Hack The Sec-Leading Resource Of Linux Tutorial: SSH

PREVENT BRUTE-FORCE SSH ATTACKS USING IPTABLES

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<u><span style="color: #990000;">PREVENT BRUTE-FORCE SSH ATTACKS</span></u></h1>
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<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;">Prevent (or at least slow down) a brute-force SSH attack.</span></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgh-J9cWc4F81gJfxyBV0kwTGX_WGJaF-PkRl_ShpUwpMgojO-YgoJLWyh3G9EnoZpTc6Fyj_8sE7VFXiH7a3cDj42F43FMN8LNvhW6BT5c1x6U05CW7d7ZUZbkMaVoICEMeKqpgPTnXtgS/s1600/Brute_Force_Login_Protection_hackthesec.co.in.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgh-J9cWc4F81gJfxyBV0kwTGX_WGJaF-PkRl_ShpUwpMgojO-YgoJLWyh3G9EnoZpTc6Fyj_8sE7VFXiH7a3cDj42F43FMN8LNvhW6BT5c1x6U05CW7d7ZUZbkMaVoICEMeKqpgPTnXtgS/s1600/Brute_Force_Login_Protection_hackthesec.co.in.jpg" /></a></div>
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<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;"><br /></span></span></div>
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<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;">By default, iptables on a CentOS 6 SSH server allows all inbound SSH traffic on port 22. See /etc/sysconfig/iptables:</span></span></div>
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<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace;"><span style="line-height: 15.6px;">*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT</span></span></pre>
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<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;">Modify /etc/sysconfig/iptables to the following in order to allow a particular IP to initiate 5 new SSH connections within a window of 60s. If additional new SSH connections are opened for that IP, then all packets from that IP will be dropped, and the incident will be logged to /var/log/messages. After 60s that IP will be able to open 5 new SSH connections, and so on. </span></span></div>
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<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace;"><span style="line-height: 15.6px;">*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -p tcp --dport 22 -m state --state NEW -m recent --set --name SSH -j ACCEPT
-A INPUT -p tcp --dport 22 -m recent --update --seconds 60 --hitcount 5 --rttl --name SSH -j LOG --log-prefix "BRUTE_FORCE_SSH"
-A INPUT -p tcp --dport 22 -m recent --update --seconds 60 --hitcount 5 --rttl --name SSH -j DROP
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT</span></span></pre>
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<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace;"><span style="line-height: 15.6px;"># service iptables stop && service iptables start</span></span></pre>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace;"><span style="line-height: 15.6px;"><a href="http://www.hackthesec.co.in/">www.hackthesec.co.in</a></span></span></pre>
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PREVENT BRUTE-FORCE SSH ATTACKS USING IPTABLES

PREVENT BRUTE-FORCE SSH ATTACKS Prevent (or at least slow down) a brute-force SSH attack. By default, iptables on a CentOS 6 SSH ...

How to Secure a CentOS Server SSH + Fail2ban + DDOS Deflate

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<span style="color: #990000; font-size: large;"><u><b>Secure a CentOS Server SSH + Fail2ban + DDOS Deflate</b></u></span><br />
<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;">Secure Shell (SSH) is a UNIX-based command interface and protocol for securely getting access to a remote computer. By default SSH run on port 22. The ideal solution is to change this default value to other port number from 1 to 65535.</span></span><br />
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<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;"><br /></span></span>
<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;"><b><u>Strengthening SSH access</u></b></span></span><br />
<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;">login to your server via SSH</span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">vi /etc/ssh/sshd_config</span></span></pre>
<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;">Uncommented line that defines the port</span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">#Port 22

set this to a port that is not already in use

port 8070</span></span></pre>
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<span style="color: #333333; font-family: verdana, geneva, sans-serif;"><span style="font-size: 15px; line-height: 26px;">Allow SSH port on Iptables Firewall</span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">vi /etc/sysconfig/iptables
-A INPUT -p tcp -m state --state NEW -m tcp --dport 8070 -j ACCEPT
service iptables restart</span></span></pre>
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;">Apply this setting</span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">/etc/init.d/sshd restart</span></span></pre>
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;">SSH will now be listening on newly set port. - 8070</span></span><br />
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<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #0b5394; font-family: verdana, geneva, sans-serif;"><b><u>Prevent bruteforce using Fail2ban and DDOS Deflate</u></b></span></span></div>
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<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><a href="http://www.fail2ban.org/wiki/index.php/Main_Page">Fail2ban </a>fails to protect against a distributed brute force attack. this is used to block selected IP addresses that may belong to hosts that are trying to breach the system's security.</span></span></div>
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><b><u>Download and Install Fail2ban </u></b></span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">rpm -ivh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-7.noarch.rpm
yum install fail2ban
vi /etc/fail2ban/jail.conf</span></span></pre>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; margin-bottom: 10px; overflow: auto; padding: 9.5px; word-break: break-all; word-wrap: break-word;"><span style="color: #333333; font-family: Menlo, Monaco, Consolas, Courier New, monospace; font-size: large;"><span style="line-height: 25.7143px;">ignoreip = 127.0.0.1
bantime  = 240
findtime  = 240
maxretry = 10</span></span></pre>
<span style="background-color: white; color: #333333; font-family: verdana, geneva, sans-serif; font-size: 15px; line-height: 26px;">You can change as your need.</span><br />
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><b><u>Start Fail2ban service</u></b></span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">service fail2ban start

/etc/init.d/fail2ban start</span></span></pre>
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<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #0b5394; font-family: verdana, geneva, sans-serif;"><b><u> DDOS Deflate</u></b></span></span></div>
<span style="background-color: white;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><a href="http://deflate.medialayer.com/" style="font-size: 15px; line-height: 26px;">DoS Deflate</a><span style="font-size: 15px; line-height: 26px;"> is a lightweight bash shell script designed to assist in the process of blocking a denial of service attack. It utilizes the command below to create a list of IP addresses connected to the server, along with their total number of connections. It is one of the simplest and easiest to install solutions at the software level.</span><br /><span style="font-size: 15px; line-height: 26px;"><b>How To Install (D)DoS Deflate :-</b></span></span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">cd /tmp
wget http://www.inetbase.com/scripts/ddos/install.sh
chmod 0700 install.sh
./install.sh</span></span></pre>
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><b>How To UnInstall (D)DoS Deflate :-</b></span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">wget http://www.inetbase.com/scripts/ddos/uninstall.ddos
chmod 0700 uninstall.ddos
./uninstall.ddos</span></span></pre>
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;">How To Edit Configuration File:-<br />you can set your email for email notification and many more option</span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">vi /usr/local/ddos/ddos.conf</span></span></pre>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; margin-bottom: 10px; overflow: auto; padding: 9.5px; word-break: break-all; word-wrap: break-word;"><span style="color: #333333; font-family: Menlo, Monaco, Consolas, Courier New, monospace;"><span style="line-height: 25.7143px;">PROGDIR="/usr/local/ddos"
PROG="/usr/local/ddos/ddos.sh"
IGNORE_IP_LIST="/usr/local/ddos/ignore.ip.list"
CRON="/etc/cron.d/ddos.cron"
APF="/etc/apf/apf"
IPT="/sbin/iptables"

