RHCSA and RHCE Chapter 7 User Administration
Let’s get straight to it, from the “Red Hat Enterprise Linux 6 Deployment Guide”:
Chapter 3. Managing Users and Groups The control of users and groups is a core element of Red Hat Enterprise Linux system administration. This chapter explains how to add, manage, and delete users and groups in the graphical user interface and on the command line, and covers advanced topics, such as enabling password aging or creating group directories.
3.1. Introduction to Users and Groups While users can be either people (meaning accounts tied to physical users) or accounts which exist for specific applications to use, groups are logical expressions of organization, tying users together for a common purpose. Users within a group can read, write, or execute files owned by that group. Each user is associated with a unique numerical identification number called a user ID (UID). Likewise, each group is associated with a group ID (GID). A user who creates a file is also the owner and group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file owner can be changed only by root, and access permissions can be changed by both the root user and file owner.
3.1.1. User Private Groups Red Hat Enterprise Linux uses a user private group (UPG) scheme, which makes UNIX groups easier to manage. A user private group is created whenever a new user is added to the system. It has the same name as the user for which it was created and that user is the only member of the user private group. User private groups make it safe to set default permissions for a newly created file or directory, allowing both the user and the group of that user to make modifications to the file or directory. The setting which determines what permissions are applied to a newly created file or directory is called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX systems, the umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator’s group, are not allowed to make any modifications. However, under the UPG scheme, this “group protection” is not necessary since every user has their own private group.
3.1.2. Shadow Passwords In environments with multiple users, it is very important to use shadow passwords provided by the shadow-utils package to enhance the security of system authentication files. For this reason, the installation program enables shadow passwords by default. The following is a list of the advantages shadow passwords have over the traditional way of storing passwords on UNIX-based systems:
- Shadow passwords improve system security by moving encrypted password hashes from the world-readable /etc/passwd file to /etc/shadow, which is readable only by the root user.
- Shadow passwords store information about password aging.
- Shadow passwords allow the /etc/login.defs file to enforce security policies.
Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /etc/shadow file, any commands which create or modify password aging information do not work. The following is a list of utilities and commands that do not work without first enabling shadow passwords:
- The chage utility.
- The gpasswd utility.
- The usermod command with the -e or -f option.
- The useradd command with the -e or -f option.
Adding a User to RHEL with useradd
3.3. Using Command Line Tools
3.3.1. Adding a New User
To add a new user to the system, typing the following at a shell prompt as root:
useradd [options] username
…where options are command line options as described in Table 3.2, “useradd command line options”. By default, the useradd command creates a locked user account. To unlock the account, run the following command as root to assign a password:
Explaining the Process
The following steps illustrate what happens if the command useradd juan is issued on a system that has shadow passwords enabled:
A new line for juan is created in /etc/passwd:
The line has the following characteristics:
- It begins with the username juan.
- There is an x for the password field indicating that the system is using shadow passwords.
- A UID greater than 499 is created. Under Red Hat Enterprise Linux, UIDs below 500 are reserved for system use and should not be assigned to users.
- A GID greater than 499 is created. Under Red Hat Enterprise Linux, GIDs below 500 are reserved for system use and should not be assigned to users.
- The optional GECOS information is left blank. The GECOS field can be used to provide additional information about the user, such as their full name or phone number.
- The home directory for juan is set to /home/juan/.
- The default shell is set to /bin/bash.
A new line for juan is created in /etc/shadow:
The line has the following characteristics:
- It begins with the username juan
- Two exclamation marks (!!) appear in the password field of the /etc/shadow file, which locks the account.
- The password is set to never expire
A new line for a group named juan is created in /etc/group:
A group with the same name as a user is called a user private group. The line created in /etc/group has the following characteristics:
- It begins with the group name juan.
- An x appears in the password field indicating that the system is using shadow group passwords.
- The GID matches the one listed for user juan in /etc/passwd
A new line for a group named juan is created in /etc/gshadow:
The line has the following characteristics:
- It begins with the group name juan.
- An exclamation mark (!) appears in the password field of the /etc/gshadow file, which locks the group.
- All other fields are blank.
A directory for user juan is created in the /home/ directory:
~]# ls -l /home total 4 drwx----- 4 juan juan 4096 Mar 3 18:23 juan
This directory is owned by user juan and group juan. It has read, write, and execute privileges only for the user juan. All other permissions are denied.
