Port knocking hides SSH from ordinary port scans by keeping the SSH firewall rule closed until a client sends a specific sequence of packets. The knockd daemon monitors packets at the link layer, validates the sequence and timeout, then runs a command that allows only the knocker’s source IP to reach SSH. A reverse sequence or timed stop command removes that temporary rule.
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This reduces unsolicited exposure; it does not replace SSH keys, updates, least-privilege accounts, logging or a recovery path. The implementation below uses the documented Debian knockd example as a starting point, then identifies the places where your interface, firewall framework and IPv4/IPv6 policy must differ.
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What port knocking changes
Without knocking, a public server normally has a firewall rule that permits TCP port 22, so scanners can identify an SSH listener and attackers can attempt authentication. With knocking, the initial policy denies SSH. The daemon watches for packets sent to several otherwise unused ports. No service has to listen on those knock ports: knockd observes the traffic and uses it as a trigger.
A valid sequence must arrive in the configured order before seq_timeout expires. When it matches, knockd runs start_command (or the profile’s command), commonly adding an iptables rule that allows port 22 only from the source address represented by %IP%. A reverse sequence or a timed stop command removes that rule.
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“This can be used to open up holes in a firewall for quick access.” — Debian
knockd(1)documentation.
Decide whether knocking fits your access model
Port knocking is useful when you need SSH reachable from changing networks but do not want it visible to routine, unsolicited scans. It is less suitable when administrators need a centrally authenticated control channel, when many users must connect concurrently, or when losing access to the daemon would be unacceptable without out-of-band recovery.
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- Use it as an additional layer. SSH public-key authentication, disabled password login where practical, timely updates, restricted accounts, logging and rate controls remain necessary.
- Plan recovery first. A malformed rule, stopped daemon, interface mismatch or changed client address can lock out every remote administrator. Keep a local console, provider console, out-of-band channel or separately secured VPN path available before enforcing the closed rule.
- Separate protocol families. The documented example is IPv4 and
iptables-oriented. Test and configure IPv6 independently; the example does not establish equivalentnftablesor IPv6 behavior.
How the packet sequence works
- The firewall starts with SSH denied.
knockdlistens on the selected network interface for TCP or UDP packets aimed at the configured knock ports.- The client sends each port in order. If the next packet does not arrive before
seq_timeout, the partial attempt expires. - After a complete match, the daemon executes the profile’s opening command. The command can substitute the knocker’s address with
%IP%. - SSH is attempted from that same source address while the temporary allow rule exists.
- A reverse sequence or an automatic stop command deletes the allow rule and returns SSH to its hidden state.
TCP flag matching is an important refinement. Setting tcpflags = syn tells the daemon to consider SYN packets, so ordinary traffic that happens to touch a configured port is less likely to corrupt or satisfy a sequence.
Prepare the host before installing knockd
Confirm the interface and firewall framework
Identify the interface that receives Internet traffic. The Debian manual’s default is eth0, but modern systems commonly use names such as ens3 or enp1s0, and a multihomed host may have more than one candidate. Set Interface explicitly instead of relying on a default.
Determine whether the host is managed by legacy iptables, an iptables-compatibility layer, or native nftables. The profile below calls /usr/sbin/iptables; it is a documented example, not a universal command for every distribution. Translate the rule into your firewall’s native syntax when necessary and test both IPv4 and IPv6 policies.
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Keep a recovery session open
Open a console or out-of-band session before changing the SSH rule. Do not close it until you have tested a successful knock, a failed knock, an SSH connection from the intended address and the closing behavior. If the daemon fails to start, that session is what lets you remove a bad rule.
Install and start the daemon
Package names and service defaults vary by distribution. On a Debian-family host, the package is commonly named knockd; verify the current package for your release, install it with the normal package manager and inspect the installed service unit before enabling it. A representative command is:
sudo apt-get update
sudo apt-get install knockdDo not assume installation enabled packet capture or selected the correct interface. Check the package’s service configuration, set the interface explicitly, then enable and start the service using your distribution’s service manager. Confirm that the daemon is running and that its log file is writable before closing your recovery console.
