Security-HOWTO
This document is a general overview of security issues that face the administrator of Linux systems. It covers general security philosophy and a number of specific examples of how to better secure your Linux system from intruders. Also included are pointers to security-related material and programs. Improvements, constructive criticism, additions and corrections are gratefully accepted. Please mail your feedback to both authors, with "Security HOWTO" in the subject.
This document covers some of the main issues that affect Linux security. General philosophy and net-born resources are discussed.
A number of other HOWTO documents overlap with security issues, and those documents have been pointed to wherever appropriate.
This document is not meant to be a up-to-date exploits document. Large numbers of new exploits happen all the time. This document will tell you where to look for such up-to-date information, and will give some general methods to prevent such exploits from taking place.
New versions of this document will be periodically posted to comp.os.linux.answers. They will also be added to the various sites that archive such information, including:
The very latest version of this document should also be available in various formats from:
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http://www.linuxsecurity.com/docs/Security-HOWTO
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http://www.tummy.com/security-howto
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All comments, error reports, additional information and criticism of all sorts should be directed to:
kevin-securityhowto@tummy.com
and
dave@linuxsecurity.com
Note: Please send your feedback to both authors. Also, be sure and include "Linux" "security", or "HOWTO" in your subject to avoid Kevin's spam filter.
No liability for the contents of this document can be accepted. Use the concepts, examples and other content at your own risk. Additionally, this is an early version, possibly with many inaccuracies or errors.
A number of the examples and descriptions use the !RedHat(tm) package layout and system setup. Your mileage may vary.
As far as we know, only programs that, under certain terms may be used or evaluated for personal purposes will be described. Most of the programs will be available, complete with source, under GNU terms.
This document is copyrighted (c)1998-2000 Kevin Fenzi and Dave Wreski, and distributed under the following terms:
whole or in part, in any medium, physical or electronic, as long as this copyright notice is retained on all copies. Commercial redistribution is allowed and encouraged; however, the authors would like to be notified of any such distributions. *
incorporating any Linux HOWTO documents must be covered under this copyright notice. That is, you may not produce a derivative work from a HOWTO and impose additional restrictions on its distribution. Exceptions to these rules may be granted under certain conditions; please contact the Linux HOWTO coordinator at the address given below. *
Linux HOWTO coordinator, at *
tjbynum@metalab.unc.edu
This document will attempt to explain some procedures and commonly-used software to help your Linux system be more secure. It is important to discuss some of the basic concepts first, and create a security foundation, before we get started.
In the ever-changing world of global data communications, inexpensive
Internet connections, and fast-paced software development, security is
becoming more and more of an issue. Security is now a basic
requirement because global computing is inherently insecure. As your
data goes from point A to point B on the Internet, for example, it may
pass through several other points along the way, giving other users
the opportunity to intercept, and even alter, it. Even other
users on your system may maliciously transform your data into
something you did not intend. Unauthorized access to your system may
be obtained by intruders, also known as "crackers", who then use
advanced knowledge to impersonate you, steal information from you, or
even deny you access to your own resources. If you're wondering
what the difference is between a "Hacker" and a "Cracker", see Eric
Raymond's document, "How to Become A Hacker", available at
http://www.tuxedo.org/esr/faqs/hacker-howto.html.
First, keep in mind that no computer system can ever be completely secure. All you can do is make it increasingly difficult for someone to compromise your system. For the average home Linux user, not much is required to keep the casual cracker at bay. However, for high-profile Linux users (banks, telecommunications companies, etc), much more work is required.
Another factor to take into account is that the more secure your system is, the more intrusive your security becomes. You need to decide where in this balancing act your system will still be usable, and yet secure for your purposes. For instance, you could require everyone dialing into your system to use a call-back modem to call them back at their home number. This is more secure, but if someone is not at home, it makes it difficult for them to login. You could also setup your Linux system with no network or connection to the Internet, but this limits its usefulness.
If you are a medium to large-sized site, you should establish a
security policy stating how much security is required by your site
and what auditing is in place to check it. You can find a well-known
security policy example at
http://www.faqs.org/rfcs/rfc2196.html. It has been recently
updated, and contains a great framework for establishing a security
policy for your company.
