docs: added article about setting up port knocking

knowledge base/knockd.md

@@ -0,0 +1,104 @@
+# knockd
+
+Port-knock server. It listens to all traffic on a network interface, looking for special _knock_ sequences of port-hits. These ports do **not** need to be open -- `knockd` listens at the link-layer level, so it can even see traffic which is destined to a closed port.
+
+When the server detects a specific sequence of port-hits, it runs a command defined in its configuration file. This can be used to open up holes in a firewall for quick access.
+
+## Table of contents <!-- omit in toc -->
+
+1. [TL:DR](#tldr)
+1. [Configuration](#configuration)
+1. [Further readings](#further-readings)
+1. [Sources](#sources)
+
+## TL:DR
+
+Server side:
+
+```sh
+# Install `knockd`.
+sudo apt install 'knockd'
+brew install 'knockd'
+sudo dnf install 'knock-server'
+sudo zypper install 'knockd'
+
+# Configure the sequence and its effects.
+sudo vim '/etc/knockd.conf'
+
+# Enable the service.
+sudo systemctl enable --now 'knockd.service'
+
+# Keep an eye on the logs to see if the sequence is working.
+sudo journalctl -xe -u 'knockd.service'
+```
+
+Client side:
+
+```sh
+# Install `knock`.
+sudo apt install 'knockd'
+brew install 'knockd'
+sudo dnf install 'knock'
+sudo zypper install 'knock'
+
+# Send the command.
+# If ':protocol' is not given, defaults to 'tcp'; defaults to 'udp' if the '-u'
+# option is specified.
+knock '123.456.789.012' '2222' '3333:udp' '4444'
+knock -vu 'example.fqdn' '2222:tcp' '3333' '4444:tcp'
+```
+
+## Configuration
+
+`knockd`'s default configuration file is `/etc/knockd.conf`.
+
+Each knock/event begins with a title marker in the form `[name]`, with it being the name of the event that will appear in the log.<br/>
+`[options]` is a special marker used to define `knockd`'s **global** options.
+
+```ini
+[options]
+	UseSyslog
+	Interface = enp0s2
+
+# Different sequences for opening and closing.
+[openSSH]
+    sequence    = 7000,8000,9000
+    seq_timeout = 10
+    tcpflags    = syn
+    command     = /usr/sbin/iptables -A INPUT -s %IP% -j ACCEPT
+[closeSSH]
+    sequence    = 9000,8000,7000
+    seq_timeout = 10
+    tcpflags    = syn
+    command     = /usr/sbin/iptables -D INPUT -s %IP% -j ACCEPT
+
+# Single sequence for opening, automatic close after 'cmd_timeout' seconds.
+# If a sequence setting contains the `cmd_timeout` statement, the `stop_command`
+# will be automatically issued after that amount of seconds.
+[openClose7777]
+	sequence      = 2222:udp,3333:tcp,4444:udp
+	seq_timeout   = 15
+	tcpflags      = syn
+	cmd_timeout   = 10
+	start_command = /usr/bin/firewall-cmd --add-port=7777/tcp --zone=public
+	stop_command  = /usr/bin/firewall-cmd --remove-port=7777/tcp --zone=public
+```
+
+Sequences can also be defined in files.
+
+Check the [`knockd(1)`][knockd man page] man page for all the information.
+
+## Further readings
+
+## Sources
+
+- [How to use port knocking to secure SSH service in Linux]
+- [Server][knockd man page]'s man page
+- [Client][knock man page]'s man page
+
+<!-- project's references -->
+<!-- internal references -->
+<!-- external references -->
+[how to use port knocking to secure ssh service in linux]: https://www.tecmint.com/port-knocking-to-secure-ssh/
+[knockd man page]: https://linux.die.net/man/1/knockd
+[knock man page]: https://linux.die.net/man/1/knock

knowledge base/set up port knocking.md

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+# Set up port knocking
+
+Technique where a daemon keeps listening on specific ports for a specific sequence of connections.<br/>
+When the correct sequence is used, the daemon issues a configured command, usually to open a defined port for the client only.
+
+This is frequently used to open the SSH port in a server for a specific client.
+
+## Further readings
+
+- [Knockd]
+
+<!-- internal references -->
+[knockd]: knockd.md

knowledge base/template.md

@@ -2,6 +2,8 @@
 
 Intro
 
+## Table of contents <!-- omit in toc -->
+
 1. [TL:DR](#tldr)
 1. [Further readings](#further-readings)
 1. [Sources](#sources)
@@ -10,6 +12,10 @@ Intro
 ## Further readings
 ## Sources
 
+All the references in the [further readings] section, plus the following:
+
 <!-- project's references -->
 <!-- internal references -->
+[further readings]: #further-readings
+
 <!-- external references -->