YoLinux.com: Linux Internet Server Security and Configuration Tutorial
Any computer connected to the internet will require steps and precautions
to be taken to reduce the exposure to hacker threats. Web, mail and DNS
servers are especially vulnerable. Large operations will hide behind a
CISCO firewall for most of their protection.
The Linux server must be configured for network security and have its
applications and services configured for security.
This tutorial covers steps and tools which can be used to
monitor and counteract hacker threats.
Simply put, it is security risk management.
This tutorial will cover basic installation and configuration for:
Prerequisites: This tutorial assumes that a computer has
Linux installed and running.
See RedHat Installation
for the basics. A connection to the internet is also assumed.
The tasks must also be performed with the root user login and password.
The computer is most vulnerable to attack through network exploits.
This tutorial covers detection and protection.
Perform the following steps to secure your web site:
See Distribution erratas and security fixes. [Red Hat Linux Errata] Update your system where appropriate.
It is best for security reasons that you reduce the number
of network services exposed. The more sevices exposed, the greater your
vulnerability. Reduce the number of network services accessible through
the xinet or inet daemon by:
inetd: (Red Hat 7.0 and earlier)
Comment out un-needed services in the /etc/initd.conf file.
Sample: (FTP is the only service I run)
ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -a
Restart the daemon to apply changes: /etc/rc.d/init.d/inetd restart
xinetd: (Red Hat 7.1 and later)
All network services are turned off by default during an upgrade. Sample file: /etc/xinetd.d/wu-ftpd:
service ftp
{
disable = yes - Default is off. This line controls xinetd service (enabled or not)
socket_type = stream
wait = no
user = root
server = /usr/sbin/in.ftpd
server_args = -l -a
log_on_success += DURATION USERID
log_on_failure += USERID
nice = 10
}
Turning on/off an xinetd service:
Edit the file: /etc/xinetd.d/service-name
Changing to the line "disable = yes" turns off an xinetd serivce.
Changing to the line "disable = no" turns on an xinetd serivce.
Xinetd configuration must be performed for each and every file in the directory
/etc/xinetd.d/ in order to configure each and every network service.
Restart the daemon to apply changes: /etc/rc.d/init.d/xinetd restart
You may also use the command:
chkconfig wu-ftpd on OR chkconfig wu-ftpd off
This will edit the appropriate file (/etc/xinetd.d/wu-ftpd)
and restart the xinetd process.
Tip:
List init settings including all xinetd controlled services: chkconfig --list
List status of services (Red Hat/Fedora Core based systems): service --status-all
Reduce the number of non-inetd network services. These will be started by
scripts in /etc/rc.d/rc*.d/ directories.
There may be no need to run sendmail (mail server),
portmap (RPC listener required by NFS),
lpd (Line printer server daemon. Hackers probe my system for this service all the time.),
innd (News server), linuxconf etc.
For example, sendmail can be removed from the boot process using the command:
chkconfig --del sendmail or by using the configuration tool ntsysv.
The service can be terminated using the command /etc/rc.d/init.d/sendmail stop.
At the very least one should run the command chkconfig --list
to see what processes are configured to be operable after boot-up.
See the YoLinux init process tutorial
Apache: Turn off modules you are not going to use. With past ssl
exploits, those using this philosophy did not get burned.
Apache 1.3.x config file /etc/httpd/conf/httpd.conf
Comment out the use of the ssl module by placing a "#" in the first column.
One can also block the https port 443 using firewall rules:
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --dport 443 -j DROP
iptables -A INPUT -p udp -s 0/0 -d 0/0 --dport 443 -j DROP
Apache: (Version 1.3+) Don't allow hackers to learn which version of the web server software you are running by inducing an error and thus an automated server response. Attacks are often version specific. Spammers also trigger errors to find email addresses.
...
ServerAdmin webmaster at megacorp dot com
ServerSignature Off
...
The response may be meaningless anyway if you are using the web server as a proxy to another.
Verify your configuration. List the open ports and processes which hold them: netstat -punta (Also try netstat -nlp)
List RPC services: [root]# rpcinfo -p localhost
Ideally you would NOT be running portmapper so not RPC services
would be available. Turn off portmapper: service portmap stop (or: /etc/init.d/portmap stop) and
remove it from the system boot sequence: chkconfig --del portmap
(Portmap is required by NFS.)
Anonymous FTP (Using wu_ftpd - Last shipped with RH 8.0. RH 9 and FC use vsftpd):
By default Red Hat comes configured for anonymous FTP.
This allows users to ftp to your server and log in with the login
anonymous and use an email address as the password. If you wish
to turn off this feature edit the file /etc/ftpaccess and change:
class all real,guest,anonymous *
to
class all real,guest *
For more on FTP configuration see: YoLinux Web server FTP configuration tutorial
Use the command chattr
and lsattr
to make a file unmodifiable over and above the usual permissions.