Before you attempt to secure your system, you should determine what level of threat you have to protect against, what risks you should or should not take, and how vulnerable your system is as a result. You should analyze your system to know what you're protecting, why you're protecting it, what value it has, and who has responsibility for your data and other assets.
attempting to access your computer. Can an intruder read or write files, or execute programs that could cause damage? Can they delete critical data? Can they prevent you or your company from getting important work done? Don't forget: someone gaining access to your account, or your system, can also impersonate you.
Additionally, having one insecure account on your system can result in your entire network being compromised. If you allow a single user to login using a .rhosts file, or to use an insecure service such as tftp, you risk an intruder getting 'his foot in the door'. Once the intruder has a user account on your system, or someone else's system, it can be used to gain access to another system, or another account.
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access to your network or computer. You must decide whom you trust to have access to your system, and what threat they could pose.
There are several types of intruders, and it is useful to keep their different characteristics in mind as you are securing your systems.
**The Curious - This type of intruder is basically interested in finding out what type of system and data you have.
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bring down your systems, or deface your web page, or otherwise force you to spend time and money recovering from the damage he has caused.
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trying to use your system to gain popularity and infamy. He might use your high-profile system to advertise his abilities.
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what data you have on your system. It might be someone who thinks you have something that could benefit him, financially or otherwise.
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setting up shop on your system and using its resources for their own purposes. He typically will run chat or irc servers, porn archive sites, or even DNS servers.
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interested in your system to use it to get into other systems. If your system is well-connected or a gateway to a number of internal hosts, you may well see this type trying to compromise your system.
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another network, and the potential for someone to gain unauthorized access.
What's at stake if someone breaks into your system? Of course the concerns of a dynamic PPP home user will be different from those of a company connecting their machine to the Internet, or another large network.
How much time would it take to retrieve/recreate any data that was lost? An initial time investment now can save ten times more time later if you have to recreate data that was lost. Have you checked your backup strategy, and verified your data lately?
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Create a simple, generic policy for your system that your users can readily understand and follow. It should protect the data you're safeguarding as well as the privacy of the users. Some things to consider adding are: who has access to the system (Can my friend use my account?), who's allowed to install software on the system, who owns what data, disaster recovery, and appropriate use of the system.
A generally-accepted security policy starts with the phrase
That which is not permitted is prohibited
This means that unless you grant access to a service for a user, that user shouldn't be using that service until you do grant access. Make sure the policies work on your regular user account. Saying, "Ah, I can't figure out this permissions problem, I'll just do it as root" can lead to security holes that are very obvious, and even ones that haven't been exploited yet.
rfc1244 is a document that describes how to create your own network security policy.
rfc1281 is a document that shows an example security policy with detailed descriptions of each step.
Finally, you might want to look at the COAST policy archive at
ftp://coast.cs.purdue.edu/pub/doc/policy to see what some
real-life security policies look like.
This document will discuss various means with which you can secure the assets you have worked hard for: your local machine, your data, your users, your network, even your reputation. What would happen to your reputation if an intruder deleted some of your users' data? Or defaced your web site? Or published your company's corporate project plan for next quarter? If you are planning a network installation, there are many factors you must take into account before adding a single machine to your network.
Even if you have a single dial up PPP account, or just a small site, this does not mean intruders won't be interested in your systems. Large, high-profile sites are not the only targets -- many intruders simply want to exploit as many sites as possible, regardless of their size. Additionally, they may use a security hole in your site to gain access to other sites you're connected to.
Intruders have a lot of time on their hands, and can avoid guessing how you've obscured your system just by trying all the possibilities. There are also a number of reasons an intruder may be interested in your systems, which we will discuss later.
Perhaps the area of security on which administrators concentrate most is host-based security. This typically involves making sure your own system is secure, and hoping everyone else on your network does the same. Choosing good passwords, securing your host's local network services, keeping good accounting records, and upgrading programs with known security exploits are among the things the local security administrator is responsible for doing. Although this is absolutely necessary, it can become a daunting task once your network becomes larger than a few machines.
Network security is as necessary as local host security. With hundreds, thousands, or more computers on the same network, you can't rely on each one of those systems being secure. Ensuring that only authorized users can use your network, building firewalls, using strong encryption, and ensuring there are no "rogue" (that is, unsecured) machines on your network are all part of the network security administrator's duties.
This document will discuss some of the techniques used to secure your site, and hopefully show you some of the ways to prevent an intruder from gaining access to what you are trying to protect.