Close SSH first, then add a temporary exception
The security property comes from the initial firewall state, not from the secrecy of the knock ports. Ensure your normal policy denies new SSH connections. If you already have a broad allow rule, place the deny or temporary rules in an order that gives the intended result; an allow rule evaluated after an unconditional drop will never work.
The opening command must be restricted to the source address that completed the sequence. Avoid a rule that opens port 22 to every address. Also account for established-connection behavior in your existing firewall: knocking should grant a narrowly scoped new connection, not silently alter unrelated traffic.
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Configure a fixed sequence with knockd
The Debian manual provides this concrete profile:
[options]
logfile = /var/log/knockd.log
[openSSH]
sequence = 7000,8000,9000
seq_timeout = 10
tcpflags = syn
command = /usr/sbin/iptables -A INPUT -s %IP% --dport 22 -j ACCEPT
[closeSSH]
sequence = 9000,8000,7000
seq_timeout = 10
tcpflags = syn
command = /usr/sbin/iptables -D INPUT -s %IP% --dport 22 -j ACCEPTThis is a documented example, not a claim that ports 7000, 8000 and 9000 or the exact rule order are correct for your host. Adapt the interface, protocol, chain order, address family and service-management settings.
What each setting controls
| Setting | Purpose | Operational implication |
|---|---|---|
logfile |
Records daemon events. | Use it while testing and protect or rotate the log according to your system policy. |
sequence |
Lists the required ports in order. | A fixed sequence can be observed and replayed if an attacker captures it. |
seq_timeout |
Sets the maximum time for completing the sequence. | A short value limits partial attempts; choose one that accommodates normal network delay without making the window unnecessarily long. |
tcpflags |
Filters packets by TCP flags. | syn limits matching to connection-opening packets and reduces accidental matches from unrelated traffic. |
command |
Runs after a valid sequence. | Use %IP% so the opening rule applies only to the successful knocker. |
Set the interface explicitly
Add the interface directive required by your installed version and set it to the Internet-facing device. The manual’s default of eth0 is not reliable on every modern host. If the server has multiple interfaces, an omitted or incorrect interface can make the daemon appear healthy while it never sees the packets arriving on the real path.
Make closing automatic
A reverse sequence is useful for deliberate closure, but an administrator can forget to send it. Prefer a timed start/stop arrangement when your installed version and firewall command support it, so the allow rule is removed automatically. Whichever method you choose, verify that the delete command exactly matches the rule that was added. Duplicate or reordered rules can make a deletion appear to succeed while another allow entry remains.
Consider one-time sequences when replay matters
A fixed sequence is not a cryptographic secret. Anyone who observes the packets can send the same ports from an address that can reach the host. The manual documents One_Time_Sequences: after a successful knock, the used sequence is invalidated and the next sequence from a sequence file must be used.
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One-time sequences improve resistance to simple replay, but introduce file management, synchronization and recovery work. Protect the sequence file, distribute unused entries through a secure channel and maintain a recovery procedure for exhausted or lost sequences. Do not claim replay-proof operation: an attacker who interferes with delivery or observes a currently valid sequence may still cause availability problems.
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Send the sequence
Install the companion knock client on the administrator workstation. Its normal workflow is the destination followed by the configured ports in order:
knock example-host.example 7000 8000 9000Replace the host name and ports with your profile. The client must use the protocol and packet characteristics expected by the profile. The manual notes that Telnet or netcat can generate simple TCP knocks, but their flags, timeouts and error handling differ; use them only for controlled diagnostics.
Wait, then connect with SSH
Allow a brief interval for the firewall command to execute, then start SSH from the same public source address that sent the knocks:
ssh [email protected]If the SSH attempt comes through a different egress address—for example, because a VPN, proxy or mobile connection changed routes—the rule tied to %IP% will not match. Test from every legitimate network that administrators actually use.
