Definition:
A server’s firewall controls what kind of Internet connections is allowed to flow in (inbound) and out (outbound) of a server.
For inbound and outbound traffic, the server’s owner can specify the traffic rule by specifying:
- What type of protocol (http, SMTP, SQL)
- What port
- A set of allowed IPs
Analogy:
- Military security checkpoint: it controls who can enter and leave the military building.