is a powerful online hash calculator is a powerful online hash calculator that enables you to quickly and easily generate hashes for your sensitive information, such as passwords and confidential data. The platform supports a wide range of hash algorithms, including MD5, SHA-1, SHA-256, SHA-384, and SHA-512.
By using, you can ensure the security of your data by converting it into a unique string of characters, known as a hash. Hashes are one-way functions that make it impossible to retrieve the original information from the hash, providing a secure method of storing and transmitting data.
One of the best things about is that it is very easy to use. Even if you do not know anything about cryptography or hash functions, it is easy to use. Simply enter the information you want to hash into the input field, select the desired algorithm, and hit the “Calculate” button. The hash will be displayed in the output field, ready to be used or stored as needed. Features

  1. Advanced encryption algorithms: utilizes the latest encryption algorithms, including MD5 and SHA-1, to provide the highest level of security for your data.
  2. User-friendly platform: Our platform is designed with the user in mind, making it easy to use and accessible from anywhere in the world. With just a few clicks, you can encrypt your sensitive information and protect it from unauthorized access.
  3. Free hash calculation tools: CMD5 provides free hash calculation tools to help you verify the authenticity of your data.
  4. Decryption services: In addition to encryption services, CMD5 also provides free decryption services to help you access your encrypted information.
  5. Dedicated support team: Our dedicated support team is always available to assist you with any questions or concerns you may have.
  6. Global access: CMD5 is accessible from anywhere in the world, making it the perfect solution for individuals and businesses alike.
  7. Privacy protection: At CMD5, we understand the importance of privacy and security. That’s why we strive to provide the most secure and user-friendly experience possible.
  8. Reliable security: Our advanced encryption algorithms ensure that your information is safe and secure, even in the event of a data breach.
  9. Easy to use: CMD5 is designed to be easy to use, so you can encrypt your sensitive information with just a few clicks.
  10. Affordable: Our platform is affordable and accessible, making it the perfect solution for individuals and businesses of all sizes.
  11. Multiple encryption options: CMD5 offers multiple encryption options, including MD5, SHA-1, and other algorithms, to provide you with the best possible protection for your data.
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  13. Secure storage: With CMD5, you can store your encrypted data securely on our servers, or you can download it to your own device for added security.
  14. Regular updates: CMD5 is constantly being updated with the latest security features and enhancements to ensure that your data remains protected.
  15. Scalable solutions: Our platform can be easily scaled to meet the needs of businesses of any size, from small startups to large enterprises.
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In addition to its hash calculation capabilities, also features an online hash database where you can search for hashes of known plaintexts. This can be useful in a variety of scenarios, such as password cracking and cyber investigations.
Whether you are a software developer, a security professional, or simply someone who wants to keep their sensitive information safe, is an important tool for data security. Try it out today and experience the power of online hash calculation for yourself!

Basic Security Steps : Linux (3)

Restrict remote login to users that really need it


Less users mean less targets, obviously. Specially, root never needs SSH access. Note that disabling login by setting the login shell to /bin/false prevents only a subset of the attacks handled by this solution.


Use the AllowUsers and/or AllowGroups directives. Users not covered by either directive are not allowed to log in via SSH.


AllowUsers user1 user2 user3 AllowGroups group1 group2 PermitRootLogin no

Disable password authentication


Password based authentication is inherently less secure than the public-key alternative. You can find an easy to follow comparison at Rongchaua’s blog.


  1. If you don’t have a GPG SSH[3] key yet, you’ll have to create one. See this GPG Quick Start guide On Unix-based systems ssh-keygen will create your SSH keypair after you answer a few simple questions[1], or see this guide for PuTTY on Windows.
  2. Once you have the private and public keys, paste the public key into the file $HOME/.ssh/authorized_keys on the server. OpenSSH is quite picky about the permissions of files, so make sure that only you can access it (“chmod 0600 $HOME/.ssh/authorized_keys“ should be good enough).
  3. Make sure you can log in without entering your login password. If you disable password authentication without setting this up correctly, you can lock yourself out of the system.
  4. Disable password login and other unsecure login methods:

    ChallengeResponseAuthentication no PasswordAuthentication no UsePAM no

Force the usage of SSH-2


The older protocol, SSH-1 is less secure, and all modern clients support the new protocol. Using only SSH-2 in sshd is also the default in modern servers[1].


Protocol 2

Use a non-standard port


Running the SSH daemon on a random port requires little effort, for a little security through obscurity decreasing the number of attackers a bit[1].

Why not:

A decent port scan will usually still find the service. It might not be worth the effort; decide for yourself.


The port number should probably be below 1024, because binding to these ports require root privileges, thus reducing the risk of port hijacking. Make sure you choose a port not already associated with an application (eg. by checking that it’s not in the nmap-services file).[1]

port $PORT

You can then log in via the command-line client using the -p switch:

$ ssh -p $PORT user@host

Take this a few steps further, and we get port knocking. It’s a technique not universally accepted as a useful solution in production environments. Setting up port knocking is out of the scope of this article, but there are plenty of resources out there.

Monitoring tools

The steps outlined above make it harder for an attacker to gain access to your system. It should be noted that this is far from being a complete guide on setting up a secure server; there are whole books dedicated to the topic for a reason.