##### frequency in minutes for running the script
##### Caution: Every time this setting is changed, run the script with --cron
#####          option so that the new frequency takes effect
FREQ=1

##### How many connections define a bad IP? Indicate that below.
NO_OF_CONNECTIONS=5

##### APF_BAN=1 (Make sure your APF version is atleast 0.96)
##### APF_BAN=0 (Uses iptables for banning ips instead of APF)
APF_BAN=1

##### KILL=0 (Bad IPs are'nt banned, good for interactive execution of script)
##### KILL=1 (Recommended setting)
KILL=1

##### An email is sent to the following address when an IP is banned.
##### Blank would suppress sending of mails
EMAIL_TO="info@hackthesec.co.in"

##### Number of seconds the banned ip should remain in blacklist.
BAN_PERIOD=600</span></span><span style="color: #333333; font-size: 12px; line-height: 19.2px;">
</span></pre>
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<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><b>How To Check The Number Of Connected Ips:-</b></span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">sh /usr/local/ddos/ddos.sh</span></span></pre>
<span style="background-color: white; font-size: 15px; line-height: 26px;"><span style="color: #333333; font-family: verdana, geneva, sans-serif;"><b>How To Restart DDos Deflate:-</b></span></span><br />
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">sh /usr/local/ddos/ddos.sh -c</span></span></pre>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><a href="http://www.hackthesec.co.in/">www.hackthesec.co.in</a></span></span></pre>
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How to Secure a CentOS Server SSH + Fail2ban + DDOS Deflate

Secure a CentOS Server SSH + Fail2ban + DDOS Deflate Secure Shell (SSH) is a UNIX-based command interface and protocol for securely gettin...

How to protect your SSH server with Fail2Ban on Ubuntu

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<span style="color: red; font-size: x-large;"><u><i>Protect your SSH server with Fail2Ban on Ubuntu</i></u></span></h1>
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<span style="font-family: Georgia, Times New Roman, serif;">The most suitable application for this purpose is <a href="http://www.hackthesec.co.in/search/label/fail2ban" target="_blank">Fail2ban</a>. It is an intrusion prevention framework written in the Python programming language. It is able to run on servers that have an interface to a packet-control system or firewall installed locally. </span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg_4cJIHMGOWul32IjKw2UHT3plp2cfuBxHMY7xFOQn3stZVpLEMarRug4jdQoFnKOtnQlH0Kr_lploQv_re2tP7msBLfE9WnJY18x5OiRKH12pqZMMiA_R0LCFsuAnMTxPffsoA6WjiHso/s1600/fail2ban_www.hackthesec.co.in.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg_4cJIHMGOWul32IjKw2UHT3plp2cfuBxHMY7xFOQn3stZVpLEMarRug4jdQoFnKOtnQlH0Kr_lploQv_re2tP7msBLfE9WnJY18x5OiRKH12pqZMMiA_R0LCFsuAnMTxPffsoA6WjiHso/s1600/fail2ban_www.hackthesec.co.in.jpg" /></a></div>
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<span style="font-family: Georgia, Times New Roman, serif;"><br /></span></div>
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<span style="font-family: Georgia, Times New Roman, serif;">It scans log files and bans IPs that show the malicious signs — too many password failures, seeking for exploits, etc. In order to install <a href="http://www.hackthesec.co.in/search/label/fail2ban" target="_blank">fail2ban</a>, make sure your repositories are up to date and simply install it:</span></div>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<strong style="background-color: white; color: #333333; font-family: Verdana, Geneva, sans-serif; font-size: 12px; line-height: 19.2px;"><span style="color: #0e0e0e; font-family: Open Sans, serif, sans-serif;"><span style="display: block; font-size: 18px; line-height: 1.4em; max-height: 200px;"></span></span></strong></div>
<ul style="text-align: left;">
<li><strong style="background-color: white; color: #333333; font-family: Verdana, Geneva, sans-serif; font-size: 12px; line-height: 19.2px;"><span style="color: #0e0e0e; font-family: Open Sans, serif, sans-serif;"><strong style="color: #333333; font-family: Verdana, Geneva, sans-serif; font-size: 12px; line-height: 19.2px;"><span style="color: #0e0e0e; font-family: Open Sans, serif, sans-serif;"><span style="font-size: 18px; line-height: 1.4em; max-height: 200px;"><a href="http://www.hackthesec.co.in/2016/01/how-to-protect-sshftp-using-fail2ban-on.html" target="_blank">How To Protect SSH/FTP using Fail2ban on CentOS/RHEL</a></span></span></strong></span></strong></li>
</ul>
<br />
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;"># apt-get update
# apt-get install fail2ban</span></code><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: inherit; padding: 0px; white-space: pre-wrap;">
</code></pre>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;">Once it is installed, it puts sample configuration file in <code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;"><b>/etc/fail2ban/jail.conf</b></code>. We should copy the sample configuration file in <code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;"><b>/etc/fail2ban/jail.local</b></code> and then, open file for editing using our favorite text editor. A lot of the possible services that we need to protect are already inside the sample file in their own section and there is DEFAULT section in the configuration file. Some of the important settings in this file are:</span></div>
<div style="background-color: #fdfdfd; box-sizing: border-box; margin-bottom: 10.5px;">
</div>
<ul style="text-align: left;">
<li><span style="font-family: Georgia, 'Times New Roman', serif; font-size: 18px; line-height: 28px;"><b>ignoreip</b>: We can use single IP, mask or a host name. IP addresses listed here will be excluded and will always be allowed to access the server</span></li>
<li><span style="font-family: Georgia, 'Times New Roman', serif; font-size: 18px; line-height: 28px;"><b>bantime</b>: The time (in seconds) that a host would be blocked from the server if they are found to be in violation of any of the defined rules. The default is set for 600 (10 minutes) but it should be increased to at least 30 minutes or more.</span></li>
<li><span style="font-family: Georgia, 'Times New Roman', serif; font-size: 18px; line-height: 28px;"><b>maxrwtry</b>: This is the number of incorrect login attempts allowed for a client before they get restricted to access the server. The default is 3.</span></li>
<li><span style="font-family: Georgia, 'Times New Roman', serif; font-size: 18px; line-height: 28px;"><b>backend</b>: specifies the backend used to get files modification. It should be set to “auto”</span></li>
<li><span style="font-family: Georgia, 'Times New Roman', serif; font-size: 18px; line-height: 28px;"><b>destemail</b>: We can specify email address where we want to get notifications when IP address is banned.</span></li>
</ul>
<br />
<div style="background-color: #fdfdfd; box-sizing: border-box; margin-bottom: 10.5px;">
</div>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;">Another important section is the ACTION section. Important lines here line describe that fail2ban will use the system iptables in order to ban suspicious IP address and specify the protocol that fail2ban will monitor which can be TCP or UDP.</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;">banaction = iptables-multiport
protocol = tcp</span></code><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: inherit; padding: 0px; white-space: pre-wrap;">
</code></pre>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;">After we are done with the default settings and actions, we should setup our <a href="http://www.hackthesec.co.in/search/label/SSH" target="_blank">SSH </a>section which will make sure that the SSH access to our server is secured. Here is how the <a href="http://www.hackthesec.co.in/search/label/SSH" target="_blank">SSH </a>section should look inside the configuration file:</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;">[ssh]

