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To block an IPv4 address or service with iptables and keep the rule after a reboot, add a correctly ordered rule to the appropriate chain, test the complete ruleset, save the working configuration to /etc/iptables/rules.v4, and have a persistence service load it at startup. A rule entered at the shell is temporary: Debian documents that iptables rules are flushed at the next reboot unless they are saved and restored.
The safest workflow is narrow and reversible. Inspect the active rules first, preserve loopback, established connections, and management access, add the DROP rule, test from a second administration session, save only the known-good rules, then verify a manual reload and a reboot.
What an iptables block rule does
iptables evaluates packets against chains in order. A matching DROP target discards the packet without sending a response. The chain determines where the packet is being considered:
| Chain | Traffic considered | Typical blocking use |
|---|---|---|
INPUT |
Traffic entering the local host | Block an incoming address or a service port exposed by this host |
OUTPUT |
Traffic leaving the local host | Prevent the host from contacting a destination address or service |
FORWARD |
Traffic routed through the host | Block traffic transiting a router or gateway |
For example, an incoming address belongs in INPUT, while traffic the machine itself must not send belongs in OUTPUT. A forwarded packet needs a rule in FORWARD. Selecting the wrong chain is a common reason a rule appears in the listing but has no effect.
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Inspect the active rules and firewall backend
Do not append a DROP rule until you know what is already loaded. Capture both the human-readable listing and the restorable form:
sudo iptables -L -v --line-numbers
sudo iptables-saveThe listing shows each chain, rule order, packet and byte counters, and targets. The saved form reveals the complete table configuration that can be restored later. Look specifically for loopback and established/related accepts, existing management rules, default chain policies, and any earlier broad accept that would prevent a later DROP from being reached.
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Also establish whether the host is using legacy iptables or an iptables-nft compatibility layer. Debian has used nftables as its default firewall framework since Debian 10 (Buster), while the iptables command may still be the interface administrators use. Do not assume that a command written for one backend behaves identically on every installed version; check the backend and the version supplied by the host before building a long-lived rules file.
Write a narrowly scoped DROP rule
Use the smallest match that answers the security requirement. These examples use 203.0.113.10, an address reserved for documentation; substitute the address or port you actually need to control.
Block an incoming IPv4 address
sudo iptables -A INPUT -s 203.0.113.10 -j DROPThis appends a rule to INPUT that drops packets whose source address matches. It affects traffic arriving at this host, not packets the host sends and not traffic merely routed through it.
Block an incoming TCP service port
sudo iptables -A INPUT -p tcp --dport 25 -j DROPThis blocks TCP destination port 25 on the local host. The protocol and destination-port match make it narrower than blocking every packet from a source address. If the service uses another protocol or port, the match must reflect that service.
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Block an outgoing destination
sudo iptables -A OUTPUT -d 203.0.113.10 -j DROPThis prevents locally generated packets destined for the address from leaving through the host. It does not block another machine from reaching that address through this host unless the traffic is routed and handled in FORWARD.
Choose between an address block and a service block
| Requirement | Match to use | Scope |
|---|---|---|
| Stop one known source from reaching this host | -A INPUT -s address -j DROP |
All protocols and ports from that source, subject to earlier rules |
| Stop everyone from reaching one local TCP service | -A INPUT -p tcp --dport port -j DROP |
That TCP destination port on this host |
| Stop this host contacting one destination | -A OUTPUT -d address -j DROP |
Locally generated traffic to that destination |
A broad address DROP can have unintended side effects if the address is shared by legitimate users or services. A port-specific rule is usually safer when the actual requirement is to disable one service.
Put safety and management rules before broad blocks
Rule order is decisive. iptables stops processing a packet when an earlier rule accepts or drops it. A broad DROP placed before your SSH or other administration accept can lock you out; an earlier accept means a later DROP will never see that packet.
A practical filter table normally keeps these categories in a deliberate order:
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- Accept packets belonging to established or related connections.
- Accept the administration service, such as SSH, from the addresses or networks that must manage the host.