Anyway, when a bad guy manages to access your system, you want to know about it, fast. Finding out about the problem from a request to stop brute-forcing another server does not qualify as fast.

fail2ban ”scans log files like /var/log/pwdfail or /var/log/apache/error_log and bans IP that makes too many password failures. It updates firewall rules to reject the IP address.” This will stop brute-force attacks, but not DDoS attacks. Dealing with denial of service attacks is still not a completely solved issue; see this article for some of the problems and solutions.

ossec is “an Open Source Host-based Intrusion Detection System. It performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response.” Chances are it will pick up the side-effects of most intrusions. Be sure to configure at least the notification address where alert emails are delivered, even if you leave other options on their default.

Basic Security Steps : Linux (2)

Man pages:

  • ssh – secure shell client (remote login program)
  • sshd – secure shell daemon (server)
  • ssh-keygen – Used to create RSA keys (host keys and user authentication keys)
  • ssh-keyscan – gather ssh public keys
  • ssh-add – adds identities for the authentication agent Used to register new keys with the agent.
  • scp – secure copy (remote file copy program)
  • slogin
  • sftp – secure file transfer program client.
  • sftp-server – secure file transfer program server.
  • ssh-agent – Authentication agent. This can be used to hold RSA keys for authentication.
  • telnet – user interface to the TELNET protocol


  • /usr/share/doc/openssh-XXX/
  • /usr/share/doc/openssh-askpass-XXX/
  • /usr/share/doc/openssl-0.XXX/


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.


rssh: Restricted shell for use with OpenSSH sftp

FTP uses clear text access to your server. This is fine if all systems in the datacenter are secure and no one can sniff the network. Router and switch configurations make it almost impossible to sniff most networks these days, but a security comprimises at the datacenter on another server can cause potential problems for your servers if you allow open un-encryped passwords used by FTP.

VsFTPd also allows one to limit the user’s view of the filesystem to their own directories. This is good. OpenSSH “sftp” does not provide this capability (until version 4.9. RHEL/CentOS 5 use OpenSSH 4.3). The “sftp” file transfer does encypt the passwords (good) but also requires shell access (bash, csh, …) for the account which allows full access to the filesystem (bad). The rssh shell can be used with sftp, scp, cvs, rsync, and rdist and can chroot users to their own directories and limit function to sftp access only (deny full shell access).

For newer systems (RHEL6/CentOS6/Fedora 11) with OpenSSH 4.9+ see the preferred chrooted sftp configuration for OpenSSH 4.9+.

The solution is to use rssh as your shell with OpenSSH “sftp”:

  • rssh Home Page
  • rssh RPMs – Dag Wieers

Installation: rpm -ivh rssh-2.3.2-1.2.el5.rf.x86_64.rpm

This installs:

  • /usr/bin/rssh
  • /etc/rssh.conf
  • also support program /usr/libexec/rssh_chroot_helper and man pages

Check installed configuration: rssh -vConfiguration:

  1. OpenSSH configuration: /etc/ssh/sshd_config… PermitUserEnvironment no … Subsystem sftp /usr/libexec/openssh/sftp-server … Security note: Also be aware of the setting AllowTcpForwarding which controls port forwarding.
  2. Add shell to list of usable shells: /etc/shells/bin/sh /bin/bash /sbin/nologin /bin/tcsh /bin/csh /bin/ksh /bin/zsh /opt/bin/ftponly /usr/bin/rssh Ubuntu: You can use the command: add-shell /usr/bin/rssh
  3. Change the user’s shell to rssh (choose one method)
    • chsh -s /usr/bin/rssh user1
    • usermod -s /usr/bin/rssh user1
    • Assign shell when creating user: useradd -m -s /usr/bin/rssh user1
    • Edit /etc/passwduser1:x:504:504::/home/user1:/usr/bin/rssh
  4. Allow execution to su: chmod u+s /usr/libexec/rssh_chroot_helper This prevents the following error in /var/log/messagesDec 20 00:23:44 nodex rssh_chroot_helper[27450]: chroot() failed, 2: Operation not permitted
  5. Set access for rssh: /etc/rssh.conflogfacility = LOG_USER allowsftp umask = 022 #chrootpath = /users/chroot user=”user1:022:00010:/home/user1” Global security allowable options include: allowscp, allowcvs, allowrdist, allowrsync Specify global chroot or omit for none. Specific user security:
    1. User login id
    2. First set of three number represent the umask
    3. Second set of five number represent the bitmask to allow1 1 1 1 1 rsync rdist cvs sftp scp
    4. Specify the global chrooted directory for all using rssh. If omitted, then not chrooted. Can be overwritten by user configuration.

    Note: User configuration overrides the shared chroot settings. Omitted user settings do not default to shared chroot settings.