enabled  = true
port     = ssh
filter   = sshd
logpath  = /var/log/auth.log
maxretry = 4</span></code><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: inherit; padding: 0px; white-space: pre-wrap;">
</code></pre>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;">This section makes <a href="http://www.hackthesec.co.in/search/label/SSH_Security" target="_blank">SSH monitoring active</a>, it listens on the SSH port for the ssh dameon, using the log file location <code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;"><b>/var/log/auth.log</b></code> and bans the IP address of the user that will have 4 failed login attempts.</span></div>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;">After all settings are made, we need to restart fail2ban and it will start monitoring the log files according to our settings, you can also check which rules are currently applied with the following <b><code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;">iptables</code> </b>command.</span></div>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;">Fail2ban is one of the most widely used applications when it comes to server security. It is very simple and effective, so you shouldn’t think twice in order to decide to install it on your server. It is a must have application.</span></div>
<div style="background-color: #fdfdfd; box-sizing: border-box; font-size: 18px; line-height: 28px; margin-bottom: 10.5px;">
<span style="font-family: Georgia, Times New Roman, serif;"><a href="http://www.hackthesec.co.in/">www.hackthesec.co.in</a></span></div>
</div>
</div>

How to protect your SSH server with Fail2Ban on Ubuntu

Protect your SSH server with Fail2Ban on Ubuntu The most suitable application for this purpose is Fail2ban . It is an intrusion prevent...

Two-step Authentication For SSH On Centos 6

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<h1 class="entry-title" style="border: 0px; clear: both; font-family: Lato, sans-serif; font-size: 33px; line-height: 1.09091; margin: 0px 0px 12px; outline: 0px; padding: 0px; text-transform: uppercase; vertical-align: baseline;">
<span style="color: red;"><u><i>TWO-STEP AUTHENTICATION FOR SSH ON CENTOS 6</i></u></span></h1>
<div>
<span style="color: black;"><a href="http://code.google.com/p/google-authenticator/" style="background-color: white; border: 0px; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Google Authenticator</a><span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;"> implements TOTP (timebased one-time-password) security tokens from RFC6238 via the Google mobile app Google Authenticator. The Authenticator provides a six digit one-time password users must provide in addition to their username and password to login, sometimes branded “two-step authentication”. Here, we install and configure a pluggable authentication module (PAM) which allows login using one-time passcodes.</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi40xdu7C_0p2adgn-P278IjXylG1Np3bMARVFPqBxvRtTD4kPiTGPcx0B4ciBJST52hgQl4hU11YfHNBt5PN0MrCV5bfuU98qN5cd6Hr4Yb4eUB1Y1VMajsVrA-Iu86p-kS0cn8hSAkh93/s1600/TwoFactorAuthenticationWith-google_authenticator_www.hackthesec.co.in.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi40xdu7C_0p2adgn-P278IjXylG1Np3bMARVFPqBxvRtTD4kPiTGPcx0B4ciBJST52hgQl4hU11YfHNBt5PN0MrCV5bfuU98qN5cd6Hr4Yb4eUB1Y1VMajsVrA-Iu86p-kS0cn8hSAkh93/s1600/TwoFactorAuthenticationWith-google_authenticator_www.hackthesec.co.in.jpg" /></a></div>
<div>
<span style="color: black;"><span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;"><br /></span></span></div>
<div>
<ul style="background-color: white; display: block; font-family: 'Open Sans', serif, sans-serif; font-size: 18.6px; font-weight: 600; text-align: left; text-decoration: none;">
<li><span style="font-size: 18.6px;"><a href="http://www.hackthesec.co.in/2016/02/secure-ssh-with-google-authenticator.html" target="_blank"><span style="color: red;">Secure SSH with Google Authenticator Two-Factor Authentication on CentOS 7</span></a></span></li>
</ul>
</div>
<div>
<h3 style="border: 0px; clear: both; font-family: Lato, sans-serif; font-size: 22px; line-height: 24px; margin: 36px 0px 12px; outline: 0px; padding: 0px; vertical-align: baseline;">
<span style="color: #0b5394;"><u>Download and Install</u></span></h3>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
At the time of this writing, only an old version of <b>libpam-google-authenticator</b> is available in the <b><a href="http://www.hackthesec.co.in/2016/01/centos-rhel-scientific-linux-6-enable.html" target="_blank">EPEL package repository</a></b>. Hence, we are going to compile it from source. First, install prerequisites:</div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;">[root@hackthesec]# yum install </span><span style="color: lime;">make gcc pam-devel</span></span></span></pre>
</div>
<div>
<span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;">TOTP (timebased one-time-password) security tokens are time sensitive. Hence, make sure that your system has ntpd running, and is configured to start the service at boot:</span></div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> service </span><span style="color: orange;">ntpd start</span><span style="color: white;">

</span><span style="color: orange;">Starting </span><span style="color: white;">ntpd:                [  OK  ]

</span></span></span><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> </span><span style="color: lime;">chkconfig  </span><span style="color: white;">ntpd on</span></span></span></pre>
</div>
<div>
<span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><br /></span></span></div>
<div>
<span style="background-color: white;"><span style="font-family: Lato, sans-serif; font-size: large;"><span style="line-height: 24px;">Then download and install libpam-google-authenticator from source installation and compilation:</span></span></span></div>
<div>
<span style="font-family: 'Courier New', Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: lime;"><br /></span></span></span></div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"> cd /tmp