One type of security that must be discussed is "security through obscurity". This means, for example, moving a service that has known security vulnerabilities to a non-standard port in hopes that attackers won't notice it's there and thus won't exploit it. Rest assured that they can determine that it's there and will exploit it. Security through obscurity is no security at all. Simply because you may have a small site, or a relatively low profile, does not mean an intruder won't be interested in what you have. We'll discuss what you're protecting in the next sections.
This document has been divided into a number of sections. They cover several broad security issues. The first, Physical Security, covers how you need to protect your physical machine from tampering. The second, Local Security, describes how to protect your system from tampering by local users. The third, Files and Filesystem Security, shows you how to setup your file systems and permissions on your files. The next, Password Security and Encryption, discusses how to use encryption to better secure your machine and network. Kernel Security discusses what kernel options you should set or be aware of for a more secure system. Network Security, describes how to better secure your Linux system from network attacks. Security Preparation, discusses how to prepare your machine(s) before bringing them on-line. Next, What To Do During and After a Break-in, discusses what to do when you detect a system compromise in progress or detect one that has recently happened. In Security Resources, some primary security resources are enumerated. The Q and A section Frequently Asked Questions, answers some frequently-asked questions, and finally a conclusion in Conclusion.
The two main points to realize when reading this document are:
/var/log/messages and keep an eye on your system, and *
current versions of software and have upgraded per security alerts. Just doing this will help make your system markedly more secure. *
The first layer of security you need to take into account is the physical security of your computer systems. Who has direct physical access to your machine? Should they? Can you protect your machine from their tampering? Should you?
How much physical security you need on your system is very dependent on your situation, and/or budget.
If you are a home user, you probably don't need a lot (although you might need to protect your machine from tampering by children or annoying relatives). If you are in a lab, you need considerably more, but users will still need to be able to get work done on the machines. Many of the following sections will help out. If you are in an office, you may or may not need to secure your machine off-hours or while you are away. At some companies, leaving your console unsecured is a termination offense.
Obvious physical security methods such as locks on doors, cables, locked cabinets, and video surveillance are all good ideas, but beyond the scope of this document. :)
Many modern PC cases include a "locking" feature. Usually this will be a socket on the front of the case that allows you to turn an included key to a locked or unlocked position. Case locks can help prevent someone from stealing your PC, or opening up the case and directly manipulating/stealing your hardware. They can also sometimes prevent someone from rebooting your computer from their own floppy or other hardware.
These case locks do different things according to the support in the motherboard and how the case is constructed. On many PC's they make it so you have to break the case to get the case open. On some others, they will not let you plug in new keyboards or mice. Check your motherboard or case instructions for more information. This can sometimes be a very useful feature, even though the locks are usually very low-quality and can easily be defeated by attackers with locksmithing.
Some machines (most notably SPARC's and macs) have a dongle on the back that, if you put a cable through, attackers would have to cut the cable or break the case to get into it. Just putting a padlock or combo lock through these can be a good deterrent to someone stealing your machine.
The BIOS is the lowest level of software that configures or manipulates your x86-based hardware. LILO and other Linux boot methods access the BIOS to determine how to boot up your Linux machine. Other hardware that Linux runs on has similar software (Open Firmware on Macs and new Suns, Sun boot PROM, etc...). You can use your BIOS to prevent attackers from rebooting your machine and manipulating your Linux system.
Many PC BIOSs let you set a boot password. This doesn't provide all that much security (the BIOS can be reset, or removed if someone can get into the case), but might be a good deterrent (i.e. it will take time and leave traces of tampering). Similarly, on S/Linux (Linux for SPARC(tm) processor machines), your EEPROM can be set to require a boot-up password. This might slow attackers down.
Another risk of trusting BIOS passwords to secure your system is the default password problem. Most BIOS makers don't expect people to open up their computer and disconnect batteries if they forget their password and have equipped their BIOSes with default passwords that work regardless of your chosen password. Some of the more common passwords include:
j262 AWARD_SW AWARD_PW lkwpeter Biostar AMI Award bios BIOS setup cmos AMI!SW1 AMI?SW1 password hewittrand shift + s y x z
I tested an Award BIOS and AWARD_PW worked. These passwords are quite
easily available from manufacturers' websites and
http://astalavista.box.sk
and as such a BIOS password cannot be considered adequate protection
from a knowledgeable attacker.
Many x86 BIOSs also allow you to specify various other good security settings. Check your BIOS manual or look at it the next time you boot up. For example, some BIOSs disallow booting from floppy drives and some require passwords to access some BIOS features.