  6. Configuring the chrooted directory: This is true for a global user chroot or individual chroot. In this example we will show a user chrooted to thier own home directory /home/user1. When chrooted, the user does not have access to the rest of the filesystem and thus is blind to all of its executables and libraries. It will therefore be necessary to copy local executables and libraries for thier local use.DescriptionUser directorySystem equivalentSystem devices /home/user1/dev /dev Configuration files /home/user1/etc /etc/etc/ /etc/*/etc/ – dynamic linker configuration/etc/nsswitch.conf/etc/passwd/etc/group/etc/hosts/etc/resolv.conf Shared libraries (32 and 64 bit) /home/user1/lib/home/user1/lib64 /lib/lib64 Executables and libraries /home/user1/usr /usr/usr/libexec/openssh/sftp-server/usr/libexec/rssh_chroot_helper Executables /home/user1/bin /bin Use scrit to add chroot required files: /opt/bin/userchroot#!/bin/bash # First and only argument ($1) is user id if [ -d /home/$1 ]; then USERDIR=/home/$1 else echo “Error: Directory /home/$1 does not exist” exit fi mkdir $USERDIR/etc mkdir $USERDIR/lib mkdir -p $USERDIR/usr/libexec/openssh mkdir -p $USERDIR/var/log mkdir $USERDIR/dev mknod -m 666 $USERDIR/dev/null c 1 3 cp -p /etc/ $USERDIR/etc # If directory exists if [ -d /etc/ ]; then cp -avRp /etc/ $USERDIR/etc fi grep $1 /etc/passwd > $USERDIR/etc/passwd cp -p /etc/ $USERDIR/etc cp -p /etc/nsswitch.conf $USERDIR/etc cp -p /etc/group $USERDIR/etc cp -p /etc/hosts $USERDIR/etc cp -p /etc/resolv.conf $USERDIR/etc cp -ap /usr/libexec/openssh/sftp-server $USERDIR/usr/libexec/openssh/sftp-server cp -ap /usr/libexec/rssh_chroot_helper $USERDIR/usr/libexec/rssh_chroot_helper # Authentication libraries required for login (32 bit and 64 bit systems) if [ -d /lib64 ]; then mkdir $USERDIR/lib64 cp -ap /lib64/ $USERDIR/lib64 cp -ap /lib64/libnss_files-*.so $USERDIR/lib64 else cp -p /lib/ $USERDIR/lib cp -p /lib/libnss_files-*.so $USERDIR/lib fi FILES=ldd /usr/libexec/openssh/sftp-server | perl -ne 's:^[^/]+::; s: \(.*\)$::; print;' for ii in $FILES do rtdir=”$(dirname $ii)” [ ! -d $USERDIR$rtdir ] && mkdir -p $USERDIR$rtdir || : /bin/cp -p $ii $USERDIR$rtdir done FILES=ldd /usr/libexec/rssh_chroot_helper | perl -ne 's:^[^/]+::; s: \(.*\)$::; print;' for ii in $FILES do rtdir=”$(dirname $ii)” [ ! -d $USERDIR$rtdir ] && mkdir -p $USERDIR$rtdir || : /bin/cp -p $ii $USERDIR$rtdir done Note:
    • Script use: /opt/bin/userchroot user1
    • The files and directories reflect the file and path names for Red Hat Enterprise Linux 5 and CentOS 5.
    • Instead of copying files, one can also use a hard link: ln /etc/ /home/user1/etc/ if the files are on the same hard drive. In that way, users recieve updates to the system. Symbolic links will not work. See symlinks and chroot for this discussion. If the user directory is on a separate drive, use the copy as defined in the script.
    • Reduce /etc/passwd to a single user (don’t have root etc):user1:x:504:504::/home/user1:/usr/bin/rssh
    • Once chroot() takes place, programs will not have access to the regular log target. Specify a chrooted syslog socket target which can be accessed. The number of sockets are limited and thus configuring rssh for each user is not a good idea for a large number of users. For use with many users, use the shared chrooted jail defined by the rssh directive: chrootpath.

Blocking FTP: Setting up rssh does not turn off or block FTP access to your system. You must still turn off vsftp: /etc/init.d/vsftpd stop. There is little point to setting up secure chrooted sftp access with rssh and also running a FTP service.


  • One can pull in the full root path by issuing an internal mount:
    • mount –bind /dev /home/user1/dev
    • mount –bind /dev /home/user1/lib
    • mount –bind /dev /home/user1/lib64
    • mount –bind /dev /home/user1/usr

    This technique can be used to narrow down the error to find which directory has the missing files. It should not be used as a final solution. Unmount when done: umount /home/user1/dev

  • If authenticating to ldap, nis, etc, pull in the appropriate libraries. You can test with all: cp -p /lib/libnss_* /home/user1/lib This can be performed for /lib64 as well.
  • Checklog files for errors: /var/log/messages

Man pages:

  • rssh man page
  • rssh.conf man page
  • sftp man page

Using gFTP as a Linux sftp client:

  • Start program through menu or command line: gftp&
  • Select “FTP” from toolbar
  • Select “Options”
  • Select “SSH” tab
  • Select “Apply” amd “Ok”
  • On the upper rigt hand side of the gftp window, select “SSH” from the pull-down menu.

Using FileZilla as a Linux sftp client:

  • Select “File” + “Site Manager”
  • Select “New Site” (bottom left)
  • Enter “Host:”
  • Choose “Servertype:” “SFTP using SSH2”
  • Select “Logontype:” “Normal”
  • Enter “User:” and click on “Connect”.


  • Multi-platform GUI client FileZilla
  • MS/Windows client WinSCP (supports sftp)


SentryTools: 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 1.2 of portsentry can issue iptables, ipchains or route commands to thwart attacks. Iptables/Ipchains is a Linux firewall system built into the Linux kernel. Linux kernel 2.6/2.4 uses iptables, kernel 2.2 (old) uses ipchains. References to ipfwadm are for even older Linux kernels. Route commands can be used by any Unix system including those non-Linux systems which do not support Iptables/Ipchains.

Steps to install and configure portsentry:

  1. Download and unzip source code
  2. Edit include file and compile
  3. Start PortSentry
  4. Read logs
  1. Download and unzip source code:
    • Download: PortSentry source code
    • Move to your source directory and unzip: tar -xzf portsentry-1.2.tar.gz
  2. Edit include file and compile: cd portsentry_beta/ Read file README.install. It details the following:
    • Edit file: portsentry_config.hSet file paths and configure separate log file for Portsentry:

      Set options:

      • CONFIG_FILE – PortSentry run-time configuration file.
      • WRAPPER_HOSTS_DENY – The path and name of TCP wrapper hosts.deny file.

