| Security GuideChapter 2. Securing Your Network2.1. Workstation SecuritySecuring a Linux environment begins with the workstation. Whether locking down a personal machine or securing an enterprise system, sound security policy begins with the individual computer. A computer network is only as secure as its weakest node. 2.1.1. Evaluating Workstation SecurityWhen evaluating the security of a Red Hat Enterprise Linux workstation, consider the following: BIOS and Boot Loader Security - Can an unauthorized user physically access the machine and boot into single user or rescue mode without a password? Password Security - How secure are the user account passwords on the machine? Administrative Controls - Who has an account on the system and how much administrative control do they have? Available Network Services - What services are listening for requests from the network and should they be running at all? Personal Firewalls - What type of firewall, if any, is necessary? Security Enhanced Communication Tools - Which tools should be used to communicate between workstations and which should be avoided?
2.1.2. BIOS and Boot Loader SecurityPassword protection for the BIOS (or BIOS equivalent) and the boot loader can prevent unauthorized users who have physical access to systems from booting using removable media or obtaining root privileges through single user mode. The security measures you should take to protect against such attacks depends both on the sensitivity of the information on the workstation and the location of the machine. For example, if a machine is used in a trade show and contains no sensitive information, then it may not be critical to prevent such attacks. However, if an employee's laptop with private, unencrypted SSH keys for the corporate network is left unattended at that same trade show, it could lead to a major security breach with ramifications for the entire company. If the workstation is located in a place where only authorized or trusted people have access, however, then securing the BIOS or the boot loader may not be necessary. The two primary reasons for password protecting the BIOS of a computer are : Preventing Changes to BIOS Settings - If an intruder has access to the BIOS, they can set it to boot from a diskette or CD-ROM. This makes it possible for them to enter rescue mode or single user mode, which in turn allows them to start arbitrary processes on the system or copy sensitive data. Preventing System Booting - Some BIOSes allow password protection of the boot process. When activated, an attacker is forced to enter a password before the BIOS launches the boot loader.
Because the methods for setting a BIOS password vary between computer manufacturers, consult the computer's manual for specific instructions. If you forget the BIOS password, it can either be reset with jumpers on the motherboard or by disconnecting the CMOS battery. For this reason, it is good practice to lock the computer case if possible. However, consult the manual for the computer or motherboard before attempting to disconnect the CMOS battery. 2.1.2.1.1. Securing Non-x86 PlatformsOther architectures use different programs to perform low-level tasks roughly equivalent to those of the BIOS on x86 systems. For instance, Intel® Itanium� computers use the Extensible Firmware Interface (EFI) shell. For instructions on password protecting BIOS-like programs on other architectures, refer to the manufacturer's instructions. 2.1.2.2. Boot Loader PasswordsThe primary reasons for password protecting a Linux boot loader are as follows: Preventing Access to Single User Mode - If attackers can boot the system into single user mode, they are logged in automatically as root without being prompted for the root password. Protecting access to single user mode with a password by editing the SINGLE parameter in the /etc/sysconfig/init file is not recommended. An attacker can bypass the password by specifying a custom initial command (using the init= parameter) on the kernel command line in GRUB. It is recommended to password-protect the GRUB boot loader as specified in Section 2.1.2.2.1, "Password Protecting GRUB". Preventing Access to the GRUB Console - If the machine uses GRUB as its boot loader, an attacker can use the GRUB editor interface to change its configuration or to gather information using the cat command. Preventing Access to Insecure Operating Systems - If it is a dual-boot system, an attacker can select an operating system at boot time (for example, DOS), which ignores access controls and file permissions.
