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Port knocking hides SSH from ordinary port scans by keeping the SSH firewall rule closed until an administrator sends a prearranged sequence of packets. A daemon such as knockd listens for that sequence without opening the knock ports as services, then adds a temporary firewall rule for the sender’s IP address. The method can reduce unsolicited exposure, but it does not replace SSH keys, patching, least privilege, logging, or a recovery path.
What port knocking changes
With a normal Internet-facing SSH host, scanners can immediately see whether TCP port 22 responds. Port knocking changes the initial state: the firewall denies SSH, while knockd watches the network interface for packets aimed at a sequence of otherwise unused TCP or UDP ports. Because the daemon observes packets at link layer, those ports do not need listening applications.
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- The client sends the configured ports in the required order and protocol.
knockdaccepts the sequence only if it arrives beforeseq_timeoutexpires.- The daemon runs a command that adds an SSH allow rule, normally restricted to the knocker’s source address through
%IP%. - A reverse sequence or a timed stop command removes that rule.
The Debian knockd(1) manual describes the purpose succinctly: “This can be used to open up holes in a firewall for quick access.” The hole is temporary only if your configuration includes a reliable close action.
Decide whether port knocking fits your threat model
Port knocking is an exposure-reduction control, not an authentication system. A fixed sequence is a shared secret sent as observable traffic. Anyone who captures it can replay it until you change the sequence. An attacker who can intercept or manipulate traffic may also interfere with the knock or the resulting connection. Ubuntu security guidance therefore treats knocking as an additional control and warns that it remains vulnerable to man-in-the-middle attacks.
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- It helps against: routine automated scans and opportunistic connection attempts that never know the sequence.
- It does not provide: cryptographic identity, confidentiality, protection from a compromised client, or a guarantee that SSH is unreachable to a determined observer.
- It can hurt availability: a daemon, firewall, interface, or sequence-file error can lock out every administrator.
Before enabling a deny rule, retain console, out-of-band management, or a separate VPN recovery path. Test that path while you can still log in; do not make port knocking your only way back into a remote machine.
Requirements and a safe starting plan
- A Linux host with a firewall you control and a public or otherwise reachable network interface.
- The
knockdpackage (the examples follow the Debian 0.7 manual; package names and service defaults can vary by distribution). - An SSH server already configured with key authentication and current security updates.
- Administrative access through an existing SSH session plus a tested console, out-of-band, or VPN fallback.
- The exact interface name that receives the knock packets. The manual’s default is
eth0, but modern systems commonly use names such asens3orenp1s0.
Record your current firewall rules before changing them. On an iptables-based host, save a copy with your distribution’s firewall-management tool or at least capture iptables -S and iptables -L -n -v. Existing rule order matters: an earlier terminal DROP can prevent a later dynamic ACCEPT from ever matching.
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On Debian or Ubuntu, install the package from the distribution repository in an already authenticated administrative session:
sudo apt update
sudo apt install knockdDo not assume that installing the package enables packet listening. Inspect the service and its distribution defaults:
systemctl status knockd
ip -br link
ip -br addrUse the interface carrying your server’s reachable address. Set it explicitly in the configuration instead of relying on the historical eth0 default, especially on hosts with multiple interfaces. Package versions may place the enable switch in a file such as /etc/default/knockd; follow the comments shipped with your package rather than copying a setting from another release.
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Build the firewall state before opening SSH dynamically
The dynamic command must insert an allow rule ahead of any blanket SSH drop. The Debian manual’s example appends an ACCEPT rule and deletes the identical rule later:
[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 documented syntax, not a universal firewall policy. If your INPUT chain ends with a DROP, use an insertion position that precedes that DROP (for example, an equivalent -I INPUT 1 command) and use the matching rule for deletion. Preserve established and related traffic, loopback, and any management exceptions required by your host. Apply equivalent policy to IPv6 if SSH is reachable over IPv6; the cited example is iptables-oriented and does not establish nftables or IPv6 behavior.
Keep port 22 closed to everyone by default. Do not create a permanent global ACCEPT merely to make testing easier. A useful test is to verify from a second network that a direct SSH attempt is refused or times out before you send a knock.
Configure the knock sequence
Create the configuration file used by your package (commonly /etc/knockd.conf) with an explicit interface and tightly scoped rules. Option names are case-insensitive in many releases, but retain the spelling shown by your installed manual.