Make a file unmodifiable: chattr +i /bin/ls
Make directories unmodifiable: chattr -R +i /bin /sbin /boot /lib
Make a file append only: chattr +a /var/log/messages
Use "tripwire"
[sourceforge: tripwire]
for security monitoring of your system for signs of unauthorized file
changes. Tripwire is offered as part of the base Red Hat 7.1
installation. For earlier releases it is available as an RPM on the Red
Hat Power tools CD. Tripwire configuration is covered below.
Watch your log files especially /var/log/messages and
/var/log/secure.
Avoid generic account names such as guest.
Use PAM network wrapper configurations to disallow passwords which can be
found easily by crack or other hacking programs.
PAM authentication can also disallow root network login access.
(Default Red Hat configuration.
You must login as a regular user and su - to obtain root access.
This is NOT the default for ssh and must be changed as noted below.)
See YoLinux Network Admin Tutorial on using PAM
Remote access should NOT be done with clear text telnet but with an encrypted connection using ssh. (Later in this tutorial)
Use Linux firewall rules to protect against attacks. (ipchains or iptables)
Access denial rules can also be imlemented on the fly by portsentry.
(Place at the end of /etc/rc.d/rc.local to be executed upon system boot, or some other appropriate script)
iptables script:
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --dport 2049 -j DROP - Block NFS
iptables -A INPUT -p udp -s 0/0 -d 0/0 --dport 2049 -j DROP - Block NFS
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --dport 6000:6009 -j DROP - Block X-Windows
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --dport 7100 -j DROP - Block X-Windows font server
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --dport 515 -j DROP - Block printer port
iptables -A INPUT -p udp -s 0/0 -d 0/0 --dport 515 -j DROP - Block printer port
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --dport 111 -j DROP - Block Sun rpc/NFS
iptables -A INPUT -p udp -s 0/0 -d 0/0 --dport 111 -j DROP - Block Sun rpc/NFS
iptables -A INPUT -p all -s localhost -i eth0 -j DROP - Deny outside packets from internet which claim to be from your loopback interface.
ipchains script:
# Allow loopback access. This rule must come before the rules denying port access!!
iptables -A INPUT -i lo -p all -j ACCEPT - This rule is essential if you want your own computer
iptables -A OUTPUT -o lo -p all -j ACCEPT to be able to access itself throught the loopback interface
ipchains -A input -p tcp -s 0/0 -d 0/0 2049 -y -j REJECT - Block NFS
ipchains -A input -p udp -s 0/0 -d 0/0 2049 -j REJECT - Block NFS
ipchains -A input -p tcp -s 0/0 -d 0/0 6000:6009 -y -j REJECT - Block X-Windows
ipchains -A input -p tcp -s 0/0 -d 0/0 7100 -y -j REJECT - Block X-Windows font server
ipchains -A input -p tcp -s 0/0 -d 0/0 515 -y -j REJECT - Block printer port
ipchains -A input -p udp -s 0/0 -d 0/0 515 -j REJECT - Block printer port
ipchains -A input -p tcp -s 0/0 -d 0/0 111 -y -j REJECT - Block Sun rpc/NFS
ipchains -A input -p udp -s 0/0 -d 0/0 111 -j REJECT - Block Sun rpc/NFS
ipchains -A input -j REJECT -p all -s localhost -i eth0 -l - Deny and log ("-l") outside packets from internet which claim to be from your loopback interface.
Note:
iptables uses the chain rule "INPUT" and ipchains uses the lower case
descriptor "input".
View rules with iptables -L or ipchains -L command.
When running an internet web server it is best from a security point of
view, that one NOT run printing, X-Window, NFS or any services which may
be exploited if a vulnerability is discovered or if misconfigured
regardless of firewall rules.
Red Hat 7.1 firewall GUI configuration tool /usr/sbin/gnome-lokkit
Use portsentry to monitor network hacker attacks. (Later in this tutorial)
A minimal and monolithic kernel might also provide a small bit of
protection (avoid trojan modules) as well as running on less common hardware
(MIPS, Alpha, etc... so buffer overflow instructions will not run.)
DDoS (Distributed Denial of Service) attacks: The only
thing you can do is have gobs of bandwidth and processing
power/firewall. Lots of processing power or a firewall are useless
without gobs of bandwidth
as the network can get sooo overloaded from a distributed attack.
Also see:
Unfortunately the packets are usually spoofed and in my case the FBI
didn't care. If the server is a remote server, have a dial-up modem or
a second IP address and route for access because the attacked route is
blocked by the flood of
network attacks. You can also request that your ISP drop ICMP traffic
to
the IP addresses of your servers. (and UDP if all you are running is a
web server. Name servers use UDP.)
For very interesting reading see "The Strange Tale" of the GRC.com DDoS attack. (Very interesing read about the anatomy of the hacker bot networks.)
User access can be restricted with the configuration files:
Remove un-needed users from the system. See /etc/passwd.
By default
Red Hat installations have many user accounts created to support
various processes. It you do not intend to run these processes, remove
the users. i.e. remove
user ids games, uucp, rpc, rpcd, ...