The files within the /etc/skel/ directory (which contain default user settings) are copied into the new /home/juan/ directory:
~]# ls -la /home/juan total 28 drwx------. 4 juan juan 4096 Mar 3 18:23 . drwxr-xr-x. 5 root root 4096 Mar 3 18:23 .. -rw-r--r--. 1 juan juan 18 Jun 22 2010 .bash_logout -rw-r--r--. 1 juan juan 176 Jun 22 2010 .bash_profile -rw-r--r--. 1 juan juan 124 Jun 22 2010 .bashrc drwxr-xr-x. 2 juan juan 4096 Jul 14 2010 .gnome2 drwxr-xr-x. 4 juan juan 4096 Nov 23 15:09 .mozilla
At this point, a locked account called juan exists on the system. To activate it, the administrator must next assign a password to the account using the passwd command and, optionally, set password aging guidelines.
Let’s go ahead and create two users called user1 and user2 and check out their settings:
[[email protected] ~]# useradd user1 [[email protected] ~]# useradd user2
Let’s see their corresponding information under /etc/passwd and /etc/shadow:
[[email protected] ~]# getent passwd user1 user1:x:500:500::/home/user1:/bin/bash [[email protected] ~]# getent passwd user2 user2:x:501:501::/home/user2:/bin/bash [[email protected] ~]# getent shadow user1 user1:!!:15744:0:99999:7::: [[email protected] ~]# getent shadow user2 user2:!!:15744:0:99999:7:::
Now let’s check out their group information:
[[email protected] ~]# getent group user1 user1:x:500: [[email protected] ~]# getent group user2 user2:x:501: [[email protected] ~]# getent gshadow user1 user1:!:: [[email protected] ~]# getent gshadow user2 user2:!::
That looks good, now let’s check out their home directories:
[email protected] ~]# tree -a /home /home ├── user1 ├── .bash_logout ├── .bash_profile └── .bashrc └── user2 ├── .bash_logout ├── .bash_profile └── .bashrc 2 directories, 6 files
That looks good, let’s check out under /etc/skel to see what files were supposed to be auto copied upon user creation:
[[email protected] ~]# ls -la /etc/skel total 20 drwxr-xr-x. 2 root root 4096 Feb 4 04:25 . drwxr-xr-x. 60 root root 4096 Feb 8 06:28 .. -rw-r--r--. 1 root root 18 Jan 27 2011 .bash_logout -rw-r--r--. 1 root root 176 Jan 27 2011 .bash_profile -rw-r--r--. 1 root root 124 Jan 27 2011 .bashrc
That looks correct. Lastly, let’s check out the settings for user creation:
[[email protected] ~]# grep -v -E '^#|^$' /etc/login.defs MAIL_DIR /var/spool/mail PASS_MAX_DAYS 99999 PASS_MIN_DAYS 0 PASS_MIN_LEN 5 PASS_WARN_AGE 7 UID_MIN 500 UID_MAX 60000 GID_MIN 500 GID_MAX 60000 CREATE_HOME yes UMASK 077 USERGROUPS_ENAB yes ENCRYPT_METHOD SHA512
We can see that all of the above settings were honored during the creation of the users.
Adding a Group to RHEL with groupadd
Now let’s move onto groups, from the same guide:
3.3.2. Adding a New Group
To add a new group to the system, type the following at a shell prompt as root:
groupadd [options] group_name
…where options are command line options as described in Table 3.3, “groupadd command line options”.
