Title: Filtering TCP connections by operating system on OpenBSD

	Title: Filtering TCP connections by operating system on OpenBSD
	Author: Solène
	Date: 06 February 2021
	Tags: openbsd security
	Description:

	# Introduction

	In this text I will explain how to filter TCP connections by operating
	system using OpenBSD Packet filter.

	OpenBSD pf.conf man page about OS Fingerprinting

	# Explanations

	Every operating system has its own way to construct some SYN packets,
	this is called Fingerprinting because it permits to identify which OS
	sent which packet. This must be clear it's not a perfect filter and
	may be easily get bypassed if you want to.
	Because if some packets required to identify the operating system, only
	TCP connections can be filtered by OS. The OS list and SYN values can
	be found in the file /etc/pf.os.

	# How to setup

	The keyword "os $value" must be used within the "from $address"
	keyword. I use it to restrict the ssh connection to my server only to
	OpenBSD systems (in addition to key authentication).

	```OpenBSD packet filter configuration file including comments
	# only allow OpenBSD hosts to connect
	pass in on egress inet proto tcp from any os OpenBSD to (egress) port 22

	# allow connections from $home IP whatever the OS is
	pass in on egress inet proto tcp from $home to (egress) port 22
	```

	This can be a very good way to stop unwanted traffic spamming logs but
	should be used with cautiousness because you may incidentally block
	legitimate traffic.