Red Hat Enterprise Linux 6 ships with the GRUB boot loader on the x86 platform. For a detailed look at GRUB, refer to the Red Hat Installation Guide. 2.1.2.2.1. Password Protecting GRUBYou can configure GRUB to address the first two issues listed in Section 2.1.2.2, "Boot Loader Passwords" by adding a password directive to its configuration file. To do this, first choose a strong password, open a shell, log in as root, and then type the following command: /sbin/grub-md5-crypt
When prompted, type the GRUB password and press Enter. This returns an MD5 hash of the password. Next, edit the GRUB configuration file /boot/grub/grub.conf. Open the file and below the timeout line in the main section of the document, add the following line: password --md5 <password-hash> Replace <password-hash> with the value returned by /sbin/grub-md5-crypt. The next time the system boots, the GRUB menu prevents access to the editor or command interface without first pressing p followed by the GRUB password. Unfortunately, this solution does not prevent an attacker from booting into an insecure operating system in a dual-boot environment. For this, a different part of the /boot/grub/grub.conf file must be edited. Look for the title line of the operating system that you want to secure, and add a line with the lock directive immediately beneath it. For a DOS system, the stanza should begin similar to the following: title DOSlock A password line must be present in the main section of the /boot/grub/grub.conf file for this method to work properly. Otherwise, an attacker can access the GRUB editor interface and remove the lock line. To create a different password for a particular kernel or operating system, add a lock line to the stanza, followed by a password line. Each stanza protected with a unique password should begin with lines similar to the following example: title DOSlockpassword --md5 <password-hash> 2.1.2.2.2. Disabling Interactive StartupPressing the I key at the beginning of the boot sequence allows you to start up your system interactively. During an interactive startup, the system prompts you to start up each service one by one. However, this may allow an attacker who gains physical access to your system to disable the security-related services and gain access to the system. To prevent users from starting up the system interactively, as root, disable the PROMPT parameter in the /etc/sysconfig/init file: PROMPT=no Passwords are the primary method that Red Hat Enterprise Linux uses to verify a user's identity. This is why password security is so important for protection of the user, the workstation, and the network. For security purposes, the installation program configures the system to use Secure Hash Algorithm 512 (SHA512) and shadow passwords. It is highly recommended that you do not alter these settings. If shadow passwords are deselected during installation, all passwords are stored as a one-way hash in the world-readable /etc/passwd file, which makes the system vulnerable to offline password cracking attacks. If an intruder can gain access to the machine as a regular user, he can copy the /etc/passwd file to his own machine and run any number of password cracking programs against it. If there is an insecure password in the file, it is only a matter of time before the password cracker discovers it. Shadow passwords eliminate this type of attack by storing the password hashes in the file /etc/shadow, which is readable only by the root user. This forces a potential attacker to attempt password cracking remotely by logging into a network service on the machine, such as SSH or FTP. This sort of brute-force attack is much slower and leaves an obvious trail as hundreds of failed login attempts are written to system files. Of course, if the cracker starts an attack in the middle of the night on a system with weak passwords, the cracker may have gained access before dawn and edited the log files to cover his tracks. In addition to format and storage considerations is the issue of content. The single most important thing a user can do to protect his account against a password cracking attack is create a strong password. 2.1.3.1. Creating Strong PasswordsWhen creating a secure password, it is a good idea to follow these guidelines: Do Not Use Only Words or Numbers - Never use only numbers or words in a password. Some insecure examples include the following: Do Not Use Recognizable Words - Words such as proper names, dictionary words, or even terms from television shows or novels should be avoided, even if they are bookended with numbers. Some insecure examples include the following: Do Not Use Words in Foreign Languages - Password cracking programs often check against word lists that encompass dictionaries of many languages. Relying on foreign languages for secure passwords is not secure. Some insecure examples include the following: cheguevara bienvenido1 1dumbKopf
Do Not Use Hacker Terminology - If you think you are elite because you use hacker terminology - also called l337 (LEET) speak - in your password, think again. Many word lists include LEET speak. Some insecure examples include the following: Do Not Use Personal Information - Avoid using any personal information in your passwords. If the attacker knows your identity, the task of deducing your password becomes easier. The following is a list of the types of information to avoid when creating a password: Some insecure examples include the following: Do Not Invert Recognizable Words - Good password checkers always reverse common words, so inverting a bad password does not make it any more secure. Some insecure examples include the following: Do Not Write Down Your Password - Never store a password on paper. It is much safer to memorize it. Do Not Use the Same Password For All Machines - It is important to make separate passwords for each machine. This way if one system is compromised, all of your machines are not immediately at risk.