So let’s add two new groups called group1 and group2 and add our users to the groups:
[[email protected] ~]# groupadd group1 [[email protected] ~]# groupadd group2 [[email protected] ~]# getent group group1 group1:x:502: [[email protected] ~]# getent group group2 group2:x:503: [[email protected] ~]# getent gshadow group1 group1:!:: [[email protected] ~]# getent gshadow group2 group2:!::
The creation went well. Now let’s add the users to these groups:
[[email protected] ~]# usermod -G group1 user1 [[email protected] ~]# usermod -G group2 user2 [[email protected] ~]# getent group group1 group1:x:502:user1 [[email protected] ~]# getent group group2 group2:x:503:user2
To get a concise view of the IDs (UID and ID) we can use the command id, like so:
[email protected] ~]# id -a user1 uid=500(user1) gid=500(user1) groups=500(user1),502(group1) [[email protected] ~]# id -a user2 uid=501(user2) gid=501(user2) groups=501(user2),503(group2)
That all looks good. Lastly let’s create a group called group3 and make both users be part of that group. This way we can share files:
[[email protected] ~]# groupadd group3 [[email protected] ~]# usermod -G group3 user1 [[email protected] ~]# usermod -G group3 user2 [[email protected] ~]# getent group group3 group3:x:504:user1,user2
That looks good. Now let’s switch to user1 and create a new directory called test and make it group writable with group3:
[[email protected] ~]# su - user1 [[email protected] ~]$ mkdir /tmp/test [[email protected] ~]$ ls -ld /tmp/test drwxrwxr-x. 2 user1 user1 4096 Feb 8 07:00 /tmp/test [[email protected] ~]$ chgrp group3 /tmp/test [[email protected] ~]$ ls -ld /tmp/test drwxrwxr-x. 2 user1 group3 4096 Feb 8 07:00 /tmp/test
Let’s do one more thing, let’s set the sgid bit on the folder, this way any file created within that directory will inherit the group ownership. So here is file creation without the bit set:
[[email protected] ~]$ touch /tmp/test/file1 [[email protected] ~]$ ls -l /tmp/test/file1 -rw-rw-r--. 1 user1 user1 0 Feb 8 07:03 /tmp/test/file1
Now let’s set the sgid bit:
[[email protected] ~]$ chmod g+s /tmp/test [[email protected] ~]$ ls -ld /tmp/test drwxrwsr-x. 2 user1 group3 4096 Feb 8 07:03 /tmp/test
notice the ‘s’ in the permissions. Now creating a second file:
[[email protected] ~]$ touch /tmp/test/file2 [[email protected] ~]$ ls -l /tmp/test/file2 -rw-rw-r--. 1 user1 group3 0 Feb 8 07:04 /tmp/test/file2
That looks perfect. Now anything created under that directory will be owned by group3. That last thing to look after is the umask. Umask controls the default permission of a newly created file. Let’s check out the umask of our user:
[[email protected] ~]$ umask 002
That means a file will be created with permission of (777 - 002 = 775). Which means that the the group has write permissions, which is good for our above setup. But what if our umask was 022? That would make our default permission be 755 and at this point the group wouldn’t have write permission and this would defeat the purpose of sharing files with our group members. So let’s change our umask and see what happens:
[[email protected] ~]$ umask 022 [[email protected] ~]$ touch /tmp/test/file3 [[email protected] ~]$ ls -l /tmp/test/file3 -rw-r--r--. 1 user1 group3 0 Feb 8 07:16 /tmp/test/file3
Now the group can’t write to the file, so if you’re sharing files with groups members ensure your umask is set appropriately.
RHEL File Permissions and Ownership
To get into permissions more, let’s check out this old “Red Hat Enterprise Linux Step By Step Guide”:
4.11. Ownership and Permissions
As a regular user, try to enter root’s home directory by entering the command cd /root/. Note the error message:
bash$ cd /root/ Permission denied
That was one demonstration of Linux security features. Linux, like UNIX, is a multi-user system and file permissions are one way the system protects against malicious tampering. One way to gain entry when you are denied permission is to enter the command su -. This is because whoever knows the root password has complete access However, switching to the superuser is not always convenient or recommended, since it is easy to make mistakes and alter important configuration files as the superuser.
All directories are owned by the person who created them. You created “foo.txt” in your login directory, so foo.txt belongs to you. That means you can specify who is allowed to read the file, write to the file, or (if it is an application instead of a text file) who can execute the file. Reading, writing, and executing are the three main settings in permissions. Since users are placed into a group when their accounts are created, you can also specify whether certain groups can read, write to, or execute a file. Take a closer look at foo.txt with the ls command using the -l option:
[[email protected] ~]$ ls -l foo.txt -rw-rw-r--. 1 user1 user1 0 Feb 8 07:24 foo.txt
A lot of detail is provided here. You can see who can read (r) and write to (w) the file as well as who created the file (user1), and to which group the owner belongs (user1). (By default, the name of your group is the same as your login name.) Other information to the right of the group includes the file size, date, and time of file creation, and file name. The first column shows current permissions; it has ten slots. The first slot represents the type of file. The remaining nine slots are actually three sets of permissions for three different categories of users. For example:
Those three sets are the owner of the file, the group in which the file belongs, and “others,” meaning other users on the system.