</span></span><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> </span><span style="color: lime;">wget </span><span style="color: white;">http://google-authenticator.googlecode.com/files/libpam-google-authenticator-1.0-source.tar.bz2</span></span></span></pre>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">
</span></span><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> </span><span style="color: lime;">bunzip2</span><span style="color: white;"> libpam-google-authenticator-1.0-source.tar.bz2
</span></span></span><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> </span><span style="color: lime;">tar xf</span><span style="color: white;"> libpam-google-authenticator-1.0-source.tar</span></span></span></pre>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">
</span></span><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> cd </span><span style="color: lime;">libpam-google-authenticator-1.0</span><span style="color: white;">

gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o google-authenticator.o google-authenticator.c
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o base32.o base32.c
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o hmac.o hmac.c
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o sha1.o sha1.c
gcc -g   -o google-authenticator google-authenticator.o base32.o hmac.o sha1.o  -ldl
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o pam_google_authenticator.o pam_google_authenticator.c
gcc -shared -g   -o pam_google_authenticator.so pam_google_authenticator.o base32.o hmac.o sha1.o -lpam
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o demo.o demo.c
gcc -DDEMO --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o pam_google_authenticator_demo.o pam_google_authenticator.c
gcc -g   -rdynamic -o demo demo.o pam_google_authenticator_demo.o base32.o hmac.o sha1.o  -ldl
gcc -DTESTING --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden        \
              -o pam_google_authenticator_testing.o pam_google_authenticator.c
gcc -shared -g   -o pam_google_authenticator_testing.so pam_google_authenticator_testing.o base32.o hmac.o sha1.o -lpam
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o pam_google_authenticator_unittest.o pam_google_authenticator_unittest.c
gcc -g   -rdynamic -o pam_google_authenticator_unittest pam_google_authenticator_unittest.o base32.o hmac.o sha1.o -lc  -ldl</span></span></span></pre>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: 'Courier New', Courier, monospace; font-size: medium; line-height: 15.6px;">[root@hackthesec]#</span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> </span><span style="color: lime;">make install</span><span style="color: white;">

cp pam_google_authenticator.so /lib64/security
cp google-authenticator /usr/local/bin</span></span></span></pre>
</div>
<div>
<h3 style="border: 0px; clear: both; font-family: Lato, sans-serif; font-size: 22px; line-height: 24px; margin: 36px 0px 12px; outline: 0px; padding: 0px; vertical-align: baseline;">
<span style="color: #0b5394;"><u>Set Up Google Authenticator</u></span></h3>
<div style="border: 0px; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
Before configuring SSH, first set up Google Authenticator. Run “google-authenticator” <span style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: inherit; font-style: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">as the user you wish to log in with via SSH</span>. You will be prompted with a few questions.</div>
</div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;">[root@hackthesec]# </span><span style="color: orange;">google-authenticator</span><span style="color: white;">

Do you want me to update your "~/.google_authenticator" file (y/n) y
 
https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/user@server%3Fsecret%3DABCD12E3FGHIJKLMN
Your new secret key is: ABCD12E3FGHIJKLMN
Your verification code is 98765432
Your emergency scratch codes are:
  01234567
  89012345
  67890123
  45678901
  23456789
 
Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases
your chances to notice or even prevent man-in-the-middle attacks (y/n) y
 
By default, tokens are good for 30 seconds and in order to compensate for
possible time-skew between the client and the server, we allow an extra
token before and after the current time. If you experience problems with poor
time synchronization, you can increase the window from its default
size of 1:30min to about 4min. Do you want to do so (y/n) y
 
If the computer that you are logging into isn't hardened against brute-force
login attempts, you can enable rate-limiting for the authentication module.
By default, this limits attackers to no more than 3 login attempts every 30s.
Do you want to enable rate-limiting (y/n) y</span></span></span></pre>
</div>
<div>
<span style="font-family: 'Courier New', Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: lime;"><br /></span></span></span></div>
<div>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
These settings are stored in the user’s <code style="border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>~/.google_authenticator</b></code> file.</div>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
Copy and paste the URL into your browser and scan the QR code that is displayed with the app Google Authenticator on your mobile device. If you can’t scan the QR code, then you can enter the information manually with the given secret key and verification code. A new verification code should be displayed every 30 seconds.</div>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
Emergency one-time use verification codes are also given for you to write down in a secure place in case you were to not have your mobile device with you.</div>
<h3 style="border: 0px; clear: both; font-family: Lato, sans-serif; font-size: 22px; line-height: 24px; margin: 36px 0px 12px; outline: 0px; padding: 0px; vertical-align: baseline;">
<u><span style="color: #0b5394;">Configure PAM</span></u></h3>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
Have PAM require Google Authenticator for SSH authentication. Modify<b> <code style="border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">/etc/pam.d/sshd</code></b> and add the line “<code style="border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>auth required pam_google_authenticator.so</b></code>” at the top.</div>
</div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;">[root@hackthesec]# vi </span><span style="color: lime;">/etc/pam.d/sshd</span><span style="color: white;">

#%PAM-1.0
</span><span style="color: orange;">auth       required     pam_google_authenticator.so
</span><span style="color: white;">auth       required     pam_sepermit.so
auth       include      password-auth
account    required     pam_nologin.so
account    include      password-auth
password   include      password-auth
# pam_selinux.so close should be the first session rule
session    required     pam_selinux.so close
session    required     pam_loginuid.so
# pam_selinux.so open should only be followed by sessions to be executed in the user context
session    required     pam_selinux.so open env_params
session    optional     pam_keyinit.so force revoke
session    include      password-auth</span></span></span></pre>
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<div>
<div style="border: 0px; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
This will require all users to use Google Authenticator for SSH authentication. To only require those users with Google Authenticator configured for their account (the <code style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>~/.google_authenticator</b></code> file exists), then instead enter “<code style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>auth required pam_google_authenticator.so</b></code><code style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; font-weight: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"> </code><code style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>nullok</b></code>“.</div>
<div style="border: 0px; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
The order in which you place items in this file matters. Given this configuration, you will first be prompted for your Google Authenticator verification code, then for your system account password when you SSH into the system.</div>
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<h3 style="border: 0px; clear: both; font-family: Lato, sans-serif; font-size: 22px; line-height: 24px; margin: 36px 0px 12px; outline: 0px; padding: 0px; vertical-align: baseline;">
<span style="color: #0b5394;"><u>Configure the SSH Service</u></span></h3>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
Modify <b><code style="border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">/etc/ssh/sshd_config</code>.</b> Verify these settings:</div>
</div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="color: lime;"><span style="line-height: 15.6px;">[root@hackthesec]# vi /etc/ssh/sshd_config