      #define CONFIG_FILE “/opt/portsentry/portsentry.conf” #define WRAPPER_HOSTS_DENY “/etc/hosts.deny” #define SYSLOG_FACILITY LOG_DAEMON – Default. Change to LOG_LOCAL6 #define SYSLOG_LEVEL LOG_NOTICE(Note: I use /opt/portsentry/ because I like to locate “optional” 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.confChange the following line by adding an extra log facility for portsentry messages which are not going to be logged to the regular syslog output file /var/log/messages. This lists what messages to filter out from /var/log/messages.

        *.info;mail.none;news.none;authpriv.none;cron.none;local6.none /var/log/messagesAdd the following line to assign a portsentry log facility:

        local6.* /var/log/portsentry.logNote: Use tab not spaces in the syslog configuration file.

        Restart syslogd: /etc/init.d/syslog restart

      • Set portsentry_config.h entry to new log facility: Change from default setting: #define SYSLOG_FACILITY LOG_DAEMON To: #define SYSLOG_FACILITY LOG_LOCAL6

      FYI: Options for the SYSLOG_FACILITY are defined in /usr/include/sys/syslog.h They include:

      SYSLOG_FACILITY Facility Name Description LOG_LOCAL0 local0 reserved for local use LOG_LOCAL1 local1 reserved for local use LOG_LOCAL2 local2 reserved for local use LOG_LOCAL3 local3 reserved for local use LOG_LOCAL4 local4 reserved for local use LOG_LOCAL5 local5 reserved for local use LOG_LOCAL6 local6 reserved for local use LOG_LOCAL7 local7 reserved for local use LOG_USER user random user-level messages LOG_MAIL mail mail system LOG_DAEMON daemon system daemons LOG_SYSLOG syslog messages generated internally by syslogd LOG_LPR lpr line printer subsystem LOG_NEWS news network news subsystem LOG_UUCP uucp UUCP subsystem LOG_CRON cron clock daemon LOG_AUTHPRIV authpriv security/authorization messages (private) LOG_FTP ftp ftp daemonOptions for the SYSLOG_LEVEL include:

      SYSLOG_LEVEL Priority Description LOG_EMERG 0 system is unusable LOG_ALERT 1 action must be taken immediately LOG_CRIT 2 critical conditions LOG_ERR 3 error conditions LOG_WARNING 4 warning conditions LOG_NOTICE 5 normal but significant condition LOG_INFO 6 informational LOG_DEBUG 7 debug-level messages

    • Edit file: portsentry.conf to set paths for configuration files and ports to monitor. TCP_PORTS=”1,11,15,20,21,23,25,69,79, … ” UDP_PORTS=”1,7,9,69,161,162,513,635, … ” … … IGNORE_FILE=”/opt/portsentry/portsentry.ignore” HISTORY_FILE=”/opt/portsentry/portsentry.history” BLOCKED_FILE=”/opt/portsentry/portsentry.blocked” #KILL_ROUTE=”/sbin/route add -host $TARGET$ reject”– Generic Unix KILL_ROUTE I prefer iptables/ipchains options below Uncomment and modify if necessary the appropriate statements. The TCP_PORTS=, UDP_PORTS= lists are ignored for stealth scan detection modes. Add common but unused services. i.e. add port 25 if the system is not accepting email as port 25 is included in most scans. 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″ – serverADVANCED_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″PAM options:

      For more on PAM see YoLinux network Admin TutorialChoose one option: (Options: network “route” or firewall command “iptables/ipchains”)

      1. For those using iptables (Linux Kernel 2.6/2.4+): 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)
      2. 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
      3. Simple method to drop network return routes if iptables or 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 “-“.

      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 your kernel is configured to use ipchain rules.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. ) Your IP address The at Home network routinely scans for news servers on port 119 from a server named Adding the IP address of this server ( greatly reduces the logging. I also added their BOOTP server. ( manually issued the iptables (kernel 2.6/2.4) 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 -j DROP

      /sbin/iptables -I INPUT -s -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)To:#               chmod 700 $(INSTALLDIR)/$(CHILDDIR)

    • Compile: make linuxFix the following compile errors in portsentry.c
      • Change printf (“Copyright 1997-2003 Craig H. Rowland <craigrowland at users dot sourceforget dot net>\n”); to one line: printf (“Copyright 1997-2003 Craig H. Rowland\n”);
      • Fix warning: warning: passing argument 3 of ‘accept’ from incompatible pointer type Separate and change declaration of “length” to: unsigned int length;
    • Install (as root): make install
  3. Run PortSentry for advanced UDP/TCP stealth scan detection:
    • portsentry -atcp
    • portsentry -audp

    OR use init scripts below in next section.

  4. 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: 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 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 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)

#!/bin/bash # Purge and re-assign chain rules ipchains -F ipchains -A input -p tcp -s 0/0 -d 0/0 2049 -y -j REJECT ipchains -A input -p udp -s 0/0 -d 0/0 2049 -j REJECT ipchains -A input -p tcp -s 0/0 -d 0/0 6000:6009 -y -j REJECT ipchains -A input -p tcp -s 0/0 -d 0/0 7100 -y -j REJECT ipchains -A input -p tcp -s 0/0 -d 0/0 515 -y -j REJECT ipchains -A input -p udp -s 0/0 -d 0/0 515 -j REJECT ipchains -A input -p tcp -s 0/0 -d 0/0 111 -y -j REJECT ipchains -A input -p udp -s 0/0 -d 0/0 111 -j REJECT ipchains -A input -j REJECT -p all -s localhost -i eth0 -l

Also see:

  • Sourceforge: Portsentry Home Page – PortSentry, Logcheck and HostSentry home page.
  • Portsentry description
  • FAQ: Firewall Forensics – Robert Graham

Other tools to detect portscans and network based hacker attacks:

  • scanlogd – Attack detection.
  • InterSect Alliance – Intrusiuon analysis. Identifies malicious or unauthorized access attempts.
  • 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.

