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Linux Security Quick Reference Guide

Security Glossary:

General Security Tips:

Linux Security Resources:

• Buffer Overflow: A condition that occurs when a user or process attempts to place

more data into a program’s storage buffer in memory and then overwrites the actual

program data with instructions that typically provide a shell owned by root on the

server. Accounted for more than 50 percent of all major security bugs leading to

security advisories published by CERT. Typically associated with set-user-ID root

binaries.

• Cryptography: The mathematical science that deals with transforming data to render

its meaning unintelligible, prevent its undetected alteration, or prevent its unauthorized

use.

• Denial of Service: Occurs when a resource is targeted by an intruder to prevent

legitimate users from using that resource. They are a threat to the availability of data

to all others trying to use that resource. Range from unplugging the network connection

to consuming all the available network bandwidth.

• IP Spoofing: An attack in which one host masquerades as another. This can be

used to route data destined for one host to antoher, thereby allowing attackers to

intercept data not originally intended for them. It is typically a one-way attack.

• Port Scanning: The process of determining which ports are active on a machine. By

probing as many hosts as possible, means to exploit the ones that respond can be

developed. It is typically the precursor to an attack.

• Packet Filtering: A method of filtering network traffic as it passes between the

firewall’s interfaces at the network level. The network data is then analyzed according

to the information available in the data packet, and access is granted or denied based

on the firewall security policy. Usually requires an intimate knowledge of how network

protocols work.

• Proxy Gateway: Also called Application Gateways, act on behalf of another

program. A host with a proxy server installed becomes both a server and a client, and

acts as a choke between the final destination and the client. Proxy servers are typically

small, carefully-written single-purpose programs that only permit specific services to

pass through it. Typically combined with packet filters.

• Set User-ID (setuid) / Set Group-ID (setgid): Files that everyone can execute as

either it's owner or group privileges. Typically, you'll find root-owned setuid files, which

means that regardless of who executes them, they obtain root permission for the

period of time the program is running (or until that program intentionally relinquishes

these privileges). These are the types of files that are most often attacked by intruders,

because of the potential for obtaining root privileges. Commonly associated with

buffer overflows.

• Trojan Horse: A program that masquerades itself as a benign program, when in fact

it is not. A program can be modified by a malicious programmer that purports to do

something useful, but in fact contains a malicious program containing hidden functions,

exploiting the privileges of the user executing it. A modified version of /bin/ps , for

example, may be used to hide the presence of other programs running on the system.

• Vulnerability: A condition that has the potential for allowing security to be

compromised. Many different types of network and local vulnerabilities exist and are

widely known, and frequently occur on computers regardless of their level of network

connectivity, processing speed, or profile.

• AutoRPM on Red Hat and apt-get on Debian can be used to download and install

any packages on your system for which there are updates. Use care when

automatically updating production servers.

• IP Masquerading enables a Linux box with multiple interfaces to act as a gateway to

remote networks for hosts connected to the Linux box on the internal network

interface. See the IP Masquerading HOWTO for implementation information.

• Install nmap to determine potential communication channels. Can determine remote

OS version, perform “stealth” scans by manipulating ICMP, TCP and UDP, and even

potentially determine the remote username running the service. Start with something

simple like:

# nmap 192.168.1.1

• Password-protect LILO for servers in public environments to require authorization

when passing LILO command-line kernel parameters at boot time. Add the password

and restricted arguments to /etc/lilo.conf , then be sure to re-run

/sbin/lilo :

image = /boot/vmlinuz-2.2.17

label = Linux

read-only

restricted

password = your-password

• The OpenWall kernel patch is a useful set of kernel security improvements that

helps to prevent buffer overflows, restrict information in /proc available to normal

users, and other changes. Requires compiling the kernel, and not for newbies.

