This is Security-HOWTO.info, produced by makeinfo version 4.0b from /tmp/sgml-tools-dir-18269/sgmltmp.Security-HOWTO18269.info.2. \input texinfo  File: Security-HOWTO.info, Node: File Permissions, Next: Integrity Checking, Prev: Umask Settings, Up: Files and File system Security File Permissions ================ 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:' * To be able to view contents of a file * To be able to read a directory `Write:' * To be able to add to or change a file * To be able to delete or move files in a directory `Execute:' * To be able to run a binary program or shell script * To be able to search in a directory, combined with read permission `Save Text Attribute: (For directories)' 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. `SUID Attribute: (For Files)' 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. `SGID Attribute: (For Files)' 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. `SGID Attribute: (For directories)' 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:' -rw-r--r-- 1 kevin users 114 Aug 28 1997 .zlogin 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: -r-------- Allow read access to the file by owner --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 -rw------T No update of "last modified time". Usually used for swap 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.  File: Security-HOWTO.info, Node: Integrity Checking, Next: Trojan Horses, Prev: File Permissions, Up: Files and File system Security Integrity Checking ================== 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: # set mailto MAILTO=kevin # run Tripwire 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/.  File: Security-HOWTO.info, Node: Trojan Horses, Prev: Integrity Checking, Up: Files and File system Security Trojan Horses ============= "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.  File: Security-HOWTO.info, Node: Password Security and Encryption, Next: Kernel Security, Prev: Files and File system Security, Up: Top Password Security and Encryption ******************************** 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 *Note "Crack" and "John the Ripper":: ) 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. * Menu: * PGP and Public-Key Cryptography:: * SSL S-HTTP and S/MIME:: * Linux IPSEC Implementations:: * `ssh' (Secure Shell) and `stelnet':: * PAM - Pluggable Authentication Modules:: * Cryptographic IP Encapsulation (CIPE):: * Kerberos:: * Shadow Passwords-:: * "Crack" and "John the Ripper":: * CFS - Cryptographic File System and TCFS - Transparent Cryptographic File System:: * X11 SVGA and display security::  File: Security-HOWTO.info, Node: PGP and Public-Key Cryptography, Next: SSL S-HTTP and S/MIME, Up: Password Security and Encryption PGP and Public-Key Cryptography =============================== 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 (http://www.rsa.com/rsalabs/newfaq/), 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.  File: Security-HOWTO.info, Node: SSL S-HTTP and S/MIME, Next: Linux IPSEC Implementations, Prev: PGP and Public-Key Cryptography, Up: Password Security and Encryption SSL S-HTTP and S/MIME ===================== 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. * `SSL:' - SSL, or Secure Sockets Layer, is an encryption 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/ * `S-HTTP:' - S-HTTP is another protocol that provides security services across the Internet. It was designed to provide confidentiality, authentication, integrity, and non-repudiability [lsqb ]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. [lsqb ] From RSA Cryptography FAQ, page 138] * `S/MIME:' - S/MIME, or Secure Multipurpose Internet Mail Extension, is an encryption standard used to encrypt electronic mail and other types of messages on the Internet. It is an open standard developed by RSA, so it is likely we will see it on Linux one day soon. More information on S/MIME can be found at http://home.netscape.com/assist/security/smime/overview.html.  