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#1 Blake


    Former Commander In Chief

  • Retired Admin
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Posted 18 August 2003 - 08:00 AM

Dave Wreski,
v0.98, 22 August 1998

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.

1. Introduction
This document covers the major security issues that affect Linux security. General philosophy and net born resources are also discussed.

A number of other HOWTO documents overlap with security issues, and those 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 point you where to look for such up to date information, and some general methods to prevent such exploits from taking place.

Additionally, while there are several resources available in various places on the Internet regarding general security, we are trying to consolidate much of this general information, and provide information a general system administrator can use as a practical guide. This should in no means substitute for reading books on the appropriate subject, and practical experience which works for you.

The US Government has several organizations devoted to computer security, and generally the information they have online is quite extensive, and very useful. A general introduction to computer security is available at http://csrc.ncsl.nis...istpubs/800-12/ which will be very useful.

See the References section for pointers to security references. It is also a tremendous advantage if you understand how TCP/IP works, and some of the common system administration functions. You might find this guide helpful in a beginner introduction http://www.sunworld....1-sysadmin.html While it is Solaris-centric, you'll find much of this information general enough to still be applicable.

You may also find this link helpful http://www.cis.ohio-...adwork/cis694q/ for another introduction to TCP, including how sequence numbers work, which is the foundation of ``man in the middle'' attacks, a description of the SYN/ACK handshake used to initiate a TCP connection, a description of a few of the problems in TCP/IP, a few other types of attacks, and how they work, as well as some solutions to these problems.

1.1 New Versions of this Document
New versions of this document will be periodically posted to comp.os.linux.answers. They will also be added to the various anonymous FTP sites who archive such information, including:

In addition, you should generally be able to find this document on the Linux Documentation Project Web home page via:

Finally, the very latest version of this document should also be available in various formats from either of the following:

1.2 Feedback
All comments, error reports, additional information and criticism of all sorts should be directed to:


1.3 Disclaimer
No liability for the contents of this documents can be accepted. Use the concepts, examples and other content at your own risk. Additionally, this is an early version, with many possibilities for inaccuracies and errors. It is provided "as is" without express or implied warranty.

Many of the examples and descriptions in this document refer specifically to the Red Hat distribution. We are very interested in incorporating other distributions as well. If you have ideas on how other distributions perform the same measures as are listed here, we would be interested in hearing from you.

1.4 Copyright Information
This document is copyrighted ©1998 Dave Wreski, and distributed under the following terms:

This document may be reproduced and distributed in 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.

All translations, derivative works, or aggregate works incorporating any Linux documents must be covered under this copyright notice. That is, you may not produce a derivative work from this document and impose additional restrictions on its distribution. Exceptions to these rules may be granted under certain conditions; please contact the author of this document for further information.

2. Overview
This document will discuss procedures and commonly used software to increase the trust level of your system. It is important to discuss the basic concepts first, and create a security foundation before we get started.

2.1 Organization of This Document
This document has been dividedinto a number of sections. They cover several broad kinds of security issues. So far these sections include:

Physical Security - covers how you need to protect your physical machine from tampering.

Files and File System Security - shows you how to setup your file-systems and permissions on your files.

Data Encryption, Cryptography and Authentication - discusses how to use encryption to better secure your machine and network.

Kernel Security - discusses what you can do at the kernel level to protect yourself, as well as improve security.

Network Security - describes how to better secure your Linux system from network attacks.

Incident Control - discusses the six stages in dealing with an incident, including the preperation before one actually occurs.
Host Security - discusses what can be done to further secure individual hosts, and what to watch out for.

Exploits - attempts to familiarize the reader with some of the most common types of exploits, so you know when and how to recognize one when it does happen.

Security Sources - Here is a list of the resources that are most usable to a Linux Security Administrator.

Firewalls and Border Patrol - discusses the various types of firewalls available for Linux, as well as pointers to general firewall information.

Glossary - Here is a list of the most frequently used acronyms and definitions that a Security Administrator should be aware of to be effective.

Frequently Asked Questions - These FAQs should help to reduce some of the more frequently encountered problems.

The two main points to realize when reading this document are:

Be aware of your system. Check system logs such as /var/log/messages and keep an eye on your system, and
Keep your system up to date by making sure you have installed the current versions of software and have upgraded per security alerts, or otherwise improved the security of any suspect programs. Just doing this will help make your system markedly more secure.

2.2 Host Security
Perhaps the area of most concentration on security is done with 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 services your hosts offer, keeping good accounting records, and upgrading programs that have 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 of machines becomes larger. It can be said that host-based security does not scale. A host-based security exploit must be repaired on each machine on your network, which requires accessing each machine individually and applying the fix.