- (rw-) (rw-) (r--) 1 user1 user1
The first item, which specifies the file type, will probably be one of the following:
- d a directory
- - a regular file (rather than directory or link)
- l a symbolic link to another program or file elsewhere on the system Others are possible, but are beyond the scope of this manual.
Beyond the first item, in each of the following three sets, you may see one of the following:
- r file can be read
- w file can be written to
- x file can be executed (if it is a program)
- - specific permission has not been assigned When you see a dash in owner, group, or others, it means that particular permission has not been granted.
Look again at the first column of foo.txt and identify its permissions.
ls -l foo.txt -rw-rw-r--. 1 user1 user1 0 Feb 8 07:24 foo.txt
The file’s owner (in this case, user1) has permission to read and write to the file. The group, user1, has permission to read and write to foo.txt, as well. It is not a program, so neither the owner or the group has permission to execute it.
The command that allows you to change permissions of a file is called chmod. From the same guide:
4.11.1. The chmod Command
Use the chmod command to change permissions. This example shows how to change the permissions on foo.txt with the chmod command. The original file looks like this, with its initial permissions settings:
-rw-rw-r--. 1 user1 user1 0 Feb 8 07:24 foo.txt
If you are the owner of the file or are logged into the root account, you can change any permissions for the owner, group, and others. Right now, the owner and group can read and write to the file. Anyone outside of the group can only read the file (r-). In the following example, you want to allow everyone (others) to write to the file, so they can read it, write notes in it, and save it. That means you must change the “others” section of the file permissions:
[[email protected] ~]$ chmod o+w foo.txt
The o+w command tells the system you want to give others write permission to the file foo.txt. To check the results, list the file’s details again. Now, the file looks like this:
[[email protected] ~]$ ls -l foo.txt -rw-rw-rw-. 1 user1 user1 0 Feb 8 07:24 foo.txt
Now, everyone can read and write to the file. Think of these settings as a kind of shorthand when you want to change permissions with chmod, because all you really have to do is remember a few symbols and letters with the chmod command.Here is a list of what the shorthand represents:
- u the user who owns the file (that is, the owner)
- g the group to which the user belongs
- o others (not the owner or the owner’s group)
- a everyone or all (u, g, and o)
- r read access
- w write access
- x execute access
- + adds the permission
- - removes the permission
- = makes it the only permission Here are some common examples of settings that can be used with
- g+w adds write access for the group
- o-rwx removes all permissions for others
- u+x allows the file owner to execute the file
- a+rw allows everyone to read and write to the file
- ug+r allows the owner and group to read the file
- g=rx allows only the group to read and execute (not write)
By adding the -R option, you can change permissions for entire directory trees. Because you can not really “execute” a directory as you would an application, when you add (or remove) the execute permission for a directory, you are really allowing (or denying) permission to search through that directory
Another way of using chmod is with octal numbers, from the same guide:
4.11.2. Changing Permissions With Numbers
Another way to change permissions uses numeric representations. Go back to the original permissions for foo.txt:
[[email protected] ~]$ ls -l foo.txt -rw-rw-r--. 1 user1 user1 0 Feb 8 07:24 foo.txt
Each permission setting can be represented by a numerical value:
- r = 4
- w = 2
- x = 1
- - = 0
When these values are added together, the total is used to set specific permissions. For example, if you want read and write permissions, you would have a value of 6; 4 (read) + 2 (write) = 6. For foo.txt, here are the numerical permissions settings:
- (rw-) (rw-) (r--)
The total for the user is six(4+2+O), the total for the group is six(4+2+O), and the total for others is four(4+O+O). The permissions setting is read as 664. If you want to change foo.txt so those in your group do not have write access, but can still read the file, remove the access by subtracting two (2) from that set of numbers. The numerical values then become six, four, and four (644). To implement these new settings, type:
[[email protected] ~]$ chmod 644 foo.txt
Now verify the changes by listing the file. Type:
[[email protected] ~]$ ls -l foo.txt -rw-r--r--. 1 user1 user1 0 Feb 8 07:24 foo.txt
Now, neither the group nor others have write permission to foo.txt.
