A firewall is a combination of hardware and software, used to provide security to a network. It is used to protect an internal network or intranet against unauthorized access from the Internet or othe

OpenSSH is a secure remote shell and tunneling tool used for encrypted administrative access, not a packet-filtering firewall.

IPTables was integrated into the Linux 2.4 kernel as part of the Netfilter project and provides stateful packet inspection, NAT, and packet filtering capabilities that define it as the standard firewall mechanism for that kernel series. It uses tables (filter, nat, mangle) and chains (INPUT, OUTPUT, FORWARD) to control traffic flow. IPChains, its predecessor, was deprecated specifically when the 2.4 kernel shipped with IPTables.

IPChains was the firewall tool for the Linux 2.2 kernel and was explicitly replaced by IPTables when the 2.4 kernel was released.

Stunnel is an SSL/TLS tunneling proxy that wraps plaintext connections in encryption, and is not a firewall or packet-filtering tool.