Firewall filtering
Firewalls are increasingly a discrete managed product/service. Particularly, they perform deep inspection of packets and go beyond OSI Layer 4 (which is explicitly downvoted at the moment).
For this topic only it's ok to include the discussion of higher OSI layer protocols.
Examples
What do I need to do in order to enable remote HTTPS management of a SonicWall NSA3500
How can I monitor SSH or HTTPS activity on Juniper SRX 240 firewall?