• Ensure system clocks are accurate. The time stamps on log files must be accurate

so security events can be correlated with remote systems. Inaccurate records make it

impossible to build a timeline. For workstations, it is enough to add a crontab entry:

0-59/30 * * * * root /usr/sbin/ntpdate -su time.timehost.com

• Install and execute the Bastille Linux hardening tool. Bastille is a suite of shell

scripts that eliminates many of the vulnerabilities that are common on default Linux

installations. It enables users to make educated choices to improve security by asking

questions as it interactively steps through securing the host. Features include basic

packet filtering, deactivating unnecessary network services, auditing file permissions,

and more. Try the non-intrusive test mode first.

• Configure sudo (superuser do) to execute privileged commands as a normal user

instead of using su . The administrator supplies his own password to execute specific

commands that would otherwise require root access. The file /etc/sudoers file

controls which users may execute which programs. To permit Dave to only manipulate

the printer on magneto :

Cmnd_Alias LPCMDS = /usr/sbin/lpc, /usr/bin/lprm

dave magneto = LPCMDS

Dave executes sudo with the authorized command and enters his own password

when prompted:

dave$ sudo /usr/sbin/lpc

Password: <password>

lpc>

• Password security is the most basic means of authentication, yet the most critical

means to protect your system from compromise. It is also one of the most overlooked

means. Without an effective well-chosen password, your system is sure to be

compromised. Obtaining access to any user account on the system is the tough part.

From there, root access is only a step away. Run password-cracking programs such as

John the Ripper or Crack regularly on systems for which you’re responsible to ensure

password security is maintained. Disable unused accounts using /usr/bin/passwd

-l . Use the MD5 password during install if your distribution supports it.

• Packet filtering isn’t just for firewalls. Using ipchains , you can provide a significant

amount of protection from external threats on any Linux box. Blocking access to a

particular service from connecting outside of your local network you might try:

# ipchains -I input -p TCP -s 192.168.1.11 telnet -j DENY -l

This will prevent incoming access to the telnet port on your local machine if the

connection originates from 192.168.1.11 . This is a very simple example. Be sure

to read the IP Chains HOWTO before implementing any firewalling.

• Apache directory and password protection

http://www.apacheweek.com/features/userauth

• Bastille Linux Project

http://www.bastille-linux.org

• BugTraq Full Disclosure Mailing List

http://www.securityfocus.com/forums/bugtraq/intro.html

• Building Internet Firewalls, Second Edition

O’Reilly & Assoc, ISBN 1565928717

• CERT Security Improvement Modules

http://www.cert.org/security-improvement

• Introduction to Linux Security

http://www.linux-mag.com/1999-10/security_01.html

• Linux Intrusion Detection Resources

http://www.linuxsecurity.com/intrusion-detection

• John the Ripper Password Cracker

http://www.openwall.com/john

• Linux and Open Source Security Advisories

http://www.linuxsecurity.com/advisories

• LinuxSecurity.com Security Reference Info

http://www.linuxsecurity.com/docs

• LinuxSecurity.com Security Discussion Lists

http://www.linuxsecurity.com/mailing-lists.html

• LinuxSecurity.com Tip of the Day

http://www.linuxsecurity.com/tips

• LinuxSecurity.com Weekly Security Newsletter

http://www.linuxsecurity.com/newsletter.html

• OpenSSH secure remote access tool

http://www.openssh.com

• OpenWall Security Project

http://www.openwall.com

Introduction:

The intent of this Quick Reference Guide is to provide a starting point for improving the security of your

system, to serve as a pointer to more in-depth security information, and to increase security awareness

and methods that can be used to improve security. It is not a substitute for reading any of the vast

amounts of Linux security documentation that already exists.

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, it may pass through several other points along the way, giving other users the opportu-

nity 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 infor-

mation from you, or even deny you access to your own resources.

Security involves defense in depth. Approaching security a step at a time, with consistency and vigi-

lance, you can mitigate the security threats, and keep the crackers at bay. Keep your system up to date

by making sure you have installed the current versions of software and are aware of all security alerts.

Doing this alone will help make your system markedly more secure.

The more secure your system is the more intrusive your security becomes. You need to decide where in

this balancing act your system will still be usable yet secure for your purposes.

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

monitor it.

Controlling File Permissions & Attributes:

Monitoring the permissions on system files is crucial to maintain host integrity.

