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To block an address or service and keep the decision after a reboot, add a narrowly matched DROP rule to the correct chain, place it after the accepts that must continue working, test the entire ruleset, save the known-good state to /etc/iptables/rules.v4, and enable a startup loader such as netfilter-persistent. A command entered with iptables changes the running kernel rules only; Debian documentation notes that rules are otherwise flushed at the next reboot.
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This guide covers IPv4 filter rules. It shows how to identify the host’s iptables backend, choose INPUT, OUTPUT, or FORWARD, avoid locking yourself out, build a complete rules.v4 file, and verify that the saved policy is loaded again.
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iptables rules are evaluated by the kernel while packets traverse the filter table. A command such as sudo iptables -A INPUT -s 203.0.113.10 -j DROP takes effect immediately, but it does not automatically become a boot-time configuration. The file /etc/iptables/rules.v4 is a persistent, text representation of the IPv4 rules that a startup service can restore.
Debian’s iptables documentation states: “As IP-Tables are not persistent, they will be deleted (“flushed”) with the next reboot.” The practical consequence is that a block which appears to work during a session can vanish after a restart unless a persistence mechanism loads it again.
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Think of the workflow as two separate states:
- Running state: the rules currently installed in the kernel, inspected with
iptables -Land exported withiptables-save. - Persistent state: the file that startup integration reads, normally
/etc/iptables/rules.v4.
Saving before the rules have been tested merely preserves a mistake. Always make the running state correct first, then export that known-good state.
Inspect the active rules and identify the backend
Start from a shell with administrative privileges and record what is already active:
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sudo iptables-saveThe listing shows chains, targets, packet and byte counters, and line numbers. The saved output shows the complete table-oriented form that can be restored later. Keep a copy of this output before editing so you know what policy you started with.
Legacy iptables versus an nftables-backed command
On contemporary Debian systems, the command named iptables may be the traditional legacy implementation or an iptables-nft compatibility layer. Debian warns that nftables has been the default firewall framework since Debian 10 (Buster). The command syntax can look identical while the underlying packet-filtering backend differs.
Do not assume that a recipe written for an older Debian release describes your host exactly. Determine which implementation is selected by the system’s alternatives and package configuration, and check the semantics against the iptables version installed on the machine. The legacy Debian manpage cited in older guidance is not a substitute for the version-specific documentation on your host.
The persistence path discussed here remains the Debian package convention for IPv4 rules, but you should avoid mixing an independently managed nftables ruleset with an iptables ruleset without understanding which service owns the firewall. A rule that is visible in one tool but not in the active backend is not a reliable control.
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A correct match in the wrong chain never blocks the packet you intended. Select the chain according to the packet’s direction relative to this machine:
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| Chain | Use it for | Typical match |
|---|---|---|
INPUT |
Traffic entering this host for a local process | Block a remote address connecting to a service on this machine |
OUTPUT |
Traffic generated by this host and leaving it | Prevent this machine from sending to a destination address |
FORWARD |
Traffic routed through this host rather than delivered locally | Control packets passing between interfaces on a router or gateway |
These chains are evaluated independently. An inbound connection to a daemon belongs in INPUT; a local program’s connection attempt belongs in OUTPUT; traffic merely routed onward belongs in FORWARD. If you are unsure, inspect interfaces and the machine’s routing role before adding a rule.
Put safety accepts before a blocking rule
iptables processes rules in order. Once a packet matches a terminating target such as ACCEPT or DROP, later rules in that chain do not get to change the result. A broad drop placed at the top can therefore cut off administration, while an accept placed first can make a later drop unreachable.
A practical order for an INPUT chain is:
- Accept loopback traffic with
-i lo. - Accept packets belonging to already established or related connections using conntrack state.
- Accept the management service, such as SSH on TCP port 22, from the addresses or networks that actually administer the host.
- Add narrow service-specific blocks, such as a single source address or destination port.
- Only then consider a broad default policy, and only when every required service has an explicit preceding accept.
This is not a universal policy file. It is an ordering model. Existing rules may already provide some of these accepts, and replacing them blindly can remove access that the host needs.