SSH: (Secure Shell)
SSH protocol suite of network connectivity tools are used to encrypt
connections accross the internet. SSH encrypts all traffic including
logins and passwords to effectively eliminate network sniffing,
connection hijacking, and other network-level attacks.
In a regular telnet session the password is transmitted across the Internet
unencrypted.
SSH is a commercial product but available freely for non-commercial use
from SSH Communications Security at
http://www.ssh.com/.
Two versions are available, SSH1 and SSH2. The newer SSH2 supports FTP and has
more options than SSH1. SSH2 can be purchased and/or downloaded from their
web site.
Note that SSH1 does have a major vulnerability issues.
The "woot-project" web site cracking and defacing gang uses this vulnerability.
DO NOT USE SSH1 PROTOCOL!!!!!
("woot-project" exploit/attack description/recovery)
OpenSSH was developed by the the
OpenBSD Project and is freely available.
OpenSSH is compatable with SSH1 and SSH2.
OpenSSH relies on the OpenSSL project for the encrypted communications layer.
Current releases of Linux come with OpenSSH/OpenSSL. (Comes with Red Hat Linux 7.x+)
Links:
OpenSSH.org - Shell. Supports SSH1 and SSH2 protocols.
If upgrading from SSH1 you may have to use the RPM option --force.
The rpm will install the appropriate binaries, configuration files and
openssh-server will install the init script /etc/rc.d/init.d/sshd
so that sshd will start upon system boot.
# $OpenBSD: ssh_config,v 1.9 2001/03/10 12:53:51 deraadt Exp $
# This is ssh client systemwide configuration file. See ssh(1) for more
# information. This file provides defaults for users, and the values can
# be changed in per-user configuration files or on the command line.
# Configuration data is parsed as follows:
# 1. command line options
# 2. user-specific file
# 3. system-wide file
# Any configuration value is only changed the first time it is set.
# Thus, host-specific definitions should be at the beginning of the
# configuration file, and defaults at the end.
# Site-wide defaults for various options
# Host *
# ForwardAgent no
# ForwardX11 no
# RhostsAuthentication no
# RhostsRSAAuthentication yes
# RSAAuthentication yes
# PasswordAuthentication yes
# FallBackToRsh no
# UseRsh no
# BatchMode no
# CheckHostIP yes
# StrictHostKeyChecking yes
# IdentityFile ~/.ssh/identity
# IdentityFile ~/.ssh/id_rsa
# IdentityFile ~/.ssh/id_dsa
# Port 22
# Protocol 2,1 - Change this line to: Protocol 2
# Cipher 3des
# Ciphers aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc
# EscapeChar ~
Host *
ForwardX11 yes
Change the line: # Protocol 2,1
to: Protocol 2
This will eliminate use of SSH1 protocol.
Uncomment the options required or accept the hard-coded defaults.
The hard coded defaults for OpenSSH client are compatable with SSH1
client files and sshd server. An upgrade to OpenSSH client will
not require any changes to the files in $HOME/.ssh/.
Server configuration file /etc/ssh/sshd_config:
Default:
# $OpenBSD: sshd_config,v 1.38 2001/04/15 21:41:29 deraadt Exp $
# This sshd was compiled with PATH=/usr/bin:/bin:/usr/sbin:/sbin
# This is the sshd server system-wide configuration file. See sshd(8)
# for more information.
Port 22
#Protocol 2,1 - Change to: Protocol 2
#ListenAddress 0.0.0.0
#ListenAddress ::
HostKey /etc/ssh/ssh_host_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
ServerKeyBits 768
LoginGraceTime 600 - Change to: LoginGraceTime 120
KeyRegenerationInterval 3600
PermitRootLogin yes - Change to: PermitRootLogin no
#
# Don't read ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes
StrictModes yes
X11Forwarding yes
X11DisplayOffset 10
PrintMotd yes
#PrintLastLog no
KeepAlive yes
# Logging
SyslogFacility AUTHPRIV
LogLevel INFO
#obsoletes QuietMode and FascistLogging
RhostsAuthentication no
#
# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
#
RSAAuthentication yes
# To disable tunneled clear text passwords, change to no here!
PasswordAuthentication yes
PermitEmptyPasswords no
# Uncomment to disable s/key passwords
#ChallengeResponseAuthentication no
# Uncomment to enable PAM keyboard-interactive authentication
# Warning: enabling this may bypass the setting of 'PasswordAuthentication'
#PAMAuthenticationViaKbdInt yes
# To change Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#AFSTokenPassing no
#KerberosTicketCleanup no
# Kerberos TGT Passing does only work with the AFS kaserver
#KerberosTgtPassing yes
#CheckMail yes
#UseLogin no
#MaxStartups 10:30:60
#Banner /etc/issue.net
#ReverseMappingCheck yes
Subsystem sftp /usr/libexec/openssh/sftp-server
Note:
Ssh protocol version 1 is not as secure, it should not take 10 minutes
to type your password and if someone logs in as root without logging in as a
particular user first then tracability is lost if there are multiple admins,
thus the changes were made as suggested above.
