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Install fwknop-server on the Ubuntu host and fwknop-client on the workstation you will use to connect. Configure the daemon in /etc/fwknop/fwknopd.conf, put the SSH authorization policy and keys in /etc/fwknop/access.conf, keep the firewall’s default-drop policy, and send a signed or keyed SPA packet before opening SSH. fwknopd then creates a temporary allow rule and removes it when the configured timeout expires.
This arrangement hides SSH from unauthenticated connection attempts while retaining normal SSH after authorization. The procedure below uses TCP port 22, the default SPA transport port UDP 62201, and a 30-second access window as an example; change those values to match your network and risk tolerance.
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Single Packet Authorization (SPA) puts the authorization request inside one encrypted packet. The client identifies the service it needs, encrypts and authenticates the request, and sends it to the host running fwknopd. The daemon passively captures the packet, checks its authentication and integrity, decrypts the request, verifies that it is not a replay, and asks the configured firewall backend to permit the requested service.
Until that process succeeds, a default-drop firewall gives an SSH scanner no usable listening service to probe. After authorization, the firewall allows the requested source and port only for the configured period. fwknopd then removes the temporary rule. SPA is therefore an authorization layer in front of SSH, not a replacement for SSH keys, strong passwords, patching, or host hardening.
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The packet and access flow
- The workstation builds an SPA request for
tcp/22. - The request is encrypted, authenticated with the selected symmetric or GnuPG material, and protected against replay.
- The packet is sent to the server’s SPA listener, normally UDP port
62201. - fwknopd validates the packet, source policy, and requested access.
- The firewall receives a short-lived rule for the approved source and service.
- You connect with SSH during that window; when it expires, the rule is deleted.
Plan the two hosts and the firewall
You need a protected Ubuntu server and a separate client workstation. The server must have a firewall backend supported by the installed fwknop package. The client only builds and sends packets; it does not need the daemon.
| Component | Install on | Purpose |
|---|---|---|
fwknop-server |
Protected Ubuntu host | Runs fwknopd, captures SPA packets, authenticates them, and changes the firewall. |
fwknop-client |
Trusted workstation | Builds and sends the SPA packet before the SSH connection. |
| Firewall | Protected Ubuntu host | Starts with a default-drop policy and receives fwknop’s temporary allow rule. |
Keep a second administrative path available while testing, such as a provider console or local console. A syntax error or a firewall-backend mismatch can otherwise lock you out before SPA has been proven.
Install the Ubuntu packages
Install the daemon on the protected host
- Refresh package metadata:
sudo apt update- Install the server package:
sudo apt install fwknop-serverUbuntu publishes the server and client as separate packages. In the Ubuntu noble package index cited for 2026, both packages are version 2.6.10-20.2build3. Package versions are release-specific, so check the release you are actually deploying before treating that number as an expectation for another Ubuntu version.
Install the client on the workstation
- Refresh metadata:
sudo apt update- Install the packet-building client:
sudo apt install fwknop-clientPackage prompts and whether a service is enabled automatically can vary by Ubuntu release. Do not assume that installation alone means the daemon is listening; inspect the installed service and its status.
Configure fwknopd and the access policy
Keep the two configuration files separate
/etc/fwknop/fwknopd.conf contains daemon-operation settings. Use it for the capture interface, packet-collection mode, listening port, firewall type, and related runtime behavior.
/etc/fwknop/access.conf contains the authorization policy. Its stanzas specify which source may request access, which ports may be opened, the timeout, and the encryption and authentication material. Access-control directives and keys do not belong in fwknopd.conf.
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Set the daemon’s interface, port, and firewall backend
Before editing values, identify the interface that actually receives traffic addressed to the server. The interface name in PCAP_INTF must match that device. A cloud host may use a name such as ens3, while another installation may use a different predictable name; copy the result from the host rather than assuming.
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Set the packet-capture mode and listening port consistently. The example policy below uses DATA_COLLECT_MODE PCAP, and fwknop’s default SPA transport is UDP 62201. If you select another port, configure both the client and fwknopd to use it. Also select the firewall type that is present and supported on the target release. A correctly formed packet cannot create access if fwknopd is watching the wrong interface, collecting on another port, or calling an unavailable firewall backend.
Create a narrowly scoped SSH stanza
Add an SSH policy to /etc/fwknop/access.conf. This is the shape of the Ubuntu example:
SOURCE trusted-client-ip-or-network;
OPEN_PORTS tcp/22;
DATA_COLLECT_MODE PCAP;
FW_ACCESS_TIMEOUT 30;Replace trusted-client-ip-or-network with the address or CIDR range that should be allowed to submit a valid request. The 30 value is seconds. It is an example of a brief authorization window, not a universal recommendation.