PasswordAuthentication yes
</span></span><span style="color: white; line-height: 15.6px;">ChallengeResponseAuthentication yes
</span><span style="color: lime; line-height: 15.6px;">UsePAM yes</span></span></pre>
</div>
<div>
<span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;">Restart the SSH service:</span></div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="background-color: transparent; line-height: 15.6px;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;">[root@hackthesec]# </span></span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;">service </span><span style="color: lime;">sshd restart</span></span></span></pre>
</div>
<div>
<span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;">When you SSH into the system as a user configured for Google Authenticator, you will have to enter the verification code that is displayed in you Google Authenticator app, and then by your system password at the next prompt:</span></div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;">login as: root
</span><span style="color: orange;">Verification code: 002536
</span><span style="color: white;">Password: *******
[root@hackthesec]#</span></span></span></pre>
</div>
<div>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
If you have any problems, look in the <code style="border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>/var/log/secure</b></code> system log file.</div>
<div style="border: 0px; color: #2b2b2b; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
If you have <b>SELinux </b>enabled, you may not be able to login, and get this error in <code style="border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>/var/log/secure</b></code>:</div>
</div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">Feb  26 23:42:50 hostname sshd(pam_google_authenticator)[1654]: Failed to update secret file "/home/hackthesec/.google_authenticator"
Feb  26 23:42:50 hostname  sshd[1652]: error: PAM: Cannot make/remove an entry for the specified session for username from 192.168.0.5</span></span></pre>
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<span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><br /></span></span></div>
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<div style="border: 0px; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
This is probably due /home/username/.google_authenticator not having an appropriate <a href="http://wiki.centos.org/HowTos/SELinux" style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: inherit; font-style: inherit; font-weight: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Type Enforcement</a> (TE):</div>
<div style="-webkit-text-stroke-width: 0px; color: black; font-family: 'Times New Roman'; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 1; word-spacing: 0px;">
</div>
<br />
<div style="orphans: auto; text-align: left; text-indent: 0px; widows: 1;">
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin: 0px 0px 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="background-color: transparent; line-height: 15.6px;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;">[root@hackthesec]#</span></span><span style="font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: white;"> </span><span style="color: lime;">ls -Z /home/hackthesec/.google_authenticator</span><span style="color: white;">
-r--------. hackthesec hackthesec unconfined_u:object_r:user_home_t:s0 /home/hackthesec/.google_authenticator</span></span></span></pre>
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<h3 style="border: 0px; clear: both; font-family: Lato, sans-serif; font-size: 22px; line-height: 24px; margin: 36px 0px 12px; outline: 0px; padding: 0px; vertical-align: baseline;">
<span style="color: red;"><u>Skip Google Authenticator Authentication if Logging in from the Local Network</u></span></h3>
<div style="border: 0px; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px; margin-bottom: 24px; outline: 0px; padding: 0px; vertical-align: baseline;">
You may trust systems on you local network enough not not require that SSH connections from them use <b>Google Authenticator</b>. If so, modify <code style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; font-family: monospace, serif; font-size: 15px; font-style: inherit; line-height: 1.6; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>/etc/pam.d/sshd</b></code> so that it looks like this:</div>
</div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;">auth [success=1 default=ignore] pam_access.so accessfile=/etc/security/access-local.conf
auth       required     pam_google_authenticator.so</span></span></pre>
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<div>
<span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;">Then add the file </span><code style="background-color: white; border: 0px; font-family: monospace, serif; font-size: 15px; line-height: 24px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><b>/etc/security/access-local.conf</b></code><span style="background-color: white; font-family: Lato, sans-serif; font-size: 16px; line-height: 24px;"> with the contents:</span></div>
<div>
<span style="font-family: 'Courier New', Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: lime;"><br /></span></span></span></div>
<div>
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"># Google Authenticator can be skipped on local network
+ : ALL : 192.168.0.0/24
+ : ALL : LOCAL
- : ALL : ALL</span></span></pre>
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<span style="color: white; font-family: Courier New, Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><br /></span></span></div>
<div>
<span style="font-family: 'Courier New', Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><a href="http://www.hackthesec.co.in/">www.hackthesec.co.in</a></span></span></div>
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<span style="font-family: 'Courier New', Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: lime;"><br /></span></span></span></div>
<div>
<span style="font-family: 'Courier New', Courier, monospace; font-size: medium;"><span style="line-height: 15.6px;"><span style="color: lime;"><br /></span></span></span></div>
</div>

Two-step Authentication For SSH On Centos 6

TWO-STEP AUTHENTICATION FOR SSH ON CENTOS 6 Google Authenticator implements TOTP (timebased one-time-password) security tokens from RFC...