The following guidelines will help you to create a strong password: Make the Password at Least Eight Characters Long - The longer the password, the better. If using MD5 passwords, it should be 15 characters or longer. With DES passwords, use the maximum length (eight characters). Mix Upper and Lower Case Letters - Red Hat Enterprise Linux is case sensitive, so mix cases to enhance the strength of the password. Mix Letters and Numbers - Adding numbers to passwords, especially when added to the middle (not just at the beginning or the end), can enhance password strength. Include Non-Alphanumeric Characters - Special characters such as &, $, and > can greatly improve the strength of a password (this is not possible if using DES passwords). Pick a Password You Can Remember - The best password in the world does little good if you cannot remember it; use acronyms or other mnemonic devices to aid in memorizing passwords.
With all these rules, it may seem difficult to create a password that meets all of the criteria for good passwords while avoiding the traits of a bad one. Fortunately, there are some steps you can take to generate an easily-remembered, secure password. 2.1.3.1.1. Secure Password Creation MethodologyThere are many methods that people use to create secure passwords. One of the more popular methods involves acronyms. For example: Think of an easily-remembered phrase, such as: "over the river and through the woods, to grandmother's house we go." Next, turn it into an acronym (including the punctuation). otrattw,tghwg.
Add complexity by substituting numbers and symbols for letters in the acronym. For example, substitute 7 for t and the at symbol (@) for a: o7r@77w,7ghwg.
Add more complexity by capitalizing at least one letter, such as H. o7r@77w,7gHwg.
Finally, do not use the example password above for any systems, ever.
While creating secure passwords is imperative, managing them properly is also important, especially for system administrators within larger organizations. The following section details good practices for creating and managing user passwords within an organization. 2.1.3.2. Creating User Passwords Within an OrganizationIf an organization has a large number of users, the system administrators have two basic options available to force the use of good passwords. They can create passwords for the user, or they can let users create their own passwords, while verifying the passwords are of acceptable quality. Creating the passwords for the users ensures that the passwords are good, but it becomes a daunting task as the organization grows. It also increases the risk of users writing their passwords down. For these reasons, most system administrators prefer to have the users create their own passwords, but actively verify that the passwords are good and, in some cases, force users to change their passwords periodically through password aging. 2.1.3.2.1. Forcing Strong PasswordsTo protect the network from intrusion it is a good idea for system administrators to verify that the passwords used within an organization are strong ones. When users are asked to create or change passwords, they can use the command line application passwd, which is Pluggable Authentication Modules ( PAM) aware and therefore checks to see if the password is too short or otherwise easy to crack. This check is performed using the pam_cracklib.so PAM module. For more information about pam_cracklib.so and its options for password checking, refer to the pam_cracklib(8) man page. Since PAM is customizable, it is possible to add more password integrity checkers, such as pam_passwdqc (available from http://www.openwall.com/passwdqc/) or to write a new module. For a list of available PAM modules, refer to http://uw714doc.sco.com/en/SEC_pam/pam-6.html. For more information about PAM, refer to the Managing Single Sign-On and Smart Cards guide. The password check that is performed at the time of their creation does not discover bad passwords as effectively as running a password cracking program against the passwords. Many password cracking programs are available that run under Red Hat Enterprise Linux, although none ship with the operating system. Below is a brief list of some of the more popular password cracking programs: Always get authorization in writing before attempting to crack passwords within an organization. Passphrases and passwords are the cornerstone to security in most of today's systems. Unfortunately, techniques such as biometrics and two-factor authentication have not yet become mainstream in many systems. If passwords are going to be used to secure a system, then the use of passphrases should be considered. Passphrases are longer than passwords and provide better protection than a password even when implemented with non-standard characters such as numbers and symbols. 2.1.3.2.3. Password AgingPassword aging is another technique used by system administrators to defend against bad passwords within an organization. Password aging means that after a specified period (usually 90 days), the user is prompted to create a new password. The theory behind this is that if a user is forced to change his password periodically, a cracked password is only useful to an intruder for a limited amount of time. The downside to password aging, however, is that users are more likely to write their passwords down. There are two primary programs used to specify password aging under Red Hat Enterprise Linux: the chage command or the graphical User Manager (system-config-users) application. Shadow passwords must be enabled to use the chage command. For more information, see the Red Hat Enterprise Linux 6 Deployment Guide. The -M option of the chage command specifies the maximum number of days the password is valid. For example, to set a user's password to expire in 90 days, use the following command: chage -M 90 <username>
