program that should be started automatically when a user logs in. Most often, it s /bin/bash, but, as discussed in the preceding section, every binary program can be referred to here as long as the complete pathname is used. As an administrator, you can manually edit /etc/passwd and the related /etc/shadow. If you intend to do this, however, don t use any editor; use vipw instead. This tailored version of the Vi editor is specifically designed for editing these critical files. Any error can have serious consequences, such as no one being able to log in. Therefore, if you make manual changes to any of these files, you should check their integrity. Besides vipw, another way to do this is to use the pwck command, which you can run without any options to see whether there are any problems you need to fix.
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