Bastille-Linux

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Original in fr Frédéric Raynal

fr to en Georges Tarbouriech

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Introduction

The project is managed by Jon Lasser (main coordinator) and Jay Beale (main developer). Many other developers, software designers and beta-testers are involved as well.

First of all, let's make it clear: Bastille-Linux is NOT a new Linux distribution! It's a set of scripts, written in perl intended to improve Linux security.

Security, means here computer security: how to avoid that unwanted people access your machine? Bastille-Linux gives a part of the answer by modifying the initial installation of your Linux distribution.

A basic task for every SysAdmin is to know the users needs, not only to comply with them, but also to avoid to keep running network unused programs... to be able to contain network security holes. One of my mentors used to say: the least you do, the better ;-] Of course, he was talking about algorithm complexity, but this is applicable to network administration: abundance of goods does harm as it gives more attack places. To reduce vulnerability just install what you really need.

Bastille-Linux tries to reduce the possibilities for an attack. To reach this goal, the software designers have a very educational approach: they explain what to do, step by step.

Presentation

Apart from its obvious usefulness, Bastille-Linux is extremely educational. The scripts ask detailed and acurate questions. Here, the developers try to teach the user. They explain the question context and the results of the available answers. This makes Bastille-Linux an easy to use tool.

For more advanced users, the source code, written in perl, is a model of clarity (yes, it's possible ;-) : every comment describes the way the action is working.r

Here are a few features of version 1.1.0 :

• The scripts now work on already used systems
• An undo function appeared : every Bastille-Linux modified file is backed up to be restored if needed
• Initially, Bastille-Linux was reserved for RedHat and Mandrake : this is not true anymore. Now a module holds the right filepath according to the distribution on which you run the scripts.

The general modules are used for:

1. Firewall installation
2. System software update
3. Audit on SUID-root programs
4. Deactivating and restricting useless services

Some security functions cascade and offer different protection levels (this is against my mentors principles ... but you have to emancipate one day ;-) You must protect every service or every potential security hole by any means. Thus, if one fails, the other ones keep protecting your system.

Installation or "let's go for a walk into the scripts while the wolf is not there"

Bastille-Linux is available as tarball .tgz at bastille-linux.sourceforge.net. This archive is only 134 Ko. Once downloaded, extract it (tar xzf Bastille-1.1.0.tgz).

Four scripts manage Bastille-Linux :

• InteractiveBastille.pl : this script asks the user various questions to determine what to do on the system. Next, it creates a config file from the given answers
• AutomatedBastille.pl : installs default configuration, offering the choice of using a firewall from a provided configuration script. This script is not maintained anymore
• BackEnd.pl : this one is in charge of system changes. It gets its parameters from the answers you first gave.
• Undo.pl : when you run BackEnd.pl, a few system files (/etc/syslog.conf,/etc/inetd.conf, /etc/hosts.*, ...) are modified. To allow to restore in case of trouble, backup copies are placed in the/root/Bastille/undo directory.

A few notes before you begin.

1. You're supposed to run the script from the /root/Bastille directory ... this will change in a future version and then you'll be able to execute it from any place (may be, it's already true by the time you read this)
2. Two modules allow account creation ... as soon as you installed shadow passwords on your system. This is the default installation, but check that /etc/passwd doesn't hold passwords, that you must be root to access /etc/shadow and that this last file really holds the passwords!
3. Bastille-Linux doesn't yet manage links between modules, like in kernel configuration (this will be available in a future version). Take care of the answers given in a module and to their consequences. Thus, you could tell IPChains module to close port 2049 and keep NFS in the list of active daemons from MiscellaneousDaemons module (NFS uses this port).

Step by step

IPChains

This script works in a very good way. It takes into account 2 network interfaces: the one communicating with the outside, and the one communicating with the local network. The goal is to determine the services available to the machines and to stop the others. The default is to refuse everything coming from the outside. Next, the script defines the rules to make some services available. Certains selected services.

The script manages TCP, UDP and ICMP protocols. It provides lists of services to watch or to avoid for each protocol. It would be too long to detail here Bastille-Linux approach to install a firewall, but reading the script and its comments help a lot. However a minimum knowledge is required to use this module.

PatchDownload

Unfortunately, this script doesn't work very well. It's rather complex since you must determine the installed packages, and among them, the ones to updated. Next, you must download the patch, check it hasn't been altered (either by hackers or the transfer) before installing it. This really depends on the distribution you use.

At the moment, Jay Beale recommends to manually execute this step but not to neglect it. A more functional version of this module is under development and should be available very soon.

FilePermissions

• Would you like to set more restrictive permissions on the administration utilities [N] ?