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Choose symmetric keys or GnuPG
fwknop deployments can use a shared symmetric key or GnuPG encryption and signing. The sources document both controls but do not establish one universally superior choice. Base the decision on how you distribute, rotate, back up, and revoke credentials.
| Choice | What the server needs | Operational consideration |
|---|---|---|
| Symmetric key | A matching secret on the server and authorized client | Simple packet generation, but every holder of the shared secret must be treated as trusted and the secret must be transferred securely. |
| GnuPG | Server key-ring path, server decryption identity, decryption password handling, and the client’s signing identity as required by the setup | Separates encryption and signing identities, but key creation, protection, rotation, and identity selection require more administration. |
For a GnuPG setup, the Ubuntu guide shows directives such as GPG_HOME_DIR, GPG_DECRYPT_ID, GPG_DECRYPT_PW, and GPG_REMOTE_ID. Use the exact identifiers and key-ring path from your own key setup. Replace every example value; never place a real private key or password in documentation, tickets, shell history, or an article.
Limit the source address
The manpage permits SOURCE ANY, which accepts a valid SPA packet regardless of its source address. That can be useful when the client’s address genuinely changes, but it broadens the set of networks from which an otherwise valid packet can be submitted. Prefer one fixed address or a narrowly defined CIDR range when your network allows it.
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If the client is behind NAT, the server may see the translated public address rather than the workstation’s private address. The client supports -R to resolve the external address, or -a ADDRESS to provide an explicit address. Make the source policy and the address carried in the SPA request agree with what the server can actually observe.
Start and observe fwknopd
Use the service mechanism installed by your Ubuntu package. First discover the unit name and inspect its state:
systemctl list-unit-files | grep -i fwknop
sudo systemctl status UNIT_NAMEReplace UNIT_NAME with the fwknop unit shown on your release, then start it through that unit. Enable it at boot only after the configuration has passed a test:
sudo systemctl start UNIT_NAME
sudo systemctl enable UNIT_NAMEIf your package uses a different service wrapper, use that wrapper’s equivalent start and status operations. The important checks are that one daemon is running, it has read the intended files, and it is attached to the real capture interface.
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Run the daemon in the foreground for diagnostics
When a packet appears to be ignored, run fwknopd in foreground mode so diagnostic messages are written to standard error. The Ubuntu manpage documents -f and --foreground:
sudo fwknopd --foregroundDo not run a second daemon against the same interface while the service instance is active. Temporarily stop the service if necessary, then restore it after testing. Watch for messages that identify packet capture, source validation, authentication, decryption, and firewall-rule creation.
Send an SPA packet for SSH
Basic client command
From the workstation with fwknop-client, send a request for TCP port 22:
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fwknop -A tcp/22 -D SERVER_IP -s-A tcp/22requests SSH access.-D SERVER_IPnames the host running fwknopd.-stells the server to authorize the packet’s source address.
Replace SERVER_IP with the server’s reachable address. If the packet must carry a specific client address, use -a ADDRESS. If the client is behind NAT and should discover its external address, use -R as supported by your client configuration.
GnuPG example
When the access policy uses GnuPG, the Ubuntu guide shows this pattern:
fwknop -A tcp/22 --gpg-recip SERVER_KEY --gpg-sign CLIENT_KEY -s -k SERVER_IPSubstitute the actual server recipient and client signing identities. The exact prompts and key names depend on your key material. Confirm that the server can decrypt with its configured identity and that it trusts or otherwise accepts the client’s signing identity. Do not copy the literal placeholders into production.
Keep the transport port aligned
By default, the client sends SPA over UDP port 62201. If you changed that value on the client or server, change it on the other side as well and permit the packet to reach the capture interface. The SPA port is not the SSH port: UDP 62201 carries the authorization packet, while TCP 22 is opened only after successful authorization.
Connect during the authorization window
Send the packet and immediately start SSH:
ssh username@SERVER_IPUse the actual account name and server address. During the configured window, the firewall should allow the approved source to reach TCP 22. After the example 30-second timeout, fwknopd should remove the temporary rule. Test both outcomes: connect successfully during the window, then wait for expiration and confirm that a new SSH attempt is refused until another valid SPA packet is sent.
