Port Knocking

About Port Knocking feature

Behind this "magical" name, Port Knocking is "a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports".

How to use it inside WebSSH?

1. Add a connection (either SSH, SFTP or TUNNEL) by pessing the + button
2. Fill all required fields in order to establish the SSH connection
3. Finally specify your port knocking sequence by using the right syntax

Port Knocking Sequence Syntax

The Port Knocking Sequence Syntax is as simple as : PORT:PROTOCOL:WAITMS

1. Fields PORT and PROTOCOL are mandatory. Field WAITMS is optional.
2. Field PORT must be an integer between 1 and 65535
3. Field PROTOCOL can have these values : TCP or UDP
4. Field WAITMS is an integer to specify the sleep in milliseconds to apply after a port knock. Default is 100 if not specified.
5. To add multiple knocks to the sequence, just add a comma between knocks

Port Knocking Examples

• 1985:TCP will knock your server on the TCP port 1985
• 1985:TCP,2020:TCP will knock your server on TCP port 1985 then 2020 (with a default sleep of 100ms)
• 1984:TCP:200,1985:UDP:300,2014:TCP:200,2016:UDP:500 will knock on TCP port 1984 and will sleep 200 ms, then will knock on UDP port 1985 and will sleep 300 ms, then will knock on TCP port 2014 and will sleep 200 ms, then will knock on UDP port 2016 and will sleep 500 ms.

Examples of server side implementation by using knockd

Debian / Ubuntu : /etc/knockd.conf

[options]
        UseSyslog

[SSH]
      sequence    = 1984,1985,2014,2016
      seq_timeout = 10
      command     = /sbin/iptables -I INPUT -s %IP% -p tcp --dport 22 -j ACCEPT
      tcpflags    = syn
      cmd_timeout   = 10
      stop_command  = /sbin/iptables -D INPUT -s %IP% -p tcp --dport 22 -j ACCEPT

• On this example, we use the iptables command to open the port 22 on the server after you have knocked the right sequence : 1984,1985,2014,2016
• After the sequence is done, the port 22 is opened for 10 seconds and then closed again

CentOS / RHEL / Fedora / AlmaLinux / RockyLinux : /etc/knockd.conf

[options]
        UseSyslog

[SSH]
      sequence      = 1984,1985,2014,2016
      seq_timeout   = 10
      start_command = /bin/firewall-cmd --zone=public --add-rich-rule 'rule family="ipv4" source address="%IP%" port protocol="tcp" port="22" accept'
      tcpflags      = syn
      cmd_timeout   = 10
      stop_command  = /bin/firewall-cmd --zone=public --remove-rich-rule 'rule family="ipv4" source address="%IP%" port protocol="tcp" port="22" accept'

• On this example, we use the firewall-cmd command to open the port 22 on the server after you have knocked the right sequence : 1984,1985,2014,2016
• After the sequence is done, the port 22 is opened for 10 seconds and then closed again

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Last update: April 26, 2023