#!/bin/bash # # Startup script for PortSentry # # chkconfig: 345 85 15 # description: PortSentry monitors TCP and UDP ports for network attacks # # processname: portsentry # pidfile: /var/run/ # config: /opt/portsentry/portsentry.conf # config: /opt/portsentry/portsentry.ignore # config: /opt/portsentry/portsentry.history # config: /opt/portsentry/portsentry.blocked # Source function library. . /etc/rc.d/init.d/functions # Source networking configuration. . /etc/sysconfig/network # Check that networking is up. [ ${NETWORKING} = “no” ] && exit 0 # See how we were called. case “$1” in start) echo -n “Starting portsentry: ” daemon /opt/portsentry/portsentry -atcp /opt/portsentry/portsentry -audp echo touch /var/lock/subsys/portsentry ;; stop) echo -n “Shutting down portsentry: ” killproc portsentry echo rm -f /var/lock/subsys/portsentry rm -f /var/run/ ;; status) status portsentry ;; restart) $0 stop $0 start ;; reload) echo -n “Reloading portsentry: ” killproc portsentry -HUP echo ;; *) echo “Usage: $0 {start|stop|restart|reload|status}” exit 1 esac exit 0

Logrotate Configuration:

Create the following file to have your logs rotate. File: /etc/logrotate.d/portsentry/var/log/portsentry.log { rotate 12 monthly errors root@localhost missingok postrotate /usr/bin/killall -HUP portsentry 2> /dev/null || true endscript }

Also see the YoLinux Sys Admin tutorial covering logrotate.


  • 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: Host: Port: 23 TCP Blocked

    The file /var/log/portsentry.log will show the action taken: portsentry[589]: attackalert: SYN/Normal scan from host: to TCP port: 23 portsentry[589]: attackalert: Host has been blocked via wrappers with string: “ALL:” portsentry[589]: attackalert: Host has been blocked via dropped route using command: “/sbin/ipchains -I input -s -j DENY -l”

  • nmap: portscanner – This is the hacker tool responsible for many of the portscans you may be recieving.Command arguments:

    ArgumentDescription-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.


    nmap -sT -F IP-address Scan nmap -sS -F IP-address SYN Scan nmap -sU -F IP-address Scan UPD ports nmap -sF -F IP-address FIN Scan nmap -O -F IP-address Determine OS nmap -p22 -F -O IP-address nmap -p 1-30,40-65535 IP-Address Scan given port ranges 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 ( )…..(The 1067 ports scanned but not shown below are in state: closed)Port State Service21/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: 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:
    • nmap-XXX.i386.rpm
    • nmap-frontend-XXX.i386.rpm


  • nmap man page
  • The Art of Port Scanning – by Fyodor
  • 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.

Use the command tripwire –version or rpm -q tripwire to determine the version.

Red Hat includes Tripwire as an optional package during install. The Ubuntu/Debian install is as easy as apt-get install tripwire. 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 on my server!

Tripwire configuration files:

  • Tripwire 2.3.0-58: (Red Hat 7.1)
    • /etc/tripwire/twcfg.txt
    • /etc/tripwire/twpol.txt

    These files are first edited and then processed by the script /etc/tripwire/ which configures Tripwire after the installation of the Tripwire RPM package.Edit and change file: /etc/tripwire/twcfg.txt

    Change:LOOSEDIRECTORYCHECKING =false to LOOSEDIRECTORYCHECKING=TRUEThis was recommended in the comments of the file twpol.txtEdit 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; 

    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/ 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 (Red Hat 6.2 Powertools): /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 -initializeThis 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:

    cp -p /root/databases/tw.db_ServerName /var/spool/tripwire/tw.db_ServerNameDon’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.

Cron and tripwire:

Cron runs tripwire:

  • Tripwire 2.3.0-58: File: /etc/cron.daily/tripwire-check#!/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/” 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.

Read tripwire report:

  • Tripwire 2.3.0-58: twprint –print-report -r /var/lib/tripwire/report/report-file.twr

Interactive mode:

  • Tripwire 1.2-3: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.

Default configuration file:

  • Tripwire 2.3.0-58: /etc/twcfg.txtROOT                   =/usr/sbin

    POLFILE                =/etc/tripwire/tw.polDBFILE                 =/var/lib/tripwire/$(HOSTNAME).twd

    REPORTFILE             =/var/lib/tripwire/report/$(HOSTNAME)-$(DATE).twrSITEKEYFILE            =/etc/tripwire/site.key

    LOCALKEYFILE           =/etc/tripwire/$(HOSTNAME)-local.keyEDITOR                 =/bin/vi



    REPORTLEVEL            =3MAILMETHOD             =SENDMAIL

    SYSLOGREPORTING        =falseMAILPROGRAM            =/usr/sbin/sendmail -oi -t

  • Tripwire 1.2-3: /etc/tw.config# Log file

    @@define LOGFILEM E+pugn# Config file

    @@define CONFM E+pinugc# Binary

    @@define BINM E+pnugsci12# Directory

    @@define DIRM E+pnug# Data file (same as BIN_M currently)