File: Security-HOWTO.info, Node: Linux IPSEC Implementations, Next: `ssh' (Secure Shell) and `stelnet', Prev: SSL S-HTTP and S/MIME, Up: Password Security and Encryption Linux IPSEC Implementations =========================== Along with CIPE, and other forms of data encryption, there are also several other implementations of IPSEC for Linux. IPSEC is an effort by the IETF to create cryptographically-secure communications at the IP network level, and to provide authentication, integrity, access control, and confidentiality. Information on IPSEC and Internet draft can be found at http://www.ietf.org/html.charters/ipsec-charter.html. You can also find links to other protocols involving key management, and an IPSEC mailing list and archives. The x-kernel Linux implementation, which is being developed at the University of Arizona, uses an object-based framework for implementing network protocols called x-kernel, and can be found at http://www.cs.arizona.edu/xkernel/hpcc-blue/linux.html. Most simply, the x-kernel is a method of passing messages at the kernel level, which makes for an easier implementation. Another freely-available IPSEC implementation is the Linux FreeS/WAN IPSEC. Their web page states, "These services allow you to build secure tunnels through untrusted networks. Everything passing through the untrusted net is encrypted by the IPSEC gateway machine and decrypted by the gateway at the other end. The result is Virtual Private Network or VPN. This is a network which is effectively private even though it includes machines at several different sites connected by the insecure Internet." It's available for download from http://www.xs4all.nl/~freeswan/, and has just reached 1.0 at the time of this writing. As with other forms of cryptography, it is not distributed with the kernel by default due to export restrictions.  File: Security-HOWTO.info, Node: `ssh' (Secure Shell) and `stelnet', Next: PAM - Pluggable Authentication Modules, Prev: Linux IPSEC Implementations, Up: Password Security and Encryption `ssh' (Secure Shell) and `stelnet' ================================== `ssh' and `stelnet' are suites of programs that allow you to login to remote systems and have a encrypted connection. `openssh' is a suite of programs used as a secure replacement for `rlogin', `rsh' and `rcp'. It uses public-key cryptography to encrypt communications between two hosts, as well as to authenticate users. It can be used to securely login to a remote host or copy data between hosts, while preventing man-in-the-middle attacks (session hijacking) and DNS spoofing. It will perform data compression on your connections, and secure X11 communications between hosts. There are several ssh implementiations now. The original commercial implementation by Data Fellows can be found at The `ssh' home page can be found at http://www.datafellows.com. The excellent Openssh implementation is based on a early version of the datafellows ssh and has been totally reworked to not include any patented or proprietary pieces. It is free and under a BSD license. It can be found at: http://www.openssh.com. There is also a open source project to re-implement ssh from the ground up called "psst...". For more information see: http://www.net.lut.ac.uk/psst/ You can also use `ssh' from your Windows workstation to your Linux `ssh' server. There are several freely available Windows client implementations, including the one at http://guardian.htu.tuwien.ac.at/therapy/ssh/ as well as a commercial implementation from DataFellows, at http://www.datafellows.com. SSLeay is a free implementation of Netscape's Secure Sockets Layer protocol, developed by Eric Young. It includes several applications, such as Secure telnet, a module for Apache, several databases, as well as several algorithms including DES, IDEA and Blowfish. Using this library, a secure telnet replacement has been created that does encryption over a telnet connection. Unlike SSH, stelnet uses SSL, the Secure Sockets Layer protocol developed by Netscape. You can find Secure telnet and Secure FTP by starting with the SSLeay FAQ, available at http://www.psy.uq.oz.au/~ftp/Crypto/. SRP is another secure telnet/ftp implementation. From their web page: "The SRP project is developing secure Internet software for free worldwide use. Starting with a fully-secure Telnet and FTP distribution, we hope to supplant weak networked authentication systems with strong replacements that do not sacrifice user-friendliness for security. Security should be the default, not an option!" For more information, go to http://www-cs-students.stanford.edu/~tjw/srp/  File: Security-HOWTO.info, Node: PAM - Pluggable Authentication Modules, Next: Cryptographic IP Encapsulation (CIPE), Prev: `ssh' (Secure Shell) and `stelnet', Up: Password Security and Encryption PAM - Pluggable Authentication Modules ====================================== Newer versions of the Red Hat Linux and Debian Linux distributions ship with a unified authentication scheme called "PAM". PAM allows you to change your authentication methods and requirements on the fly, and encapsulate all local authentication methods without recompiling any of your binaries. Configuration of PAM is beyond the scope of this document, but be sure to take a look at the PAM web