2.3 Network Security
Network security is as necessary as local host security. With your single system, or a distributed computing network, the Internet, or hundreds, if not thousands or more computers on the same network, you can't rely on each one of those systems being secure. Making sure authorized users are the only ones permitted to use your network resources, building firewalls, using strong encryption, and ensuring there are no rogue, or 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.

2.4 Security Through Obscurity
One type of security that must be discussed is ``security through obscurity''. This means that by doing something like changing the login name from 'root' to 'toor', for example, to try and obscure someone from breaking into your system as root may be thought of as a false sense of security, and can result in very unpleasant and unexpected consequences.

However, it can also be used to your benefit if done properly. If you tell all the users who are authorized to use the root account on your machines to use the root equivilent instead, entries in the /var/log/secure for the real root user would surely indicate an attempted break-in, giving you some advance notice. You'll have to decide if this advantage outweighs the additional administration overhead.

In most cases, though, any system attacker will quickly see through such empty security measures. Simply because you may have a small site, or relatively low profile does not mean an intruder won't be interested in what you have. We'll discuss what your protecting in the next sections.

2.5 Why Do We Need Security?
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, your data. 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 still wondering what the difference is between a ``Hacker'' and a ``Cracker'', see Eric Raymond's document, ``How to Become A Hacker'', available at http://sagan.earthsp...cker-howto.html.

2.6 How Vulnerable Are We?
While it is difficult to determine just how vulnerable a particular system is, there are several indications we can use:

The Computer Emergency Response Team consistently reports an increase in computer vulnerabilities and exploits.

TCP and UDP, the protocols that comprise the Internet, were not written with security as their first priority when it was created more than 30 years ago.

A version of software on one host has the same vulnerabilities as the same version of software on another host. Using this information, an intruder can exploit multiple systems using the same attack method.

Many administrators don't even take simple security measures necessary to protect their site, or don't understand the ramifications of implementing some services. Many administrators are not given the additional time necessary to integrate the necessary security measures.

2.7 How Secure Is Secure?
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. 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 is still 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 makes it harder to surf the web.

If you have more than one person logging on to your machine, or machines, 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 It has been recently updated, and contains a great framework for establishing a security policy for your company.

It is even advisable to generate a security policy for systems with just two users, or even a desktop machine, used for normal Internet dialup access.

While developing your security policy, you will have to decide on that balance between security and ease-of-use. You will also need to determine the current level of security on your systems. Ask yourself questions such as these:

How often do you change your passwords?
How would you improve security?
How many guessable passwords are there on your system?
Do you have any intentional backdoors to your system?
Improving security at your site will have to be a progressive process -- you can not secure your systems overnight, and most likely your users will be reluctant to change, because they feel they will be losing usability. Also, don't discount the possibility that there are several packages and binaries on your system that are not even used, and can be removed without affecting functionality, yet improving security by limiting the available exploits.

2.8 What Are You Trying to Protect?
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.

Risk is the possibility that an intruder may be successful in attempting to access your computer. Can an intruder read, write files, or execute programs that could cause damage? Can they delete critical data? 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. A single user that is allowed to login using an rhosts file, or through the use of an insecure service, increases the ability for the intruder using this to ``get his foot in the door''. Once the intruder has even a normal user account on your system, or someone else's system, the likelihood it can be used to gain access to another system, or another account is quite high.

Threat is typically from someone with motivation to gain unauthorized access to your network, or computer. You must decide who you trust to have access to your system, and what threat they could impose.
There are several types of intruders, and it is useful to keep the 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.
The Malicious - This type of intruder is out to either bring down your systems, or deface your web page, or otherwise cause you time and money to recover.
The High-Profile Intruder - This type of intruder is trying to use your system to gain popularity and infamy. He might use your high-profile system to advertise his abilities.
The Competition - This type of intruder is interested in what data you have on your system. It might be someone who thinks you have something that could benefit him financially, or otherwise.

Vulnerability - describes how well protected your computer is from another network, and the potential for someone gaining unauthorized access.
What's at stake if someone breaks into your system? How much is it worth? When making the evaluation, you should consider items such as computer hardware and software, intellectual property, employee's, resources, such as network bandwidth, disk space, etc.

Of course the concerns of a dynamic PPP home user will be different than 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?

2.9 Developing A Security Policy
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 expressly 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 this permissions problem out, 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.

Additionally, there are several questions you will need to answer to successfully develop a security policy:

What level of security do your users expect?
How much is there to protect, and what is it worth?
Can you afford the down-time of an intrusion?
Should there be different levels of security for different groups?
Do you trust your internal users?
Have you found the balance between acceptable risk and secure?
You should develop a plan on who to contact when there is a security problem that needs attention.