Setting "PermitRootLogin no" mandates that remote
logins use an undetermined user login. This removes root, a known login
on all Linux systems, from the list of dictionary atttacks available.
It is a good idea to change the "Banner" so that a login greeting and
legal disclaimer is presented
to the user. i.e. change file /etc/issue.net contents to:
Access is granted to this server only to authorized personel of Mega Corp.
By default, the /etc/issue.net message presents to the hacker
the OS name, kernel release and information which can be used to
determine potential vulnerabilities.
Public keys generated: chmod 644 /etc/ssh/ssh_host_dsa_key.pub /etc/ssh/ssh_host_rsa_key.pub
For SELinux:
/sbin/restorecon /etc/ssh/ssh_host_rsa_key.pub
/sbin/restorecon /etc/ssh/ssh_host_dsa_key.pub
Generate user keys:
Client:
Use the command: /usr/bin/ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user-id/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user-id/.ssh/id_rsa.
Your public key has been saved in /home/user-id/.ssh/id_rsa.pub.
The key fingerprint is:
XXXblablablaXXXaf:90:8f:dc:65:0d:XXXXXXXXXXXXXX user-id@node-name
Using ssh: On client use the following command
and login as you normally would with a telnet session:
ssh name-of server
The first time you use ssh it will issue the following message:
The authenticity of host 'node.your-domain.com (XXX.XXX.XXX.XXX)' can't be established.
RSA key fingerprint is XXXXblablablaXXX1:81:29:00:3a:c5:fb:XXXXXXXXXXX.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'node.your-domain.com,XXX.XXX.XXX.XXX' (RSA) to the list of known hosts.
user@node.your-domain.com's password:
Answer yes. It won't ask again.
To use a different user name for the login, state it on the command line:
ssh -l username name-of server
Note: You can now also use the command sftp for secure ftp file transfers
using ssh.
The sshd should not be started using xinetd/inetd due to time necessary to perform
calculations when it is initailized.
ssh client will suid to root. sshd on the server is run as root.
Root privileges are required to communicate on ports lower than 1024.
The -p option may be used to run SSH on a different port.
RSA is used for key exchange, and a conventional cipher (default Blowfish)
is used for encrypting the session.
Encryption is started before authentication, and no passwords or other
information is transmitted in the clear.
Authentication:
Login is invoked by the user.
The client tells the server the public key that the user wishes to
use for authentication.
Server then checks if this public key is admissible.
If yes then random number is generated and encrypts it with the public
key and sends the value to the client.
The client then decrypts the number with its private key and computes a
checksum. The checksum is sent back to the server
The server computes a checksum from the data and compares the checksums.
Authentication is accepted if the checksums match.
SSH will use $HOME/.rhosts (or $HOME/.shosts)
To establish a secure network connection on another TCP port, use
"tunneling" options with the ssh command:
Forward TCP local port to hostport on the remote-host:
ssh remote-host -L port:localhost:hostport command
Specifying ports lower than 1024 will require root access.
FTP opens various ports and thus is not a good candidate. Port 21 is only
used to establish the connection.
The network sniffer Ethereal (now Wireshark) was used to sniff network transmissions
between the client and server for both telnet and ssh
with the following results:
Test telnet clear text login: (port 23)
The text sent by the client is green text on a black background.
The rest of the text was transmitted by the server.
Note that both the login ("JoeUser") and password ("super-secret-password") were captured.
Test ssh encrypted login: (port 22)
Note that the entire login and password exchange was encrypted.
PortSentry:
This tool will monitor the network probes and attacks against your server.
It can be configured to log and counter these probes and attacks.
PortSentry can modify your /etc/hosts.deny (PAM module) file and
issue IP firewall commands automatically to block hackers.
PortSentry can be loaded as an RPM but this tutorial covers compiling
PortSentry from source to configure a more preferable system logging.
Note: Version 1.1 of portsentry can issue iptables, ipchains or route commands
to thwart attacks. Linux Kernel 2.2 (Red Hat 6.x and 7.0) uses ipchains.
Linux kernel 2.4 (Red Hat 7.1) uses iptables but can also use ipchains but
NOT both. Route commands can be used by any Unix system.
Steps to install and configure portsentry:
Download and unzip source code
Edit include file and compile
Start PortSentry
Read logs
Download and unzip source code:
Download: PortSentry source code
(Note: Portsentry version 1.1 includes a bug fix required for Red Hat 7.1 kernel 2.4)
Move to your source directory and unzip: tar -xzf portsentry-1.1.tar.gz
Edit include file and compile:
cd portsentry-1.1/
Read file README.install. It details the following:
(Note: I use /opt/portsentry/ because I like to locate custom
files/software there.
It allows for an easy backup by separating it from the OS.