site for more information. http://www.kernel.org/pub/linux/libs/pam/index.html. Just a few of the things you can do with PAM: * Use encryption other than DES for your passwords. (Making them harder to brute-force decode) * Set resource limits on all your users so they can't perform denial-of-service attacks (number of processes, amount of memory, etc) * Enable shadow passwords (see below) on the fly * allow specific users to login only at specific times from specific places Within a few hours of installing and configuring your system, you can prevent many attacks before they even occur. For example, use PAM to disable the system-wide usage of `.rhosts' files in user's home directories by adding these lines to `/etc/pam.d/rlogin': # # Disable rsh/rlogin/rexec for users # login auth required pam_rhosts_auth.so no_rhosts  File: Security-HOWTO.info, Node: Cryptographic IP Encapsulation (CIPE), Next: Kerberos, Prev: PAM - Pluggable Authentication Modules, Up: Password Security and Encryption Cryptographic IP Encapsulation (CIPE) ===================================== The primary goal of this software is to provide a facility for secure (against eavesdropping, including traffic analysis, and faked message injection) subnetwork interconnection across an insecure packet network such as the Internet. CIPE encrypts the data at the network level. Packets traveling between hosts on the network are encrypted. The encryption engine is placed near the driver which sends and receives packets. This is unlike SSH, which encrypts the data by connection, at the socket level. A logical connection between programs running on different hosts is encrypted. CIPE can be used in tunnelling, in order to create a Virtual Private Network. Low-level encryption has the advantage that it can be made to work transparently between the two networks connected in the VPN, without any change to application software. Summarized from the CIPE documentation: The IPSEC standards define a set of protocols which can be used (among other things) to build encrypted VPNs. However, IPSEC is a rather heavyweight and complicated protocol set with a lot of options, implementations of the full protocol set are still rarely used and some issues (such as key management) are still not fully resolved. CIPE uses a simpler approach, in which many things which can be parameterized (such as the choice of the actual encryption algorithm used) are an install-time fixed choice. This limits flexibility, but allows for a simple (and therefore efficient, easy to debug...) implementation. Further information can be found at http://www.inka.de/~bigred/devel/cipe.html As with other forms of cryptography, it is not distributed with the kernel by default due to export restrictions.  File: Security-HOWTO.info, Node: Kerberos, Next: Shadow Passwords-, Prev: Cryptographic IP Encapsulation (CIPE), Up: Password Security and Encryption Kerberos ======== Kerberos is an authentication system developed by the Athena Project at MIT. When a user logs in, Kerberos authenticates that user (using a password), and provides the user with a way to prove her identity to other servers and hosts scattered around the network. This authentication is then used by programs such as `rlogin' to allow the user to login to other hosts without a password (in place of the `.rhosts' file). This authentication method can also used by the mail system in order to guarantee that mail is delivered to the correct person, as well as to guarantee that the sender is who he claims to be. Kerberos and the other programs that come with it, prevent users from "spoofing" the system into believing they are someone else. Unfortunately, installing Kerberos is very intrusive, requiring the modification or replacement of numerous standard programs. You can find more information about kerberos by looking at the kerberos FAQ, and the code can be found at http://nii.isi.edu/info/kerberos/. [lsqb ]From: Stein, Jennifer G., Clifford Neuman, and Jeffrey L. Schiller. "Kerberos: An Authentication Service for Open Network Systems." USENIX Conference Proceedings, Dallas, Texas, Winter 1998.] Kerberos should not be your first step in improving security of your host. It is quite involved, and not as widely used as, say, SSH.  