Note: If you have a server machine, and you set up a boot password, your machine will not boot up unattended. Keep in mind that you will need to come in and supply the password in the event of a power failure. ;(
The various Linux boot loaders also can have a boot password set. LILO, for example, has password and restricted settings; password requires password at boot time, whereas restricted requires a boot-time password only if you specify options (such as single) at the LILO prompt.
From the lilo.conf man page:
password=password The per-image option `password=...' (see below) applies to all images. restricted The per-image option `restricted' (see below) applies to all images. password=password Protect the image by a password. restricted A password is only required to boot the image if parameters are specified on the command line (e.g. single).
Keep in mind when setting all these passwords that you need to remember them. :) Also remember that these passwords will merely slow the determined attacker. They won't prevent someone from booting from a floppy, and mounting your root partition. If you are using security in conjunction with a boot loader, you might as well disable booting from a floppy in your computer's BIOS, and password-protect the BIOS.
Also keep in mind that the /etc/lilo.conf will need to be mode "600" (readable and writing for root only), or others will be able to read your passwords!
If anyone has security-related information from a different boot loader, we would love to hear it. (grub, silo, milo, linload, etc).
Note: If you have a server machine, and you set up a boot password, your machine will not boot up unattended. Keep in mind that you will need to come in and supply the password in the event of a power failure. ;(
If you wander away from your machine from time to time, it is nice to be able to "lock" your console so that no one can tamper with, or look at, your work. Two programs that do this are: xlock and vlock.
xlock is a X display locker. It should be included in any Linux distributions that support X. Check out the man page for it for more options, but in general you can run xlock from any xterm on your console and it will lock the display and require your password to unlock.
vlock is a simple little program that allows you to lock some or all of the virtual consoles on your Linux box. You can lock just the one you are working in or all of them. If you just lock one, others can come in and use the console; they will just not be able to use your virtual console until you unlock it. vlock ships with !RedHat Linux, but your mileage may vary.
Of course locking your console will prevent someone from tampering with your work, but won't prevent them from rebooting your machine or otherwise disrupting your work. It also does not prevent them from accessing your machine from another machine on the network and causing problems.
More importantly, it does not prevent someone from switching out of the X Window System entirely, and going to a normal virtual console login prompt, or to the VC that X11 was started from, and suspending it, thus obtaining your privileges. For this reason, you might consider only using it while under control of xdm.
If you have a webcam or a microphone attached to your system, you should consider if there is some danger of a attacker gaining access to those devices. When not in use, unplugging or removing such devices might be an option. Otherwise you should carefully read and look at any software with provides access to such devices.
The first thing to always note is when your machine was rebooted. Since Linux is a robust and stable OS, the only times your machine should reboot is when you take it down for OS upgrades, hardware swapping, or the like. If your machine has rebooted without you doing it, that may be a sign that an intruder has compromised it. Many of the ways that your machine can be compromised require the intruder to reboot or power off your machine.
Check for signs of tampering on the case and computer area. Although many intruders clean traces of their presence out of logs, it's a good idea to check through them all and note any discrepancy.
It is also a good idea to store log data at a secure location, such as a dedicated log server within your well-protected network. Once a machine has been compromised, log data becomes of little use as it most likely has also been modified by the intruder.
The syslog daemon can be configured to automatically send log data to a central syslog server, but this is typically sent unencrypted, allowing an intruder to view data as it is being transferred. This may reveal information about your network that is not intended to be public. There are syslog daemons available that encrypt the data as it is being sent.
Also be aware that faking syslog messages is easy -- with an exploit program having been published. Syslog even accepts net log entries claiming to come from the local host without indicating their true origin.
Some things to check for in your logs:
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We will discuss system log data later in the HOWTO.
The next thing to take a look at is the security in your system against attacks from local users. Did we just say local users? Yes!
Getting access to a local user account is one of the first things that system intruders attempt while on their way to exploiting the root account. With lax local security, they can then "upgrade" their normal user access to root access using a variety of bugs and poorly setup local services. If you make sure your local security is tight, then the intruder will have another hurdle to jump.
Local users can also cause a lot of havoc with your system even (especially) if they really are who they say they are. Providing accounts to people you don't know or for whom you have no contact information is a very bad idea.
You should make sure you provide user accounts with only the minimal requirements for the task they need to do. If you provide your son (age 10) with an account, you might want him to only have access to a word processor or drawing program, but be unable to delete data that is not his.
