About me: My name is Solène Rapenne, pronouns she/her. I like learning and sharing knowledge. Hobbies: '(BSD OpenBSD Qubes OS Lisp cmdline gaming security QubesOS internet-stuff). I love percent and lambda characters. OpenBSD developer solene@. No AI is involved in this blog.
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Written by Solène, on 06 February 2021.
Tags:
#openbsd
#security
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
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.
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).
# 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.