There are quite a few documents available on developing a Site Security Policy. You can start with this one from Sun Microsystems

2.10 Means of Securing Your Site
This document will discuss various means in which you can secure the assets you have worked hard for: your local machine, data, users, network, even your reputation. What would happen to your reputation if an intruder deleted some of your user's 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 dialup 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 several reasons an intruder may be interested in your systems, which we will discuss later.

See the Host Security and Network Security sections for further information on steps to perform to secure your hosts.

2.11 Temporary Changes
Changes made for supposedly brief periods of time are also a great security risk. Subverting your firewall so you can dial-in from home to your workstation also allows an attacker to do the same. Also, temporary changes easily become permanent, as we quickly forget about such changes.

Remember, the weakest link in the security implementation is likely to be exploited first.

3. Network Security
Network security is becoming more and more important as people spend more and more time connected. Compromising network security is often much easier than physical or local, and is much more common.

There are a number of good tools to assist with network security, and more and more of them are shipping with Linux distributions.

3.1 Windows Networking
Most likely your network will also include Microsoft clients, presumably using either NetBIOS or other inheriently insecure networking protocols.

Among other things, NetBIOS is the protocol Microsoft uses to publicize share names, user names, and host names within the network.

Disabling NetBIOS on any Windows workstations is a prudent idea, as is blocking TCP and UDP ports 137 through 139 on your border routers or firewalls.

A detailed discussion on the actual reasons for this insecurity is available in a paper written by Hobbit, and can be found at his site here

Unfortunately, disabling NetBIOS also will disable any Remote Access Service it may be offering, as well as browsing (Network Neighborhood). If you must retain your NT server on your network, you may consider two NICs in the machine, one outbound via TCP/IP and one internal only. Disable NetBIOS binding to the TCP/IP side. This keeps enterprising folks from poking into the network via TCP/IP, then using various NET commands to gather network information.

The hacker group called l0pht have written a utility similar to how Crack works on UNIX, called l0phtcrack and is available at their site as is other generally useful information.

The file security_level.txt, distributed with SAMBA, discusses the various security levels that can be set using SAMBA, including encrypted passwords, server security, share-level security, and user security. It does a good job of explaining the general security concerns you must deal with.

The security research group called Rhino9, have also put together in depth information on the NetBIOS protocol and interface. You can find it at

Internet Security Systems also produces a document on Windows file sharing security, and is available here This document, titled File Sharing: Unknown Dangers on Your Network, helps to describe some of the security issues you should be aware of, and just how insecure Windows 95 really is. It is a good overview, whereas Hobbit's document is more of a low-level description at the protocol level.

3.2 Identify Gateway Machines
Special attention should be paid to gateway or firewall systems, as they usually control access to the services running on the entire network. Such gateways should be identified, its function within the network shouild be assessed and owners or administrators should be identified. These hosts, often referred to as ``bastion hosts'' are a prime target for an intruder. They should be some of the most fortified machines on the network.

Be sure to regularly review the current access policies and security of the system itself.

These ``systems'' should absolutely only be running the services necessary to perform it's operation. Your firewall should not be your mail server, web server, contain user accounts, etc. Some of the things you should check for, and absolutely fortify on these hosts include:

Turn off access to all but necessary services.
Depending on the type of firewall, disable IP Forwarding, preventing the system from routing packets unless absolutely instructed to do so.
Update machine by installing vendor patches immediately.
Restrict network management utilities, such as SNMP, ``public'' communities, and write access.
Be sure firewall policy includes mechanisms for preventing common attacks such as IP Spoofing, Fragmentation attacks, Denial of Service, etc.
Monitor status very closely. You should develop a reference point in which the machine normally operates to be able to detect variations which may indicate an intrusion.
Develop a comprehensive firewall model. Firewalls should be treated as a security system, not just a program that runs on a machine and has an access control list. Firewall administration should be centrally controlled and evaluation of firewall policies should be done prior to actual firewall deployment.

3.3 Network Monitoring
It is important to keep aware of the status of your network, so you can not only detect when there is an intrusion, but when there is abnormal system activity, such as system load, increased disk usage, slower network, etc. There are many tools available for network monitoring, most of which were developed on other platforms first, then ported to Linux.

See the COAST archives, available at http://www.cs.purdue.../coast/hotlist/ for network monitoring tools.

Matthew Franz has put together a Linux distribution that runs on two or three floppies, and includes many of the tools necessary to probe a network and the services it has available. This sounds like a great method in which to test your current security policy, as well as find otherwise unknown vulnerabilities. You can find the latest version, as well as more information, at

3.4 Network Configuration Files
Improperly configured network services and configuration files can lead to a system lacking full control over its services. You can configure your systems to be secure, yet still offer the services necessary. As a general rule:

Remove the /etc/hosts.equiv file. A properly configured system, using TCP Wrappers, offers much better control over which hosts and users are allowed access to the other machines on your network.