Check fwknopd’s service logs or foreground output while performing the test. A useful successful sequence includes receipt of the packet, source acceptance, authentication and decryption, creation of the firewall rule, and later removal of that rule. If SSH succeeds without a preceding authorization packet, inspect the base firewall policy; the host may still be exposing port 22 independently of fwknop.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshoot failures by symptom
| Symptom | Likely cause | What to check |
|---|---|---|
| No diagnostic message when the client sends | Wrong capture interface, blocked or mismatched UDP port, or daemon not running | Confirm the service state, verify PCAP_INTF, compare both sides’ SPA port, and run fwknopd --foreground. |
| Packet is seen but rejected for source | SOURCE does not match the address the server sees, or the client’s NAT address was not resolved |
Use a correct fixed address or CIDR, or use the client’s -R or -a ADDRESS option as appropriate. |
| Authentication or decryption failure | Different symmetric secrets, wrong GnuPG identity, unavailable key ring, or incorrect password configuration | Compare key identifiers and key-ring paths, confirm permissions, and ensure every placeholder was replaced. |
| Authorization succeeds but SSH remains unreachable | Firewall backend mismatch, wrong requested port, or an independent firewall rule still blocks traffic | Check the selected firewall type, the OPEN_PORTS value, and the rule-creation message in the daemon output. |
| SSH remains open after the timeout | The base firewall already permits it, or the temporary rule was not removed | Inspect the effective firewall policy and daemon logs; SPA cannot conceal a service that another rule exposes. |
| Repeated packet appears ineffective | Replay protection is rejecting a duplicate SPA packet | Generate a fresh packet instead of resending a captured datagram. fwknop records packet digests and rejects duplicates. |
Do not confuse an SSH failure with an SPA failure
First establish whether fwknopd accepted the packet and created a rule. If it did, test TCP reachability and SSH authentication separately. A wrong username, an unavailable SSH key, or an sshd configuration problem occurs after SPA and requires a different fix.
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Security and operational choices
Keep default deny in place
SPA works because the ordinary firewall policy denies unsolicited access. Do not replace that policy with a broad allow rule merely to make troubleshooting easier. The temporary rule should be the only new path to SSH, and its source and lifetime should be as narrow as practical.
Protect and rotate credentials
Treat symmetric secrets, GnuPG private keys, and decryption passwords as credentials. Transfer matching client and server material through a secure channel, restrict file permissions, and plan how to replace a key if a workstation is lost. A valid packet is an authorization credential; anyone who obtains the required material may be able to request access within the policy’s limits.
Choose the timeout deliberately
A short timeout reduces exposure if a rule is left in place, but it can expire while a user is still preparing an SSH connection. Start with a window that supports the real workflow, measure the time needed to connect, and reduce it once operations are reliable. The 30-second value shown here is only an example.
Account for changing client networks
Fixed source addresses provide tighter restriction. Mobile, residential, and NAT-heavy clients may require -R, an explicit -a ADDRESS, or a carefully considered broader source policy. Do not select SOURCE ANY solely to avoid diagnosing address translation; use it only when the deployment genuinely requires packets from changing networks.
Keep versions and backends aligned
The noble package listing cited for 2026 shows fwknop version 2.6.10-20.2build3 for both packages, but another Ubuntu release may ship a different build. Recheck package documentation, service-unit names, supported firewall integration, and configuration syntax when moving between releases.
Deployment options at a glance
| Decision | Narrower or simpler choice | When another choice may fit |
|---|---|---|
| Key management | Symmetric key with controlled distribution | GnuPG when separate encryption and signing identities or an existing key-management process are important. |
| Client source | Specific address or CIDR in SOURCE |
NAT-aware -R, explicit -a ADDRESS, or broader policy when the client’s public address changes. |
| Packet collection | PCAP, with the interface explicitly set | Another mode supported by the installed release and matching your network design. |
| Firewall integration | The backend installed and supported on the Ubuntu host | A different supported backend only when its rules, service behavior, and package integration have been verified. |
There is no single combination that fits every network. The deciding constraints are how keys can be distributed, whether the client’s address is stable, which interface receives the packet, and which firewall backend the Ubuntu release supports.
Quick Recap
Final validation checklist
- The server has
fwknop-server; the sending workstation hasfwknop-client. - The daemon reads
/etc/fwknop/fwknopd.conffor interface, capture mode, SPA port, and firewall behavior. - The SSH policy is in
/etc/fwknop/access.confwith a deliberate source,tcp/22, and timeout. - Client and server agree on UDP port
62201, unless both were intentionally changed. - Encryption and authentication material matches, is protected, and contains no unreplaced example values.
- The base firewall denies SSH before authorization.
- A fresh SPA packet produces a temporary rule and permits SSH.
- The rule disappears after the timeout and SSH is refused until another valid packet arrives.
- Foreground output or service logs show enough detail to distinguish capture, source, key, and firewall errors.
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