In the above command, replace <username> with the name of the user. To disable password expiration, it is traditional to use a value of 99999 after the -M option (this equates to a little over 273 years). For more information on the options available with the chage command, refer to the table below. Table 2.1. chage command line options | Option | Description |
|---|
-d days | Specifies the number of days since January 1, 1970 the password was changed. | -E date | Specifies the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used. | -I days | Specifies the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not locked after the password expires. | -l | Lists current account aging settings. | -m days | Specify the minimum number of days after which the user must change passwords. If the value is 0, the password does not expire. | -M days | Specify the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account. | -W days | Specifies the number of days before the password expiration date to warn the user. |
You can also use the chage command in interactive mode to modify multiple password aging and account details. Use the following command to enter interactive mode: chage <username>
The following is a sample interactive session using this command: ~]# chage juanChanging the aging information for juanEnter the new value, or press ENTER for the defaultMinimum Password Age [0]: 10Maximum Password Age [99999]: 90Last Password Change (YYYY-MM-DD) [2006-08-18]:Password Expiration Warning [7]:Password Inactive [-1]:Account Expiration Date (YYYY-MM-DD) [1969-12-31]: 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
To assign a null password instead, use the following command: passwd -d username
Using a null password, while convenient, is a highly insecure practice, as any third party can log in first and access the system using the insecure username. Always make sure that the user is ready to log in before unlocking an account with a null password. 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. You can also use the graphical User Manager application to create password aging policies, as follows. Note: you need Administrator privileges to perform this procedure. Click the menu on the Panel, point to and then click to display the User Manager. Alternatively, type the command system-config-users at a shell prompt. Click the Users tab, and select the required user in the list of users. Click Properties on the toolbar to display the User Properties dialog box (or choose on the menu). Click the Password Info tab, and select the check box for Enable password expiration. Enter the required value in the Days before change required field, and click OK.
2.1.4. Administrative ControlsWhen administering a home machine, the user must perform some tasks as the root user or by acquiring effective root privileges via a setuid program, such as sudo or su. A setuid program is one that operates with the user ID (UID) of the program's owner rather than the user operating the program. Such programs are denoted by an s in the owner section of a long format listing, as in the following example: ~]$ ls -l /bin/su-rwsr-xr-x. 1 root root 34904 Mar 10 2011 /bin/su The s may be upper case or lower case. If it appears as upper case, it means that the underlying permission bit has not been set. For the system administrators of an organization, however, choices must be made as to how much administrative access users within the organization should have to their machine. Through a PAM module called pam_console.so, some activities normally reserved only for the root user, such as rebooting and mounting removable media are allowed for the first user that logs in at the physical console (refer to Managing Single Sign-On and Smart Cards for more information about the pam_console.so module.) However, other important system administration tasks, such as altering network settings, configuring a new mouse, or mounting network devices, are not possible without administrative privileges. As a result, system administrators must decide how much access the users on their network should receive. 2.1.4.1. Allowing Root AccessIf the users within an organization are trusted and computer-literate, then allowing them root access may not be an issue. Allowing root access by users means that minor activities, like adding devices or configuring network interfaces, can be handled by the individual users, leaving system administrators free to deal with network security and other important issues. On the other hand, giving root access to individual users can lead to the following issues: Machine Misconfiguration - Users with root access can misconfigure their machines and require assistance to resolve issues. Even worse, they might open up security holes without knowing it. Running Insecure Services - Users with root access might run insecure servers on their machine, such as FTP or Telnet, potentially putting usernames and passwords at risk. These services transmit this information over the network in plain text. Running Email Attachments As Root - Although rare, email viruses that affect Linux do exist. The only time they are a threat, however, is when they are run by the root user. Keeping the audit trail intact - Because the root account is often shared by multiple users, so that multiple system administrators can maintain the system, it is impossible to figure out which of those users was root at a given time. When using separate logins, the account a user logs in with, as well as a unique number for session tracking purposes, is put into the task structure, which is inherited by every process that the user starts. When using concurrent logins, the unique number can be used to trace actions to specific logins. When an action generates an audit event, it is recorded with the login account and the session associated with that unique number. Use the aulast command to view these logins and sessions. The --proof option of the aulast command can be used suggest a specific ausearch query to isolate auditable events generated by a particular session.