Disable the use of $HOME/.rhosts files. By properly configuring PAM, you can eliminate the risk of a user subverting system security by allowing unauthorized access from a remote system via a .rhosts file. This should be replaced by the functionally equivilent SSH file called .shosts. If this is not possible, Wietse Venema wrote a more secure rsh and rlogin daemon replacement, available in the logdaemon package. You can find this at

Verify the /etc/exports configuration. Be sure if you are exporting filesystems using NFS, be sure to configure /etc/exports with the most restrictive access possible. This means not using wildcards, not allowing root access, and exporting read-only wherever possible. Verify who can mount these filesystems using /usr/sbin/showmount -e localhost.

Secure access to your console. Check the /etc/securetty file for the list of tty's that root is permitted to login from. This should only include the local tty's, and never including pseudo-ttys (from a remote location). The absense of this file indicates root is permitted to login from anywhere. Use /bin/su or sudo, available at

Be sure to review your /etc/inetd.conf and see what services are being offered by your inetd. Disable any that you do not need by commenting them out (# at the beginning of the line), and then sending your inetd process a SIGHUP. All services running from inetd should be wrapped using TCP Wrappers.

Disable all services such as the ``r-utilities'' including exec (used by rsh, login (used by rlogin), and shell, (used by rcp) should be disabled immediately from being started in /etc/inetd.conf. These protocols are extremely insecure and have been the cause of exploits in the past.

Disable all unnecessary RPC services. Disable any non-essential services that are registered with the portmapper. RPC services are generally insecure, and have typically been replaced by newer forms of an equivilent service. Use rpcinfo -p hostname to find the list of RPC services running on hostname.
The best method of configuration here is to only enable the services in which the box is intended to serve. Network-based exploits are equally as common as other forms of exploits, and they are performed by finding weaknesses in services, or poorly configured services.

3.5 Check for Poor Topology Configuration
Poor network configurations can also lead to a very difficult intrusion to track. Protecting the ``front door'' with a very well configured firewall will not prevent someone from entering through the ``back door'' via the modem bank with poor authorization.

3.6 Disable Unnecessary and Unauthorized Services
Before you put your Linux system on ANY network the first thing to look at is what services you need to offer. Services that you do not need to offer should be disabled so that you have one less thing to worry about and attackers have one less place to look for a hole.

You should check your /etc/rc.d/rcN.d, where N is your systems run level and see if any of the servers started in that directory are not needed. The files in /etc/rc.d/rcN.d are actually symbolic links to the directory /etc/rc.d/init.d. Renaming the files in the init.d directory has the effect of disabling all the symbolic links in /etc/rc.d/rcN.d. If you only wish to disable a service for a particular runlevel, rename the appropriate file with a lower-case ``s'', instead of the upper-case ``S'', such as in S45dhcpd.

If you have BSD style rc files, you will want to check /etc/rc* for programs you don't need. The Red Hat distribution includes tksysv, a graphical program to change what runlevel a particular server runs in. The newer distributions also include linuxconf, which can also do this.

Additionally, machines on your network running unauthorized services can create an opportunity for a cracker to gain access to the system. Regular port scanning of your machines, as well as running network security scanning tools, can help to find these potential exploits before an intruder does.

3.7 Monitoring Network Services with TCP Wrappers
Most Linux distributions ship with tcp_wrappers ``wrapping'' all your TCP services. A tcp_wrapper (known as /usr/sbin/tcpd) is invoked from /sbin/inetd instead of the real service, such as telnet or ftp. tcpd then checks the host that is requesting the service and either executes the real server or denies access from that host. tcpd allows you to restrict access to your tcp services. You should make a /etc/hosts.allow and add in only those hosts that need to have access to your machines services.

By making simple changes to the inetd configuration file, /etc/inetd.conf you can monitor and control incoming requests to network services. Such a modification might look like the following:


telnet stream tcp nowait root /usr/sbin/in.telnetd

TCP Wrappers
telnet stream tcp nowait root /usr/sbin/tcp /usr/etc/in.telnetd

In default mode the wrappers report the name of the client host and of the requested service. Be sure you have syslogd configure properly to ensure correct logging.

As no information is exchanged between the wrappers and the client or server applications there is no overhead on the actual conversation between the client and server applications occurs.

Additionally, you can configure:

Access control to restrict what systems can connect to what network daemons.

Client user name lookups with the RFC 931 (ident) protocol.

Additional protection against hosts that pretend to have someone else's host name.

Additional protection against hosts that pretend to have someone else's host address.