File: Security-HOWTO.info, Node: Shadow Passwords-, Next: "Crack" and "John the Ripper", Prev: Kerberos, Up: Password Security and Encryption Shadow Passwords- ================= Shadow passwords are a means of keeping your encrypted password information secret from normal users. Recent versions of both Red Hat and Debian Linux use shadow passwords by default, but on other systems, encrypted passwords are stored in `/etc/passwd' file for all to read. Anyone can then run password-guesser programs on them and attempt to determine what they are. Shadow passwords, by contrast, are saved in `/etc/shadow', which only privileged users can read. In order to use shadow passwords, you need to make sure all your utilities that need access to password information are recompiled to support them. PAM (above) also allows you to just plug in a shadow module; it doesn't require re-compilation of executables. You can refer to the Shadow-Password HOWTO for further information if necessary. It is available at http://metalab.unc.edu/LDP/HOWTO/Shadow-Password-HOWTO.html It is rather dated now, and will not be required for distributions supporting PAM.  File: Security-HOWTO.info, Node: "Crack" and "John the Ripper", Next: CFS - Cryptographic File System and TCFS - Transparent Cryptographic File System, Prev: Shadow Passwords-, Up: Password Security and Encryption "Crack" and "John the Ripper" ============================= If for some reason your `passwd' program is not enforcing hard-to-guess passwords, you might want to run a password-cracking program and make sure your users' passwords are secure. Password cracking programs work on a simple idea: they try every word in the dictionary, and then variations on those words, encrypting each one and checking it against your encrypted password. If they get a match they know what your password is. There are a number of programs out there...the two most notable of which are "Crack" and "John the Ripper" (http://www.openwall.com/john/) . They will take up a lot of your CPU time, but you should be able to tell if an attacker could get in using them by running them first yourself and notifying users with weak passwords. Note that an attacker would have to use some other hole first in order to read your `/etc/passwd' file, but such holes are more common than you might think. Because security is only as strong as the most insecure host, it is worth mentioning that if you have any Windows machines on your network, you should check out L0phtCrack, a Crack implementation for Windows. It's available from http://www.l0pht.com  File: Security-HOWTO.info, Node: CFS - Cryptographic File System and TCFS - Transparent Cryptographic File System, Next: X11 SVGA and display security, Prev: "Crack" and "John the Ripper", Up: Password Security and Encryption CFS - Cryptographic File System and TCFS - Transparent Cryptographic File System ================================================================================ CFS is a way of encrypting entire directory trees and allowing users to store encrypted files on them. It uses an NFS server running on the local machine. RPMS are available at http://www.zedz.net/redhat/, and more information on how it all works is at ftp://ftp.research.att.com/dist/mab/. TCFS improves on CFS by adding more integration with the file system, so that it's transparent to users that the file system that is encrypted. More information at: http://www.tcfs.it/. It also need not be used on entire file systems. It works on directory trees as well.  File: Security-HOWTO.info, Node: X11 SVGA and display security, Prev: CFS - Cryptographic File System and TCFS - Transparent Cryptographic File System, Up: Password Security and Encryption X11 SVGA and display security ============================= * Menu: * X11:: * SVGA:: * GGI (Generic Graphics Interface project)::  File: Security-HOWTO.info, Node: X11, Next: SVGA, Up: X11 SVGA and display security X11 --- It's important for you to secure your graphical display to prevent attackers from grabbing your passwords as you type them, reading documents or information you are reading on your screen, or even using a hole to gain root 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 use `xhost' to specify the hosts that are allowed access to your display. This is not very secure at all, because if someone has access to your machine, they can `xhost + `their machine" and get in easily. Also, if you have to allow access from an untrusted machine, anyone there can compromise your display. When using `xdm' (X Display Manager) to log in, you get a much better access method: MIT-MAGIC-COOKIE-1. A 128-bit "cookie" is generated and stored in your `.Xauthority' file. 