2.1.4.2. Disallowing Root AccessIf an administrator is uncomfortable allowing users to log in as root for these or other reasons, the root password should be kept secret, and access to runlevel one or single user mode should be disallowed through boot loader password protection (refer to Section 2.1.2.2, "Boot Loader Passwords" for more information on this topic.) The following are four different ways that an administrator can further ensure that root logins are disallowed: - Changing the root shell
To prevent users from logging in directly as root, the system administrator can set the root account's shell to /sbin/nologin in the /etc/passwd file. Table 2.2. Disabling the Root Shell | Effects | Does Not Affect |
|---|
Prevents access to the root shell and logs any such attempts. The following programs are prevented from accessing the root account: login
gdm
kdm
xdm
su
ssh
scp
sftp
| Programs that do not require a shell, such as FTP clients, mail clients, and many setuid programs. The following programs are not prevented from accessing the root account: sudo
FTP clients Email clients
|
- Disabling root access via any console device (tty)
To further limit access to the root account, administrators can disable root logins at the console by editing the /etc/securetty file. This file lists all devices the root user is allowed to log into. If the file does not exist at all, the root user can log in through any communication device on the system, whether via the console or a raw network interface. This is dangerous, because a user can log in to their machine as root via Telnet, which transmits the password in plain text over the network. By default, Red Hat Enterprise Linux's /etc/securetty file only allows the root user to log in at the console physically attached to the machine. To prevent the root user from logging in, remove the contents of this file by typing the following command at a shell prompt as root: echo > /etc/securetty
To enable securetty support in the KDM, GDM, and XDM login managers, add the following line: auth [user_unknown=ignore success=ok ignore=ignore default=bad] pam_securetty.so to the files listed below: A blank /etc/securetty file does not prevent the root user from logging in remotely using the OpenSSH suite of tools because the console is not opened until after authentication. Table 2.3. Disabling Root Logins | Effects | Does Not Affect |
|---|
Prevents access to the root account via the console or the network. The following programs are prevented from accessing the root account: | Programs that do not log in as root, but perform administrative tasks through setuid or other mechanisms. The following programs are not prevented from accessing the root account: |
- Disabling root SSH logins
To prevent root logins via the SSH protocol, edit the SSH daemon's configuration file, /etc/ssh/sshd_config, and change the line that reads: #PermitRootLogin yes to read as follows: PermitRootLogin no Table 2.4. Disabling Root SSH Logins | Effects | Does Not Affect |
|---|
Prevents root access via the OpenSSH suite of tools. The following programs are prevented from accessing the root account: | Programs that are not part of the OpenSSH suite of tools. |
- Using PAM to limit root access to services
PAM, through the /lib/security/pam_listfile.so module, allows great flexibility in denying specific accounts. The administrator can use this module to reference a list of users who are not allowed to log in. To limit root access to a system service, edit the file for the target service in the /etc/pam.d/ directory and make sure the pam_listfile.so module is required for authentication. The following is an example of how the module is used for the vsftpd FTP server in the /etc/pam.d/vsftpd PAM configuration file (the \ character at the end of the first line is not necessary if the directive is on a single line): auth required /lib/security/pam_listfile.so item=user \ sense=deny file=/etc/vsftpd.ftpusers onerr=succeed This instructs PAM to consult the /etc/vsftpd.ftpusers file and deny access to the service for any listed user. The administrator can change the name of this file, and can keep separate lists for each service or use one central list to deny access to multiple services. If the administrator wants to deny access to multiple services, a similar line can be added to the PAM configuration files, such as /etc/pam.d/pop and /etc/pam.d/imap for mail clients, or /etc/pam.d/ssh for SSH clients. For more information about PAM, refer to the chapter titled Using Pluggable Authentication Modules (PAM) in the Managing Single Sign-On and Smart Cards guide. Table 2.5. Disabling Root Using PAM | Effects | Does Not Affect |
|---|
Prevents root access to network services that are PAM aware. The following services are prevented from accessing the root account: login
gdm
kdm
xdm
ssh
scp
sftp
FTP clients Email clients Any PAM aware services
| Programs and services that are not PAM aware. |
