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Systems & Data Center · Intermediate

iptables

The legacy Linux packet-filtering command that manages netfilter tables and chains such as INPUT, OUTPUT and FORWARD.

Definition

iptables rules are evaluated in order within chains of the filter, nat, mangle and raw tables, with targets such as ACCEPT, DROP and REJECT. Rules are not persistent unless saved. Modern distributions use nftables, often with an iptables-nft compatibility layer.

Example

iptables -A INPUT -p tcp --dport 22 -j ACCEPT allows inbound SSH.

Exam tip

Rule order matters: the first matching rule wins, so place specific allows before a catch-all drop.

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