Secure SSH with Google Authenticator Two-Factor Authentication on CentOS 7

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<h1 style="border-bottom-color: rgb(195, 202, 213); border-bottom-style: solid; border-bottom-width: 1px; color: #314d7c; font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 26px; margin: 0px 0px 15px; padding: 20px 0px;">
Google Authenticator Two-Factor Authentication on CentOS 7</h1>
<span style="background-color: white; font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">SSH access is always critical and you might want to find ways to improve the security of your SSH access. In this article we will see how we can secure SSH with simple two factor authentication by using Google Authenticator. Before using it you have to integrate the SSH daemon on your server with Google Authenticator one time password protocol TOTP and another restriction is that you must have your android phone with you all the time or at least the time you want <b>SSH access</b></span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEituG_vrQj2qbbwgkfPTj-hr01-z_SbSoir2VjHmPUmfztQDCX-xzh9UoCT-i_CI0PuUB6UeR69vAnWiVDbVFIo39trNQWXj3s743gRcnt_8tiuJyUV9pcfaOrbiM7g1_Z6vkAz7FYmH-jO/s1600/google-authenticator-www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEituG_vrQj2qbbwgkfPTj-hr01-z_SbSoir2VjHmPUmfztQDCX-xzh9UoCT-i_CI0PuUB6UeR69vAnWiVDbVFIo39trNQWXj3s743gRcnt_8tiuJyUV9pcfaOrbiM7g1_Z6vkAz7FYmH-jO/s320/google-authenticator-www.hackthesec.co.in.png" width="179" /></a></div>
<span style="background-color: white; font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;"><br /></span>
<span style="background-color: white; font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;"><br /></span>
<span style="background-color: white; font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">First of all we will install the open source <b>Google Authenticator PAM module</b> by executing the following command on the shell.</span><br />
<div style="background-color: white;">
<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="line-height: 15.6px;"><span style="color: white;">yum install </span><span style="color: lime;">google-authenticator</span></span></span></pre>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">This command will install Google authenticator on you <a href="http://www.hackthesec.co.in/search/label/centos7" target="_blank">Centos 7</a> Server. The next step is to get the verification code. It's a very simple command to get the verification code and scratch codes by just answering simple questions of server which he will ask you. You can do that step by running the following command:</span></div>
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<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); line-height: normal; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: cyan; font-family: Courier New, Courier, monospace; font-size: large;"><span style="line-height: 15.6px;">google-authenticator</span></span></pre>
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<span style="color: white; font-family: Courier New, Courier, monospace;"><span style="line-height: 15.6px;"><br /></span></span></div>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">You will get an output like the following screenshot which is being displayed to help you step by step as this step is very important and crucial. Write down the emergency scratch codes somewhere safe, they can only be used one time each, and they're intended for use if you lose your phone.</span></div>
<div style="font-family: Verdana, Geneva, sans-serif; font-size: 12px; line-height: 19.2px;">
<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;"><br /></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgRa0sksNiUe3yuPtSYG_BLap7nPIgsV6_P0q-6kVhQCFK3dChx6VkIRhdC-C-WIE1RQCHRQVm5H8NVrwtwGfpfH1htsLoIRq58e8IS9GhJE9NCXnFJJoPj6AWfRKTKfa9IMZSthekDMbzE/s1600/ssh_two_factor_authentication-www.hackthesec.co.in.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgRa0sksNiUe3yuPtSYG_BLap7nPIgsV6_P0q-6kVhQCFK3dChx6VkIRhdC-C-WIE1RQCHRQVm5H8NVrwtwGfpfH1htsLoIRq58e8IS9GhJE9NCXnFJJoPj6AWfRKTKfa9IMZSthekDMbzE/s1600/ssh_two_factor_authentication-www.hackthesec.co.in.jpg" /></a></div>
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Now download Google authenticator application on your Mobile phone, the app exists for Android and Iphone. Well I have Android so I will download it from Google Play Store where I searched it out just by typing "<b>google authenticator</b>".</div>
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The next step is to change some files which we will start by first changing<b> /etc/pam.d/sshd</b>. Add the following line to the bottom of line:</div>
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<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="line-height: 15.6px;"><span style="color: white;">auth required pam_</span><span style="color: lime;">google_authenticator.so</span></span></span></pre>
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<span style="color: white; font-family: Courier New, Courier, monospace;"><span style="line-height: 15.6px;"><br /></span></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFXc2rN8O_YBWlLUgUrTPMcjlPm0Pa6x27jFRvQH1STaXipAVhisNBIQzJwSDiNJPkM5ALSbNj4y2JnOgHxe3zsimBEAVGyT0mpZ8YoN6pYGryQy6lCTtmylqK_dAnFLYkuX95JSIop3sm/s1600/ssh_two_factor_authentication2-www.hackthesec.co.in.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFXc2rN8O_YBWlLUgUrTPMcjlPm0Pa6x27jFRvQH1STaXipAVhisNBIQzJwSDiNJPkM5ALSbNj4y2JnOgHxe3zsimBEAVGyT0mpZ8YoN6pYGryQy6lCTtmylqK_dAnFLYkuX95JSIop3sm/s1600/ssh_two_factor_authentication2-www.hackthesec.co.in.jpg" /></a></div>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">Change the next file which is <b>/etc/ssh/sshd_config</b>. Add the following line in the file and if its already placed then change the parameter to "yes":</span></div>
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<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="line-height: 15.6px;"><span style="color: orange;">ChallengeResponseAuthentication </span><span style="color: white;">yes</span></span></span></pre>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEikQFBGqygMhqOpxkF1AXZFqfPYK6kcwT7Y5xtZTdk5ulwiYnmpAKpDBkMDvAGUX2-YGvZEAJzWxqtgRd8A9ypImFboal5LMOVzXgFy_sl-4UY5ZJrVFyuSCTnr7bggeQRKen21a0hARb9d/s1600/ssh_two_factor_authentication3-www.hackthesec.co.in.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEikQFBGqygMhqOpxkF1AXZFqfPYK6kcwT7Y5xtZTdk5ulwiYnmpAKpDBkMDvAGUX2-YGvZEAJzWxqtgRd8A9ypImFboal5LMOVzXgFy_sl-4UY5ZJrVFyuSCTnr7bggeQRKen21a0hARb9d/s1600/ssh_two_factor_authentication3-www.hackthesec.co.in.jpg" /></a></div>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;"><br /></span></div>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">Now restart the service of ssh by the following command:<br /></span></div>
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<pre style="background-attachment: scroll; background-clip: initial; background-color: #3c3c3c; background-image: none; background-origin: initial; background-position: 0px 0px; background-repeat: repeat; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="line-height: 15.6px;"><span style="color: white;">service </span><span style="color: lime;">sshd </span><span style="color: white;">restart</span></span></span></pre>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">Last step is to test the service by connecting with SSH to the server to see if it will require verification code. You can see the following screenshot which shows the verification code(FROM MY ANDROID MOBILE GOOGLE Authenticator App) that keeps on changing time after time and you have to login with it:<br /></span></div>
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<span style="font-family: Tahoma, Helvetica, Arial, sans-serif; font-size: 14px; line-height: 17.92px;">So we have successfully configured SSH authentication based on <b>Google Authenticator</b>. Now your <b>SSH </b>is secure and no brute attack can invade your server unless someone has your verification code which will require access to your phone as well.</span></div>
<div style="font-family: Verdana, Geneva, sans-serif; font-size: 12px; line-height: 19.2px;">
<br /><a href="http://www.hackthesec.co.in/">www.hackthesec.co.in</a></div>
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Secure SSH with Google Authenticator Two-Factor Authentication on CentOS 7

Google Authenticator Two-Factor Authentication on CentOS 7 SSH access is always critical and you might want to find ways to improve the s...

