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Port Knocking Server and Securing SSH connection for CentOS 7

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knockd

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In this guide, you’ll learn how to use Port Knocking Daemon knockd to secure SSH Connection and Hardening you server.

Scenario and Abstraction

Port knocking as an Idea and Technique, is a method to externally open ports that, by default, the firewall keeps closed.

It works by issues requiring connection attempts to (

a series of predefined closed ports,

&& in a specific sequence and specific connection protocol (ie TCP/UDP). )

When the correct sequence of port “knocks” connection attempts is received by server side,
the server firewall will opens certain port(s) (we’ll focus here on ssh service.) to allow a connection.

This Port Knocking technique can be implemented using only IPtables in advanced way,or using Port-Knocking daemon knockd which we will learn here in this article.

Note

  1. This guide, runs on Updated CentOS 7 that build with Latest 64 bit kernel.
  2. Running FirewallD, you can review Introduction to FirewallD on CentOS linode tutorial here.

We’ll Firewall-ed and deny all public SSH connection service at Server-Side,

but just gives SSH pass and and open the gate to allow connection,
only for knocker IP address who do correct knocking (sending packets) for a predefined and specific sets of closed ports (in correct sequence and correct connection protocol) .

so for right knocker the knockd service will fire a open gate firewall-cmd rule for its IP address ,
also can fire closed gate rule for it.

Note

default settings for firewalld is to allowing ssh service for public-zone, so you may need to make sure to remove ssh form allowed service.

Remove ssh access service permanent from firewall public-zone “assumed as our work default zone”.

firewall-cmd --zone=public --remove-service=ssh --permanent
firewall-cmd --reload

Steps Server side

Installation

for install Port Knocking on CentOS 7, I would recommend using rpm package of knock-server from nux-dextop repository. but suing direct link for rpm package just as below

rpm -ivh http://li.nux.ro/download/nux/dextop/el7Server/x86_64/knock-server-0.7-1.el7.nux.x86_64.rpm

Note

for more information about nux-dextop and centos repositories, you can review Available Repositories for CentOS.
if you need to install nux-dextop repository, and because its coexist with Fedora EPEL, you must install both repos and for EL7 use:
yum -y install epel-release && rpm -Uvh http://li.nux.ro/download/nux/dextop/el7/x86_64/nux-dextop-release-0-5.el7.nux.noarch.rpm

Configuring Knock-server

the configuration file has the name knockd.conf and is located at etc directory /etc/knockd.conf.
our configuration will be settings like below file.

file:/etc/knockd.conf

 

[options]
UseSyslog
logfile = /var/log/knockd.log
[OpenSSH]
Sequence = 3333,4444,5555
Seq_timeout = 15
Tcpflags = syn
Command = /bin/firewall-cmd --zone=public --add-rich-rule="rule family="ipv4" source address="%IP%" service name="ssh" accept"

[CloseSSH]
Sequence = 6666,7777,8888
Seq_timeout = 15
Tcpflags = syn
Command = /bin/firewall-cmd --zone=public --remove-rich-rule="rule family="ipv4" source address="%IP%" service name="ssh" accept"

in the above configuration settings for knockd we used two knocks. The first will allow the knocker to connect sshd, and the second will close the connection, when the knocker is complete.

  • The option UseSyslog: mean that, the knockd will logging message using rsyslogd inserting into /var/log/knockd.log file.
  • Seq_Timeout is the time in seconds that to wait for a sequence to complete, or will ignore received knocks and start over.
  • tcpflags Only pay attention to packets that have this flag set.
  • Sequence Specify the sequence of ports in the special knock. Optionally, you can define the protocol to be used on a per-port basis (but default is TCP) ie sequence = 2222:udp,3333:tcp,4444:udp.
  • start_command Specify the command to be executed when a client makes the correct port-knock. All instances of %IP% will be replaced with the knocker’s IP address.
  • command Specify the command to be executed when when a client makes the correct port-knock to close the connection.