    @@define DATAM E+pnugsci12# Device files

    @@define DEVM E+pnugsc# exclude all of /proc

    =/proc E#=/dev @@DIRM

    /dev @@DEVM#=/etc @@DIRM

    /etc @@CONFM# Binary directories

    #=/usr/sbin @@DIRM/usr/sbin @@BINM

    #=/usr/bin @@DIRM/usr/bin @@BINM

    #=/sbin @@DIRM/sbin @@BINM

    #=/bin @@DIRM/bin @@BINM

    #=/lib @@DIRM/lib @@BINM

    #=/usr/lib @@DIRM/usr/lib @@BINM

    =/usr/src E=/tmp @@DIRM



    /var/named @@CONFM – If you are running Bind DNS slave /home/httpd/cgi-bin @@BINM Delete/comment out:#/dev @@DEVM

    This eliminated the reporting of too much junk due to a reboot of the system.

Basic Security Steps : Linux (1)

Perform the following steps to secure your web site

    • Red Hat/CentOS:
      • yum check-update (Print list of packages to be updated.)
      • yum update

      Note that this can be automated using the /etc/init.d/yum-updatesd service (RHEL/CentOS 5) or create a cron job /etc/cron.daily/yum.cron#!/bin/sh /usr/bin/yum -R 120 -e 0 -d 0 -y update yum /usr/bin/yum -R 10 -e 0 -d 0 -y update

    • Ubuntu/Debian:
      • apt-get update (Update package list to the latest version associated with that release of the OS.)
      • apt-get upgrade
  • Reduce the number of network services exposed. These will be started by scripts in /etc/rc.d/rc*.d/ directories. (See full list of services in:/etc/init.d/) 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
  • 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 find command to locate vulnerabilities – find suid and guid files (which can execute with root privileges) as well as world writable files and directories. For example:
    • find / -xdev \( -perm -4000 -o -perm -2000 \) -type f -print Remove suid privileges on executable programs with the command: chmod -s filename
    • find / -xdev \( -nouser -o -nogroup \) -print Find files not owned by a valid user or group.
  • Use the command chattr and lsattr to make a sensitive security 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 and Ubuntu distributions. 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)
  • Proc file settings for defense against attackes. This includes protective measures against IP spoofing, SYN flood or syncookie attacks.
  • 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:
    • Turn off ICMP (look invisible to network scans)
    • Monitor the attack with tcpdump

    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. DNS name servers use UDP.) For very interesting reading see “The Strange Tale” of the DDoS attack. (Very interesing read about the anatomy of the hacker bot networks.)

  • User access can be restricted with the following configuration files:
    • /etc/security/limits.conf
    • /etc/security/group.conf
    • /etc/security/time.conf

    See YoLinux SysAdmin tutorial – restrict users

  • 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, …



  • It is best for security reasons that you reduce the number of inetd 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.


      • List init settings including all xinetd controlled services: chkconfig –list
      • List status of services (Red Hat/Fedora Core based systems): service –status-all


Kernel Configuration:

  • Use Linux firewall rules to protect against attacks. (ipchains: kernel 2.6, 2.4 or iptables: kernel 2.2) 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 interfaceipchains -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.


    • iptables uses the chain rule “INPUT” and ipchains uses the lower case descriptor “input”.
    • View rules with iptables -L or ipchains -L command.
    • iptables man page
    • 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.

    Also see:

    • YoLinux Internet Gateway Tutorial
    • Red Hat 7.1 firewall GUI configuration tool /usr/sbin/gnome-lokkit
  • Use portsentry to monitor network hacker attacks and dynamically assign firewall rules to thwart attackers. (Later in this tutorial)
  • A monolithic and minimal 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.)
  • Kernel Security Enhancements:
    • Red Hat/CentOS SELinux: National Security Agency (NSA): Security-Enhanced Linux – Altered for increased security. For more see the Systems Admin and Web site configuration tutorials.
    • Ubuntu Apparmor community wiki
  • Enable ExecShield: this is enabled by default on Red Hat EL 5/CentOS 5. ExecShield is a Linux kernel feature which protects the system agains buffer overflow exploits. This feature is performed by random placement of stack memory, prevention of execution of memory used to hold data and text buffer handling. ExecShield can be enabled in the Red Hat/CentOS configuration file /etc/sysctl.conf by adding the following two lines:kernel.exec-shield = 1 kernel.randomize_va_space = 1 The current system configuration can be checked:
    • cat /proc/sys/kernel/exec-shield
    • cat /proc/sys/kernel/randomize_va_space

    Both should be “1”. (System default)Note: Intel XD/AMD NX 32 bit x86 processors only (not x86_64 which can address more that 4Gb): Enable AMD NX or Intel XD support by use of the PAE (Physical Address Extension) kernel. The PAE memory extension is required to access the XD/NX bit. To see if your processor supports NX or XD PAE, use the command: cat /proc/cpuinfo | grep flags to show a field with “pae” and “nx”. Install a Linux kernel (2.6.8+) with PAE support with the command yum install kernel-PAE. The boot loader will also have to specify the PAE kernel for boot. The BIOS will also have to be configured to support it as well. This kernel should only be installed on a system with a x86 32 bit processor which offers this support. The 64 bit x86_64 processors which can natively interact with the XD/NX bit do not need the PAE kernel.


Firewall Rules to Block Bad IP Blocks:

It is well known that there are various blocks of IP addresses where nefarious hackers and spam bots reside. These IP blocks were often once owned by legitimate corporations and organizations but have fallen into an unsupervised realm or have been highjacked and sold to criminal spammers. These IP blocks should be blocked by firewall rules.