Some programs must not be made available to everyone. Bastille-Linux provides you with an impressive list of programs that can be changed to rights 0750.
• Next, it's about SUID bit. This bit allows a program to be run by a user having root permission. ('s' in position 4, 7 and 10)
>>ls -l /bin/ping
-rwsr-xr-x 1 root root 17968 Mar 6 15:57 /bin/ping
This bit, it improves ease of use, reduces security. That is, if one discovers a vulnerability (let's say a buffer overflow ), he will be able to become root .
Bastille-Linux suggests to remove this bit from some programs among which mount/umount, ping, dump/restore, cardctl, ... Among others, this list contains ping and traceroute that are two utilities allowing to check a server state. Removing their SUID bit corresponds to a paranoiac mode, but doesn't prevent from the standard use of a machine.
In that list of SUID-root programs, you can find as well the r-commands. It concerns programs such as rcp, rlogin or rsh. These utilities don't encrypt the data sent to the network. Furthermore, they only use the IP address as means of authentication. This is inadequate (spoofing possibility).

AccountSecurity

• Would you like to set up a second UID 0 account [N] ?

Answering yes to this question creates another root account on your system. In that case, you shouldn't use the original root account anymore, but the one you just created. This gives twice more possibilities to crack a root password... but allows you to see who uses the root identity from the files in /var/log.
• May we take strong steps to disallow the dangerous r-protocols? [Y]

As above mentioned r-commands are a strong mean of getting a new account on a machine: the passwords are transferred in clear text! On such a network, a sniffer offers interesting aspects...
Bastille-Linux prohibits the use of r-commands via PAM (Pluggable Authentication Modules), removes execute permission and stops the servers found in tcp_wrapper (rlogind, rexecd and rshd). They are configured from /etc/inetd.conf where you just have to put them as comments.
• Would you like to enforce password aging? [Y]

Passwords then shall be changed every 180 days. Accounts where password don't change are deactivated.
• Would you like to create a non-root user account? [N]

Use your root account the least possible. As a matter of fact, a single command like rm -rf / can make a disaster as root ... but a bit less as a standard user. Only administrative tasks should be done as root.
•  Would you like to restrict the use of cron to administrative accounts? [Y]

cron allows to automatically run tasks. For instance, an administrator can use it to check files integrity on a regular basis, or to search the files in /var/log. On the other hand, to give access to every user is too much of a privilege. As a workaround, Bastille-Linux creates a file /etc/cron.allow containing the users allowed to use this service.

BootSecurity

Obviously, that isn't enough. To physically protect a computer, the BIOS must be password-protected, the hard disk must be the only boot device, the box must be locked to prevent someone from adding his own hard disk... This is of course the paranoid behaviour and you don't need to do that without a very good reason.

From a sofware point of view, some restrictions provide a good compromise compared to the above mentioned:

• Would you like to password-protect the LILO prompt? [N]

Answering "Yes" prevents unallowed people from accessing the system after reboot.
• Would you like to reduce the LILO delay time to zero? [N]

That way, someone is unable to provide parameters at boot time. If your computer is running various OSes, you must not do this, since the default OS would be the only one working!
• Do you ever boot Linux from the hard drive? [Y]

If you answered yes to one of the previous questions and LILO is installed on your disk, then you must also answer yes to this one to write the LILO changes on the disk.
• Would you like to write the LILO changes to a boot floppy? [N]

If you own a boot floppy, either for emergency reason or because it's your way to boot Linux, then you must change it.
• Would you like to disable CTRL-ALT-DELETE rebooting? [N]

This is intended for preventing someone from rebooting the machine. However, this is useless as soon as the electrical plug is not protected ;-)
• Would you like to password protect single-user mode? [Y]

As we already saw, it looks like a good idea to prevent any people from becoming root on the machine.

SecureInetd

For instance, a mistake in RedHat6.0 DNS allows to remotely become root. Deactivating this service or reducing privileges protects from this annoyance.

Some protocols, such as the already mentioned r-commands but ftp or telnet too, are quite vulnerable. Others allow to get information (finger or identd for example) on the machine accounts, etc ... Many of these services are managed by tcp_wrapper  which allows to control who accesses a given service (by means of /etc/hosts.{allow, deny}) files. Then, once the wrapper decided the client was allowed to access the service, it sends the request to the corresponding server.

This part is still a bit rigid and should be changed in future versions.

• Would you like to modify inetd.conf and /etc/hosts.allow to optimize use of Wrappers? [Y]

Bastille-Linux installs those two files. It can be interesting to have a look at them as to pass them more accurate parameters according to your needs.
• Would you like to set sshd to accept connections only from a small list of IP addresses? [N]

sshd is a daemon allowing to connect in a secure way (keys exchanging, passwords and data crypting, ...).It's a full replacement for telnet, rlogin, rsh, rcp and ftp. Let's mention an equivalent version to ssh under BSD license: OpenSSH. Back on this later.
• Would you like to make "Authorized Use" banners? [Y]

Every person trying to connect to your machine will see a warning message asking if she's allowed to connect through this service. This message can be found in the /etc/motd file.