How to Hide Application Port Using knockd in Linux

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<h1 class="post-title single" style="background-color: white; margin: 0px 0px 5px; padding: 0px;">
<span style="color: red; font-size: large;"><u><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="letter-spacing: -1px; line-height: 1.2em;">Hide </span><span style="letter-spacing: -1px; line-height: 38.4px;">Application</span><span style="letter-spacing: -1px; line-height: 1.2em;"> Port Using knockd in Linux</span></span></u></span></h1>
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As a system administrator, we should do everything to secure our server from attackers. As the internet grows, threats to our server is also growing. One of the popular entrances to attack our server is through the port on your server that open. If your SSH server is running on </div>
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your machine, then usually the SSH port is listening. Which means it is open, waiting for the connection.<br />
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Leaving the port open for 24 hours, is not recommended because it is vulnerable. Because we can scan the machine to see the open port. <strong>Nmap</strong> is one of the most popular port scanner that can be used by anyone to scan your machine.</div>
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How if we can open the on demand and close the port when it’s not used? Sounds interesting. Now we can do it using knockd application.</div>
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<span style="color: #0b5394;"><u>What is knockd</u></span></h1>
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Knockd is a port-knock server. It listens to all traffice on an ethernet (or PPP) interface, looking for special “knock” sequences of port-hits. (Source : <a href="http://www.zeroflux.org/projects/knock" style="outline: none; text-decoration: none;" title="knockd">http://www.zeroflux.org/projects/knock</a>)</div>
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<u><span style="color: #0b5394;">How it works</span></u></h1>
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Every application needs a port as a “door” for “listening” requests from other clients. This port usually on open state or close state. There are a lot of ports that available on the server. But there are some ports that agreed by consensus, such as <a href="http://www.hackthesec.co.in/search/label/SSH" target="_blank">SSH </a>(22), Web (80) and <a href="http://www.hackthesec.co.in/search/label/ftp" target="_blank">FTP </a>(21).</div>
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A basic rule of server security is to open only used ports and close the rest. You may have some ports that are sometimes used and sometimes not. Leaving those ports open while is not being used is not recommended.</div>
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When you install knockd, you can let the client “knock” the server with pattern. The knocking sequence can be custom by you. So this knocking pattern will be unique to each other. If the pattern is match, then the port you need will be opened for a period of time and the request can enter your server.</div>
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Once you have done with the application, you can close the port manually or automatically.</div>
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<span style="color: #0b5394;"><u>How to install knockd</u></span></h1>
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On this article, we are using Zorin 9 OS which based on Ubuntu 14.04 LTS. If you are using another distribution please adjust it to the installation method of your distribution.</div>
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Knockd is available on Ubuntu repository. Then we can use <b>apt-get to install knockd</b>.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">$ sudo </span><span style="color: lime;">apt-get</span><span style="color: white;"> install </span><span style="color: lime;">knockd</span></span></span></pre>
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Just wait for a few minutes, then your knockd is already setup.</div>
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<span style="color: #0b5394;"><u>Configure knockd</u></span></h1>
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Knock configuration file is located in <strong>/etc/knockd.conf</strong><br />
The sample configuration is simple and easy to understand.</div>
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We can see that the configuration is divided into three sections. The <strong>[options]</strong> section, <strong>[openSSH]</strong> section to open SSH port and <strong>[closeSSH]</strong> section to close SSH port.</div>
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By default <strong>[options]</strong> section contain only 1 row. It tell us that the knockd log will be recorded using the operating system log application. On Ubuntu, we will see the log in <strong>/var/log/syslog</strong>; folder.</div>
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Of course we can choose not to use SysLog. We can change it into this line, if we want to use custom log.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">logfile = /var/log/</span><span style="color: lime;">knockd.log</span></span></span></pre>
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The above line, will put knock log file in <strong>/var/log/knockd.log</strong></div>
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[openSSH] sections has identical commands with [closeSSH] section.</div>
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<strong>sequence = 1200,1300,1400</strong><br />
This is the knock pattern. It will trigger the command below in the section. The value of this parameter is fully customized. We can choose another random number.</div>
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<strong>seq_timeout = 10</strong><br />
This will tell knockd about how long the knock pattern must be completed in.</div>
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<strong>command = /sbin/iptables -I INPUT -s %IP% -p tcp --dport 22 -j ACCEPT</strong><br />
This parameter will open SSH port on port 22</div>
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<strong>tcpflags = syn</strong><br />
This parameter indicates that the Client will sends a TCP SYNchronize packet to the server</div>
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<span style="color: #0b5394;"><u>Setting up the firewall</u></span></h1>
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As we know before, knockd will open particular port temporarily. So we have to make sure firewall is running on the server. Basically, we will close all ports. We are using iptables syntax to do it. Here are the steps.</div>
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The 1st command, will allow the current on-going session through firewall.<br />
The 2nd command, will allow the server is able to ping by another machine.<br />
The 3rd command, will reject every requests.</div>
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To test the knockd service, we expect that our will firewall will drop all ssh connection. Then knockd will open it temporarily on demand.</div>
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<span style="color: #0b5394;"><u>Test the knockd on server side</u></span></h1>
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Once the firewall and knockd are setup, next we can test them.<br />
To test the firewall, try remote the server via SSH from another machine. (<em>on this article, client IP is 10.1.6.14 and server IP 10.0.76.224</em>)</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">$ ssh -l </span><span style="color: lime;">pungki </span><span style="color: white;">10.0.76.224</span></span></span></pre>
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With :<br />
<div style="font-size: 14px;">
<strong>-l</strong> = login name</div>
<div style="font-size: 14px;">
<strong>pungki</strong> = the user name on the destination server</div>
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If the firewall works, then we will get <strong>“Connection refused”</strong> error message.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjfRafJqFuFGUE3uaD6U5pfEqKmV8CwgJAc_b6-XdxHSaAoThuavWut-Q8G8Tm590Evw3g9mQkxHS9Zer5iE6Mme7HRMnyKw9fLpuObT6APc5xAgHKO44mQERwZwVKov6nZ-9RU0t_BL04u/s1600/connection_refused_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjfRafJqFuFGUE3uaD6U5pfEqKmV8CwgJAc_b6-XdxHSaAoThuavWut-Q8G8Tm590Evw3g9mQkxHS9Zer5iE6Mme7HRMnyKw9fLpuObT6APc5xAgHKO44mQERwZwVKov6nZ-9RU0t_BL04u/s1600/connection_refused_www.hackthesec.co.in.png" /></a></div>