There are various friendly services which seek and discover these IP blocks to firewall and deny and they share this information with us. Thanks!

The Spamhaus drop list: This is a script to download the total drop list and generate an iptables filter script to block these very IP addresses:

#!/bin/bash # Blacklist of hacker zones and bad domains from FILE=drop.lasso /bin/rm -f $FILE wget$(cat $FILE | egrep -v ‘^;’ | awk ‘{ print $1}’) echo “#!/bin/bash” > for ipblock in $blocks do echo “iptables -I INPUT -s $ipblock -j DROP” >> done chmod ugo+x echo “…Done” To block the IP addresses just execute the script on each of your servers: ./

At the very minimum, these blocks of IP addresses should be denied by all servers.

Block or allow by country: One can deny access by certain countries or the inverse, allow only certain countries to access your server.

See these sites to generate lists:

  • – generates Apache htaccess or iptables rules
  • Country IP block list generator
  • CIDR lists

Block forum and comment list spammers: Use the list generated from honeypots operated by

#!/bin/bash # Big list of IP adresses to block # IPs gathered from the last 30 days # Over 100k IP addresses rm -f wget rm -f listed_ip_30.txt unzip echo “#!/bin/bash” > cat ./listed_ip_30.txt | awk ‘{print “/sbin/iptables -I INPUT -s ” $1 ” -j DROP”}’ >> chmod ugo+x

To block the IP addresses just execute the script: ./

Be aware that this is an extremely long list and can take hours to run. It is also a rapidly changing list which is updated constantly.

[Potential Pitfall]: You may get the following error:iptables: Unknown error 18446744073709551615 I found that by slowing down the execution of the script, I can avoid this error. I added a bash echo to write each line to the screen and it behaved much better although also much slower.#!/bin/bash set -x verbose /sbin/iptables -I INPUT -s XX.XX.XX.XX -j DROP …


Apache web server:

  • Apache modules: Turn off modules you are not going to use. With past ssl exploits, those using this philosophy did not get burned.
    • Red Hat EL 5/CentOS 5 Apache 2.2: The configuration file /etc/httpd/conf.d/ssl.conf enables SSL by default. This file is picked up from the line Include conf.d/*.conf in the file /etc/httpd/conf/httpd.conf Rename the file /etc/httpd/conf.d/ssl.conf to ssl.conf_OFF to turn off SSL (any file ending with “.conf” is included in the web server configuration).
    • Ubuntu 8.04: a2dismod ssl This will disable the loading of SSL. The Ubuntu distribution has a fairly frugal use of modules by default. The default configuration has SSL turned off.
    • Apache 1.3.x config file /etc/httpd/conf/httpd.conf#<IfDefine HAVE_SSL> #LoadModule ssl_module modules/ #</IfDefine> … … #<IfDefine HAVE_SSL> #AddModule mod_ssl.c #</IfDefine> … … <IfDefine HAVE_SSL> Listen 80 #Listen 443 </IfDefine> … … #<IfModule mod_ssl.c> #… #… … #<VirtualHost _default_:443> #… #… … 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 exposure: (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.
  • Block hackers and countries which will never use your website. Use the Apache directive Deny from to block access.<Directory /home/projectx/public_html> … … … Order allow,deny # Block form bots Deny from allow from all </Directory> For extensive lists of IP addresses to block, see the block list


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 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+)


  • – Shell. Supports SSH1 and SSH2 protocols.
    • – Encrypted network layer
    • – SSH for other platforms
  • SSH:
    • – Secure shell
    • – SSh for other platforms
  • Secure Shell IETF working group – (Internet Engineering Task Force)


  • Download:
    • Download OpenSSH RPM’s (sourceforge) – statically linked with OpenSSL 0.9.5 – Pick this one for an easy complete RPM install
    • Download OpenSSH source (tgz)
    • Red Hat Linux 6.x Open SSL RPM downloads ( (SSL only)

    Note: SSH and SSL are included with Red Hat Linux 7.0+

  • Installation:
    • Common to Client and Server:
      • Red Hat/Fedora/CentOS: rpm -ivh openssh-2.9p2-8.7.i386.rpm
      • Ubuntu/Debian: apt-get install ssh
    • Client:
      • Red Hat/Fedora/CentOS: rpm -ivh openssh-askpass-2.9p2-8.7.i386.rpm rpm -ivh openssh-clients-2.9p2-8.7.i386.rpm rpm -ivh openssh-askpass-gnome-2.9p2-8.7.i386.rpm – Gnome desktop users
      • Ubuntu/Debian: apt-get install openssh-client ssh-askpass-gnome
    • Server:
      • Red Hat/Fedora/CentOS: rpm -ivh openssh-server-2.9p2-8.7.i386.rpm
      • Ubuntu/Debian: apt-get install openssh-server

    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.

  • Configuration:
    • Client configuration file /etc/ssh/ssh_config: (Default)# $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 #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/ #ReverseMappingCheck yes Subsystem sftp /usr/libexec/openssh/sftp-server
      • If changes are made to the configuration file, restart the “sshd” daemon to pick up the new configuration: Ubuntu: /etc/init.d/ssh restart Red Hat: /etc/init.d/sshd restart or service sshd restart
      • 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.netcontents to:     Access is granted to this server only to authorized personel of Mega Corp.

        By default, the /etc/ message presents to the hacker the OS name, kernel release and information which can be used to determine potential vulnerabilities.