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 access to only that connection. See the Remote-X-Apps mini-howto, available at http://metalab.unc.edu/LDP/HOWTO/mini/Remote-X-Apps.html. You can also use `ssh' (see *Note `ssh' (Secure Shell) and `stelnet':: , above) to allow secure X connections. This has the advantage of also being transparent to the end user, and means that no unencrypted data flows across the network. You can also disable any remote connections to your X server by using the '-nolisten tcp' options to your X server. This will prevent any network connections to your server over tcp sockets. Take a look at the `Xsecurity' man page for more information on X security. The safe bet is to use `xdm' to login to your console and then use `ssh' to go to remote sites on which you wish to run X programs.  File: Security-HOWTO.info, Node: SVGA, Next: GGI (Generic Graphics Interface project), Prev: X11, Up: X11 SVGA and display security SVGA ---- SVGAlib programs are typically SUID-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.  File: Security-HOWTO.info, Node: GGI (Generic Graphics Interface project), Prev: SVGA, Up: X11 SVGA and display security 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. http://synergy.caltech.edu/~ggi/  File: Security-HOWTO.info, Node: Kernel Security, Next: Network Security, Prev: Password Security and Encryption, Up: Top Kernel Security *************** This is a description of the kernel configuration options that relate to security, and an explanation of what they do, and how to use them. As the kernel controls your computer's networking, it is important that it be very secure, and not be compromised. To prevent some of the latest networking attacks, you should try to keep your kernel version current. You can find new kernels at urlnam (ftp://ftp.kernel.org) or from your distribution vendor. There is also a international group providing a single unified crypto patch to the mainstream Linux kernel. This patch provides support for a number of cryptographic subsystems and things that cannot be included in the mainstream kernel due to export restrictions. For more information, visit their web page at: http://www.kerneli.org * Menu: * 2-0 Kernel Compile Options:: * 2-2 Kernel Compile Options:: * Kernel Devices::  File: Security-HOWTO.info, Node: 2-0 Kernel Compile Options, Next: 2-2 Kernel Compile Options, Up: Kernel Security 2-0 Kernel Compile Options ========================== For 2.0.x kernels, the following options apply. You should see these options during the kernel configuration process. Many of the comments here are from `./linux/Documentation/Configure.help', which is the same document that is referenced while using the Help facility during the `make config' stage of compiling the kernel. * Network Firewalls (CONFIG[lowbar]FIREWALL) This option should be on if you intend to run any firewalling or masquerading on your Linux machine. If it's just going to be a regular client machine, it's safe to say no. * IP: forwarding/gatewaying (CONFIG[lowbar]IP[lowbar]FORWARD) If you enable IP forwarding, your Linux box essentially becomes a router. If your machine is on a network, you could be forwarding data from one network to another, and perhaps subverting a firewall that was put there to prevent this from happening. Normal dial-up users will want to disable this, and other users should concentrate on the security implications of doing this. Firewall machines will want this enabled, and used in conjunction with firewall software. You can enable IP forwarding dynamically using the following command: root# echo 1 > /proc/sys/net/ipv4/ip_forward and disable it with the command: root# echo 0 > /proc/sys/net/ipv4/ip_forward Keep in mind the files in /proc are "virtual" files and the shown size of the file might not reflect the data output from it. * IP: syn cookies (CONFIG[lowbar]SYN[lowbar]COOKIES) a "SYN Attack" is a denial of service (DoS) attack that consumes all the resources on your machine, forcing you to reboot. We can't think of a reason you wouldn't normally enable this. In the 2.2.x kernel series this config option merely allows syn cookies, but does not enable them. To enable them, you have to do: root# echo 1 > /proc/sys/net/ipv4/tcp_syncookies

* IP: Firewalling (CONFIG[lowbar]IP[lowbar]FIREWALL) This option is necessary if you are going to configure your machine as a firewall, do masquerading, or wish to protect your dial-up workstation from someone entering via your PPP dial-up interface. * IP: firewall packet logging (CONFIG[lowbar]IP[lowbar]FIREWALL[lowbar]VERBOSE) This option gives you information about packets your firewall received, like sender, recipient, port, etc. * IP: Drop source routed frames (CONFIG[lowbar]IP[lowbar]NOSR) This option should be enabled. Source routed frames contain the entire path to their destination inside of the packet. This means that routers through which the packet goes do not need to inspect it, and just forward it on. This could lead to data entering