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The reason we get the error message is the 10.1.6.14 is not allowed to enter the server. If we use this command, we will see no result.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">$ </span><span style="color: lime;">sudo /sbin/iptables -L</span><span style="color: white;"> -n |grep 10.1.6.14</span></span></span></pre>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhlW_3AQMtYbLz_q36ogYB6i0YJjh4ge86K41UwVbalkoKmgBrhnHVhob46hi21H8dr7xaHH_b1G4gc3LK4fwoWEYVtbPg6LhccN3CDsm2CxXbxQJT3HCRSYXNN36U9COiy7LuHd9-2wmmC/s1600/iptables_before_knock_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhlW_3AQMtYbLz_q36ogYB6i0YJjh4ge86K41UwVbalkoKmgBrhnHVhob46hi21H8dr7xaHH_b1G4gc3LK4fwoWEYVtbPg6LhccN3CDsm2CxXbxQJT3HCRSYXNN36U9COiy7LuHd9-2wmmC/s1600/iptables_before_knock_www.hackthesec.co.in.png" /></a></div>
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Later, we will see the difference after knockd implemented.</div>
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Next step, is to test the knockd service.</div>
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To run knockd, we need to change knockd default file which located in /etc/default/knockd. Change the value of <strong>START_KNOCKD parameter from 0 to 1</strong>.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaZe0IJykHyWEfqgWsgqhoIjObun1UVhtKQaU8RUIYT6aK8Bf6Y0TVRZgvWb1oYiLvL2oMsJq0UJYuFczFGTS83J7Z-zJqdSX5VvoRNYTCfkCNoOFfVhU5m7dTzPWSKiNZij9_HvXRUjhT/s1600/default_knockd_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaZe0IJykHyWEfqgWsgqhoIjObun1UVhtKQaU8RUIYT6aK8Bf6Y0TVRZgvWb1oYiLvL2oMsJq0UJYuFczFGTS83J7Z-zJqdSX5VvoRNYTCfkCNoOFfVhU5m7dTzPWSKiNZij9_HvXRUjhT/s1600/default_knockd_www.hackthesec.co.in.png" /></a></div>
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Save the file. Then type :</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">$ sudo </span><span style="color: lime;">service knockd</span><span style="color: white;"> start</span></span></span></pre>
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*<em>note : I tried to run the service using /etc/init.d/knockd start , but it always fail to start</em></div>
<h1 style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 20pt; letter-spacing: -1px; line-height: 1.2em; margin: 0px 0px 15px; padding: 0px;">
<span style="color: #0b5394;"><u>Test knockd from Client side</u></span></h1>
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On the client side, we need knock client to “knock” the server. On the client side, we use Centos 5.2. Then we install knock-client from <a href="http://pkgs.repoforge.org/knock/knock-0.5.3.el5.rf.i386.rpm" style="outline: none; text-decoration: none;" title="knock client">http://pkgs.repoforge.org/knock/knock-0.5.3.el5.rf.i386.rpm</a></div>
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Then run the command below to knock the server :</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;">$ knock -v 10.0.76.224 1200 1300 1400</span></span></pre>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiT-KXni4C1INDyp91QdgH7yvG91kZqKOEcYMcWhZdz5fsUlTNUldddqAee8sHvviRBFJBgYewr2odXHyNyXjcTQ_5-bCUELvpd05hsNe0CItVvTYgegGHi5rhxYso1PmqIB7Mnxcn_nBYI/s1600/knock_open_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiT-KXni4C1INDyp91QdgH7yvG91kZqKOEcYMcWhZdz5fsUlTNUldddqAee8sHvviRBFJBgYewr2odXHyNyXjcTQ_5-bCUELvpd05hsNe0CItVvTYgegGHi5rhxYso1PmqIB7Mnxcn_nBYI/s1600/knock_open_www.hackthesec.co.in.png" /></a></div>
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<strong>-v</strong> = verbose<br />
<strong>10.0.76.224</strong> = Server IP<br />
<strong>1200 1300 1400</strong> = knock sequence which defined in the knockd configuration</div>
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After knock the server, now we will see that the client IP is now allowed to enter the server.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">$ sudo /sbin/</span><span style="color: lime;">iptables </span><span style="color: white;">-L -n |grep 10.1.6.14</span></span></span></pre>
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Then we can run SSH to remote the server.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjINYEuu7LmyDs7fvdRsVd38bC5K8Z15OTpi7ZNei7wNDzoWW4jSmV9023OiY4aI0ZfDP5XMeFreENSehDFjnRlBCHQCJjj-hoa-5vx_Bk51mmIEIJYJMT5E9phhW3X_GL67zhkaYP9UPtZ/s1600/ssh_success_after_knock_1_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjINYEuu7LmyDs7fvdRsVd38bC5K8Z15OTpi7ZNei7wNDzoWW4jSmV9023OiY4aI0ZfDP5XMeFreENSehDFjnRlBCHQCJjj-hoa-5vx_Bk51mmIEIJYJMT5E9phhW3X_GL67zhkaYP9UPtZ/s1600/ssh_success_after_knock_1_www.hackthesec.co.in.png" /></a></div>
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As we can see on the above image, the host name of are different. After SSH to the remote machine established then the host name is changed from @web01 into @dev-machine.</div>
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<span style="color: #0b5394;"><u>Close the port</u></span></h1>
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After the client done to remote the server, the client need to close the port. To do that, we can use the command below :</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"><span style="color: white; font-size: small;"><span style="font-size: large; line-height: 15.6px;"><span style="color: white;">$ </span><span style="color: lime;">knock </span><span style="color: white;">-v 10.0.76.224 1400 1300 1200</span></span></span></pre>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjseKC2KePBKaJufOpIsrsJfwjiU8L08ZMVOMFPEEGBsIcRr-pUpfbtcDurAkGDY-0XYN_hGxf1JUPlAnPg7hF4wVCI7dG1q3-nnq6sWwkUN_a8IKq3OG3nibdSX93H5a_QdfGOrQILERhP/s1600/knock_close_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjseKC2KePBKaJufOpIsrsJfwjiU8L08ZMVOMFPEEGBsIcRr-pUpfbtcDurAkGDY-0XYN_hGxf1JUPlAnPg7hF4wVCI7dG1q3-nnq6sWwkUN_a8IKq3OG3nibdSX93H5a_QdfGOrQILERhP/s1600/knock_close_www.hackthesec.co.in.png" /></a></div>
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<em><strong>Please be careful</strong>, to close the port, we put the knock sequence in the opposite order.</em></div>
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After knock the server, we will see a prompt again. To check if the knock was success, we use iptables command again. If it was success, we will see that IP 10.1.6.14 will disappeared.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg10Hwu4L6yAHomwQ2jSZ3ZCHYjJfWKC431IPW1b-qU2qyrUw3WGasrdHXFk5gvu0rfzXlobAOaA63L9_blX2lc6Dv3dztXyMRuquceOPguOa_G2UD32-rEM-a-Ltt8hKUU8oQZ7BXBB8l-/s1600/iptables_after_knock_closed_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg10Hwu4L6yAHomwQ2jSZ3ZCHYjJfWKC431IPW1b-qU2qyrUw3WGasrdHXFk5gvu0rfzXlobAOaA63L9_blX2lc6Dv3dztXyMRuquceOPguOa_G2UD32-rEM-a-Ltt8hKUU8oQZ7BXBB8l-/s1600/iptables_after_knock_closed_www.hackthesec.co.in.png" /></a></div>
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On the previous one, after knock to open the port, we saw that the client IP Address - 10.1.6.14 - is allowed to enter the server by firewall. Now, after we knock to close the port, if we check with the same iptables command, the rule was deleted.</div>
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<span style="color: #0b5394;"><u>Close the port Automatically</u></span></h1>
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Since the close port activity is triggered by client, we will have the possibility that the client forget to close the port. We don’t want it to happen. So we can configure knockd to close the port automatically.</div>
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In order to do that, <strong>we need to customize</strong> the knockd configuration file. Here is the sample of modified knockd configuration file.</div>
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The command still looks identical. The difference with the previous configuration is we put [openSSH] section and [closeSSH] section in the same block.</div>
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Then we add <strong>cmd_timeout = 10</strong> line to tell the server <strong>to execute the stop_command 10 seconds after start_command is executed</strong>. The port will be automatically closed, but the established connection remain connected.</div>
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How to Hide Application Port Using knockd in Linux

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