If you prefer, you can use /etc/portsentry/ for
configurations files and follow the Linux/Unix file system logic)
The above default, "LOG_DAEMON", will log messages to the /var/log/messages file.
To log to a separate file dedicated to PortSentry logging:
(This will eliminate logging clutter in the main system logging file)
Add logging directives to syslogd configuration file: /etc/syslog.conf
Change the following line to reflect that portsentry messages are
not going to be logged to the regular syslog output
file /var/log/messages
Uncomment and modify if necessary the appropriate statements. The
TCP_PORTS=, UDP_PORTS= lists are ignored for stealth scan detection
modes. I added UDP port 68 (BOOTP) and TCP 21 (ftp), 22 (ssh), 25 (smtp
mail), 53 (dns bind), 80 (http web server), 119 (news) to the
ADVANCED_EXCLUDE_UDP and ADVANCED_EXCLUDE_TCP statements respectively.
ADVANCED_EXCLUDE_TCP="21,22,25,53,80,110,113,119" - server
ADVANCED_EXCLUDE_UDP="21,22,53,110,520,138,137,68,67"
OR
ADVANCED_EXCLUDE_TCP="113,139" - workstation
ADVANCED_EXCLUDE_UDP="520,138,137,68,67"
Route deny options: (Options: network "route" or firewall command "iptables/ipchains")
Simple method to drop network return routes if ipchains are not compiled into your kernel:
KILL_ROUTE="/sbin/route add -host $TARGET$ reject"
You can check the addresses dropped with the command: netstat -rn They will be routed to interface "-".
For Linux 2.2.x kernels (version 2.102+) using ipchains: (Best option)
KILL_ROUTE="/sbin/ipchains -I input -s $TARGET$ -j DENY -l" OR KILL_ROUTE="/sbin/ipchains -I input -s $TARGET$ -j DENY" Note: The second option is without the "-l" or logging option so ipchains won't keep logging the portscan in /var/log/messages
For those using iptables (RH 7.1+ Linux Kernel 2.4+):
KILL_ROUTE="/sbin/iptables -I INPUT -s $TARGET$ -j DROP"
(Note: The default used in portsentry.conf uses the
incorrect path for Red Hat.
Change /usr/local/bin/iptables
to /sbin/iptables)
Note on Red Hat 7.1: During installation/upgrade the firewall configuration
tool /usr/bin/gnome-lokkit may be invoked. It will configure
a firewall using ipchains and will add this to your boot process.
To see if ipchains and the Lokkit configuration is invoked during system boot,
use the command: chkconfig --list | grep ipchains.
You can NOT use portsentry to issue iptables rules if ipchain rules have
been issued previously.
More info on iptables and ipchains support/configuration in Red Hat 7.1 and kernel 2.4.
Edit file: portsentry.ignore (contains IP addresses to ignore. )
127.0.0.1 0.0.0.0 Your IP address
The at Home network routinely scans for news servers on port 119 from
a server named authorized-scan1.security.home.net.
Adding the IP address of this server (24.0.0.203)
greatly reduces the logging. I also added their BOOTP server.
(24.9.139.130)
I manually issued the iptables (RH 7.1 kernel 2.4)
commands on my workstation
to drop the hosts and deny their scans.
At Home users may add the commands to the file
/etc/rc.d/rc.local
/sbin/iptables -I INPUT -s 24.0.0.203 -j DROP /sbin/iptables -I INPUT -s 24.9.139.130 -j DROP
Edit file: Makefile
INSTALLDIR = /opt
And remove the line under "uninstall": (dangerous line!!)
# /bin/rmdir $(INSTALLDIR)
And remove the line under "install": (troublesome line!!)
# chmod 700 $(INSTALLDIR)
Compile: make linux
Install (as root): make install
Run PortSentry for advanced UDP/TCP stealth scan detection:
portsentry -atcp
portsentry -audp
OR use init scripts below in next section.
Check logfile for hacker attacks. See: /var/log/messages
or /var/log/portsentry.log if you are logging to a dedicated file.
Also check /etc/hosts.deny to see a list of IP addresses that
PortSentry has deamed attackers.
Check the "HISTORY_FILE" /opt/portsentry/portsentry.history
Note: Is is possible to have all logging sent to a logging daemon on a single
server. This will allow the administrator to check the logs on only one server
rather than individually on many.
Note on Red Hat 7.1: Red Hat Powertools 7.1 now includes portsentry 1.0.
I reccomend using version 1.1 configured as above.
Powertools RPM layout:
/usr/sbin/portsentry - (chmod 700) executable
/etc/portsentry/ - (chmod 700) Directory used for configuration files.
/etc/portsentry/portsentry.conf (chmod 600)
/etc/portsentry/portsentry.ignore (chmod 600)
/var/portsentry/portsentry.history
/var/portsentry/portsentry.blocked
Instead of using a firewall command (ipchains/iptables),
a false route is used: /sbin/route add -host $TARGET$ gw 127.0.0.1.