Notification upon usage of specific services, such as may be used to set trap doors for attempted intrusion.

3.8 Running Services in a chroot Environment
Several network services can now be configured to run in a restricted environment, called a ``chroot jail''. This is an isolated environment seperated from the ``real'' operating system. Services such as Apache or bind can be operated in this environment. A special root directory is created, with a complete installation of all programs and libraries necessary to execute the service. The intention is to prevent someone from obtaining root privilege on the ``real'' operating system, due to a bug in the service that is operating in the chroot jail.

This should not be treated as a panacea, however. It may help to restrict a process' filesystem access, but it doesn't affect its ability to make privileged system calls (e.g. init_module, modify_ldt, bind to a priviliged port, etc.) So ultimately a root process can break out of a chroot environment; it just makes the necessary shellcode more involved than just ``exec("/bin/sh")''. You can find more information on it's advantages and disadvantages at This isn't explicitly a chroot discussion, but is helpful, nevertheless.

3.9 Domain Name Service (DNS) Security
Keeping up-to-date DNS information about all hosts on your network can help to increase security. In the event of an unauthorized host becomes connected to your network, you can recognize it by its lack of a DNS entry. Many services can be configured to not accept connections from hosts that do not have valid DNS entries.

Descriptive hostnames are just as useful to attackers as they are to internal users. Host names such as ``'' is obvious to an attacker, as is ``''. These are likely to be prime targets to an attacker. A machine named ``'' likely indicates a normal user's PC, which is also least likely to have an updated security mechanism installed, making it also a prime target.

Keep conscious of possible DNS spoofing. You can find more information on this in the Exploits section of this document.

Further information on securing DNS can be obtained from http://www.psionic.c.../dns-linux.html

Cricket Liu and Paul Albitz, the authors of the famed DNS and BIND O'Reilly book, contributed an article on Sun World with hints on how to secure DNS. You can find it, as well as some other excellent general security information at http://www.sunworld....ol-11-bind.html which discusses information on how to prevent being DNS spoofed.

Additonally, BIND can now successfully be run in a chroot() environment. John A. Martin <> has put together a set of Red Hat packages that can be used to install BIND in a chroot jail. You can find more information on this available at

Be sure to configure a separate user for BIND. This not only restricts the amount of damage an intruder can do after exploiting a security hole in BIND, but also allows administration of the zone files without having to be root. This is generally a good practice, and more packages are configured for doing this more easily than before possible.

3.10 Network File System (NFS) Security
NFS is a very widely used file sharing protocol. It allows servers running nfsd(8) and mountd(8) to ``export'' entire filesystems to other machines with nfs filesystem support built-in to their kernels (or some other client support if they are non Linux machines). mountd(8) keeps track of mounted filesystems in /etc/mtab, and can display them with showmount(8).

Many sites use NFS to serve home directories to users, so that no matter what machine in the cluster they login to, they will have all their home files.

There is some small amount of ``security'' allowed in exporting filesystems. You can make your nfsd map the remote root user (uid=0) to the nobody user, denying them total access to the files exported. However, since individual users have access to their own (or at least the same uid) files, the remote superuser can login or su to their account and have total access to their files. This is only a small hindrance to an attacker that has access to mount your remote filesystems.

If you must use NFS, make sure you export to only those machines that you really need to export only. Never export your entire root directory, export only directories you need to export and export read-only wherever possible.

Filter TCP port 111, UDP port 111 (portmapper), TCP port 2049, and UDP port 2049 (nfsd) on your firewall or gateway to prevent external access.

The NFS HOWTO also discusses some of the security issues with NFS, and it is available at http://sunsite.unc.e.../NFS-HOWTO.html for more information on NFS.

3.11 Network Information Service (NIS)
Network Information service (formerly YP) is a means of distributing information to a group of machines. The NIS master holds the information tables and converts them into NIS map files. These maps are then served over the network, allowing NIS client machines to get login, password, home directory and shell information (all the information in a standard /etc/passwd file), among other information.

NIS is not at all secure. It was never meant to be. It was meant to be handy and useful. Not only was it not intended to be secure, it also has characteristics which inherently make it insecure. Among these are:

Lack of access control for contents of NIS maps
Negation of password shadowing
Rogue servers acting as authentic ones
Anyone that can guess the name of your NIS domain (anywhere on the net) can get a copy of your passwd file, and use Crack against your users passwords.

If you must use NIS, make sure you are aware of the dangers.

Control the use of /etc/netgroup file for NIS systems. Explicitly define which hosts and which users can connect from a known list of machines.

There is a much more secure replacement for NIS, called NIS+. Check out the NIS HOWTO for more information, available at http://sunsite.unc.e.../NIS-HOWTO.html

3.12 File Transport Protocol (FTP)
The Washington University FTP server is the default server on Linux distributions. It has the ability to run in a chroot environment, thus (theoretically) protecting the real root environment, limiting the damage an exploit can do.