The last thing that we should cover is the ‘sticky’ bits. From the old “Red Hat Enterprise Linux 4 Introduction To System Administration”:
There are three such special permissions within Red Hat Enterprise Linux. They are:
- setuid — used only for binary files (applications), this permission indicates that the file is to be executed with the permissions of the owner of the file, and not with the permissions of the user executing the file (which is the case without setuid). This is indicated by the character s in the place of the x in the owner category. If the owner of the file does not have execute permissions, a capital S reflects this fact.
- setgid — used primarily for binary files (applications), this permission indicates that the file is executed with the permissions of the group owning the file and not with the permissions of the group of the user executing the file (which is the case without setgid). If applied to a directory, all files created within the directory are owned by the group owning the directory, and not by the group of the user creating the file. The setgid permission is indicated by the character s in place of the x in the group category. If the group owning the file or directory does not have execute permissions, a capital S reflects this fact.
- sticky bit — used primarily on directories, this bit dictates that a file created in the directory can be removed only by the user that created the file. It is indicated by the character t in place of the x in the everyone category. If the everyone category does not have execute permissions, the T is capitalized to reflect this fact. Under Red Hat Enterprise Linux, the sticky bit is set by default on the /tmp/ directory for exactly this reason.
Hopefully the above helps with permissions.
Sharing Files with Group Members in RHEL
We discussed sharing files with group members by using the sgid bit on a folder and with appropriate umask settings. Another way of sharing files is to setup a group password and allow users to login to that group. From “Red Hat Enterprise Linux 4 Introduction To System Administration”:
184.108.40.206 . /etc/gshadow
The /etc/gshadow file is readable only by the root user and contains an encrypted password for each group, as well as group membership and administrator information. Just as in the /etc/group file, each group’s information is on a separate line. Each of these lines is a colon delimited list including the following information:
- Group name — The name of the group. Used by various utility programs as a human-readable identifier for the group.
- Encrypted password — The encrypted password for the group. If set, non-members of the group can join the group by typing the password for that group using the newgrp command. If the value of this field is !, then no user is allowed to access the group using the newgrp command. A value of !! is treated the same as a value of ! — however, it also indicates that a password has never been set before. If the value is null, only group members can log into the group.
- Group administrators — Group members listed here (in a comma delimited list) can add or remove group members using the gpasswd command.
- Group members — Group members listed here (in a comma delimited list) are regular, nonadministrative members of the group. Here is an example line from /etc/gshadow:
general:!!:shelley:juan,bobThis line shows that the general group has no password and does not allow non-members to join using the newgrp command. In addition, shelley is a group administrator, and juan and bob are regular, non-administrative members.
So let’s make user1 be an administrator of group1:
[[email protected] ~]# gpasswd -A user1 group1 [[email protected] ~]# getent gshadow group1 group1:!:user1:
Also let’s add user1 as a member as well:
[[email protected] ~]# gpasswd -a user1 group1 Adding user user1 to group group1 [[email protected] ~]# getent gshadow group1 group1:!:user1:user1
Now let’s switch to that user and set the password for the group:
[[email protected] ~]# su - user1 [[email protected] ~]$ gpasswd group1 Changing the password for group group1 New Password: Re-enter new password: [[email protected] ~]$ getent group group1 group1:x:502:user1
We now see an ‘x’ instead of an ‘!’, so we know the password is set. Now let’s create a new directory called test2 and make the group owner group1:
[[email protected] ~]$ mkdir /tmp/test2 [[email protected] ~]$ chgrp group1 /tmp/test2 [[email protected] ~]$ ls -ld /tmp/test2 drwxrwxr-x. 2 user1 group1 4096 Feb 9 01:19 /tmp/test2
That looks good, now let’s become user2 and login to group1 with the password that we just set:
[[email protected] ~]# su - user2 [[email protected] ~]$ id -a uid=501(user2) gid=501(user2) groups=501(user2),503(group2)
Just checking out current gid:
[[email protected] ~]$ id -g 501
We can see that currently we are part of user2 (our private group) and group2, which is what we setup above. Now let’s login to group1:
[[email protected] ~]$ newgrp group1 Password: [[email protected] ~]$ id -g 502
Now creating a new file under our tmp directory:
[[email protected] ~]$ umask 0002 [[email protected] ~]$ touch /tmp/test2/file [[email protected] ~]$ ls -l /tmp/test2/file -rw-rw-r-- 1 user2 group1 0 Feb 9 04:18 /tmp/test2/file
We accomplished the same thing without using the sgid bit.