• Regularly audit your systems for any unauthorized and unnecessary use of the setuid or setgid

permissions. “Set-user-ID root” programs run as the root user, regardless of who is executing them,

and are a frequent cause of buffer overflows. Many programs are setuid and setgid to enable a

normal user to perform operations that would otherwise require root, and can be removed if your

users do not need such permission. Find all setuid and setgid programs on your host and

descriminately remove the setuid or setgid permissions on a suspicious program with chmod :

root# find / -type f -perm +6000 -ls

59520 30 -rwsr-xr-x 1 root root 30560 Apr 15 1999 /usr/bin/chage

59560 16 -r-sr-sr-x 1 root lp 15816 Jan 6 2000 /usr/bin/lpq

root# chmod -s /usr/bin/chage /usr/bin/lpq

root# ls -l /usr/bin/lpq /usr/bin/chage

-rwxr-xr-x 1 root root 30560 Apr 15 1999 /usr/bin/chage

-r-xr-xr-x 1 root lp 15816 Jan 6 2000 /usr/bin/lpq

• World-writable files are easily altered or removed. Locate all world-writable files on your system:

root# find / -perm -2 ! -type l -ls

In the normal course of operation, several files will be world-writable, including some from /dev and

the /tmp directory itself.

• Locate and identify all files that do not have an owner or belong to a group. Unowned files may also

be an indication an intruder has accessed your system.

root# find / -nouser -o -nogroup

• Using the lsattr and chattr commands, administrators can modify characteristics of files and

directories, including the ability to control deletion and modification above what normal chmod

provides. The use of “append-only” and “immutable” attributes can be particularly effective in

preventing log files from being deleted, or Trojan Horses from being placed on top of trusted

binaries. While not a guarantee a system file or log won’t be modified, only root has the ability to

remove this protection. The chattr command is used to add or remove these properties, while the

lsattr can be used to list them.

Log files can be protected by only permitting appending to them. Once the data has been written, it

cannot be removed. While this will require modifications to your log rotation scripts, this can provide

additional protection from a cracker attempting to remove his tracks. Once rotated, they should be

changed to immutable. Files suitable for these modifications include /bin/login , /bin/rpm ,

/etc/shadow , and others that should not change frequently.

# chattr +i /bin/login

# chattr +a /var/log/messages

# lsattr /bin/login /var/log/messages

----i--- /bin/login

-----a-- /var/log/messages

• There should never be a reason for user’s to be able to run setuid programs from their home

directories. Use the nosuid option in /etc/fstab for partitions that are writable by others than

root. You may also wish to use the nodev and noexec on user’s home partitions, as well as /var ,

which prohibits execution of programs, and creation of character or block devices, which should

never be necessary anyway. See the mount man page for more information.

• Network Time Protocol information

http://www.ntp.org

• nmap Port Scanner

http://www.insecure.org/nmap

• Practical UNIX & Internet Security, Second Ed.

O’Reilly & Assoc, ISBN 1565921488

• rsync Incremental File Transfer Utility

http://rsync.samba.org

• Secure Shell FAQ

http://www.employees.org/~satch/ssh/faq/

• Security-related HOWTOs and FAQs

http://www.linuxsecurity.com/docs

• Site Security Handbook (RFC2196)

http://www.linuxsecurity.com/docs/rfcs/rfc2196.txt

• sudo root access control tool

http://www.courtesan.com/sudo

• Snort Network Intrusion Detection System

http://www.snort.org

• Tripwire file integrity tool

http://www.tripwiresecurity.com

• Using Snort

http://www.linuxsecurity.com/using-snort.html

Kernel Security:

Several kernel configuration options are available to improve security through the

/proc pseudo-filesystem. Quite a few of the files in /proc/sys are directly related to

security. Enabled if contains a 1 and disabled if it contains a 0 . Many of the options

available in /proc/sys/net/ipv4 include:

• icmp_echo_ignore_all : Ignore all ICMP ECHO requests. Enabling this option will

prevent this host from responding to ping requests.

• icmp_echo_ignore_broadcasts : Ignore ICMP echo requests with a broadcast/

multicast destination address. Your network may be used as an exploder for denial of

service packet flooding attacks to other hosts.