Several good rules of thumb when allowing other people legitimate access to your Linux machine:
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using the 'last' command and/or checking log files for any activity by the user. *
to ease account maintenance, and permits easier analysis of log data. *
accounts also provide accountability, and this is not possible with group accounts. *
Many local user accounts that are used in security compromises have not been used in months or years. Since no one is using them they, provide the ideal attack vehicle.
The most sought-after account on your machine is the root (superuser) account. This account has authority over the entire machine, which may also include authority over other machines on the network. Remember that you should only use the root account for very short, specific tasks, and should mostly run as a normal user. Even small mistakes made while logged in as the root user can cause problems. The less time you are on with root privileges, the safer you will be.
Several tricks to avoid messing up your own box as root:
non-destructive way...especially commands that use globing: e.g., if you want to do rm foo*.bak, first do ls foo*.bak and make sure you are going to delete the files you think you are. Using echo in place of destructive commands also sometimes works. *
confirmation for deletion of files. *
* Only become root to do single specific tasks. If you find yourself trying to figure out how to do something, go back to a normal user shell until you are sure what needs to be done by root. *
path (that is, the PATH environment variable) specifies the directories in which the shell searches for programs. Try to limit the command path for the root user as much as possible, and never include . (which means "the current directory") in your PATH. Additionally, never have writable directories in your search path, as this can allow attackers to modify or place new binaries in your search path, allowing them to run as root the next time you run that command. *
as root. They are subject to many sorts of attacks, and are downright dangerous when run as root. Never create a .rhosts file for root. *
login from. By default (on Red Hat Linux) this is set to only the local virtual consoles(vtys). Be very wary of adding anything else to this file. You should be able to login remotely as your regular user account and then su if you need to (hopefully over ssh or other encrypted channel), so there is no need to be able to login directly as root. *
affect a lot of things. Think before you type! *
If you absolutely positively need to allow someone (hopefully very trusted) to have root access to your machine, there are a few tools that can help. sudo allows users to use their password to access a limited set of commands as root. This would allow you to, for instance, let a user be able to eject and mount removable media on your Linux box, but have no other root privileges. sudo also keeps a log of all successful and unsuccessful sudo attempts, allowing you to track down who used what command to do what. For this reason sudo works well even in places where a number of people have root access, because it helps you keep track of changes made.
Although sudo can be used to give specific users specific privileges for specific tasks, it does have several shortcomings. It should be used only for a limited set of tasks, like restarting a server, or adding new users. Any program that offers a shell escape will give root access to a user invoking it via sudo. This includes most editors, for example. Also, a program as innocuous as /bin/cat can be used to overwrite files, which could allow root to be exploited. Consider sudo as a means for accountability, and don't expect it to replace the root user and still be secure.
A few minutes of preparation and planning ahead before putting your systems on-line can help to protect them and the data stored on them.
SUID/SGID programs to be run from there. Use the nosuid option in /etc/fstab for partitions that are writable by others than root. You may also wish to use nodev and noexec on users' home partitions, as well as /var, thus prohibiting execution of programs, and creation of character or block devices, which should never be necessary anyway. *
/etc/exports with the most restrictive access possible. This means not using wild cards, not allowing root write access, and exporting read-only wherever possible. *
possible. See umask settings. *
NFS, be sure to configure /etc/exports with suitable restrictions. Typically, using `nodev', `nosuid', and perhaps `noexec', are desirable. *
default. You can control the per-user limits using the resource-limits PAM module and /etc/pam.d/limits.conf. For example, limits for group users might look like this:
@users hard core 0 @users hard nproc 50 @users hard rss 5000
This says to prohibit the creation of core files, restrict the number of processes to 50, and restrict memory usage per user to 5M.
You can also use the /etc/login.defs configuration file to set the same limits.
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for all users on your system. Their integrity must be maintained because they can be used to determine when and from where a user (or potential intruder) has entered your system. These files should also have 644 permissions, without affecting normal system operation.
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overwriting a file that must be protected. It also prevents someone from creating a hard link to the file. See the chattr(1) man page for information on the immutable bit.
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* SUID and SGID files on your system are a potential security risk, and should be monitored closely. Because these programs grant special privileges to the user who is executing them, it is necessary to ensure that insecure programs are not installed. A favorite trick of crackers is to exploit SUID-root programs, then leave a SUID program as a back door to get in the next time, even if the original hole is plugged.