My init script calls the portsentry executable twice with the apropriate
command line arguments to monitor tcp and udp ports. The Red Hat 7.1
init script uses the file /etc/portsentry/portsentry.modes
and a for loop in the
init script to call portsentry the appropriate number of times.
Their init script also recreates the portsentry.ignore file each time
portsentry is started by including the IP addresses found with ifconfig
and the addresses 0.0.0.0 and localhost.
Persistent addresses must be placed above a line stating:
Do NOT edit below this otherwise it is not included in the creation
of the new file.
The Red Hat 7.1 Powertools portsentry version logs everything to
/var/log/messages. My configuration avoids log clutter by logging
to a separate file.
Notes on DOS (Denial of Service) possibility:
If portsentry is configured to shut down an
attack with firewall rules, an attacker may use this feature to slow down
your machine over time by creating a huge set of firewall rules.
It would require the hacker to use (or spoof) a new IP address each time.
It is probably a good idea to monitor or even clear the firewall rules from
time to time.
iptables:
List firewall rules: iptables -L
Clear firewall rules: iptables -F
ipchains:
List firewall rules: ipchains -L
Clear firewall rules: ipchains -F
Clean-up script: /etc/cron.monthly/reset-chainrules
(-rwx------ 1 root root)
This script is run automatically once a week by cron. (The presence of this
script in this directory for the Red Hat configuration makes it so)
snort - Instead of monitoring
a single server with portsentry, snort monitors the network, performing
real-time traffic analysis and packet logging on IP networks for the
detection of an attack or probe.
Also see: YoLinux IDS and Snort links
Using an init script to start and stop the portsentry program.
Init configuration: /etc/rc.d/init.d/portsentry
The init script needs to be executable: chmod a+x /etc/rc.d/init.d/portsentry
After adding the following script, enter it into the init process with
the command: chkconfig --add portsentry or
chkconfig --level 345 portsentry on
See YoLinux Init Tutorial for more information.
Portscan your workstation - Use your web browser to go to this site. Select "Probe my ports" and it will
scan you. You can then look at the file /opt/portsentry/portsentry.blocked.atcp
to see that portsentry dropped the scanning site:
The file /var/log/portsentry.log will show the action taken:
portsentry[589]: attackalert: SYN/Normal scan from host: shieldsup.grc.com/207.71.92.221 to TCP port: 23
portsentry[589]: attackalert: Host 207.71.92.221 has been blocked via wrappers with string: "ALL: 207.71.92.221"
portsentry[589]: attackalert: Host 207.71.92.221 has been blocked via dropped route using command:
"/sbin/ipchains -I input -s 207.71.92.221 -j DENY -l"
nmap: portscanner - This is
the hacker tool responsible for many of the portscans you may be recieving.
Command arguments:
Argument
Description
-sO
IP scan. Find open ports.
-sT
TCP scan. Full connection made.
-sS
SYN scan (half open scan). This scan is typically not logged on receiving system.
-sP
Ping ICMP scan.
-sU
UDP scan.
-P0
Don't ping before scan.
-PT
Use ping to determine which hosts are available.
-F
Fast scan. Scan for ports listed in configuration.
-T
Set timing of scan to use values to avoid detection.
-O
Determins operating system.
-p 1000-1999,5000-5999
Scan port ranges specified.
Also see: nmap man page for a full listing of nmap command line arguments.
Add the option -v (verbose) or -vv (super verbose) for more info.
The ports will be determined to be open, filtered or firewalled.
Sample output from command: nmap -sS -F -O IP-Address
Starting nmap V. 2.54BETA7 ( www.insecure.org/nmap/ ) ... .. (The 1067 ports scanned but not shown below are in state: closed) Port State Service 21/tcp open ftp 22/tcp open ssh 25/tcp open smtp 53/tcp open domain 111/tcp open sunrpc - Shut down the portmap (RPC) daemon: /etc/rc.d/init.d/portmap stop
137/tcp filtered netbios-ns - Turn off netbios services: /etc/rc.d/init.d/smb stop
138/tcp filtered netbios-dgm
139/tcp filtered netbios-ssn
TCP Sequence Prediction: Class=random positive increments
Difficulty=2727445 (Good luck!)
Remote operating system guess: Linux 2.1.122 - 2.2.16
Nmap run completed -- 1 IP address (1 host up) scanned in 36 seconds
nmap/nmapfe: nmapfe = nmap front end
- GUI front end to nmap.
It's an amazingly easy and usefull tool which will help
you make discoveries about your servers before the hackers do.
Nmap and nmapfe are available with distribution or on the Red Hat Powertools CD for older (7.1) releases:
ndiff - Compares two nmap scans and outputs the differences. Monitor network for changes.
Tripwire: (security monitoring)
Tripwire monitors your file system for changes. Tripwire is used to create an
initial database of information on all the system files then runs periodically
(cron) to compare the system to the database.