DisableUserTools

Then, if this machine is only a server on which no one has to compile anything, remove the compiler.

ConfigureMiscPAM

PAM stands for "Pluggable Authentification Module". It's a library allowing the SysAdmin to select the type of users authentication for each application, the rights they have, the resources they can access, and so on.

• Would you like to put limits on system resource usage? [Y]

The /etc/security/limits.conf file contains system limits. Bastille-Linux changes them that way:
• core files are limited to 0 ;
• each user is limited to 150 processes ;
• the maximum file size is limited to 40Mb.
Each value can be modified later directly from the configuration file.
• Should we restrict console access to a small group of user accounts? [N]

On RedHat6.0/6.1, users connecting from the console have some privileges such as mounting the CD-ROM. It's possible to restrict the console access to a group of trusted users. This question allows to define those users if you feel like implementing such a policy.

Logging

It's remarkable that if you have a minimal number of running services, every problem will be pointed out quickly from /var/log files. On the other hand, if your system runs a lot of useless servers the /var/log files become very big and accordingly more difficult to manage (you then need to implement dedicated scripts).

This module adds new checks in the /etc/syslog.conf file.

• Would you like to add additional logging? [Y]

Bastille-Linux creates a /var/log/kernel file, for messages coming from the kernel and recording serious problems (firewall messages make part of this category). Some information is sent to 2 terminals (TTY 7 and 8). A new file named /var/log/loginlog records users connecting to the system.
• Do you have a remote logging host? [N]

Unless you own another machine to which you can send your messages, the answer is no.
• Would you like to set up process accounting? [N]

Under Linux, it's possible to record the executed commands, who did it and when. If it's useful to check the machine activity, this log quickly becomes enormous. Of course, it uses a lot of resources and it's then better to deactivate this feature unless you really need it.

MiscellaneousDaemons

Sendmail

sendmail as we said before, doesn't need to be run as a daemon to send and receive mail. For home users sendmail is probably quite useless since you can use any mail client (netscape, rmail, pine, mutt, etc...) to send your mail. To receive your mail, you can activate sendmail on a regular basis to check your mailbox.
 

• Do you want to leave sendmail running in daemon mode? [Y]

We have just seen in most cases it's useless and dangerous, then it's much better to deactivate it.
• Would you like to run sendmail via cron to process the queue? [N]

Using this, sendmail checks the mail queue every 15 minutes. You can change this parameter from the /etc/sysconfig/sendmail file.
• Would you like to disable the VRFY and EXPN sendmail commands? [Y]

These commands provide useful information to hackers or for spamming.

RemoteAcces

You can use a software in which the session key length doesn't exceed 128 bit. Let's explain what a session key is. It's the key used to crypt the data.  This session key is built step by step by the client and the server: it comes from a key exchange protocol (Diffie-Hellman in most cases).  It consists in building a key from a piece of every member's key. Next, this session key is used to crypt the data according to a symetric algorithm (i.e.   the same key is used to encrypt and uncrypt the data). Thus, DES, used to encrypt Unix passwords, is a symetric algorithm with a 56 bit key.

Now, is a 128 bit key big enough to ensure transaction confidentiality and security: YES! Even if today DES is said "not so secure", the best attacks are not within range of CPU power that most people have. On the other hand, it's a mistake to believe a key of 2k length is twice more difficult to find than a key of k length. As a matter of fact, if the difficulty is exponential, it grows much faster than the key size. For a key of k length, it exists 2^k possible keys (and so 2^2k for a key of 2k length). Then, multiplying the key size by  2, we add  possible keys! When you notice the difficulty to break DES (56 bit), you can expect 128 bit keys to be inviolable (as soon as the encrypting/uncrypting algorithm doesn't hold security hole).  From an attack point of view,  to increase this limit  only makes the difficulty go from an "impossible level" to an "even more impossible level".

4 different software packages  provide with similar services :

The following modules still concern services. For them the policy may look surprising: you begin with privileges restriction and then you stop them. In spite of appearances, these two measures are not conflicting. These services can be reactivated, either incidentally or by undesirable people... then it seems better to restrict them.
 

DNS

Let's add a few technical details before explaining Bastille-Linux behaviour. The daemon managing this service is called named. Its configuration comes from  the /etc/named.conf file.
 

• Would you like to chroot named and set it to run as a non-root user? [N]

To do that, Bastille-Linux creates a new user called dns, who has no shell,  but his own directory /home/dns.  In that directory you build a classical system architecture adding usual directories (/usr, /etc, /var, etc ...). Next, you have to copy configuration files and libraries needed by the daemon. Some more changes have still to be done (for syslog for example - check the script DNS.pm). Now, the DNS has its own environment :)
• Would you like to deactivate named, at least for now? [Y]

Most people don't need a name server on their machine, as the ISP provides them with this service. The DNS-HOWTO describes cache installation for name resolution, but even that can be a source of problem.