• ip_forward : Enable or disable the forwarding of IP packets between interfaces.

Default value is dependent on whether the kernel is configured as host or router.

• ip_masq_debug : Enable or disable debugging of IP masquerading.

• tcp_syncookies : Protection from the “SYN Attack”. Send syncookies when the SYN

backlog queue of a socket overflows.

• rp_filter : Determines if source address verification is enabled. Enable this option on

all routers to prevent IP spoofing attacks against the internal network.

• secure_redirects : Accept ICMP redirect messages only for gateways listed in default

gateway list.

• log_martians : Log packets with impossible addresses to kernel log.

• accept_source_route : Determines whether source routed packets are accepted or

declined. Should be disabled unless specific reason requires it.

The file /etc/sysctl.conf on recent Red Hat contains a few default settings and is

processed at system startup. The /sbin/sysctl program can be used to control these

parameters. It is also possible to configure their values using /bin/echo . For example,

to disable IP forwarding, as root run:

echo “0” > /proc/sys/net/ipv4/ip_forward

This must written to a system startup file or /etc/sysctl.conf on Red Hat to occur

after each reboot. More information is available in proc.txt file in the kernel

Documentation/ directory.

Network Intrusion Detection:

Intrusion detection devices are an integral part of any network. The Internet is

constantly evolving, and new vulnerabilities and exploits are found regularly. They

provide an additional level of protection to detect the presence of an intruder, and help

to provide accoutability for the attacker's actions.

The snort network intrusion detection tool performs real-time traffic analysis,

watching for anamolous events that may be considered a potential intrusion attempt.

Based on the contents of the network traffic, at either the IP or application level, an

alert is generated. It is easily configured, utilizes familiar methods for rule

development, and takes only a few minutes to install. Snort currently includes the

ability to detect more than 1100 potential vulnerabilities. It is quite feature-packed out

of the box:

• Detect and alert based on pattern matching for threats including buffer overflows,

stealth port scans, CGI attacks, SMB probes and NetBIOS queries, NMAP and

other portscanners, well-known backdoors and system vulnerabilities, DDoS

clients, and many more;

• Can be used on an existing workstation to monitor a home DSL connection, or on

a dedicated server to monitor a corporate web site.

Implementation By Dave Wreski

Concept By Benjamin Thomas

Permission to distribute granted

http://www.GuardianDigital.com

v1.1

Apache Security:

Disable Unnecessary Services:

Using RPM and dpkg:

DNS Security:

• Limit Apache to listen only on local interface by configuring

/etc/httpd/conf/httpd.conf to read:

Listen 127.0.0.1:80

• Use the following to disable access to the entire filesystem by default, unless

explicitly permitted. This will disable printing of indexes if no index.html

exists, server-side includes, and following symbolic links. Disabling symlinks

may impact performance for large sites.

• Zone transfers should only be permitted by master name servers to update the zone

(domain) information in their slave servers. Failure to do so may result in IP numbers and

hostnames being revealed to unauthorized users. Restrict queries to only public domains.

Suitable for name servers with both public and private zones.

// Allow transfer only to our slave name server. Allow queries

// only by hosts in the 192.168.1.0 network.

zone “mydomain.com” {

type master;

file “master/db.mydomain.com”;

allow-transfer { 192.168.1.6; };

allow-query { 192.168.1.0/24; };

};

• Deny and log queries for our version number except from the local host. The ability to

determine the bind version enables an attacker to find the corresponding exploit for that

version.

// Disable the ability to determine the version of BIND running

zone “bind” chaos {

type master;

file “master/bind”;

allow-query { localhost; };

};

The ./master/bind file should then contain:

$TTL 1d

@ CHAOS SOA localhost. root.localhost. (

1 ; serial

3H ; refresh

15M ; retry

1W ; expire

1D ) ; minimum

NS localhost.

