iptables Rule Generator

A firewall rule is both a packet matcher and a decision placed at a specific point in an ordered chain. The text can be valid while the policy is wrong: an allow rule below an earlier drop may never see traffic, a broad accept can expose a service, and a remote-host mistake can cut off the same administrative session needed for recovery.

The default filter table has three built-in paths. INPUT handles packets addressed to the local host, OUTPUT handles packets created by the host, and FORWARD handles packets routed through it. The correct chain depends on the packet path, not simply on whether the rule feels inbound or outbound from the operator's perspective.

Rules are evaluated in chain order. When a packet does not match, evaluation continues to the next rule. Terminal targets such as ACCEPT and DROP decide the packet's fate immediately; RETURN leaves the current chain according to the calling context. LOG records a match but does not stop traversal, so a later rule or the chain policy still decides the packet.

IPv4 and IPv6 use separate command families. A service reachable over both protocols needs equivalent policy on both sides, and each side needs address-family-correct source and destination matches. A dual-stack shortcut can generate two commands only when the address scope is unrestricted; family-specific CIDRs require separate rules so an IPv4 range is never mistaken for an IPv6 range.

The command name does not always reveal the kernel policy owner. On many current systems, iptables may use an nftables-compatible backend; other hosts retain legacy xtables or delegate policy to firewalld, nftables, a distribution service, or an orchestration system. Mixing direct rules with a manager that later reloads its own policy can erase or contradict a valid runtime change.

Runtime changes and persistent policy are separate decisions. An append, insert, or delete changes the active ruleset immediately when executed, while a save or manager handoff controls what returns after reboot or reload. Persist only a rule that has passed remote testing, chain-order review, and a rollback rehearsal.

Firewall generation cannot determine business authorization, service ownership, routing, interface topology, existing chain order, or the safety of a maintenance window. Keep a second administrative session, verify the exact target host, and use host-native policy tooling whenever it owns the ruleset.

How to Use This Tool:

Draft one bounded filter-table rule, then compare the command with the live chain, backend ownership, and persistence plan before running anything.

1. Choose a Rule preset as a starting point, then set Address family, Backend profile, distribution, persistence target, and review strictness for the actual host.
2. Select the rule operation, built-in chain, target action, and protocol. Use Insert with a verified position when an earlier broad rule would otherwise shadow the new rule.
3. Narrow the match with source and destination addresses, port direction and ports, interfaces, ICMP type, and connection state. Use any only when broad scope is intentional.
4. For TCP or UDP, enter single ports, comma-separated ports, or ranges. Keep every value within 1–65,535 and remember that the multiport matcher allows at most 15 entries, with each range counting as two.
5. Add a concise rule comment and choose whether the command needs sudo. For REJECT, confirm that the selected reject response matches the protocol and address family.
6. Stop if production strictness blocks generation. Narrow broad accepts, add service or connection-state scope, choose a durable ownership path, add an audit comment, and pin the host backend as required by the blocking message.
7. Run the generated duplicate check first, compare the apply command with the numbered live chain, keep a rollback path, and execute only in an approved change window. Test from the expected network path before saving the rules.

Interpreting Results:

Ready means the local rule model found no remaining review items. Review needed means a command exists but host-specific or policy-sensitive findings remain. A blocked production review produces an input error instead of a command, because the unresolved finding is treated as a deployment stop.

Technical Details:

An iptables rule is assembled from an operation, chain, zero or more match clauses, and a target. Match extensions are evaluated in command order and short-circuit when a condition fails. If every match succeeds, the target decides whether traversal stops, continues, or returns to a calling chain.

The generated command uses the default filter table and only the built-in INPUT, OUTPUT, and FORWARD chains. It does not model NAT, packet marking, raw-table behavior, custom chains, rate limiting, sets, owner matches, or every extension available on a particular kernel and userspace release.

Rule Core:

Validation and Review Core:

Blocking validation protects command syntax and family consistency. Review rules call out operational risk that can still be intentional. Production strictness promotes selected review findings to blockers before any command is returned.

Dual-stack mode emits one IPv4 and one IPv6 command but accepts only unrestricted source and destination scope. This constraint prevents one family's CIDR from being copied into the other command. Use separate family-specific drafts when either side needs a narrow address range.

Persistence output is advisory. Commands that redirect iptables-save into a root-owned path may require a root shell or a privilege-aware write method because shell redirection is evaluated separately from sudo. Distribution packages and active managers determine the real save, test, reload, and boot-restore workflow.

Limitations and Responsible Use:

The commands are specimens for review, not authorization to change a firewall. They do not read the live ruleset, select a safe insertion point, test reachability, inspect counters, identify the active policy manager, or guarantee recovery after a remote lockout.

• Keep an out-of-band or second administrative path before changing remote access rules. Confirm the rollback command before the apply command.
• List the target chain with line numbers and counters, check for an existing equivalent rule, and place the rule before any earlier terminal decision that would shadow it.
• Test from another host on the intended network path. A local socket check does not prove that packets cross the expected interface and chain.
• Save only after runtime verification. A persistent bad rule can make the same outage return at boot.

Worked Examples:

References:

• iptables and ip6tables manual page, Linux man-pages project.
• iptables-extensions manual page, Linux man-pages project.
• How to allow a port with iptables, Simplified Guide.
• How to save iptables rules permanently, Simplified Guide.