I will cover Tripwire version 1.2-3 (Red Hat Powertools 6.2) and version
2.3.0-58 (Red Hat 7.1). Use the command tripwire --version or
rpm -q tripwire to determine the version.
Red Hat 7.1 includes Tripwire as an optional package during install.
Tripwire for earlier releases is available from the RedHat Powertools CD in
RPM format.
Upon installation it will proceed to scan your entire filesystem to create
a default database of what your system looks like. (files and sizes etc)
It took about ten minutes to run!
Tripwire configuration files:
Tripwire 2.3.0-58:
/etc/tripwire/twcfg.txt
/etc/tripwire/twpol.txt
These files are first edited and then processed by the script
/etc/tripwire/twinstall.sh which configures Tripwire after the
installation of the Tripwire RPM package.
Edit and change file: /etc/tripwire/twcfg.txt
Change:
LOOSEDIRECTORYCHECKING =false
to
LOOSEDIRECTORYCHECKING=TRUE
This was recommended in the comments of the file twpol.txt
Edit and change file: /etc/tripwire/twpol.txt
Change:
severity = $(SIG_XXX) to severity = $(SIG_XXX),
emailto = root@localhost or severity = $(SIG_XXX),
emailto = root@localhost;admin@isp.com
where XXX is the severity level.
This will cause Tripwire to email a report of discrepancies for the
rule edited. Set the email address to one appropriate for you.
I also added:
"User binaries" rule: directory /opt/bin
"Libraries" rule: directory /opt/lib
I removed/commented out:
the rule "System boot changes" as it reports changes due to system boot.
Rule: "Root config files": Many of the non-existant
files listed under /root were commented out to reduce the number of
errors reported.
Rule "File System and Disk Administraton Programs": Many
of the non-existant binaries listed under /sbin were commented out to
reduce the number of errors reported.
After configuration files have been edited run the script: /etc/tripwire/twinstall.sh
The script will ask for a "passphrase" for the site and local system.
This is a similar concept to a password - remember it!
If at any point you want to
make configuration/policy changes, edit these files and re-run the
configuration script. The script will generate the true configuration
files used by Tripwire:
/etc/tripwire/tw.cfg
(View with command: twadmin --print-cfgfile)
/etc/tripwire/tw.pol
(View with command: twadmin --print-polfile)
/etc/tripwire/site.key
/etc/tripwire/ServerName-a-local.key
These files are binary and not human readable.
Tripwire 1.2-3: /etc/tw.config
Tripwire initialization:
If at any time you change the configuration file to monitor your system
differently or install an upgrade (changes a whole lot of files which
will "trip" tripwire into reporting all changes) you may want to generate
a new database.
Tripwire 2.3.0-58: /usr/sbin/tripwire --init
You will be prompted for your "local passphrase".
This will generate a tripwire database file: /var/lib/tripwire/ServerName-a.twd
Tripwire 1.2-3: /usr/sbin/tripwire -initialize
This will generate a tripwire database file: ./databases/tw.db_ServerName
If you are in root's home directory, this will create the file /root/databases/tw.db_ServerName
At this point copy it to a useable location:
Don't change /etc/tw.config
without first running tripwire -initialize
otherwise it will show differences due to settings in tw.config file rather
than true differences.
#!/bin/sh HOST_NAME=`uname -n` if [ ! -e /var/lib/tripwire/${HOST_NAME}.twd ] ; then echo "**** Error: Tripwire database for ${HOST_NAME} not found. ****" echo "**** Run "/etc/tripwire/twinstall.sh" and/or "tripwire --init". ****" else test -f /etc/tripwire/tw.cfg && /usr/sbin/tripwire --check fi
You may move this cron script to the directory /etc/cron.weekly/
to reduce reporting from a daily to a weekly event.
Tripwire reports will be written to: /var/lib/tripwire/report/HostName-Date.twr
Tripwire 1.2-3:
File: /etc/cron.daily/tripwire.verify
script which runs the command: /usr/sbin/tripwire -loosedir -q
Note: You may want to move the script to /etc/cron.weekly/tripwire.verify
to reduce email reporting to root.
Update tripwire database - run:
tripwire -interactive
This will allow you to respond Y/N to files if they should be permanently
updated in the tripwire database. This will still run tripwire against the
whole file system.
I ran it from /root and it updated
/root/databases/tw.db_ServerName
You must then cp -p to /var/spool/tripwire/
to update the tripwire database.
CHKROOTKIT: Performing a trojan/worm/virus file scan.
Tripwire will monitor your filesystems for intrusion or addition of a file
so you may determine what changes have occured on your system in sensitive
areas. Chkrootkit will scan your system for known exploits,
trojan commands, and worms used to compromise a system.
Download chkrootkit from http://www.chkrootkit.org. It is a shell script which should be run as root as well as a
small collection of C programs.
Installation:
make sense (Compile C programs)
./chkrootkit (Run shell script and call programs.)
This software is constantly being upgraded and updated to include
scans for new exploits.
If running portsentry, chkrootkit may return a false error while performing the bindshell test.