Apache

1. to host a site: for this you need a fixed IP address. ISPs have such addresses but it isn't normally true for their clients
2. to check your own web pages: in that case, you just have to launch the server (/etc/rc.d/init.d/httpd start) when needed.

• Would you like to deactivate the Apache web server? [Y]

Since we don't always need it and in a minimization spirit, we deactivate it.
• Would you like to bind the web server to listen only to the localhost? [N]

It's possible to bind the daemon httpd to a specific address. Here, Bastille-Linux suggests to connect it to the localhost address, 127.0.0.1. This allows to have a server running on a machine to test your own web pages.
You can access them from: http://localhost/ (or http://localhost/raynal to access my own pages) That is Apache uses the loopback interface (lo) to work.
• Would you like to bind the web server to a particular interface? [N]

Answering yes cancels the previous answer. We said there we wanted to use the web server with the loopback ... and this question allows to give another interface (Ethernet for example) with an IP address bound to it.
• Would you like to deactivate the following of symbolic links? [Y]

You must do that. As for the DNS working in a restricted space, you must not allow Apache to leave its universe (i.e. /home/httpd). For instance, if one of the users has a link to root / in his web server directory, then everybody can access all the files... especially the configuration files such as passwd and others.
• Would you like to deactivate server-side includes? [Y]

As Jay Beale says, if you don't know what it is, you don't need it! Enough to say that this can be configured as to allow users to execute any program on the server (I'm afraid so... fear that sysadmin friends... that was a dream for you ;-)
•  Would you like to disable CGI scripts, at least for now? [Y]

Writing a CGI script (Common Gateway Interface) is not so difficult ... but implies to be careful (for instance to avoid stack overloads generating core files on the server). Many methods used to commit a system rely on errors in these scripts.
• Would you like to disable indexes? [N]

Without index.html file in a directory, Apache lists all the files in that directory. This is less a problem than allowing symbolic links reading... but, just imagine if a directory contains sensitive data.

Printing

FTP

Furthermore, lately security holes have been found in wu-ftpd. If you need to leave that server running, Bastille-Linux allows to restrict some features.

The file managing the FTP server access is /etc/ftpacces.

• Would you like to disable user privileges on the FTP daemon ? [N]

One of FTP problems is to allow anonymous connection (you know, that user having an email address as a password... very often a ghost one ;-). An other problem is file upload (i.e. to send files to the server and fill the disk or upload programs to break into the server). Many attacks use these weaknesses. Another trouble comes from the fact the transaction is not encrypted.
• Would you like to disable anonymous download? [N]

This prohibits connection to your server with anonymous login.

A few more tricks and tips

To do that, just backup the file containing the questions. Next, edit the Questions.txt file. Every module begins with the FILE keyword... just keep what you need.

/root/Bastille >> cp Questions.txt Questions.txt-orig
/root/Bastille >> emacs Questions.txt BackEnd.pl &
/root/Bastille >> ./InteractiveBastille

Of course Bastille-Linux measures are not enough to secure your system:

1. no system is 100% secure;
2. more measures are required to complete Bastille-Linux "work".

It's a long and difficult way to secure a system. You must keep your self informed about security holes (by means of mailing lists such as bugtraq from securityfocus site, for example).

Conclusion

The two fundamental and general concepts are minimalism and depth. Less running services means less security holes. For each of these services, various protections are better than one... but this is double-edged since a bad configured protection (or the conjunction of various) can turn against you.

Last, let's mention the next version will be named BUS (Bastille Unix Security). It will be a clever mixture of Bastille-Linux and Msec (Mandrake Security Project), the later one being renamed to Usec (Unix Security Project).

References

• http://bastille-linux.sourceforge.net :  Bastille-Linux official site
• Bastille-Linux: A Walkthrough : on www.SecurityFocus.com site, by Jay Beale, main author of Bastille-Linux, presents the software and its installation. It helped me a lot to write the article you are reading ;-)
• http://www.sans.org : the site of the SANS group, (Jay Beale collaborates with them).  A true information mine about computer security.
• http://www.securityfocus.com/frames/?content=/vdb/stats.html:  still at SecurityFocus, an interesting study about OSes and their bugs. OS et leurs bugs. First places are for a well known "OS" family, next comes Linux.
• www.kitetoa.com : a site testing web servers... and finding interesting things.  At the same time funny and pathetic (for sysadmin caught in the act) -French only-
• http://www.ssh.com : secured shell. ssh official site; freely usable for evaluation or non commercial use or for universities
• http://www.openssh.org : same as ssh ... but under BSD license  :)