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To whitelist an IP address in iptables, add an ACCEPT rule that matches the trusted source and place it before any broad DROP or REJECT rule. For a single IPv4 host, use a /32 mask; for one IPv6 host, use /128. Limit the rule to SSH or another required port whenever possible, verify the live rule order, and check whether UFW or another firewall manager controls the VPS before making direct changes.
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What an iptables whitelist rule actually does
iptables has no separate “whitelist” object. A whitelist is simply an ACCEPT rule whose match criteria identify traffic you trust. The Linux kernel’s netfilter subsystem evaluates packets against ordered chains and applies the target of the first matching rule, such as ACCEPT, DROP, or REJECT.
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For inbound traffic, the relevant chain is usually INPUT. A source-only rule allows every inbound service from that address. A rule that also specifies a protocol and destination port allows only that service. The second form is safer for most VPS administration because an address that may use SSH does not automatically need access to databases, control panels, or other listening ports.
Before changing the firewall
Confirm the address you intend to trust
Use the public address that the VPS will see as the packet source. A single IPv4 address is written with a /32 mask, for example 203.0.113.10/32. A subnet uses its network address and mask, such as 198.51.100.0/24. The -s or --source option accepts a host, network, or CIDR value.
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If your connection comes through a corporate gateway, VPN, bastion host, or carrier-grade NAT, the server may see that gateway’s address rather than the address shown by your workstation. Test from the actual route you will use to connect. Do not whitelist an entire dynamic ISP range merely because your personal address can change; use a controlled VPN or bastion address when a stable source is required.
Check which firewall owns the rules
Ubuntu identifies UFW as its default firewall configuration tool. Other distributions, VPS images, hosting panels, or orchestration systems may also generate netfilter rules. A direct rule entered with iptables can be overwritten by a manager reload, conflict with its policy, or disappear after a reboot.
Inspect the active configuration and management layer before proceeding. If UFW or another manager is authoritative, make the allowlist change through that manager’s documented interface, or deliberately coordinate the direct rule with it. Do not assume that a command accepted by the iptables binary will remain the source of truth.
Keep a recovery path open
- Keep an existing SSH session open while testing a new rule.
- Have the VPS provider’s console, serial terminal, or rescue access available in case you lock yourself out.
- Record the current rules with
iptables -Sand, if appropriate, save the output for review before editing. - Know how to remove the exact rule you add; a mistaken broad rule can expose more services than intended.
Choose the scope of the allowlist
| Need | Match pattern | Effect |
|---|---|---|
| One IPv4 host, every inbound service | -s 203.0.113.10/32 |
Allows matching traffic from that host to the INPUT chain, regardless of destination port. |
| One IPv4 host, SSH only | -p tcp -s 203.0.113.10/32 --dport 22 |
Allows TCP connections from that host to destination port 22. |
| An IPv4 subnet | -s 198.51.100.0/24 |
Allows the addresses covered by that network mask. |
| One IPv6 host, every inbound service | -s 2001:db8::10/128 with ip6tables |
Allows matching IPv6 traffic from one host. |
The source-only form is broader than most administrative requirements. Add -p tcp and --dport for SSH, or the appropriate protocol and port for another service. If the service listens on a nonstandard port, use that destination port instead of 22.
Allow one IPv4 address to reach the VPS
Broad source-only exception
Use this when the trusted host genuinely needs access to all inbound services covered by the INPUT chain:
sudo iptables -I INPUT 1 -s 203.0.113.10/32 -j ACCEPT-I INPUT 1 inserts the rule at position 1 in the INPUT chain. -s matches the packet source, and -j ACCEPT accepts a packet that matches. Inserting at the top gives the exception priority over a later broad deny rule.
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Recommended SSH-only exception
If the purpose is remote administration, narrow the match to TCP port 22:
sudo iptables -I INPUT 1 -p tcp -s 203.0.113.10/32 --dport 22 -j ACCEPTThis rule matches TCP traffic from 203.0.113.10 whose destination port is 22. It does not authorize that source to reach other ports. Replace 22 if the SSH daemon uses a different port.
Allow a trusted subnet
When several controlled addresses need the same access, use the subnet CIDR:
sudo iptables -I INPUT 1 -p tcp -s 198.51.100.0/24 --dport 22 -j ACCEPTA /24 covers many addresses, so verify that every possible member is trusted. A smaller mask or individual /32 rules reduces exposure when the network does not need to be broadly admitted.
Why insertion position matters
iptables evaluates a chain in order. If a preceding rule drops or rejects the packet, a later allow rule is never reached. Appending an exception after a broad deny is therefore a common reason an apparently correct whitelist “does not work.”
The -I INPUT 1 form explicitly inserts the exception before existing rules. If you choose another position, confirm that every earlier rule permits the same traffic. Do not move a rule above established-connection handling or other carefully designed policy without understanding the chain’s complete order.
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Inspect and verify the live rule set
List rules with counters and line numbers
sudo iptables -L INPUT -n -v --line-numbersThis view shows the chain’s rule numbers, packet and byte counters, numeric addresses and ports, and targets. Confirm that the new ACCEPT rule appears before a broad DROP or REJECT. After an allowed connection attempt, its packet and byte counters should increase.
Display rules in command form
sudo iptables -S INPUTThe command-form output makes the exact match and target easier to copy for auditing or removal. Check that the source mask, protocol, and destination port are what you intended, rather than relying on a truncated human-readable listing.