RHEL Password Aging
Getting back to the “Red Hat Enterprise Linux 6 Deployment Guide”:
3.3.3. Enabling Password Aging
For security reasons, it is advisable to require users to change their passwords periodically. This can either be done when adding or editing a user on the Password Info tab of the User Manager application, or by using the chage command. To configure password expiration for a user from a shell prompt, run the following command as root:
chage [options] username
…where options are command line options as described in Table 3.4, “chage command line options”. When the chage command is followed directly by a username (that is, when no command line options are specified), it displays the current password aging values and allows you to change them interactively.
You can configure a password to expire the first time a user logs in. This forces users to change passwords immediately.
Set up an initial password. There are two common approaches to this step: you can either assign a default password, or you can use a null password. To assign a default password, type the following at a shell prompt as root:
passwd username juan:x:501:501::/home/juan:/bin/bash
To assign a null password instead, use the following command:
passwd -d username
Force immediate password expiration by running the following command as root:
chage -d 0 username
This command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place. Upon the initial log in, the user is now prompted for a new password.
So let’s check out the policies our user1:
[[email protected] ~]# chage -l user1 Last password change : Feb 08, 2013 Password expires : never Password inactive : never Account expires : never Minimum number of days between password change : 0 Maximum number of days between password change : 99999 Number of days of warning before password expires : 7
All of the above settings are stored in /etc/shadow. Let’s set the password to expire today, and set a warning message to be displayed when the password is expiring one day before the expiration date:
[[email protected] ~]# date Sat Feb 9 01:55:28 MST 2013 [[email protected] ~]# chage -E 2013-02-09 -M 1 user1 [[email protected] ~]# chage -l user1 Last password change : Feb 09, 2013 Password expires : Feb 10, 2013 Password inactive : never Account expires : Feb 09, 2013 Minimum number of days between password change : 0 Maximum number of days between password change : 1 Number of days of warning before password expires : 7
Now let’s switch user to user1, and see if we get the message:
[[email protected] ~]$ su - user1 Password: Warning: your password will expire in 1 day
You can also manually lock user accounts by using the passwd utility:
[[email protected] ~]# getent shadow user1 user1:$6$IPcp2gLd$mhQKFrXYQDGFPDEDDXeOfz5ObWCMpAKvK4X/3fTUknO:15745:0::7::: [[email protected] ~]# passwd -l user1 Locking password for user user1. Success [[email protected] ~]# getent shadow user1 user1:!!$6$IPcp2gLd$mhQKFrXYQDGFPDEDDXeOfz5ObWCMpAKvK4X/3fTUkn0:15745:0::7:::
Notice the ‘!!’ in front of the hash of the password, signifying that the account is locked.
RHEL Check Integrity of Password and Group Files
The other two utilities mentioned in the guide were pwck and gpwck. Both utilities just check to see if /etc/passwd, /etc/shadow and /etc/group, /etc/gshadow have proper syntax and are in sync. Here the commands run on the test machines:
[[email protected] ~]# pwck user 'adm': directory '/var/adm' does not exist user 'uucp': directory '/var/spool/uucp' does not exist user 'gopher': directory '/var/gopher' does not exist user 'ftp': directory '/var/ftp' does not exist user 'saslauth': directory '/var/empty/saslauth' does not exist pwck: no changes [[email protected] ~]# grpck [[email protected] ~]#
It looks like for some users the home directory doesn’t exist, but that is okay and my groups and group password are okay.
RHEL System Authentication
Lastly there is “System Authentication”, from the deployment guide:
Chapter 10. Configuring Authentication
Authentication is the way that a user is identified and verified to a system. The authentication process requires presenting some sort of identity and credentials, like a username and password. The credentials are then compared to information stored in some data store on the system. In Red Hat Enterprise Linux, the Authentication Configuration Tool helps configure what kind of data store to use for user credentials, such as LDAP. For convenience and potentially part of single sign-on, Red Hat Enterprise Linux can use a central daemon to store user credentials for a number of different data stores. The System Security Services Daemon (SSSD) can interact with LDAP, Kerberos, and external applications to verify user credentials. The Authentication Configuration Tool can configure SSSD along with NIS, Winbind, and LDAP, so that authentication processing and caching can be combined.