NESSUS: Performing a network vulnerability scan/security assessment of your system.
Let me start by saying that this should only be performed on your own systems.
It is considered and attack to run this against the systems of others
and legal action may be taken against you for performing such an audit.
This is not a scan like NMAP. NESSUS will search and locate vulnerabilities
on your system by actively trying to perform known exploits against the
system.
Nessus is amazingly complete and effective. In fact it is awesome!!
It will identify services on your system and try to exploit them.
If a vulnerability is found it will make recomendations about upgrades,
configuration changes and where to find patches. It will also explain
any causes for concern in detail and explain why your system is vulnerable.
And that's not all! It can output reports in various formats including HTML
with pie charts and bar charts!! The HTML reports will have hyperlinks to the
security reports, upgrades and patches. (I'm impressed)
It can scan Unix, Linux and Windows systems for vulnerabilities.
Note:
Running "Dangerous Plugins" may cause a crash of the system being audited!!
The NESSUS software is available from http://Nessus.org.
If compiling source:
Edit file: nessus-core/include/config.h (Set USE_AF_UNIX to define socket type)
nessus-server-....rpm : Nessus plugins which are used to perform
the various checks. (Scripts in nasl scripting language) Note that the
RPM installs an init script which starts nessusd during boot. Disable
with chkconfig --del nessusd
nessus-devel-....rpm : Nessus developement librairies and headers.
Running NESSUS:
Add a NESSUS user:
/usr/sbin/nessus-adduser
Login : admindude
Authentication method (cipher/plaintext) [cipher] :
Is "admindude" a local user on this machine [ |n]? y
New pass phrase:
...
Start server daemon: /usr/sbin/nessusd
or /etc/rc.d/init.d/nessusd start
Start client program: /usr/bin/nessus
First enter your "Login" id and select the "Log in" button.
In this example I am running the nessusd server on node "localhost".
Enter the appropriate nessus server node name or IP address if
different.
You will then be placed in the "Plugins" panel. Note that
"dangerous" plug-ins may crash a server.
Select the "Target selection" tab and enter the name or IP address
of the server to audit.
Select "Start the scan" and wait. (It takes about 15 minutes to
audit one computer.)
The results may be reviewed by selecting the node from the collumn in
the left window.
LIDS.org - Kernel patch to enhance Linux security.
Openwall.com - Owl
(security enhanced Linux) and security patches. This kernel patch makes
the stack of a process non-executable so instructions loaded during a
buffer overflow attack will not run.
"Linux Firewalls"
by Robert L. Ziegler, Carl Constaintine
ISBN #0735710996, New Riders 10/2001
This is the newer version. It includes updates on the Linux 2.4 kernel,
VPN's and SSH.
"Linux Firewalls"
Robert L. Ziegler
ISBN #0-7357-0900-9, New Riders 11/1999
Most complete Linux firewall/security book in publication.
Covers ipchains, bind and a complete review of possible firewall configurations.
"Hack Proofing Linux : A Guide to Open Source Security"
by James Stanger, Patrick T. Lane
ISBN #1928994342, Syngress
"Real World Linux Security: Intrusion Prevention, Detection and Recovery"
by Bob Toxen
ISBN #0130281875, Prentice Hall
"Hacking Linux Exposed"
by Brian Hatch, James B. Lee, George Kurtz
ISBN #0072225645, McGraw-Hill (2nd edition)
From the same authors of "Hacking Exposed".
"Maximum Linux Security: A Hacker's Guide to Protecting Your Linux Server and Workstation"
by Anonymous and John Ray
ISBN #0672321343, Sams
Covers not only audit and protection methods but also investigates and
explains the attacks and how they work.
"Network Intrusion Detection: An Analyst's Handbook"
by Stephen Northcutt, Donald McLachlan, Judy Novak
ISBN #0735710082, New Riders Publishing
"SSH, the Secure Shell : The Definitive Guide"
by Daniel J. Barrett, Richard Silverman
ISBN #0596000111, O'Reilly & Associates
"Nessus Network Auditing (Jay Beale's Open Source Security)"
by Renaud Deraison, Noam Rathaus, HD Moore, Raven Alder, George Theall, Andy Johnston, Jimmy Alderson
ISBN #1931836086, Syngress
"Computer Security Incident Handling Step by Step"
by Stephen Northcutt
ISBN #0967299217
"Security Assessment: Case Studies for Implementing the NSA IAM"
by Russ Rogers, Greg Miles, Ed Fuller, Ted Dykstra
ISBN #1932266968, Syngress
"Network Security Assessment"
by Chris McNab
ISBN #059600611X, O'Reilly
"A Practical Guide to Security Assessment"
by Sudhanshu Kairab
ISBN #0849317061, Auerbach Publications
"Aggressive Network Self-defense"
by NEIL R. WYLER
ISBN #1931836205, Syngress Publishing
Security Source Magazine
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continue to learn about valuable security topics and strategies in each
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