2.1.4.3. Enabling Automatic LogoutsWhen the user is logged in as root, an unattended login session may pose a significant security risk. To reduce this risk, you can configure the system to automatically log out idle users after a fixed period of time: Make sure the screen package is installed. You can do so by running the following command as root: yum install screen
For more information on how to install packages in Red Hat Enterprise Linux, refer to Red Hat Enterprise Linux 6 Deployment Guide. As root, add the following line at the beginning of the /etc/profile file to make sure the processing of this file cannot be interrupted: trap "" 1 2 3 15 Add the following lines at the end of the /etc/profile file to start a screen session each time a user logs in to a virtual console or remotely: SCREENEXEC="screen"if [ -w $(tty) ]; then trap "exec $SCREENEXEC" 1 2 3 15 echo -n 'Starting session in 10 seconds' sleep 10 exec $SCREENEXECfi Note that each time a new session starts, a message will be displayed and the user will have to wait ten seconds. To adjust the time to wait before starting a session, change the value after the sleep command. Add the following lines to the /etc/screenrc configuration file to close the screen session after a given period of inactivity: idle 120 quit autodetach off This will set the time limit to 120 seconds. To adjust this limit, change the value after the idle directive. Alternatively, you can configure the system to only lock the session by using the following lines instead: idle 120 lockscreen autodetach off This way, a password will be required to unlock the session.
The changes take effect the next time a user logs in to the system. 2.1.4.4. Limiting Root AccessRather than completely denying access to the root user, the administrator may want to allow access only via setuid programs, such as su or sudo. For more information on su and sudo, refer to the Red Hat Enterprise Linux 6 Deployment Guide and the su(1) and sudo(8) man pages. Users may need to leave their workstation unattended for a number of reasons during everyday operation. This could present an opportunity for an attacker to physically access the machine, especially in environments with insufficient physical security measures (see Section 1.1.3.1, "Physical Controls"). Laptops are especially exposed since their mobility interferes with physical security. You can alleviate these risks by using session locking features which prevent access to the system until a correct password is entered. The main advantage of locking the screen instead of logging out is that a lock allows the user's processes (such as file transfers) to continue running. Logging out would stop these processes. 2.1.5.1. Locking GNOME Using gnome-screensaver-commandThe default desktop environment for Red Hat Enterprise Linux 6, GNOME, includes a feature which allows users to lock their screen at any time. There are several ways to activate the lock: Press the key combination specified in ⤍ ⤍ ⤍ ⤍ . The default combination is Ctrl+Alt+L. Select ⤍ on the panel. Execute the following command from a command line interface: gnome-screensaver-command -l
All of the techniques described have the same result: the screen saver is activated and the screen is locked. Users can then press any key to deactivate the screen saver, enter their password and continue working. Keep in mind that this function requires the gnome-screensaver process to be running. You can check whether this is the case by using any command which provides information about processes. For example, execute the following command from the terminal: pidof gnome-screensaver
If the gnome-screensaver process is currently running, a number denoting its identification number (PID) will be displayed on the screen after executing the command. If the process is not currently running, the command will provide no output at all. Refer to the gnome-screensaver-command(1) man page for additional information. The means of locking the screen described above rely on manual activation. Administrators should therefore advise their users to lock their computers every time they leave them unattended, even if only for a short period of time. 2.1.5.1.1. Automatic Lock on Screen Saver ActivationAs the name gnome-screensaver-command suggests, the locking functionality is tied to GNOME's screen saver. It is possible to tie the lock to the screen saver's activation, locking the workstation every time it is left unattended for a set period of time. This function is activated by default with a five minute timeout. To change the automatic locking settings, select ⤍ ⤍ on the main panel. This opens a window which allows setting the timeout period (the Regard the computer as idle after slider) and activating or deactivating the automatic lock (the Lock screen when screensaver is active check box). Disabling the Activate screensaver when computer is idle option in the Screensaver Preferences dialog prevents the screen saver from starting automatically. Automatic locking is therefore disabled as well, but it is still possible to lock the workstation manually using the techniques described in Section 2.1.5.1, "Locking GNOME Using gnome-screensaver-command". 2.1.5.1.2. Remote Session LockingYou can also lock a GNOME session remotely using ssh as long as the target workstation accepts connections over this protocol. To remotely lock the screen on a machine you have access to, execute the following command: ssh -X <username>@<server> "export DISPLAY=:0; gnome-screensaver-command -l"