your system that may be a potential exploit. * IP: masquerading (CONFIG[lowbar]IP[lowbar]MASQUERADE) If one of the computers on your local network for which your Linux box acts as a firewall wants to send something to the outside, your box can "masquerade" as that host, i.e., it forewords the traffic to the intended destination, but makes it look like it came from the firewall box itself. See http://www.indyramp.com/masq for more information. * IP: ICMP masquerading (CONFIG[lowbar]IP[lowbar]MASQUERADE[lowbar]ICMP) This option adds ICMP masquerading to the previous option of only masquerading TCP or UDP traffic. * IP: transparent proxy support (CONFIG[lowbar]IP[lowbar]TRANSPARENT[lowbar]PROXY) This enables your Linux firewall to transparently redirect any network traffic originating from the local network and destined for a remote host to a local server, called a "transparent proxy server". This makes the local computers think they are talking to the remote end, while in fact they are connected to the local proxy. See the IP-Masquerading HOWTO and http://www.indyramp.com/masq for more information. * IP: always defragment (CONFIG[lowbar]IP[lowbar]ALWAYS[lowbar]DEFRAG) Generally this option is disabled, but if you are building a firewall or a masquerading host, you will want to enable it. When data is sent from one host to another, it does not always get sent as a single packet of data, but rather it is fragmented into several pieces. The problem with this is that the port numbers are only stored in the first fragment. This means that someone can insert information into the remaining packets that isn't supposed to be there. It could also prevent a teardrop attack against an internal host that is not yet itself patched against it. * Packet Signatures (CONFIG[lowbar]NCPFS[lowbar]PACKET[lowbar]SIGNING) This is an option that is available in the 2.2.x kernel series that will sign NCP packets for stronger security. Normally you can leave it off, but it is there if you do need it. * IP: Firewall packet netlink device (CONFIG[lowbar]IP[lowbar]FIREWALL[lowbar]NETLINK) This is a really neat option that allows you to analyze the first 128 bytes of the packets in a user-space program, to determine if you would like to accept or deny the packet, based on its validity.  File: Security-HOWTO.info, Node: 2-2 Kernel Compile Options, Next: Kernel Devices, Prev: 2-0 Kernel Compile Options, Up: Kernel Security 2-2 Kernel Compile Options ========================== For 2.2.x kernels, many of the options are the same, but a few new ones have been developed. Many of the comments here are from `./linux/Documentation/Configure.help', which is the same document that is referenced while using the Help facility during the `make config' stage of compiling the kernel. Only the newly- added options are listed below. Consult the 2.0 description for a list of other necessary options. The most significant change in the 2.2 kernel series is the IP firewalling code. The `ipchains' program is now used to install IP firewalling, instead of the `ipfwadm' program used in the 2.0 kernel. * Socket Filtering (CONFIG[lowbar]FILTER) For most people, it's safe to say no to this option. This option allows you to connect a user-space filter to any socket and determine if packets should be allowed or denied. Unless you have a very specific need and are capable of programming such a filter, you should say no. Also note that as of this writing, all protocols were supported except TCP. * Port Forwarding Port Forwarding is an addition to IP Masquerading which allows some forwarding of packets from outside to inside a firewall on given ports. This could be useful if, for example, you want to run a web server behind the firewall or masquerading host and that web server should be accessible from the outside world. An external client sends a request to port 80 of the firewall, the firewall forwards this request to the web server, the web server handles the request and the results are sent through the firewall to the original client. The client thinks that the firewall machine itself is running the web server. This can also be used for load balancing if you have a farm of identical web servers behind the firewall. Information about this feature is available from http://www.monmouth.demon.co.uk/ipsubs/portforwarding.html (to browse the WWW, you need to have access to a machine on the Internet that has a program like lynx or Netscape). For general info, please see ftp://ftp.compsoc.net/users/steve/ipportfw/linux21/ * Socket Filtering (CONFIG[lowbar]FILTER) Using this option, user-space programs can attach a filter to any socket and thereby tell the kernel that it should allow or disallow certain types of data to get through the socket. Linux socket filtering works on all socket types except TCP for now. See the text file `./linux/Documentation/networking/filter.txt' for more information. * IP: Masquerading The 2.2 kernel masquerading has been improved. It provides additional support for masquerading special protocols, etc. Be sure to read the IP Chains HOWTO for more information.  