- Accept any other explicitly required services.
- Apply narrow DROP rules for unwanted sources, destinations, or ports.
Debian’s examples include loopback, established/related state, outbound traffic, and service-port rules. Adapt those principles to the host instead of copying a blanket policy. If you administer the machine remotely, keep a second management session open while testing so that a mistake can be corrected through the other session.
Build a complete rules.v4 file
/etc/iptables/rules.v4 is a complete IPv4 ruleset, not a fragment containing only the new DROP line. A minimal filter-table file has table and chain declarations, the rules in their intended order, and a terminating COMMIT:
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*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p tcp --dport 22 -j ACCEPT
-A INPUT -s 203.0.113.10 -j DROP
COMMITThe example permits loopback, established and related traffic, and TCP port 22 before dropping the selected source. The chain policies shown are ACCEPT; changing a policy to DROP is a separate, much broader design decision and should not be inferred from this example. Include every rule the host needs, not just the one being added.
Because this file is for IPv4, it does not by itself govern IPv6 traffic. If the host is reachable over IPv6, it needs a corresponding IPv6 firewall policy managed through the host’s chosen firewall framework.
Apply and test the complete configuration
There are two safe ways to reach a known-good file: add a command-line rule, inspect the resulting complete ruleset, then save it; or edit a prepared file, restore it, and test before saving anything over the previous copy. In either case, restore the complete file with:
sudo iptables-restore < /etc/iptables/rules.v4Restoring a complete file makes the ordering explicit and lets you test the same configuration that will be loaded at boot. From a remote session, retain a second session and test both the intended block and required administration access. Generate a packet that should match the DROP, then confirm that legitimate traffic still works. Counters in the verbose listing provide evidence that the expected rule is seeing packets:
sudo iptables -L -v --line-numbersCheck the chain, line number, target, and packet/byte counters. A zero counter does not prove a rule is wrong; it can mean the test traffic used a different protocol, port, direction, address family, or chain. Conversely, a counter increasing on an earlier ACCEPT explains why a later DROP is ineffective.
Save the known-good active rules to rules.v4
Once the complete ruleset has passed testing, serialize the rules currently loaded in the kernel:
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sudo iptables-save | sudo tee /etc/iptables/rules.v4This writes the active IPv4 configuration to the conventional persistent path. Using tee avoids the common shell-redirection mistake in which sudo applies to iptables-save but not to the file-opening redirection. A root shell with a redirection also works, but the command above makes the privilege boundary explicit.
Save only after testing. If you save a locked-out or syntactically incorrect configuration first, you have replaced the known-good startup file with the problem you are trying to diagnose.
Load rules automatically at startup
iptables rules in memory are not persistent. Debian states: "As IP-Tables are not persistent, they will be deleted ("flushed") with the next reboot." The Debian persistence package provides startup integration for the rules file.
Save through netfilter-persistent
When the persistence package is installed, its save operation invokes plugins to store the currently loaded rules:
sudo netfilter-persistent saveThis is an alternative to calling iptables-save directly and is useful when the package’s plugin system manages more than one firewall backend.
Load the saved file
The package’s start operation loads persistent rules:
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sudo netfilter-persistent startUse the startup integration supplied by the Debian package for the host’s release. Debian’s package template documents that current IPv4 rules can be saved to /etc/iptables/rules.v4 and loaded automatically during startup; it also points administrators to iptables-save(8) for ongoing updates. Rules may be saved automatically during package installation, but you should still inspect and verify the resulting file rather than assuming it contains the policy you intended.
Verify persistence without waiting for a reboot
A manual reload exercises the same file-reading path used for recovery:
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sudo iptables -L -v --line-numbers. - Reload the file with
sudo iptables-restore < /etc/iptables/rules.v4. - Run the verbose, numbered listing again.
- Confirm that the expected chains, order, targets, and management accepts are present.
A restore normally resets the active state to what the file describes, so do not treat it as a harmless display command. Perform it from a console or from the protected second management session.