Replace <username> with your user name and <server> with the IP address of the workstation you wish to lock. 2.1.5.2. Locking Virtual Consoles Using vlockUsers may also need to lock a virtual console. This can be done using a utility called vlock. To install this utility, execute the following command as root: ~]# yum install vlock After installation, any console session can be locked using the vlock command without any additional parameters. This locks the currently active virtual console session while still allowing access to the others. To prevent access to all virtual consoles on the workstation, execute the following: vlock -a
In this case, vlock locks the currently active console and the -a option prevents switching to other virtual consoles. Refer to the vlock(1) man page for additional information. There are several known issues relevant to the version of vlock currently available for Red Hat Enterprise Linux 6: The program does not currently allow unlocking consoles using the root password. Additional information can be found in BZ# 895066. Locking a console does not clear the screen and scrollback buffer, allowing anyone with physical access to the workstation to view previously issued commands and any output displayed in the console. Refer to BZ# 807369 for more information.
2.1.6. Available Network ServicesWhile user access to administrative controls is an important issue for system administrators within an organization, monitoring which network services are active is of paramount importance to anyone who administers and operates a Linux system. Many services under Red Hat Enterprise Linux 6 behave as network servers. If a network service is running on a machine, then a server application (called a daemon), is listening for connections on one or more network ports. Each of these servers should be treated as a potential avenue of attack. 2.1.6.1. Risks To ServicesNetwork services can pose many risks for Linux systems. Below is a list of some of the primary issues: Denial of Service Attacks (DoS) - By flooding a service with requests, a denial of service attack can render a system unusable as it tries to log and answer each request. Distributed Denial of Service Attack (DDoS) - A type of DoS attack which uses multiple compromised machines (often numbering in the thousands or more) to direct a coordinated attack on a service, flooding it with requests and making it unusable. Script Vulnerability Attacks - If a server is using scripts to execute server-side actions, as Web servers commonly do, a cracker can attack improperly written scripts. These script vulnerability attacks can lead to a buffer overflow condition or allow the attacker to alter files on the system. Buffer Overflow Attacks - Services that connect to ports numbered 0 through 1023 must run as an administrative user. If the application has an exploitable buffer overflow, an attacker could gain access to the system as the user running the daemon. Because exploitable buffer overflows exist, crackers use automated tools to identify systems with vulnerabilities, and once they have gained access, they use automated rootkits to maintain their access to the system.
The threat of buffer overflow vulnerabilities is mitigated in Red Hat Enterprise Linux by ExecShield, an executable memory segmentation and protection technology supported by x86-compatible uni- and multi-processor kernels. ExecShield reduces the risk of buffer overflow by separating virtual memory into executable and non-executable segments. Any program code that tries to execute outside of the executable segment (such as malicious code injected from a buffer overflow exploit) triggers a segmentation fault and terminates. Execshield also includes support for No eXecute ( NX) technology on AMD64 platforms and eXecute Disable ( XD) technology on Itanium and Intel® 64 systems. These technologies work in conjunction with ExecShield to prevent malicious code from running in the executable portion of virtual memory with a granularity of 4KB of executable code, lowering the risk of attack from stealthy buffer overflow exploits. To limit exposure to attacks over the network, all services that are unused should be turned off. 2.1.6.2. Identifying and Configuring ServicesTo enhance security, most network services installed with Red Hat Enterprise Linux are turned off by default. There are, however, some notable exceptions: cupsd - The default print server for Red Hat Enterprise Linux.
lpd - An alternative print server.
xinetd - A super server that controls connections to a range of subordinate servers, such as gssftp and telnet.
sendmail - The Sendmail Mail Transport Agent ( MTA) is enabled by default, but only listens for connections from the localhost. sshd - The OpenSSH server, which is a secure replacement for Telnet.