FTP sites are easily misconfigured, and doing so can lead to a false sense of security, as well as easily exploitable holes. Attackers can use a misconfigured site to transfer pirate software, gain remote access, corrupt downloadable files, cause a denial of service, among other misuses.

Be sure to disable FTP entirely if you don't have any reason to leave it enabled (such as replacing it with ssh) and definately enable quotas on the FTP filesystem. Additionally, disable anonymous FTP access if it is not necessary.

3.13 Simple Mail Transport Protocol (SMTP)
One of the most important services you can provide is a mail server. Unfortunately, it is also one of the most vulnerable to attack, simply due to the number of tasks it must perform and the privileges it typically needs.

If you are using sendmail, it is very important to keep up on current versions. Sendmail has a long long history of security exploits. Always make sure you are running the most recent version.

An alternative to sendmail is qmail, which alledges to be more secure, and easier to configure. qmail was designed with security in mind from the ground up. It's reported that it's fast, stable and secure. You can find it at

Wietse Venema <> is writing a mail server that is still in testing stages, but also promotes improved security. You can find out more about vmail at

Significant improvements in preventing unsolicited bulk email (spam) have been made with recent versions of the available SMTP servers. Starting with sendmail-8.9, anti-relaying is enabled by default, which prevents a remote host from using your network and mail servers for forwarding mail to other hosts. Additional filters are also available for preventing spam.

4. Host Security
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 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 have no contact information for is a very bad idea.

4.1 Delete Unnecessary Packages
If you know you are not going to use some particular package, you can also delete it entirely. /bin/rpm -e <packagename> under the Red Hat distribution will erase an entire package. Under debian /bin/dpkg likely does the same thing.

If you are configuring a new machine to be installed on the network, only initially install the packages that are necessary for its normal operation.

Removing unnecessary setuid and setgid binaries should be a priority. You should always be aware of which ones are available on your system. You can do this using the following:

user@myhost$ find / -type f -perm +6000

This will find all the setuid and setgid binaries on your system. You can find more about the setuid and setgid permissions in the File System Security section.

4.2 Default System Configuration
The default Linux system installation is generally far more secure than other operating systems, due to not having to conform to older standards and traditions.

However, installing any operating system, and connecting it to the network is a foolish idea. Many system defaults are still more lenient than is intended to be used in a production network system.

Spend some time to customize it to your environment. Be sure to follow these guidelines, as well as the ones refered to herein, including disabling any services that are not necessary, configuring auditing, etc, before connecting a machine to the network.

4.3 Make a Full Backup of Your Machine
Discussion of backup methods and storage is beyond the scope of this document, but a few words relating to backups and security:

If you have less than 650Mb of data to store on a partition, a CD-R copy of your data is a good way to go (as it's hard to tamper with later, and if stored properly can last a long time). Tapes and other re-writable media should be write protected as soon as your backup is complete and verified to prevent tampering. Make sure you store your backups in a secure off line area. A good backup will ensure that you have a known good point to restore your system from.

A six-tape cycle is an easy one to maintain. This includes four tapes for during the week, one tape for even Friday's, and one tape for odd Friday's. Perform an incremental backup every day, and a full backup on the appropriate Friday tape. If you make some particular important changes or add some important data to your system, a backup might well be in order.

4.4 Backup Your Red Hat or Debian File Database
In the event of an intrusion, you can use your RPM database like you would use tripwire, but only if you can be sure it too hasn't been modified. You should copy the RPM database and /bin/rpm executable to a floppy or Zip disk, and keep this copy off-line at all times. The Debian distribution likely has something similar. (Would someone fill me in here, until I get Debian re-installed?) See the section on Integrity Checking for further information, and instructions on how to do this.

4.5 Make Use of Your System Accounting Data
It is very important that the information that comes from your system accounting files has not been compromised, and is installed and configured properly. Making the files in /var/log, /var/run/utmp, and /var/log/wtmp readable, and writable only by the root user is a good start. Knowing which tools to use at what times is a good practice.

You can find more information on this in the User and System Accounting section.

4.6 Apply All New System Updates
Most Linux users install from a CDROM. Due to the fast paced nature of security fixes, new (fixed) programs are always being released. Before you connect your machine to the network, it's a good idea to check with your distribution's ftp site ( for example) and get all the updated packages since you received your distribution CDROM. Many times these packages contain important security fixes, so it's a good idea to get them installed.

4.7 Creating New Accounts
You should make sure to provide user accounts with only the minimal requirements for the task they need to do. If you provide your secretary, or another general user, with an account, you might want them to only have access to a word processor or drawing program, but be unable to delete data that is not his or hers.