[options]
Interface = ens3
LogFile = /var/log/knockd.log
[openSSH]
Sequence = 7000,8000,9000
Seq_Timeout = 10
TCPFlags = syn
Command = /usr/sbin/iptables -I INPUT 1 -p tcp -s %IP% --dport 22 -j ACCEPT
[closeSSH]
Sequence = 9000,8000,7000
Seq_Timeout = 10
TCPFlags = syn
Command = /usr/sbin/iptables -D INPUT -p tcp -s %IP% --dport 22 -j ACCEPTThe three ports and ten-second timeout are the manual’s example values, not magic numbers. Choose ports that do not collide with services or monitoring rules on your host, and keep the timeout short enough that a partial sequence expires quickly. Matching the TCP syn flag prevents unrelated traffic from being counted as a knock; use the protocol and flag behavior documented for your installed version.
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%IP% is important: it limits the temporary SSH rule to the address that completed the sequence. Opening port 22 to all sources defeats the principal exposure benefit. If you use a command alias such as start_command or stop_command, verify the exact option names in the local knockd(1) manual; releases differ in whether they call the action command or use explicit start and stop fields.
Make the opening rule close automatically
A reverse sequence lets an administrator close access deliberately, but it is easy to forget. Prefer a timed start/stop pair when your version supports it: run the start command after a valid sequence, then run the stop command after a defined interval. Keep a reverse close sequence as an emergency operator action.
When a close action deletes a rule, its match must be byte-for-byte equivalent to the rule that was inserted: interface/chain, protocol, source address, destination port, and target. If those details differ, the delete command can fail while the SSH hole remains open. Check the live rules after both operations rather than trusting a successful process exit alone.
Use one-time sequences when replay matters
The Debian manual supports One_Time_Sequences. After a sequence is accepted, it is invalidated and replaced by the next sequence in the configured file; the manual states: “After each successful knock, the used sequence will be invalidated and the next sequence from the sequence file has to be used for a successful knock.” This improves resistance to capture-and-replay attacks, but it creates state that must be backed up and replenished.
Enable this mode only after you understand the sequence-file format and permissions documented by your installed package. Keep the file readable and writable only by the daemon’s required account, protect a spare copy offline, and document how to load a replacement sequence through console access. A lost or exhausted file can lock out every administrator even when the firewall and SSH daemon are healthy.
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Enable the daemon and verify its logs
- Validate the file for syntax errors using the checking or foreground option documented by your package, if available.
- Enable the service at boot and start it:
sudo systemctl enable --now knockd. - Confirm the process is listening on the intended interface with
systemctl status knockdand your system log or the configured logfile. - From a separate network, confirm that direct SSH is blocked.
- Send one complete sequence, then inspect the firewall immediately for an ACCEPT rule containing your source address.
- Connect with SSH from that same source address, close the rule with the reverse sequence, and verify that the rule disappears.
If your distribution uses a different init system, service unit, logfile location, or firewall manager, use its shipped defaults. The Debian manual and package behavior are the reference for the syntax above; they do not guarantee identical paths on every Linux release.
Client workflow
The companion knock utility sends the ports in order. A typical session is:
knock -v server.example 7000 8000 9000
ssh admin@server.example
knock -v server.example 9000 8000 7000Use the protocol expected by the server configuration. The manual notes that simple TCP knocks can also be generated with tools such as telnet or netcat, but those alternatives make it easier to send the wrong flags or protocol. Wait briefly for the firewall rule to be installed before starting SSH, and ensure both commands use the same public source address. If your client changes networks between the knock and the SSH connection, %IP% will not match and the connection remains blocked.
Test the packet path, rule ordering, and timeout
Confirm packets reach the selected interface
Run a packet capture during a test, restricted to the chosen ports and interface, and verify that packets arrive in the expected order. A capture that shows nothing usually indicates the wrong interface, an upstream firewall, or a client using UDP when the server expects TCP (or the reverse).
Confirm the sequence window
Deliberately pause longer than Seq_Timeout between two knocks. The partial sequence should expire and must not open SSH. Then send the complete sequence inside the window. This proves that the timeout is active rather than merely documented.
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Confirm rule precedence
Inspect counters and order with iptables -L INPUT -n -v --line-numbers. The temporary source-specific ACCEPT must be evaluated before a general SSH DROP. If it is below that DROP, change the command to insert the rule earlier and retest the deletion command.
Confirm automatic closure
After the timed interval or reverse sequence, list the rules again. A disappearing rule is the evidence that closure works; a log message alone does not prove that the firewall state changed.
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Common failures and recovery
The knock is logged but SSH remains blocked
- Check that the source address in the dynamic rule is the address the server actually sees, especially behind NAT.
- Check rule order: a preceding DROP, another firewall framework, or a cloud security group can override iptables.
- Check that the SSH daemon listens on the port and address you are testing; the example assumes TCP 22.