Test from the trusted and untrusted paths
- From the whitelisted source, connect to the intended service. For SSH, use the server’s address and the configured SSH port.
- Keep the original administrative session open while the new session is tested.
- Observe the rule counters with
sudo iptables -L INPUT -n -v --line-numbers. - From a source that should not be allowed, test only when you have console recovery available. A failed connection may appear as a timeout for
DROPor an immediate refusal forREJECT, depending on the rule reached.
A counter that remains at zero indicates that the test traffic did not match the rule, reached a different interface or address family, was handled by another chain or manager, or never arrived at the VPS.
Whitelist IPv6 separately
iptables and ip6tables administer separate IPv4 and IPv6 rule sets. An IPv4 allow rule does not permit the same client’s IPv6 connection.
For one IPv6 source, use a /128 mask and the IPv6 tool:
sudo ip6tables -I INPUT 1 -s 2001:db8::10/128 -j ACCEPTFor SSH-only IPv6 access, add the protocol and port just as with IPv4:
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sudo ip6tables -I INPUT 1 -p tcp -s 2001:db8::10/128 --dport 22 -j ACCEPTInspect the IPv6 chain with:
sudo ip6tables -L INPUT -n -v --line-numbers
sudo ip6tables -S INPUTTest both address families if the VPS has IPv4 and IPv6 connectivity. A hostname may resolve to either family, so an SSH client can appear to bypass an IPv4 rule by choosing IPv6.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Diagnose an allow rule that appears not to work
The rule is below a deny
List the chain with line numbers. If a DROP or REJECT matches first, insert the allow rule ahead of it or adjust the policy through the firewall manager that owns the chain.
The source address is different from the one you entered
Proxies, VPNs, NAT gateways, and bastions change the source address visible to the server. Check the connection path and use the address observed by the VPS. A workstation’s local address, such as a private RFC 1918 address, is normally not the public source seen across the internet.
The connection uses IPv6
An iptables rule cannot match IPv6 packets. Add the corresponding ip6tables rule or test explicitly over IPv4. Verify which address family the client selected.
The service is not listening on the expected port
An accepted packet still cannot connect to a service that is stopped, bound only to localhost, listening on another port, or blocked by a separate host or provider firewall. Confirm the daemon’s listen address and port independently of the iptables rule.
Another firewall manager rewrote the chain
UFW or a provider-managed firewall may regenerate rules and remove direct edits. Check its status and configuration, then make the exception in the authoritative layer. Mixing unmanaged commands with generated rules makes order and persistence unpredictable.
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The packet reaches another chain or interface policy
The INPUT chain handles traffic destined for the VPS itself. Forwarded traffic uses different paths, and interface-specific rules may impose additional conditions. Inspect the complete ruleset when the packet counters on the expected rule do not move.
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Delete by matching the complete rule
Use the same match criteria with -D to remove a rule. For example:
sudo iptables -D INPUT -p tcp -s 203.0.113.10/32 --dport 22 -j ACCEPTFor a broad source-only rule, remove the corresponding form:
sudo iptables -D INPUT -s 203.0.113.10/32 -j ACCEPTAlternatively, delete by the line number shown by --line-numbers. Re-list the chain afterward because line numbers shift when rules are deleted.
Correct an overly broad exception
If you initially allowed every service but only need SSH, remove the source-only rule and add the protocol-and-port rule. Do not leave both rules in place: the broad rule would continue to grant access and make the narrow rule irrelevant.
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An interactive iptables change affects the running ruleset. Whether it survives a reboot depends on the Linux distribution, installed persistence service, VPS image, and firewall manager. There is no single save-and-restore command that is safe to present for every environment.
Identify the system’s documented persistence mechanism and test it in a maintenance window. If UFW or another manager owns the firewall, save the change through that manager rather than creating a competing restore process. After a reboot or firewall reload, verify the rule order, counters, and both address families again.
Quick Recap
Practical decision checklist
- One host or a network? Use
/32for one IPv4 host,/128for one IPv6 host, and a subnet CIDR only when every covered address is trusted. - All services or one service? Prefer protocol and destination-port matches for SSH or another specific service.
- IPv4 or IPv6? Configure and test each family separately with
iptablesandip6tables. - Who owns the firewall? Check UFW, provider controls, and other managers before issuing direct commands.
- Where is the rule placed? Insert it before broad deny rules and verify with numbered output.
- Can you recover? Keep an existing session and console access until testing is complete.
- Will it persist? Follow the distribution or manager’s documented persistence method and verify after reload or reboot.
Example: a complete SSH allowlist workflow
- Determine the public source address used by the administrator, for example
203.0.113.10. - Confirm that the VPS uses direct iptables administration rather than UFW or another generated configuration.
- Insert the narrow rule:
sudo iptables -I INPUT 1 -p tcp -s 203.0.113.10/32 --dport 22 -j ACCEPT. - List the chain with
sudo iptables -L INPUT -n -v --line-numbersand verify the rule is above any broad deny. - Open a second SSH session from the trusted address while keeping the first session active.
- Check the counters and command-form output to confirm that the expected packets matched.
- If administrators also connect over IPv6, add and verify the equivalent
ip6tablesrule. - Document and implement the environment-specific persistence method, then recheck the rules after a controlled reload or reboot.
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