Several good rules of thumb when allowing other people legitimate access to your Linux machine:

Limit access privileges given to new users.

Be aware when/where they login from, or should be logging in from.

Make sure to remove inactive accounts

The use of the same user-ID on all computers and networks is advisable to ease account maintenance, as well as permit easier analysis of log data (but I'm sure someone will dispute this). However, it's practically essential if using NFS. There are several other protocols that use UIDs for local and remote access as well.

The creation of group user-IDs should be absolutely prohibited. User accounts also provide accountability, and this is not possible with group accounts.

Be sure shadow passwords are enabled. See the Password Security section for more information.

Regularly audit user accounts for invalid or unused accounts, expired accounts, etc.

Check for repeated login failures

Be sure to enable quotas, to prevent denial of service attacks involving filling disk partitions, or appending exploits to group-writable files.

Disable group accounts, and unused system accounts, such as sys or uucp. These accounts should be locked, and given non-functional shells.
Many local user accounts that are used in security compromises are ones that have not been used in months or years. Since no one is using them they provide the ideal attack vehicle.

4.8 Root Security
The most sought-after account on your machine is the 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. Running as root all the time is a very very very bad idea.

Several tricks to avoid messing up your own box as root:

When doing some complex command, try running it first in a non destructive way...especially commands that use globbing: e.g., you are going to do a rm foo*.bak, instead, 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.

Provide your users with a default alias to the /bin/rm command to ask for 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.

The command path for the root user is very important. The command path, or the PATH environment variable, defines the location the shell searches for programs. Try and limit the command path for the root user as much as possible, and never use '.', meaning 'the current directory', in your PATH statement. 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.

Never use the rlogin/rsh/rexec (called the ``r-utilities'') suite of tools as root. They are subject to many sorts of attacks, and are downright dangerous run as root. Never create a .rhosts file for root.

The /etc/securetty file contains a list of terminals that root can login from. By default (on Red Hat Linux) this is set to only the local virtual consoles (vtys). Be very careful of adding anything else to this file. You should be able to login remotely as your regular user account and then use 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.

Always be slow and deliberate running as root. Your actions could affect a lot of things. Think before you type!
If you absolutely positively need to allow someone (hopefully very trusted) to have superuser 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. sudo 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, but use sudo so you can 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 the user root access. 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 yet be secure.

4.9 Workstations and DialUp Security
User of computers to connect to the Internet via a dial-up line, or workstations that otherwise offer no services to external hosts can also improve their security with relatively easy modifications to the stock Linux installation.

If there is never have a need to connect to your machine from another one on the network, the quickest solution is to simply disable /usr/sbin/inetd from even being started. This is the master Internet daemon, which controls some normal server services, such as telnet, ftp, etc. If you retrieve your mail from a remote host, and your Internet Service Provider is hosting your web page, then most likely there is not a need to enable these services.

On stock Red Hat systems, the file /etc/rc.d/rc3.d/S50inet controls the starting and stopping of the inetd server. Simply rename the S50inet file to s50inet to disable it, or see your Red Hat administration manual for further information.

Alternatively, if you are a home dialup user, it is also possible to deny all incoming connections using TCP Wrappers. TCP Wrappers, /usr/sbin/tcpd, also logs failed attempts to access services, so this can give you an idea that you are under attack. If you add new services, you should be sure to configure it to use tcp_wrappers TCP based. For example, a normal dial-up user can prevent outsiders from connecting to your machine, yet still have the ability to retrieve mail, and make network connections to the Internet. To do this, you might add the following to your /etc/hosts.allow:

ALL: 127.

(including the ending period) And of course /etc/hosts.deny would contain:


which will prevent external connections to your machine, yet still allow you from the inside to connect to servers on the Internet. TCP Wrappers can be combined with several other services, such as sendmail and sshd to give even further control over access. See the respective documentation for further information.

4.10 X11, SVGA and display security

It's important for you to secure your graphical display to prevent attackers from doing things such as grabbing your passwords as you type them without you knowing it, reading documents or information you are reading on your screen, or even using a hole to gain superuser access. Running remote X applications over a network also can be fraught with peril, allowing sniffers to see all your interaction with the remote system.

X has a number of access control mechanisms. The simplest of them is host based. You can use xhost to specify what hosts are allowed access to your display. This is not very secure at all. If someone has access to your machine they can xhost + their machine and get in easily.

When using xdm (X Display Manager) to login, you get a much better access method: MIT-MAGIC-COOKIE-1. A 128bit cookie is generated and stored in your .Xauthority file. These cookies need to be transferred in confidence, and you really don't gain anything if your home directory is shared via NFS. If you need to allow a remote machine access to your display, you can use the xauth command and the information in your .Xauthority file to provide only that connection access. See the Remote-X-Apps mini-howto, available at http://sunsite.unc.e...ote-X-Apps.html.

