Set port knocking with knockd and iptables

This document describes a stealth method to externally open ports that, by default, are kept closed by the firewall.

Server side

1. Install knockd

apt-get install knockd

2. Configure knockd

vim /etc/knockd.conf
 
[options]
        UseSyslog
 
[OpenClosePort]
        sequence    = 2123:udp,3543:tcp,6454:udp
        seq_timeout = 5
        Start_Command     = /sbin/iptables -I INPUT -s %IP% -p tcp --dport PORT -j ACCEPT
        tcpflags    = syn
        Cmd_timeout = 3600
        Stop_Command     = /sbin/iptables -D INPUT -s %IP% -p tcp --dport PORT -j ACCEPT
 
:wq

Notes:

  • sequence - the sequence required to open desired port
  • seq_timeout - time to wait for a sequence to complete
  • Start_Command - command to be executed when a client makes the correct port-knock
  • Stop_Command - command to be executed when Cmd_Timeout seconds have passed since Start_Command has been executed
  • Cmd_Timeout - time to wait between Start_Command and Stop_Command in seconds
  • PORT - port to be opened

3. Enable knockd

 

vim /etc/default/knockd
 
:%s/START_KNOCKD=0/START_KNOCKD=1/g
:%s/#KNOCKD_OPTS="-i eth1"/KNOCKD_OPTS="-i eth0"/g
:wq

 

4. Start knockd

/etc/init.d/knockd restart

Client side

1. Knock the port

nmap -Pn --host_timeout 201 --max-retries 0 -sU -p 2123 host; nmap -Pn --host_timeout 201 --max-retries 0 -p 3543 host; nmap -Pn --host_timeout 201 --max-retries 0 -sU -p 6454 host

2. Check if the port is open

telnet host PORT

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