Find all SUID/SGID programs on your system, and keep track of what they are, so you are aware of any changes which could indicate a potential intruder. Use the following command to find all SUID/SGID programs on your system:
root# find / -type f \( -perm -04000 -o -perm -02000 \)
The Debian distribution runs a job each night to determine what SUID files exist. It then compares this to the previous night's run. You can look in /var/log/setuid* for this log.
You can remove the SUID or SGID permissions on a suspicious program with chmod, then restore them back if you absolutely feel it is necessary.
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hole if a cracker gains access to your system and modifies them. Additionally, world-writable directories are dangerous, since they allow a cracker to add or delete files as he wishes. To locate all world-writable files on your system, use the following command:
root# find / -perm -2 ! -type l -ls
and be sure you know why those files are writable. In the normal course of operation, several files will be world-writable, including some from /dev, and symbolic links, thus the ! -type l which excludes these from the previous find command.
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Unowned files may also be an indication an intruder has accessed your system. You can locate files on your system that have no owner, or belong to no group with the command:
root# find / -nouser -o -nogroup -print
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administration duties, as these files should not be permitted on your system. Remember, a cracker only needs one insecure account to potentially gain access to your entire network. You can locate all .rhosts files on your system with the following command:
root# find /home -name .rhosts -print
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Finally, before changing permissions on any system files, make sure you understand what you are doing. Never change permissions on a file because it seems like the easy way to get things working. Always determine why the file has that permission before changing it.
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The umask command can be used to determine the default file creation mode on your system. It is the octal complement of the desired file mode. If files are created without any regard to their permissions settings, the user could inadvertently give read or write permission to someone that should not have this permission. Typical umask settings include 022, 027, and 077 (which is the most restrictive). Normally the umask is set in /etc/profile, so it applies to all users on the system. The file creation mask can be calculated by subtracting the desired value from 777. In other words, a umask of 777 would cause newly-created files to contain no read, write or execute permission for anyone. A mask of 666 would cause newly-created files to have a mask of 111. For example, you may have a line that looks like this:
umask 033
Be sure to make root's umask 077, which will disable read, write, and execute permission for other users, unless explicitly changed using chmod. In this case, newly-created directories would have 744 permissions, obtained by subtracting 033 from 777. Newly-created files using the 033 umask would have permissions of 644.
If you are using Red Hat, and adhere to their user and group ID creation scheme (User Private Groups), it is only necessary to use 002 for a umask. This is due to the fact that the default configuration is one user per group.
It's important to ensure that your system files are not open for casual editing by users and groups who shouldn't be doing such system maintenance.
Unix separates access control on files and directories according to three characteristics: owner, group, and other. There is always exactly one owner, any number of members of the group, and everyone else.
A quick explanation of Unix permissions:
Ownership - Which user(s) and group(s) retain(s) control of the permission settings of the node and parent of the node
Permissions - Bits capable of being set or reset to allow certain types of access to it. Permissions for directories may have a different meaning than the same set of permissions on files.
Read:
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Write:
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Execute:
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The "sticky bit" also has a different meaning when applied to directories than when applied to files. If the sticky bit is set on a directory, then a user may only delete files that the he owns or for which he has explicit write permission granted, even when he has write access to the directory. This is designed for directories like /tmp, which are world-writable, but where it may not be desirable to allow any user to delete files at will. The sticky bit is seen as a t in a long directory listing.
This describes set-user-id permissions on the file. When the set user ID access mode is set in the owner permissions, and the file is executable, processes which run it are granted access to system resources based on user who owns the file, as opposed to the user who created the process. This is the cause of many "buffer overflow" exploits.
If set in the group permissions, this bit controls the "set group id" status of a file. This behaves the same way as SUID, except the group is affected instead. The file must be executable for this to have any effect.
If you set the SGID bit on a directory (with chmod g+s directory), files created in that directory will have their group set to the directory's group.