You can also use ssh (see ssh, below) to allow secure X connections. This has the advantage of also being transparent to the end user, and means that no un-encrypted data flows across the network.

Take a look at the Xsecurity(1) man page for more information on X security. The safe bet is to use xdm(1) to login to your console and then use ssh to go to remote sites you wish to run X programs off of.

SVGAlib programs are typically setuid-root in order to access all your Linux machine's video hardware. This makes them very dangerous. If they crash, you typically need to reboot your machine to get a usable console back. Make sure any SVGA programs you are running are authentic, and can at least be somewhat trusted. Even better, don't run them at all.

GGI (Generic Graphics Interface project)
The Linux GGI project is trying to solve several of the problems with video interfaces on Linux. GGI will move a small piece of the video code into the Linux kernel, and then control access to the video system. This means GGI will be able to restore your console at any time to a known good state. They will also allow a secure attention key, so you can be sure that there is no Trojan horse login program running on your console.

4.11 identd
identd is a small program that typically runs out of your inetd. It keeps track of what user is running what tcp service, and then reports this to whoever requests it.

Many people misunderstand the usefulness of identd, and so they disable it or block all off site requests for it. identd is not there to help out remote sites. There is no way of knowing if the data you get from the remote identd is correct or not. There is no authentication in identd requests.

Why would you want to run it then? Because it helps you out, and is another data-point in tracking. If your identd has not been compromised, then you know it is telling remote sites the user-name or user-ID of people using TCP services. If the admin at a remote site comes back to you and tells you a user was trying to hack into their site, you can easily take action against that user at your site who is misusing a service. If you are not running identd, you will have to look at lots and lots of logs, figure out who was on at the time, and in general take a lot more time to track down the user.

The identd that ships with most distributions is more configurable than many people think. You can disable identd for specific users (they can make a .noident file), you can log all identd requests, which is recommended, and identd can return a uid instead of a user name or even NO-USER. Keep in mind it is really only useful is on a network where nobody hostile has root access. Then it can help in catching mail forgeries, for instance.

5. User, System, and Process Accounting
All Linux systems support system-wide process, user, and system accounting, and it is wise to take advantage of it. You will need this information when troubleshooting a possible security incident, and your ability to address all aspects of a specific incident strongly depends on the success of this analysis.

There are quite a few things, as the Security Administrator, of which you should be aware. These include at least the following:

Login activity
Authorization information
Authentication information
Commands users have run
Restarts and shutdowns of the system
Network transactions records

5.1 Using Syslog
The system daemon called syslog is the program used to log system events such as kernel messages, login or logout messages, general system messages, etc.

Be sure to keep an eye on its normal operation and what gets written to it's log files, especially under the ``auth'' facility. Multiple login failures, for example, can indicate an attempted break-in. Keep in mind that the lack of information does not indicate the opposite.

Where to look for your log file will depend on your distribution. In a Linux system that conforms to the ``Linux File-system Standard'', such as Red Hat, you will want to look in /var/log and check messages, mail.log, and others.

You can find out where your distribution is logging to by looking at your /etc/syslog.conf file. This is the file that tells /usr/sbin/syslogd (the system logging daemon) where to log various messages.

You might also want to configure your log-rotating script or daemon to keep logs around longer so you have time to examine them. Take a look at the logrotate package in recent Red Hat distributions. Other distributions likely have a similar process. It seems that many distributions default to only logging the most basic information, so you should spend some time and customize it for your environment.

If your log files have been tampered with, see if you can determine when the tampering started, and what sort of things they appeared to tamper with. Are there large periods of time that cannot be accounted for? Checking backup tapes (if you have any) for untampered log files is a good idea.

Log files are typically modified by the intruder in order to cover his tracks, but they should still be checked for strange happenings. You may notice the intruder attempting to gain entrance, or exploit a program in order to obtain the root account. You might see log entries before the intruder has time to modify them.

You should also be sure to seperate the ``authpriv'' facility from other log data, including attempts to switch users using /bin/su, login attempts, and other user accounting information.

Storing Log Data Securely
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. It most likely of little value in a criminal investigation. It helps if the log data, which has been stored remotely, indicates when root access was gained so that logs before that point are okay.

The syslogd daemon can be configured to automatically send log data to a central syslogd server, but this is typically sent in cleartext data, allowing an intruder to view data as it is being transferred. This may reveal information abo

Also tagged with one or more of these keywords: cryptography, denial of service, distributed, spoofing, patch, port scan, sniffer, spam, backdoor