• Control which interfaces named listens on. Restricting the interfaces on which named

runs can limit the exposure to only the necessary networks.

listen-on { 192.168.1.1; };

• Use Access Control Lists to classify groups of hosts with differing degrees of trust. The

“internal” ACL label might be used to describe internal hosts that are permitted a greater

degree of access to the information than other hosts might be. Before it can be used it

must be defined:

acl “internal” {

{ 192.168.1.0/24; 192.168.2.11; };

};

It can then be used in “zone” statements or the main “options” statement:

zone “inside.mynet.com” {

type master;

file “master/inside.mynet.com”;

allow-query { “internal”; };

};

• Configure BIND to run as a normal user. Once BIND has been started, it has the ability

to relinquish its privileges, and run as a user with limited abilities instead of root .

# useradd -M -r -d /var/named -s /bin/false named

# groupadd -r named

This account should be used for nothing other than running the name server. Ensure the

zone files are readable by the named user. It is then necessary to modify the default

named init script, typically found in /etc/rc.d/init.d/named on Red Hat or

/etc/init.d/named on Debian:

/usr/sbin/named -u named -g named

It is also possible to run named in a “chroot jail” which helps to restrict the damage that

can be done should named be subverted.

Disabling or removing unused programs and services from your host is the most

effective way to limit threats originating from a remote host. Use your distributions

package management tools to scan the list of installed packages, then remove

those that are unnecessary.

• Many of the services running from inetd are legacy programs, which are hardly

ever required, yet typically enabled by default. The file /etc/inetd.conf is

used to specify which services are offered. Disable all services that you do not

want to provide by commenting them out using the # character in the first column

of the line.

• The /etc/rc*.d or /etc/rc.d/rc* directories contains shell scripts that

control the execution of network and system services during runlevels. Rename or

otherwise disable any that are not required or remove the package entirely. Red

Hat users can use /sbin/chkconfig --list to list which services run in which

runlevel, and /sbin/chkconfig --del <name> to disable a service.

If you don’t understand what a particular service does, disable it until you find out.

Use netstat and ps to confirm they have not been started after a reboot. Use

/bin/netstat -a -p --inet to determine which are available and the

process ID associated with them. A port scanner should also be used to get a view

of what remote hosts see.

The /bin/rpm program on Red Hat and derivitives and the /usr/bin/dpkg on

Debian and derivitives are used to control the management of packages.

• Remove a package

# rpm -e <package-name>

# dpkg -r <package-name>

• List contents of entire package

# rpm -qvl <package-name.rpm>

# dpkg -c <package-name.deb>

• List all installed packages with info about each

# rpm -qvia

# dpkg -l

• List contents of a package

# rpm -qvpl <package-name.rpm>

# dpkg -c <package-name.deb>

• Print information about a package

# rpm -qpi <package-name.rpm>

# dpkg -I <package-name.deb>

• Verify package characteristics (basic integrity check)

# rpm -Va

# debsums -a

• Determine to which package a file belongs

# rpm -qf </path/to/file>

# dpkg -S </path/to/file>

• Install new package

# rpm -Uvh <package-name.rpm>

# dpkg -i <package-name.deb>

Options None

AllowOverride None

Order deny,allow

Deny from all

</Directory>

• Use the following to control access to the server from limited

addresses in /etc/httpd/conf/access.conf to read:

# Deny all accesses by default

Order deny,allow

# Allow access to local machine

Allow from 127.0.0.1

# Allow access to entire local network

Allow from 192.168.1.

# Allow access to single remote host

Allow from 192.168.5.3

# Deny from everyone else

Deny from all

</Directory>

• Use the following to require password authentication when attempting to

access a specific directory in /etc/httpd/conf/access.conf :

Order Deny,Allow

Deny from All

Allow from 192.168.1.11

AuthName “Private Information”

AuthType Basic

AuthUserFile /etc/httpd/conf/private-users

AuthGroupFile /etc/httpd/conf/private-groups

require group <group-name>

</Directory>

Create the private-groups file using the following format:

group-name: user1 user2 user...