Write narrowly scoped DROP rules
DROP discards packets that match. The examples below use the documentation address 203.0.113.10; substitute the address or port that is appropriate for your environment.
Block one source address from reaching local services
sudo iptables -A INPUT -s 203.0.113.10 -j DROPThis appends a rule to INPUT matching packets whose source address is 203.0.113.10. It affects traffic entering the host, not traffic that the host initiates and not packets being routed between other interfaces.
Block a TCP destination port on this host
sudo iptables -A INPUT -p tcp --dport 25 -j DROPThe protocol selector limits the match to TCP and --dport 25 selects the destination service port. A UDP service on the same number would require its own UDP rule if it also needs to be blocked.
Prevent this host from contacting an address
sudo iptables -A OUTPUT -d 203.0.113.10 -j DROPThis controls locally generated traffic whose destination is the specified address. It does not prevent another machine from contacting this host.
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Decide whether an address or a service is the real requirement
- Use a source-address match when one sender must be denied regardless of which local service it targets.
- Use a destination-port match when the service must be closed to incoming clients but other services should remain reachable.
- Use an output destination match when the concern is where this host connects, rather than who connects to it.
- Use the forwarding chain only when this machine is routing the traffic.
Keep matches as specific as the requirement allows. A narrow rule is easier to audit and less likely to interfere with unrelated traffic.
Build a complete rules.v4 file
A persistent file is not just a list of isolated commands. It has a table header, built-in chain policies, rules, and a terminating COMMIT. A minimal filter-table example that preserves loopback, established connections, and SSH before blocking one source is:
*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 counters in the chain declarations start at zero in this example. The three built-in policies are shown as ACCEPT so that the file is intentionally conservative; changing a policy to DROP is a separate design decision that requires accepts for every service and management path you need.
Do not copy this sample as a blanket firewall without adapting it. Confirm the actual management port, interface layout, existing state rules, and services on the host. A rule file should represent the complete policy you intend to load, not only the one line that motivated the change.
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When the file is ready, restore it into the running firewall:
sudo iptables-restore < /etc/iptables/rules.v4This command changes the active rules immediately. It is therefore a test of the whole file, not merely a syntax exercise. If you are connected remotely, keep a second management session open. One session can verify connectivity while the other applies the change, giving you a path to correct an ordering mistake before losing access.
Check the result and counters
sudo iptables -L -v --line-numbersConfirm that:
- the intended chain contains the rule;
- the rule appears after the required loopback, established/related, and management accepts;
- the target is
DROPand the source, protocol, and port are exactly the values you intended; - packet and byte counters increase when matching traffic is generated; and
- unrelated services still work.
A counter of zero does not prove that the rule is wrong; it may simply mean that no matching traffic has arrived yet. Conversely, a counter increasing on an unexpected rule is evidence that an earlier match is handling the traffic before it reaches the block.
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Save the known-good running state to rules.v4
After testing, export the active IPv4 rules to the Debian persistence path:
sudo iptables-save | sudo tee /etc/iptables/rules.v4The pipe sends the complete current ruleset to tee, which has administrative privileges and writes the file. This avoids the common shell-redirection mistake in which sudo applies only to iptables-save while the unprivileged shell attempts to open the destination. A root-only redirection is another valid way to write the same file.
Save only after you have verified the running state. If you save first and discover that SSH was blocked or a needed accept was ordered too late, the persistence file will faithfully preserve that failure and reload it at the next boot.
Enable boot-time loading with netfilter-persistent
The Debian persistence package supplies startup integration for saved netfilter rules. Its netfilter-persistent command operates through plugins:
netfilter-persistent saveasks the plugins to save the currently loaded rules to persistent storage.netfilter-persistent startloads the persistent rules during a service start, including boot-time startup when the package integration is enabled.netfilter-persistent stopandflushprovide lifecycle operations defined by the command’s manual.
With the Debian package installed, use the package’s startup integration rather than assuming that merely creating /etc/iptables/rules.v4 is sufficient. The package template documents that the current IPv4 rules can be saved to this path and loaded automatically at startup; it also notes automatic saving during package installation and directs ongoing updates to iptables-save(8).