File: Security-HOWTO.info, Node: Kernel Devices, Prev: 2-2 Kernel Compile Options, Up: Kernel Security Kernel Devices ============== There are a few block and character devices available on Linux that will also help you with security. The two devices `/dev/random' and `/dev/urandom' are provided by the kernel to provide random data at any time. Both `/dev/random' and `/dev/urandom' should be secure enough to use in generating PGP keys, `ssh' challenges, and other applications where secure random numbers are required. Attackers should be unable to predict the next number given any initial sequence of numbers from these sources. There has been a lot of effort put in to ensuring that the numbers you get from these sources are random in every sense of the word. The only difference between the two devices, is that `/dev/random' runs out of random bytes and it makes you wait for more to be accumulated. Note that on some systems, it can block for a long time waiting for new user-generated entropy to be entered into the system. So you have to use care before using `/dev/random'. (Perhaps the best thing to do is to use it when you're generating sensitive keying information, and you tell the user to pound on the keyboard repeatedly until you print out "OK, enough".) `/dev/random' is high quality entropy, generated from measuring the inter-interrupt times etc. It blocks until enough bits of random data are available. `/dev/urandom' is similar, but when the store of entropy is running low, it'll return a cryptographically strong hash of what there is. This isn't as secure, but it's enough for most applications. You might read from the devices using something like: root# head -c 6 /dev/urandom | mimencode This will print six random characters on the console, suitable for password generation. You can find `mimencode' in the `metamail' package. See `/usr/src/linux/drivers/char/random.c' for a description of the algorithm. Thanks to Theodore Y. Ts'o, Jon Lewis, and others from Linux-kernel for helping me (Dave) with this.  File: Security-HOWTO.info, Node: Network Security, Next: Security Preparation (before you go on-line), Prev: Kernel Security, Up: Top 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 compromising physical or local security, 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. * Menu: * Packet Sniffers:: * System services and tcp[lowbar]wrappers:: * Verify Your DNS Information:: * identd:: * Configuring and Securing the Postfix MTA:: * SATAN ISS and Other Network Scanners:: * sendmail qmail and MTA's:: * Denial of Service Attacks:: * NFS (Network File System) Security-:: * NIS (Network Information Service) (formerly YP)-:: * Firewalls:: * IP Chains - Linux Kernel 2-2-x Firewalling:: * Netfilter - Linux Kernel 2-4-x Firewalling:: * VPNs - Virtual Private Networks::  File: Security-HOWTO.info, Node: Packet Sniffers, Next: System services and tcp[lowbar]wrappers, Up: Network Security Packet Sniffers =============== One of the most common ways intruders gain access to more systems on your network is by employing a packet sniffer on a already compromised host. This "sniffer" just listens on the Ethernet port for things like `passwd' and `login' and `su' in the packet stream and then logs the traffic after that. This way, attackers gain passwords for systems they are not even attempting to break into. Clear-text passwords are very vulnerable to this attack. Example: Host A has been compromised. Attacker installs a sniffer. Sniffer picks up admin logging into Host B from Host C. It gets the admins personal password as they login to B. Then, the admin does a `su' to fix a problem. They now have the root password for Host B. Later the admin lets someone `telnet' from his account to Host Z on another site. Now the attacker has a password/login on Host Z. In this day and age, the attacker doesn't even need to compromise a system to do this: they could also bring a laptop or pc into a building and tap into your net. Using `ssh' or other encrypted password methods thwarts this attack. Things like APOP for POP accounts also prevents this attack. (Normal POP logins are very vulnerable to this, as is anything that sends clear-text passwords over the network.)