You - The owner of the file
Group - The group you belong to
Everyone - Anyone on the system that is not the owner or a member of the group
File Example:
1st bit - directory? (no) 2nd bit - read by owner? (yes, by kevin) 3rd bit - write by owner? (yes, by kevin) 4th bit - execute by owner? (no) 5th bit - read by group? (yes, by users) 6th bit - write by group? (no) 7th bit - execute by group? (no) 8th bit - read by everyone? (yes, by everyone) 9th bit - write by everyone? (no) 10th bit - execute by everyone? (no)
The following lines are examples of the minimum sets of permissions that are required to perform the access described. You may want to give more permission than what's listed here, but this should describe what these minimum permissions on files do:
--w------- Allows the owner to modify or delete the file (Note that anyone with write permission to the directory the file is in can overwrite it and thus delete it) ---x------ The owner can execute this program, but not shell scripts, which still need read permission ---s------ Will execute with effective User ID = to owner --------s- Will execute with effective Group ID = to group
files ---t------ No effect. (formerly sticky bit)
Directory Example:
drwxr-xr-x 3 kevin users 512 Sep 19 13:47 .public_html/ 1st bit - directory? (yes, it contains many files) 2nd bit - read by owner? (yes, by kevin) 3rd bit - write by owner? (yes, by kevin) 4th bit - execute by owner? (yes, by kevin) 5th bit - read by group? (yes, by users 6th bit - write by group? (no) 7th bit - execute by group? (yes, by users) 8th bit - read by everyone? (yes, by everyone) 9th bit - write by everyone? (no) 10th bit - execute by everyone? (yes, by everyone)
The following lines are examples of the minimum sets of permissions that are required to perform the access described. You may want to give more permission than what's listed, but this should describe what these minimum permissions on directories do:
dr-------- The contents can be listed, but file attributes can't be read d--x------ The directory can be entered, and used in full execution paths dr-x------ File attributes can be read by owner d-wx------ Files can be created/deleted, even if the directory isn't the current one d------x-t Prevents files from deletion by others with write access. Used on /tmp d---s--s-- No effect
System configuration files (usually in /etc) are usually mode 640 (-rw-r-----), and owned by root. Depending on your site's security requirements, you might adjust this. Never leave any system files writable by a group or everyone. Some configuration files, including /etc/shadow, should only be readable by root, and directories in /etc should at least not be accessible by others.
; SUID Shell Scripts:
SUID shell scripts are a serious security risk, and for this reason the kernel will not honor them. Regardless of how secure you think the shell script is, it can be exploited to give the cracker a root shell.
Another very good way to detect local (and also network) attacks on your system is to run an integrity checker like Tripwire, Aide or Osiris. These integrety checkers run a number of checksums on all your important binaries and config files and compares them against a database of former, known-good values as a reference. Thus, any changes in the files will be flagged.
It's a good idea to install these sorts of programs onto a floppy, and then physically set the write protect on the floppy. This way intruders can't tamper with the integrety checker itself or change the database. Once you have something like this setup, it's a good idea to run it as part of your normal security administration duties to see if anything has changed.
You can even add a crontab entry to run the checker from your floppy every night and mail you the results in the morning. Something like:
MAILTO=kevin
15 05 * * * root /usr/local/adm/tcheck/tripwire
will mail you a report each morning at 5:15am.
Integrity checkers can be a godsend to detecting intruders before you would otherwise notice them. Since a lot of files change on the average system, you have to be careful what is cracker activity and what is your own doing.
You can find the freely available unsusported version of
Tripwire at
http://www.tripwire.org,
free of charge. Manuals and support can be purchased.
Aide can be found at
http://www.cs.tut.fi/rammer/aide.html.
Osiris can be found at
http://www.shmoo.com/osiris/.
"Trojan Horses" are named after the fabled ploy in Homer's "Iliad". The idea is that a cracker distributes a program or binary that sounds great, and encourages other people to download it and run it as root. Then the program can compromise their system while they are not paying attention. While they think the binary they just pulled down does one thing (and it might very well), it also compromises their security.
You should take care of what programs you install on your machine. !RedHat provides MD5 checksums and PGP signatures on its RPM files so you can verify you are installing the real thing. Other distributions have similar methods. You should never run any unfamiliar binary, for which you don't have the source, as root! Few attackers are willing to release source code to public scrutiny.
Although it can be complex, make sure you are getting the source for a program from its real distribution site. If the program is going to run as root, make sure either you or someone you trust has looked over the source and verified it.
One of the most important security features used today are passwords. It is important for both you and all your users to have secure, unguessable passwords. Most of the more recent Linux distributions include passwd programs that do not allow you to set a easily guessable password. Make sure your passwd program is up to date and has these features.
In-depth discussion of encryption is beyond the scope of this document, but an introduction is in order. Encryption is very useful, possibly even necessary in this day and age. There are all sorts of methods of encrypting data, each with its own set of characteristics.