      • [Potential Pitfall]: Slow ssh logins – If you get the “login” prompt quickly but the “password” prompt takes 30 seconds to a minute, then you have a DNS lookup delay. Set UseDNS no in the config file /etc/ssh/sshd_config and then restart sshd. The IP address of eth0 (or the NIC used) should also refer to your own hostname in /etc/hosts
  • Generate system keys: /etc/ssh/
    • ssh-keygen -q -t rsa -f /etc/ssh/ssh_host_rsa_key -C ” -N ”
    • ssh-keygen -q -t dsa -f /etc/ssh/ssh_host_dsa_key -C ” -N ”
    • Private keys generated: chmod 600 /etc/ssh/ssh_host_dsa_key /etc/ssh/ssh_host_rsa_key
    • Public keys generated: chmod 644 /etc/ssh/ /etc/ssh/
    • For SELinux:
      • /sbin/restorecon /etc/ssh/
      • /sbin/restorecon /etc/ssh/
  • Generate user keys:
    • Client: Use the command: /usr/bin/ssh-keygen -t rsaGenerating 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/ The key fingerprint is:XXXblablablaXXXaf:90:8f:dc:65:0d:XXXXXXXXXXXXXX user-id@node-nameFiles generated: $HOME/.ssh/id_rsa – binary $HOME/.ssh/ – ssh-rsa …223564257432 email address – Multiple keys/lines allowd.
    • Server:
      • FTP the file $HOME/.ssh/ to the server
      • cd $HOME/.ssh/
      • cat >> authorized_keys
  • Using ssh: On client use the following command and login as you normally would with a telnet session: ssh name-of serverThe first time you use ssh it will issue the following message:The authenticity of host ‘ (XXX.XXX.XXX.XXX)’ can’t be established. RSA key fingerprint isXXXXblablablaXXX1:81:29:00:3a:c5:fb:XXXXXXXXXXX. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added ‘,XXX.XXX.XXX.XXX’ (RSA) to the list of known hosts.’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.

OpenSSH Man Pages:

  • ssh – OpenSSH SSH client (remote login program)
  • sshd – OpenSSH ssh daemon
  • ssh-keygen – Used to create RSA keys (host keys and user authentication keys)
  • ssh_config – OpenSSH SSH client configuration file
  • sshd_config – OpenSSH SSH daemon configuration file
  • ssh-add – adds RSA or DSA identities for the authentication agent. Used to register new keys with the agent.
  • scp – secure copy (remote file copy program)
  • ssh-agent – authentication agent This can be used to hold RSA keys for authentication.
  • sftp – Secure file transfer program
  • sftp-server – SFTP server subsystem

Other OpenSSH Links:

  • Red Hat Open SSH Guide – Also scp, sftp, Gnome ssh-agent
  • Linux Journal: OpenSSH Part I

SSH for MS/Windows Links:

  • PuTTY. Also see PuTTY configuration
  • Tera Term

SSH Notes:

  • 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.

Best WordPress Security Plugins

Top Seventeen Security choices For Your Computer

In this day and age it’s inevitable that if you permit net access on your laptop or your net business you’re about to would like some style of security system of some sort to do and facilitate remove bugs and viruses that you simply could and doubtless can encounter. If you allow yourself unprotected it’s around as dumb as about to a gun fight with a knife. this text contains an inventory of the foremost downloaded choices within the market these days.

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The Top 10 

The top three most downloaded and used choices square measure captcha, a straightforward security scan and a information manager. A captcha could be a terribly easy manner for businesses to avoid spam users on their web site, in different words users that aren’t AN actual person and is simply a trojan horse fishing around creating bother and things tougher for actual customers or shoppers. It forces you to browse and enter a distorted word combination, that are a few things a trojan horse alone couldn’t do. A security scan is one amongst the foremost easy and staple items a web user (weather skilled for a business or simply a private electronic computer owner) will install and use. It searches your computer’s system for any attainable break downs in security and the way to resolve it. A information manager is very helpful in rental you have got the flexibility to repair and restore your information itself. conjointly it permits you to copy your most significant knowledge just in case the worst happens.

Numbers four through seven square measure all just about ways in which you’ll be able to give a boost to your existing security. that if you run an internet business you’d be additional doubtless to require and want these, making an attempt to create your web site as secure as attainable for the peace of mind and protection of your clients/customers and for the continuity of your own business. nobody goes to require to use your web site if they understand their personal info is taken at a moment’s notice with hardly any effort.

The eighth and ninth place spots visit programs that square measure centered on reducing and interference any potential spam that you simply may receive, whereas variety ten is concentrated additional on backup. variety eight will this by adding AN antivirus aspect to your web site that permits you to be protected against things like malware and spam. . variety nine will this by involving captcha in additional places on your web site than the afore mentioned possibility. The tenth most downloaded possibility is only simply AN choice to copy your most significant info, no matter which will be.

Additional Security Plugins

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In spots eleven through fifteen square measure chiefly centered on providing you with a inordinateness of security choices starting from protection up your account once a such variety of failing go browsing tries, scanning your system and providing you with a grade supported what it finds, and having the ability to prevent spam with a lot of various ways in which to log in.

And last however not least, spots sixteen and seventeen square measure engineered around providing you with some basic tools to assist you management your web site and its security. primarily sort of a bundled package for those those that aren’t quite certain what they have or most need therefore it offers you a little little bit of everything.

There square measure several, more security systems and tools out there for your laptop, however this text mentions simply a number of of them that square measure at the highest of their field. If you actually rely on it they’re there at the highest for one reason, which is reason is that they work!