Create password entries for each user in the above list:

# htpasswd -cm /etc/httpd/conf/private-users user1

New password: <password>

Re-type new password: <password>

Adding password for user user1

Be sure to restart apache and test it. This will result in the enabling of double

reverse lookups to verify the identity of the remote host. Remove the -c

option to htpasswd after the first user has been added. Be sure the password

file you create is not located within the DocumentRoot to prevent it from

being downloaded.

Checking Package Integrity:

The md5sum command is used to compute a 128-bit fingerprint that is strongly

dependant upon the contents of the file to which it is applied. It can be used to

compare against a previously-generated sum to determine whether the file has

changed. It is commonly used to ensure the integrity of updated packages

distributed by a vendor:

# md5sum package-name

995d4f40cda13eacd2beaf35c1c4d5c2 package-name

The string of numbers can then be compared against the MD5 checksum

published by the packager. While it does not take into account the possibility that

the same person that may have modified a package also may have modified the

published checksum, it is especially useful for establishing a great deal of

assurance in the integrity of a package before installing it.

Configuring Syslog:

The syslogd is responsible for capturing logging information generated by

system processes. The klogd is responsible for capturing logging information

generated by the kernel. System logs provide the primary indication of a potential

problem.

• Fine-tune the default /etc/syslog.conf to send log information to specific

files for easier analysis.

# Monitor authentication attempts

auth.*;authpriv.*

Install and Configure OpenSSH:

OpenSSH is a replacement for telnet and ftp that eliminates eavesdropping,

connection hijacking, and encrypts all communication between hosts. One of the

most indepensible free security tools in existence.

• Install the OpenSSH and OpenSSL Packages:

openssh-<current-version>.rpm

openssh-server-<current-version>.rpm

openssh-clients-<current-version>.rpm

openssl-<current-version>.rpm

• Generate Public/Private Key Pair:

OpenSSH uses public key cryptography to provide secure authorization.

Generating the public key, which is shared with remote systems, and the private

key which is kept on the local system, is done first to configure OpenSSH.

orion$ ssh-keygen

Generating RSA keys: ...ooooooO....ooooooO

Key generation complete.

Enter file in which to save the key (/home/dave/.ssh/identity):

Created directory '/home/dave/.ssh'.

Enter passphrase (empty for no passphrase): <passphrase>

Enter same passphrase again: <passphrase>

Your identification has been saved in /home/dave/.ssh/identity.

Your public key has been saved in /home/dave/.ssh/identity.pub.

The key fingerprint is:

ac:42:11:c8:0d:b6:7e:b4:06:6a:a3:a7:e8:2c:b0:12 dave@orion

• Copy Public Key to Remote Host:

host2$ mkdir -m 700 ~dave/.ssh

host2$ cp /mnt/floppy/identity.pub ~dave/.ssh/authorized_keys

• Log in to Remote Host:

The SSH client ( /usr/bin/ssh ) is a drop-in replacement for rlogin and rsh . It

can be used to securely login to a remote host:

orion$ ssh host2

Enter passphrase for RSA key 'dave@orion': <passphrase>

Last login: Sat Aug 15 17:13:01 2000 from orion

No mail.

host2$

• Copy Files to Remote Host:

The OpenSSH package also includes scp , a secure and improved replacement for

rcp . This allows you to securely copy files over a network.

orion$ scp /tmp/file.tar.gz host2:/tmp

Enter passphrase for RSA key 'dave@orion:

file.tar.gz 100% |***************************| 98304 00:00

It is also possible to encapsulate ordinarily insecure protocols such as IMAP and

POP within SSH to prevent transmitting clear text passwords to your mail server.

Additionally, the rsync incremental file transfer utility can use SSH to securely

synchronize two hosts or backup data to a log server securely. SSH can even be

used to securely connect two subnets across the Internet, effectively creating a

virtual private network. Disable remote root logins and emtpy password ability.

