Hack The Sec-Leading Resource Of Linux Tutorial: SSH

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<span style="color: #990000;"><u><i>Deny/Allow Access for Users & Groups in OpenSSH</i></u></span></h2>
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<span style="font-family: Georgia, Times New Roman, serif;">For security purposes many times we required to restrict or allow for SSH access for specific Users or Groups. To make any changes edit <b>OpenSSH </b>configuration file <span style="color: red;">/etc/ssh/sshd_config</span> and do required changes for allowing or denying any user or group.</span></div>
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<span style="color: #0b5394;">Allow/Deny Users and Groups:</span></h2>
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<span style="font-family: Georgia, Times New Roman, serif;">To allow or deny any user or group on OpenSSH, first edit configuration file<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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><span style="color: red;">/etc/ssh/sshd_config</span></span> in your favorite editor and do changes as following examples.</span></div>
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<span style="font-family: Georgia, Times New Roman, serif;"><u><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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><span style="color: #0b5394;">1. Deny Users:</span></span> </u>To restrict for block specific user for SSH on server add the following rules. For example to restrict users raj, john and sarah.</span></div>
<pre class="pretty" style="background: none 0px 0px repeat scroll rgb(240, 240, 240); border-left-color: rgb(116, 183, 63); border-left-width: 3px; border-radius: 5px; border-style: none none none solid; line-height: 1.5em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 8px 20px 8px 18px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><orange><span style="color: #990000;">DenyUsers</span></orange>  raj  john  sarah
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<span style="font-family: Georgia, Times New Roman, serif;"><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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><span style="color: #0b5394;"><u>2. Deny Groups:</u></span></span> In place of restricting specific users we can also restrict groups in OpenSSH. For example to restrict support and marketing groups from ssh add following rules.</span></div>
<pre class="pretty" style="background: none 0px 0px repeat scroll rgb(240, 240, 240); border-left-color: rgb(116, 183, 63); border-left-width: 3px; border-radius: 5px; border-style: none none none solid; line-height: 1.5em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 8px 20px 8px 18px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="color: #990000;"><orange>DenyGroups</orange>  </span>support  marketing</span><span style="font-size: 0.9em;">
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<span style="font-family: Georgia, Times New Roman, serif;"><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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><span style="color: #0b5394;"><u>3. Allow Users:</u></span></span> By default all users are allowed to login to OpenSSH server. But we can specifically allow any user, For example hackthesec and linuxos belongs to support group and we already have restricted that group members. So we can specify that users in Allow users to allow them specifically.</span></div>
<pre class="pretty" style="background: none 0px 0px repeat scroll rgb(240, 240, 240); border-left-color: rgb(116, 183, 63); border-left-width: 3px; border-radius: 5px; border-style: none none none solid; line-height: 1.5em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 8px 20px 8px 18px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="color: #990000;"><orange>AllowUsers</orange>  </span>hackthesec  linuxos
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<span style="font-family: Georgia, Times New Roman, serif;"><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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><span style="color: #0b5394;"><u>4. Allow Groups:</u></span></span> Similarly we can allow all the members of specific group which may be denied in any other group. For example user andrew and peter belongs to marketing group as well as admin group. So we can allow access to both users by allowing admin group.</span></div>
<pre class="pretty" style="background: none 0px 0px repeat scroll rgb(240, 240, 240); border-left-color: rgb(116, 183, 63); border-left-width: 3px; border-radius: 5px; border-style: none none none solid; line-height: 1.5em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 8px 20px 8px 18px; vertical-align: baseline;"><span style="font-family: Courier New, Courier, monospace; font-size: large;"><span style="color: #990000;"><orange>AllowGroups</orange>  </span>admin
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<span style="color: #0b5394;"><u>Restart OpenSSH:</u></span></h2>
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<span style="font-family: Georgia, Times New Roman, serif;">After doing any changes in configuration file, make sure to restart <b>OpenSSH </b>service as following.</span></div>
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or
</span><span style="color: lime;"># systemd restart sshd</span></span></pre>
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How to Deny/Allow Access for Users & Groups in OpenSSH

Deny/Allow Access for Users & Groups in OpenSSH For security purposes many times we required to restrict or allow for SSH access fo...