No knock appears in the log
- Verify the configured interface and its routing address.
- Verify TCP versus UDP and the required TCP flags.
- Check upstream firewalls and network ACLs that may discard the knock ports.
The rule will not close
- Compare the delete command with the inserted rule, including protocol and source match.
- Look for duplicate rules created by repeated successful knocks; remove only the intended source-specific entries through console access.
- If a timed stop action failed, use the reverse sequence or an out-of-band firewall session immediately.
Administrators are locked out
Use the retained console, out-of-band channel, or VPN to stop the daemon and restore the previously saved firewall rules. Do not repeatedly guess sequences over the Internet; that can worsen a lockout and leave unclear firewall state. After recovery, test from the actual administrative network and add monitoring for daemon and rule-change failures.
Security controls that still matter
- Use SSH public-key authentication and disable password authentication where your access policy permits.
- Patch the operating system, SSH server, kernel, and firewall components.
- Run administrative accounts with least privilege and require elevation for privileged actions.
- Log SSH authentication and firewall changes, and alert on unexpected rule additions.
- Restrict the temporary rule to the source IP and expose it only for the shortest practical interval.
- Keep a tested recovery channel independent of the knock daemon.
Knocking can hide SSH from ordinary scanners, but it does not encrypt the knock sequence or authenticate the sender cryptographically. SSH’s own encryption and key verification remain the primary defense for the session itself.
Fixed sequences, one-time sequences, VPNs, and single-packet authentication
| Design | Visibility to unsolicited scans | Replay resistance | Operational complexity | Control-channel authentication | Recovery considerations |
|---|---|---|---|---|---|
| Fixed knockd sequence | SSH can remain absent from ordinary scans until the sequence succeeds. | Weak: an observed sequence can be replayed. | Lowest; a small daemon configuration and firewall commands. | None beyond possession of the sequence. | Simple to reset, but a daemon or rule error can lock out administrators. |
| One-time knockd sequences | Same scan-hiding behavior. | Stronger against replay because accepted sequences are invalidated. | Higher; sequence-file state, permissions, backup, and replenishment are required. | Still not cryptographic identity. | Protect against exhausted, lost, or desynchronized sequence files. |
| VPN-gated SSH | SSH can be limited to the VPN address space. | Depends on the VPN’s keys and configuration. | Higher; separate service, client setup, routing, and key management. | Can provide cryptographic peer authentication. | Maintain an independent administrative path and VPN recovery procedure. |
| Single-packet-authentication design | SSH remains closed until an authenticated packet is accepted. | Depends on its cryptographic construction and key handling. | Higher than a fixed sequence and requires separate deployment research. | Designed to authenticate the control packet cryptographically. | Document key rotation and a console fallback before enforcement. |
The table compares design properties, not a benchmark. Choose a fixed sequence when simplicity and scan reduction are the goal; choose one-time sequences when replay is a material concern and you can operate sequence state; choose a VPN or authenticated single-packet design when you need a cryptographically authenticated control channel and can support its additional infrastructure.
IPv6, nftables, and cloud firewalls
The documented commands use IPv4 iptables. If the host has an AAAA record or an IPv6 address, test IPv6 separately and create equivalent controls with the firewall framework actually enforcing that traffic. Do not assume that an IPv4 rule, a knock observed on one interface, or an iptables command controls IPv6 packets. Likewise, systems managed by nftables, firewalld, UFW, or a cloud security group may translate or override iptables changes. Verify the effective ruleset and document which component owns it before putting a temporary command in production.
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- Identify and explicitly configure the receiving interface.
- Save the existing firewall rules and confirm a recovery channel.
- Keep SSH denied by default and place the dynamic ACCEPT ahead of the terminal DROP.
- Use a short sequence timeout and match the intended protocol and TCP flags.
- Scope the temporary rule to
%IP%; never open port 22 globally. - Automate closure with a timed stop action and retain a reverse close sequence.
- Consider one-time sequences only with a documented, backed-up sequence-file process.
- Do not enable DNS lookup casually; the Debian manual warns that resolving addresses can reveal the first port of a sequence and make a supposedly stealthy host identifiable.
- Test direct blocking, successful opening, SSH access, timeout expiry, and closure over both IPv4 and IPv6 where applicable.
- Monitor daemon health, firewall-rule changes, and SSH logs after deployment.
Bottom line
Port knocking is a practical way to keep an SSH daemon out of routine scans: deny SSH, listen for an ordered packet sequence, insert a source-specific rule, and remove it automatically. Treat the sequence as a convenience secret rather than authentication, use one-time sequences or a cryptographically authenticated alternative when replay matters, and never deploy the rule without a tested recovery path.
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