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Close and verify
Send the reverse sequence:
knock example-host.example 9000 8000 7000Alternatively, wait for the timed stop command. From a separate test network, confirm that a new SSH connection is refused after closure while an already established session behaves according to your existing firewall’s stateful rules. Review the daemon log and firewall rule list after each test.
Troubleshoot failures systematically
The daemon runs but never opens SSH
- Check that
Interfacenames the interface receiving the packets. - Confirm the client sent ports in the exact order and within
seq_timeout. - Verify that the client and profile agree on TCP versus UDP and on required TCP flags.
- Read the daemon log for malformed configuration or command errors.
- Inspect firewall order. A broad drop rule before the temporary accept rule can prevent the intended match.
- Confirm that the source address in the temporary rule is the address the server actually sees, especially behind NAT.
SSH opens but does not close
- Check whether the reverse sequence is configured and sent in reverse order.
- Compare the deletion command character-for-character with the rule that was added.
- Look for duplicate allow rules from repeated successful knocks.
- Verify that the timed stop action is enabled and that its command has permission to modify the firewall.
Ordinary traffic appears to trigger or disrupt sequences
Use a short timeout and appropriate TCP flag filtering. Avoid publishing the knock ports as services, and investigate any monitoring, load-balancing or NAT device that may retransmit, reorder or transform packets. A sequence is an ordered packet event, not an authenticated session.
You are locked out
Use the console or out-of-band path you retained before deployment. Remove or correct the temporary firewall rule, restore a known-good SSH allow policy, fix the interface or command, and test again from a controlled client before re-enabling the closed state.
What port knocking protects—and what it cannot
| Design | Visibility to ordinary scans | Replay resistance | Administration | Control-channel authentication | Recovery concern |
|---|---|---|---|---|---|
Fixed knockd sequence |
SSH remains closed until the sequence succeeds. | Low: an observed sequence can be reused. | Simple profile and firewall command. | No cryptographic authentication of the knock itself. | Daemon, interface or rule errors can lock out administrators. |
| One-time sequences | SSH remains closed until an unused sequence succeeds. | Better against reuse of a consumed sequence. | Requires protected sequence-file handling and synchronization. | Still depends on sequence secrecy and packet delivery. | Lost, exhausted or desynchronized entries require recovery. |
| VPN-controlled SSH | SSH can be limited to the VPN network rather than the public Internet. | Depends on the VPN’s authentication and key management. | Separate VPN deployment and client administration. | Potentially stronger, but implementation-specific. | VPN failure can remove the normal administration path. |
| Single-packet-authentication design | Can keep the service closed until an authenticated packet is accepted. | Depends on its cryptographic design and key handling. | Separate software and key-management work. | Designed to authenticate the control packet. | Requires an independently tested emergency path. |
Port knocking therefore reduces exposure; it does not make SSH invisible to a determined observer, and it does not encrypt or authenticate the SSH login beyond SSH’s own security. Ubuntu guidance specifically warns that knocking remains vulnerable to man-in-the-middle attacks. The principal protection for the session is still SSH encryption and strong authentication.
Operational checklist
- Record the actual Internet-facing interface and configure it explicitly.
- Keep SSH denied until a valid sequence is received.
- Use a short sequence timeout and appropriate TCP flag matching.
- Scope the temporary rule to
%IP%; never open port 22 globally as the knock response. - Ensure the temporary allow rule is evaluated before any unconditional SSH drop.
- Provide a reverse sequence or automatic stop command, preferably with a timeout.
- Test IPv4 and IPv6 separately and document which firewall framework each uses.
- Protect logs and, if used, the one-time sequence file.
- Retain console, out-of-band or separately secured VPN recovery access.
- Continue using SSH keys, updates, least privilege, monitoring and sensible login controls.
Limitations of the documented example
The core profile comes from the Debian knockd 0.7 manual, whose source was last updated on 2016-10-08. Package defaults, service units, firewall compatibility layers, cloud networking and IPv6 behavior can change by distribution and release. Treat the sample ports, interface default and iptables commands as values to adapt and verify, not as universal defaults.
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