When determining whether to leave these services running, it is best to use common sense and err on the side of caution. For example, if a printer is not available, do not leave cupsd running. The same is true for portmap. If you do not mount NFSv3 volumes or use NIS (the ypbind service), then portmap should be disabled. 2.1.6.3. Insecure ServicesPotentially, any network service is insecure. This is why turning off unused services is so important. Exploits for services are routinely revealed and patched, making it very important to regularly update packages associated with any network service. Refer to Section 1.5, "Security Updates" for more information. Some network protocols are inherently more insecure than others. These include any services that: Transmit Usernames and Passwords Over a Network Unencrypted - Many older protocols, such as Telnet and FTP, do not encrypt the authentication session and should be avoided whenever possible. Transmit Sensitive Data Over a Network Unencrypted - Many protocols transmit data over the network unencrypted. These protocols include Telnet, FTP, HTTP, and SMTP. Many network file systems, such as NFS and SMB, also transmit information over the network unencrypted. It is the user's responsibility when using these protocols to limit what type of data is transmitted. Remote memory dump services, like netdump, transmit the contents of memory over the network unencrypted. Memory dumps can contain passwords or, even worse, database entries and other sensitive information. Other services like finger and rwhod reveal information about users of the system.
Examples of inherently insecure services include rlogin, rsh, telnet, and vsftpd. FTP is not as inherently dangerous to the security of the system as remote shells, but FTP servers must be carefully configured and monitored to avoid problems. Refer to Section 2.2.6, "Securing FTP" for more information about securing FTP servers. Services that should be carefully implemented and behind a firewall include: The next section discusses tools available to set up a simple firewall. 2.1.7. Personal FirewallsAfter the necessary network services are configured, it is important to implement a firewall. You should configure the necessary services and implement a firewall before connecting to the Internet or any other network that you do not trust. Firewalls prevent network packets from accessing the system's network interface. If a request is made to a port that is blocked by a firewall, the request is ignored. If a service is listening on one of these blocked ports, it does not receive the packets and is effectively disabled. For this reason, care should be taken when configuring a firewall to block access to ports not in use, while not blocking access to ports used by configured services. For most users, the best tool for configuring a simple firewall is the graphical firewall configuration tool which ships with Red Hat Enterprise Linux: the Firewall Configuration Tool (system-config-firewall). This tool creates broad iptables rules for a general-purpose firewall using a control panel interface. For advanced users and server administrators, manually configuring a firewall with iptables is probably a better option. Refer to Section 2.8, "Firewalls" for more information. Refer to Section 2.8.9, "IPTables" for a comprehensive guide to the iptables command. 2.1.8. Security Enhanced Communication ToolsAs the size and popularity of the Internet has grown, so has the threat of communication interception. Over the years, tools have been developed to encrypt communications as they are transferred over the network. Red Hat Enterprise Linux 6 ships with two basic tools that use high-level, public-key-cryptography-based encryption algorithms to protect information as it travels over the network. OpenSSH - A free implementation of the SSH protocol for encrypting network communication. Gnu Privacy Guard (GPG) - A free implementation of the PGP (Pretty Good Privacy) encryption application for encrypting data.
OpenSSH is a safer way to access a remote machine and replaces older, unencrypted services like telnet and rsh. OpenSSH includes a network service called sshd and three command line client applications: ssh - A secure remote console access client.
scp - A secure remote copy command.
sftp - A secure pseudo-ftp client that allows interactive file transfer sessions.
Although the sshd service is inherently secure, the service must be kept up-to-date to prevent security threats. Refer to Section 1.5, "Security Updates" for more information. GPG is one way to ensure private email communication. It can be used both to email sensitive data over public networks and to protect sensitive data on hard drives. |
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