After a planned reboot, repeat the numbered listing and test traffic in both directions. If the rules disappear, the problem is persistence loading rather than the DROP syntax: inspect whether the file exists at the exact path, whether the persistence service is installed and enabled for that release, and whether the service’s backend matches the command used to create the file.
Command-line rules versus a declarative rules.v4 file
| Approach | Strength | Risk or limitation | Best use |
|---|---|---|---|
| Immediate command-line rule | Fast response to a live incident | Exists only in memory until explicitly saved; appending can put it after an earlier accept | Short, controlled changes followed by inspection and a save |
Declarative rules.v4 file |
Records the full order and can be restored consistently | A malformed or unsafe file can replace the active policy when restored | Repeatable configuration and recovery |
| Manual restore | Explicit control over when a file is loaded | Someone must run it after a change or reboot | Testing, rollback, and maintenance windows |
Boot-time netfilter-persistent loading |
Restores rules automatically at startup | Depends on the package, plugins, service configuration, and matching backend | Hosts that must retain filtering across reboots |
| Legacy iptables | Uses the traditional implementation and rule syntax | May differ from the host’s current firewall framework | Systems deliberately running the legacy backend |
| iptables-nft compatibility layer | Lets existing iptables commands operate with nftables underneath | Backend-specific behavior and mixed tooling can confuse troubleshooting | Debian systems using the modern default while retaining iptables commands |
The important distinction is not whether a rule was typed or stored in a file; it is whether the final, ordered ruleset is known-good and whether the same backend loads it after a restart.
Troubleshoot common failures
The blocked traffic still succeeds
- Confirm the packet direction. Incoming traffic belongs in
INPUT, locally generated traffic inOUTPUT, and routed traffic inFORWARD. - Check that the address, protocol, and port match the packet you are generating.
- Inspect line numbers and counters. An earlier ACCEPT may be terminating evaluation before the DROP.
- Check whether the traffic is IPv6; an IPv4 rule in
rules.v4cannot match it. - Verify that the command and the listing refer to the same iptables backend used by the host.
SSH or another management service was locked out
Use the still-open second session or an out-of-band console to restore the known-good file. The immediate cause is usually a broad DROP before the management accept, or a source-address DROP that includes the administrator. Reorder the complete file so loopback, established/related state, and the required management rule appear before the broad block, restore it, verify access, and only then save it.
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The rules vanish after reboot
- Check that the final rules were written to
/etc/iptables/rules.v4, not merely left in memory. - Confirm that the Debian persistence package and its startup integration are installed for the release.
- Use
sudo netfilter-persistent startto test the loader manually. - Inspect the post-start listing and counters to determine whether the file loaded and whether traffic is reaching the intended rule.
iptables-restore rejects the file
Compare the file with the expected complete format: a table declaration such as *filter, chain declarations, correctly formed rules, and a final COMMIT. Check spelling and module options against the iptables version installed on the host. A legacy Debian manpage may not describe every behavior of a newer installation, so use the local command’s documentation when a match or target is rejected.
The file loads but behavior changes unexpectedly
Read the entire restored listing rather than examining only the new line. A file can contain an earlier accept, a different default policy, or a rule in another chain that explains the result. Compare the saved output from iptables-save with the numbered live listing and test one direction at a time.
Quick Recap
Operational checklist
- Inspect
iptables -L -v --line-numbersandiptables-savebefore editing. - Identify whether the command uses legacy iptables or an iptables-nft compatibility layer.
- Choose INPUT, OUTPUT, or FORWARD from the packet’s actual path.
- Match only the source, destination, protocol, or port that must be blocked.
- Place loopback, established/related, and required management accepts before broad DROP rules.
- Keep a second administration session during remote changes.
- Restore and test the complete file, not just an isolated command.
- Save the known-good active rules to
/etc/iptables/rules.v4. - Use the persistence service’s save and start operations where installed.
- Reload manually and verify order, targets, and counters before relying on a reboot.
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