Note

IN EL7/CentOS 7, FirewallD is a frontend controller and wrapper for iptables, you can review the very nice article Introduction to FirewallD on CentOS at

caution

Above configuration settings are for testing of knockd.config, you must change at least the ports numbers and sequence and customize it for your server.
while testing will be good to run Lish for emergency.

Run Knockd as Daemon

because of knockd is not a native service you can use to start knockd at boot

chkconfig knockd on

start the knockd service

service knockd start

Note

you may get error message like Starting knockd: could not open eth0: eth0: No such device exist
so you can adjustment the Ethernet device interface card name from the config file /etc/sysconfig/knockd

Change eth interface device if needed /etc/sysconfig/knockd.

file:/etc/sysconfig/knockd

OPTIONS="-i eth0"

Using from Client side

Installation

Installing Port Knocking knokd client package as:

rpm -ivh http://li.nux.ro/download/nux/dextop/el7Server/x86_64/knock-0.7-1.el7.nux.x86_64.rpm

Run Knockd and Make send a sequence

Now we have to use of knock command from our client we issue the command:
knock -v 3333 4444 5555

  • You can examine the logs by accessing the server where knockd-server is located to see the logs about the processes we are doing. you can see logs like followings and notice that, Stage 1, 2 and 3 are passed and the necessary command is executed. So the server is ready for ssh access.

Firing knocks form Client Device (local IP: ‘192.168.1.10’) to Server (local IP: ‘192.168.1.103’):

[root@server ~]# knock -v 192.168.1.103 3333 4444 5555
hitting tcp 192.168.1.103:3333
hitting tcp 192.168.1.103:4444
hitting tcp 192.168.1.103:5555

and Server side log will be as following

[2017-01-01 01:33] starting up, listening on eno16777736
[2017-01-01 01:33] 192.168.1.10: OpenSSH: Stage 1
[2017-01-01 01:33] 192.168.1.10: OpenSSH: Stage 2
[2017-01-01 01:33] 192.168.1.10: OpenSSH: Stage 3
[2017-01-01 01:33] 192.168.1.10: OpenSSH: OPEN SESAME
[2017-01-01 01:33] OpenSSH: running command: /bin/firewall-cmd --zone=public --add-rich-rule="rule family="ipv4" source address="192.168.1.10" service name="ssh" accept"

and for check firewall rules changes to allow my client IP.

[root@localhost ~]# firewall-cmd --list-all
public (active)
target: default
icmp-block-inversion: no
interfaces: eth0
sources:
services:
ports:
protocols:
masquerade: no
forward-ports:
sourceports:
icmp-blocks:
rich rules:
rule family="ipv4" source address="192.168.1.10" service name="ssh" accept

and so:

  • To open the port on the server side
    knock -v 3333 4444 5555
  • To access your server side ssh
    ssh [user]@ -p [SSHPortNumber]
  • To close the connections mean that, to remove the IPtable rule that related to your IP address.
    knock -v 6666 7777 8888

Note

from any client you can use same knock-client technique to open or close gates for you, for our scenario here, all we need is to send 3 tcp packet to a specific Ports targets server IP address.
That can be doing using many network application ie nmap, windows knockknock, netcat,telnet, or others.
for using nmap to knock server and open/close gate you can use the below comman suitable with our scenario/example.
but I think you many need to extend the Seq_timeout value

nmap -Pn -p 6666 192.168.1.103 >/dev/null && nmap -Pn -p 7777 192.168.1.103 >/dev/null && nmap -Pn -p 8888 19 2.168.1.103 >/dev/null

from nmap command, now you send 3 tcp packets (knocks) to the server targets the specific predefined port sequence
and now you get the gate closed after you,and also may use nmap to open it and makes a ssh connection again.

Others Port knocking

Port Knocking as technique and policy to open gate for correct knocker who are knocking in predefined sequence, can be presented suing IPtables

Next Steps

First, remember to be sure that you have followed linode guide to Securing Your Server.
Second, you may need to apply two-factor authentication for more secure SSH connection and hardening your server, you can review this linode Ubunto|debian guide about how to Use Google Authenticator to enable two-factor authentication for SSH connections..

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