/var/log/authlog

# Monitor all kernel messages

kern.* /var/log/kernlog

# Monitor all warning and error messages

*.warn;*.err /var/log/syslog

# Send a copy to remote loghost. Configure syslogd init

# script to run with -r -s domain.com options on log

# server. Ensure a high level of security on the log

# server!

*.info @loghost

auth.*;authpriv.* @loghost

• Restrict access to log directory and syslog files for normal users using:

# chmod 751 /var/log /etc/logrotate.d

# chmod 640 /etc/syslog.conf /etc/logrotate.conf

# chmod 640 /var/log/*log

Configuring TCP Wrappers:

Frequently used to monitor and control access to services listed in

/etc/inetd.conf . The in.ftpd service might be wrapped using:

ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -L -i -o

Before the in.telnetd daemon is spawned, tcpd first determines if the

source is a permitted host. Connection attempts are sent to syslogd . All

services should be disabled by default in /etc/hosts.deny using the

following:

ALL: ALL

To send an email to the admin and report failed connection attempt:

ALL: ALL: /bin/mail \

-s “%s connection attempt from %c” admin@mydom.com

Enable specific services in /etc/hosts.allow using the service name

followed by the host:

sshd: magneto.mydom.com, juggernaut.mydom.com

in.ftpd: 192.168.1.

Trailing period indicates entire network should be permitted. Use tcpdchk to

verify your access files. A syslog entry will be created for failed attempts.

Access control is performed in the following order:

Install and Configure Tripwire:

Critical System Files:

Tripwire is a program that monitors file integrity by maintaining a database of

cryptographic signatures for programs and configuration files installed on the

system, and reports changes in any of these files.

A database of checksums and other characteristics for the files listed in the

configuration file is created. Each subsequent run compares any differences to

the reference database, and the administrator is notified.

The greatest level of assurance that can be provided occurs if Tripwire is run

immediately after Linux has been installed and security updates applied, and

before it is connected to a network.

A text configuration file, called a policy file, is used to define the characteristics for

each file that are tracked. Your level of paranoia determines the frequency in

which the integrity of the files are checked. Administration requires constant

attention to the system changes, and can be time-consuming if used for many

systems. Available in unsupported commercial binary for Red Hat and similar.

# Create policy file from text file

/usr/TSS/bin/twadmin -m P policy.txt

# Initialize database according to policy file

/usr/TSS/bin/tripwire —init

# Print database

/usr/TSS/bin/twprint -m d

# Generate daily report file

/usr/TSS/bin/tripwire -m c -t 1 -M

# Update database according to policy file and report file

/usr/TSS/bin/tripwire --update --polfile policy/tw.pol \

--twrfile report/<hostname>-<date>.twr

File/Directory

Description

Perms

/var/log

/var/log/messages

/etc/crontab

/etc/syslog.conf

/etc/logrotate.conf

/var/log/wtmp

/var/log/lastlog

/etc/ftpusers

/etc/passwd

/etc/shadow

/etc/pam.d

/etc/hosts.allow

/etc/hosts.deny

/etc/lilo.conf

/etc/securetty

/etc/shutdown.allow

/etc/security

/etc/rc.d/init.d

/etc/init.d

/etc/sysconfig

/etc/inetd.conf

/etc/cron.allow

/etc/cron.deny

/etc/ssh

/etc/sysctl.conf

751

644

600

640

660

640

600

644

600

750

600

400

700

750

751

600

400

750

400

Directory containing all log files

System messages

System-wide crontab file

Syslog daemon configuration file

Controls rotating of system log files

Who is logged in now. Use who to view

Who has logged in before. Use last to view

List of users that cannot FTP

List of the system’s user accounts

Contains encrypted account passwords

PAM configuration files

Access control file

Boot loader configuration file

TTY interfaces that allow root logins

Users permitted to ctrl-alt-del

System access security policy files

Program start-up files on Red Hat systems

Program start-up files on Debian systems

System and network config files on Red Hat

Internet SuperServer configuration file

List of users permitted to use cron

List of users denied access to cron

Secure Shell configuration files

Contains kernel tunable options on recent Red Hat

• Access will be granted when a daemon/client pair matches an entry in

the /etc/hosts.allow file.

• Otherwise, access will be denied when a daemon/client pair matches

an entry in the /etc/hosts.deny file.

• Otherwise, access will be granted.

A non-existing access control file is treated as if it were an empty file. Thus,

access control will be turned off if no access control files are present!

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