10.1. Configuring System Authentication When a user logs into a Red Hat Enterprise Linux system, that user presents some sort of credential to establish the user identity. The system then checks those credentials against the configured authentication service. If the credentials match and the user account is active, then the user is authenticated. (Once a user is authenticated, then the information is passed to the access control service to determine what the user is permitted to do. Those are the resources the user is authorized to access.) The information to verify the user can be located on the local system or the local system can reference a user database on a remote system, such as LDAP or Kerberos. The system must have a configured list of valid account databases for it to check for user authentication. On Red Hat Enterprise Linux, the Authentication Configuration Tool has both GUI and command-line options to configure any user data stores. A local system can use a variety of different data stores for user information, including Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind. Additionally, both LDAP and NIS data stores can use Kerberos to authenticate users.
First let’s check out NIS:
10.1.2.2. Configuring NIS Authentication
Install the ypbind package. This is required for NIS services, but is not installed by default.
[[email protected] ~]# yum install ypbind
When the ypbind service is installed, the portmap and ypbind services are started and enabled to start at boot time … …
10.1.4 .3. Configuring NIS User Stores
To use a NIS identity store, use the -enablenis. This automatically uses NIS authentication, unless the Kerberos parameters are explicitly set, so it uses Kerberos authentication. The only parameters are to identify the NIS server and NIS domain; if these are not used, then the authconfig service scans the network for NIS servers.
authconfig --enablenis --nisdomain=EXAMPLE --nisserver=nis.example.com --update
The authconfig command line can get pretty advanced, so I decided to use authconfig-tui just for ease and time saving. So setup NIS authentication on our test machine:
[[email protected] ~]# yum install ypbind ... ... Installed: ypbind.i686 3:1.20.4-29.el6 Dependency Installed: libgssglue.i686 0:0.1-11.el6 libtirpc.i686 0:0.2.1-3.el6 rpcbind.i686 0:0.2.0-8.el6 yp-tools.i686 0:2.9-10.el6 Complete!
Let’s enable both daemons to be started on boot:
[[email protected] ~]# chkconfig rpcbind on [[email protected] ~]# chkconfig ypbind on
Now let’s configure the machine to authenticate with a NIS server (itself for now):
If I had actually setup a NIS server, the rest would just fall into place. After I hit next authconfig-tui tried to start the appropriate services, but failed, cause there was no NIS server:
[[email protected] ~]# authconfig-tui Starting rpcbind: rpcbind Starting NIS service: ypbind Binding NIS service: ..................... Shutting down NIS service: ypbind
To confirm the settings were applied, we can check out the nsswitch.conf file:
[[email protected] ~]# grep -E '^passwd|^shadow|^group' /etc/nsswitch.conf passwd: files nis shadow: files nis group: files nis
We can also see the NIS settings by looking at /etc/yp.conf:
[[email protected] ~]# grep -v -E '^#|^$' /etc/yp.conf domain local.com server 192.168.1.110
Similar steps can be taken to setup LDAP authentication. From the same guide:
10.1.2.1. Configuring LDAP Authentication
Either the openldap-clients package or the sssd package is used to configure an LDAP server for the user database. Both packages are installed by default.
10.1.4 .2. Configuring LDAP User Stores
To use an LDAP identity store, use the -enableldap. To use LDAP as the authentication source, use -enableldapauth and then the requisite connection information, like the LDAP server name, base DN for the user suffix, and (optionally) whether to use TLS. The authconfig command also has options to enable or disable RFC 2307bis schema for user entries, which is not possible through the Authentication Configuration UI. Be sure to use the full LDAP URL, including the protocol (ldap or ldaps) and the port number. Do not use a secure LDAP URL (ldaps) with the -enableldaptls option.
authconfig --enableldap --enableldapauth --ldapserver=ldap://ldap.example.com:389,ldap://ldap2.example.com:389 --ldapbasedn="ou=people,dc=example,dc=com" --enableldaptls --ldaploadcacert=https://ca.server.example.com/caCert.crt --update
- RHCSA and RHCE Chapter 10 - The Kernel
- RHCSA and RHCE Chapter 9 - System Logging, Monitoring, and Automation
- RHCSA and RHCE Chapter 8 Network Installs
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