Most Unicies (and Linux is no exception) primarily use a one-way encryption algorithm, called DES (Data Encryption Standard) to encrypt your passwords. This encrypted password is then stored in (typically) /etc/passwd (or less commonly) /etc/shadow. When you attempt to login, the password you type in is encrypted again and compared with the entry in the file that stores your passwords. If they match, it must be the same password, and you are allowed access. Although DES is a two-way encryption algorithm (you can code and then decode a message, given the right keys), the variant that most Unixes use is one-way. This means that it should not be possible to reverse the encryption to get the password from the contents of /etc/passwd (or /etc/shadow).
Brute force attacks, such as "Crack" or "John the Ripper" (see Section crack ) can often guess passwords unless your password is sufficiently random. PAM modules (see below) allow you to use a different encryption routine with your passwords (MD5 or the like). You can use Crack to your advantage, as well. Consider periodically running Crack against your own password database, to find insecure passwords. Then contact the offending user, and instruct him to change his password.
You can go to
http://consult.cern.ch/writeup/security/security_3.html for
information on how to choose a good password.
Public-key cryptography, such as that used for PGP, uses one key for encryption, and one key for decryption. Traditional cryptography, however, uses the same key for encryption and decryption; this key must be known to both parties, and thus somehow transferred from one to the other securely.
To alleviate the need to securely transmit the encryption key, public-key encryption uses two separate keys: a public key and a private key. Each person's public key is available by anyone to do the encryption, while at the same time each person keeps his or her private key to decrypt messages encrypted with the correct public key.
There are advantages to both public key and private key cryptography, and you can read about those differences in the RSA Cryptography FAQ, listed at the end of this section.
PGP (Pretty Good Privacy) is well-supported on Linux. Versions 2.6.2
and 5.0 are known to work well. For a good primer on PGP and how to
use it, take a look at the PGP FAQ:
http://www.pgp.com/service/export/faq/55faq.cgi
Be sure to use the version that is applicable to your country. Due to export restrictions by the US Government, strong-encryption is prohibited from being transferred in electronic form outside the country.
US export controls are now managed by EAR (Export Administration Regulations). They are no longer governed by ITAR.
There is also a step-by-step guide for configuring PGP on Linux
available at
http://mercury.chem.pitt.edu/angel/!LinuxFocus/English/November1997/article7.html.
It was written for the international version of PGP, but is easily
adaptable to the United States version. You may also need a patch for
some of the latest versions of Linux; the patch is available at
ftp://metalab.unc.edu/pub/Linux/apps/crypto.
There is a project maintaining a free re-implementation of pgp with
open source. GnuPG is a complete and free replacement for PGP. Because
it does not use IDEA or RSA it can be used without any
restrictions. GnuPG is in compliance with
OpenPGP.
See the GNU Privacy Guard web page for more information:
http://www.gnupg.org/.
More information on cryptography can be found in the RSA cryptography
FAQ, available at
http://www.rsa.com/rsalabs/newfaq/. Here you will find
information on such terms as "Diffie-Hellman", "public-key
cryptography", "digital certificates", etc.
Often users ask about the differences between the various security and encryption protocols, and how to use them. While this isn't an encryption document, it is a good idea to explain briefly what each protocol is, and where to find more information.
method developed by Netscape to provide security over the Internet.
It supports several different encryption protocols, and provides
client and server authentication. SSL operates at the transport
layer, creates a secure encrypted channel of data, and thus can
seamlessly encrypt data of many types. This is most commonly seen
when going to a secure site to view a secure online document with
Communicator, and serves as the basis for secure communications with
Communicator, as well as many other Netscape Communications data
encryption. More information can be found at
http://www.consensus.com/security/ssl-talk-faq.html.
Information on Netscape's other security implementations, and a good
starting point for these protocols is available at
http://home.netscape.com/info/security-doc.html. It's also
worth noting that the SSL protocol can be used to pass many other
common protocols, "wrapping" them for security. See
http://www.quiltaholic.com/rickk/sslwrap/
*
security services across the Internet. It was designed to provide confidentiality, authentication, integrity, and non-repudiability [cannot be mistaken for someone else? while supporting multiple key-management mechanisms and cryptographic algorithms via option negotiation between the parties involved in each transaction. S-HTTP is limited to the specific software that is implementing it, and encrypts each message individually.
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