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If you change rules interactively later, either run netfilter-persistent save or repeat the explicit iptables-save | sudo tee /etc/iptables/rules.v4 export. Otherwise the running firewall and the file can diverge.
Verify a manual reload and a reboot
Before relying on a reboot, exercise the same load operation that startup will use:
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- Restore the file with
sudo iptables-restore < /etc/iptables/rules.v4. - Inspect the result with
sudo iptables -L -v --line-numbers. - Confirm the block rule, ordering, targets, and counters.
- After a planned reboot, run the listing again and verify that the same rules are present.
The post-reboot check matters because a rule can work interactively while the persistence service is disabled, misconfigured, or loading a different file. Verification should examine the complete chain rather than looking only for the one DROP line.
Troubleshoot the failures that matter most
Remote administration stopped working
The usual cause is a broad drop before the management accept, or a source-specific drop that also matches your administration address. Use the still-open management session or console access to restore a known-good file, then inspect line numbers and move the necessary SSH or other management accept ahead of the blocking rule. Do not keep appending exceptions indefinitely; rebuild the order so the intended decision is obvious.
The rule is present but traffic is not blocked
Check the chain first. Incoming local traffic belongs in INPUT, locally generated traffic in OUTPUT, and routed traffic in FORWARD. Then check the match fields: source versus destination address, TCP versus UDP, and destination port. Finally inspect counters. If an earlier ACCEPT matches, the packet will never reach a later DROP.
The rule disappears after reboot
Confirm that the tested rules were exported to exactly /etc/iptables/rules.v4 and that the persistence package’s startup integration is enabled. Run netfilter-persistent start manually and inspect the chains afterward. If the file loads manually but not at boot, the problem is service integration rather than the DROP syntax.
The file and the command output do not agree
The file may be stale because rules were edited interactively after the last save. Treat the active rules shown by iptables-save as the source for a new snapshot, test that snapshot with iptables-restore, and save again only after verification.
Backend behavior is unexpected
Recheck whether the host uses legacy iptables or an iptables-nft compatibility layer. Debian’s nftables-default warning applies from Debian 10 Buster onward, while older manpages may describe legacy behavior. Use the installed command and package documentation as the authority for that host instead of assuming that a rule written for another release maps identically.
Quick Recap
Command, file, and policy approaches compared
| Decision | Advantages | Risk or limitation |
|---|---|---|
| Immediate command-line rule | Fast way to change the running firewall and observe the result. | Not persistent by itself; the rule can be flushed at reboot. |
Declarative rules.v4 file |
Records the complete IPv4 policy in a repeatable form and can be restored as one unit. | A malformed or overly broad file can affect many chains at once; test before saving it as the known-good state. |
| Manual restore | Useful for controlled reloads and recovery with iptables-restore. |
Someone must run it after a reboot or change unless startup integration is configured. |
Boot-time netfilter-persistent loading |
Restores the saved rules automatically through package startup integration. | It loads what was saved; stale or incorrect contents are reproduced reliably. |
| Legacy iptables versus iptables-nft compatibility | Both expose familiar iptables commands on systems that support them. | The underlying backend and version affect semantics and administration; identify the implementation first. |
| Narrow match-specific DROP | Limits collateral impact to a source, destination, protocol, or port. | Requires precise matching and the correct chain. |
| Broad chain policy | Can provide a deny-by-default posture when every required accept is explicitly defined first. | Easy to lock out administration or break services if ordering and exceptions are incomplete. |
A repeatable checklist
- Inspect
sudo iptables -L -v --line-numbersand export a baseline withsudo iptables-save. - Identify whether the host uses legacy iptables or an iptables-nft compatibility layer.
- Choose
INPUT,OUTPUT, orFORWARDfrom the packet’s path. - Preserve loopback, established/related traffic, and management accepts before adding a drop.
- Write the narrowest matching rule for the source, destination, protocol, or port.
- Place the complete policy in
/etc/iptables/rules.v4and restore it. - Test from a second management session when working remotely.
- Inspect order, targets, and counters with
iptables -L -v --line-numbers. - Save the verified active state with
iptables-save | sudo tee /etc/iptables/rules.v4ornetfilter-persistent save. - Ensure startup integration is enabled, then verify the same rules after a reload and reboot.
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