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<span style="color: #990000;">Protect SSH/FTP using Fail2ban on <span style="color: #990000;"><a href="http://www.hackthesec.co.in/search/label/Centos" target="_blank">CentOS</a>/<a href="http://www.hackthesec.co.in/search/label/RHEL" target="_blank">RHEL</a></span></span></h2>
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<span style="background-color: white; border: 0px; font-family: 'Roboto Slab'; font-size: 16px; font-weight: 700; line-height: 28px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Fail2ban</span><span style="background-color: white; font-family: 'Roboto Slab'; font-size: 16px; line-height: 28px;"> is very useful application for you, if you are managing security of server, or you are running your own vps or physical server. Fail2ban scan log files created on system and has ability to ban ips which found malicious bassed on configuration rules. We can use it for monitoring various system services logs like Apache, SSH and blog the ips which are trying to breach the system’s security.</span><br />
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<span style="background-color: white; color: #0b5394; font-family: 'Roboto Slab'; font-size: 24px; font-weight: bolder; line-height: 34px;">Step 1: Install EPEL RPM Repository</span><br />
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<span style="font-size: large;"><span style="line-height: 28px;">Fail2ban is available in <a href="http://www.hackthesec.co.in/2016/01/how-to-install-epel-repo-on-centos-and.html" target="_blank">EPEL</a> repository, So make sure you have installed <a href="http://www.hackthesec.co.in/2016/01/how-to-install-epel-repo-on-centos-and.html" target="_blank">EPEL</a> repository in your system or you can use </span><a href="http://www.hackthesec.co.in/2016/01/how-to-install-epel-repo-on-centos-and.html" style="line-height: 28px; outline: none; text-decoration: none;" target="_blank">these instructions</a><span style="line-height: 28px;"> to install it.</span></span></div>
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<span style="line-height: 28px;"><br /></span><span style="font-size: 24px; font-weight: bolder; line-height: 34px;"><span style="color: #0b5394;">Step 2: Install Fail2ban Package</span></span></div>
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After installing required repository use yum package manager to install Fail2ban rpm package using following command.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); color: white; line-height: 1.3em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"># yum install fail2ban<span style="font-size: 0.9em;">
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<span style="color: #0b5394;">Step 3: Setup Fail2ban Default Configuration</span></h2>
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Fail2ban provides its own security configuration file <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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">/etc/fail2ban/jail.conf</span>, but we need to create a copy of this file as <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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">jail.local</span>.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); color: white; line-height: 1.3em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"># cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local<span style="font-size: 0.9em;">
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Now we need to make necessory changes in jail.local file to create ban rules. Edit this file in your favorite editor and make changes in <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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">[DEFAULT]</span> section.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); color: white; line-height: 1.3em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;"># "ignoreip" can be an IP address, a CIDR mask or a DNS host. Fail2ban will use as always allowed, 
# so add your system ip to protect your ip from banned.
<orange style="color: orange;">ignoreip = 127.0.0.1/8 192.168.1.0/24 11.22.33.44</orange>

# "bantime" is the total number of seconds that a host is banned (3600sec = 1 Hour ).
<orange style="color: orange;">bantime  = 3600</orange>

# A host is banned if it has generated "maxretry" during the last "findtime" seconds. as per below 
# settings, 2 minutes
<orange style="color: orange;">findtime  = 120</orange>

# "maxretry" is the number of failures before a host get banned.
<orange style="color: orange;">maxretry = 3 </orange><span style="font-size: 0.9em;">
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<span style="color: #0b5394;">Step 4: Protect SSH/SFTP</span></h2>
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After completing default configuration, go down in the same file jail.local and update <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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">[ssh-iptables]</span> section as below.</div>
<pre style="background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); color: white; line-height: 1.3em; margin-bottom: 20px; outline: 0px; overflow: auto; padding: 20px; vertical-align: baseline;">enabled  = true
filter   = sshd
action   = iptables[name=SSH, port=22, protocol=tcp]
           sendmail-whois[name=SSH, dest=root, sender=admin@hackthesec.co.in, sendername="Fail2Ban"]
logpath  = /var/log/secure
maxretry = 3<span style="font-size: 0.9em;">
</span></pre>
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<span style="color: #0b5394;">Step 5: Protect FTP (vsFTPd) Server</span></h2>
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Let’s protect your FTP (vsFTPd) server, Find the below entry of <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-size: inherit; font-weight: 700; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">[vsftpd-iptables]</span> section and make changes as below. If you are not using vsFTPd, you can skip this section.</div>
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enabled  = true
filter   = vsftpd
action   = iptables[name=VSFTPD, port=21, protocol=tcp]
           sendmail-whois[name=VSFTPD, dest=admin@hackthesec.co.in]
logpath  = /var/log/vsftpd.log
maxretry = 5
bantime  = 1800<span style="font-size: 0.9em;">
</span></pre>
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<span style="color: #0b5394;">Step 6: Restart Fail2ban Service</span></h2>
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After making all the changes save your file and restart Fail2ban service using following command.</div>
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<pre style="-webkit-text-stroke-width: 0px; background: none 0px 0px repeat scroll rgb(60, 60, 60); border-radius: 4px; border: 1px solid rgb(229, 229, 229); color: white; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 1.3em; margin: 0px 0px 20px; orphans: auto; outline: 0px; overflow: auto; padding: 20px; text-align: start; text-indent: 0px; text-transform: none; vertical-align: baseline; widows: 1; word-spacing: 0px;"># service fail2ban restart
# chkconfig fail2ban on<span style="font-size: 0.9em;">
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How To Protect SSH/FTP using Fail2ban on CentOS/RHEL

Protect SSH/FTP using Fail2ban on CentOS / RHEL Fail2ban is very useful application for you, if you are managing security of server, ...