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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsTo hide SSH behind Single Packet Authorization (SPA), install fwknop-server on the Ubuntu host, install fwknop-client on the machine you will connect from, define the permitted source and port in /etc/fwknop/access.conf, and configure the daemon in /etc/fwknop/fwknopd.conf. Send one authenticated SPA packet over UDP 62201, connect during the resulting firewall rule window, and let fwknop remove that rule automatically after the configured timeout.
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What fwknop changes about SSH exposure
fwknop implements Single Packet Authorization for strong service concealment. Instead of leaving TCP port 22 reachable and relying only on the SSH daemon to reject unwanted connections, you keep the host firewall at a default-drop policy. The fwknop daemon passively receives an SPA packet, authenticates and decrypts it, verifies that it has not been replayed, and then adds a temporary firewall rule for the requested service.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteAn SPA packet carries the access request in encrypted form, includes an authentication code, and is designed to be non-replayable. A port scanner therefore sees no normally reachable SSH service while no valid authorization exists. The concealment is not a replacement for SSH keys, a patched operating system, or a correctly configured firewall: it is an additional authorization layer that briefly exposes the service to an approved source.
The packet-to-connection sequence
- The client constructs an SPA request for a service such as
tcp/22. - The request is encrypted and authenticated with the key material configured on both sides.
fwknopdcaptures the packet on its configured interface and UDP port.- The daemon checks the source policy, decrypts and authenticates the request, and rejects a duplicate digest.
- The configured firewall backend receives a temporary allow rule.
- You establish SSH, then the rule is removed when
FW_ACCESS_TIMEOUTexpires.
Requirements and Ubuntu package roles
You need administrative access to the protected Ubuntu server, a second machine from which to send the SPA packet, and a firewall policy that does not already expose SSH to the whole internet. Keep an existing console or out-of-band session open while testing so a configuration mistake cannot lock you out.
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| Package | Install on | Purpose | Ubuntu noble listing |
|---|---|---|---|
fwknop-server |
Protected Ubuntu host | Runs fwknopd, captures SPA packets, and changes the firewall |
2.6.10-20.2build3 in the 2026 package index |
fwknop-client |
Workstation or administration host | Builds and sends SPA packets | 2.6.10-20.2build3 in the 2026 package index |
Those version numbers are specific to Ubuntu noble’s 2026 package listing. Other Ubuntu releases can publish a different build, so confirm the candidate version for the release you are actually administering before relying on package-specific behavior. The server package uses Ubuntu’s firewall and Netfilter libraries; the available backend must match the firewall implementation on your host.
Install the server and client
Install fwknop-server on Ubuntu
- Refresh the package index:
sudo apt update - Install the daemon package:
sudo apt install fwknop-server - Read any package prompt carefully. Service activation and prompt wording can vary by Ubuntu release; do not assume that installation alone means the daemon is collecting packets.
Install fwknop-client on the sending machine
On the workstation that will initiate access, run:
sudo apt update
sudo apt install fwknop-clientThe client and server do not have to run on the same computer. In the normal arrangement, only the server package is installed on the protected host, while the client package is installed wherever you administer the host from.
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Separate daemon settings from access policy
fwknop uses two configuration files with different responsibilities. Mixing them up is a common reason for a packet that looks correctly formed to be ignored.
| File | What belongs there | What does not belong there |
|---|---|---|
/etc/fwknop/fwknopd.conf |
Daemon behavior: capture interface, packet-collection/listening settings, firewall type, and related runtime options | Per-client source permissions and encryption keys |
/etc/fwknop/access.conf |
Accepted source addresses, requested ports, timeout, and encryption/authentication material | General daemon interface and backend behavior |
Configure fwknopd.conf on the server
Open the daemon configuration as root:
sudoedit /etc/fwknop/fwknopd.confSet the packet-capture interface to the interface that actually receives traffic. On a multi-homed host, do not copy an interface name from another machine. Check the names with:
ip -br link
ip route get 198.51.100.24Use the resulting interface when setting PCAP_INTF. Also verify the daemon’s capture/listening port and firewall type. The default SPA transport is UDP port 62201; if you deliberately change it, the client and daemon must use the same value. Select the firewall backend supported by the Ubuntu release and enabled on the host. A backend mismatch can let the daemon authenticate a packet without creating the rule you expect.
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After editing, save the file with permissions that do not expose any sensitive settings. The daemon configuration establishes how packets are collected and how rules are applied; the authorization stanza described next determines who may request SSH.
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Create an SSH access stanza in access.conf
Edit the access policy:
sudoedit /etc/fwknop/access.confThe following example authorizes one administration address and opens SSH for 30 seconds. The documentation-network addresses are illustrative; replace them with the real client address, server address, and account used in your deployment.
SOURCE 198.51.100.24/32;
OPEN_PORTS tcp/22;
DATA_COLLECT_MODE PCAP;
FW_ACCESS_TIMEOUT 30;SOURCE is the address or CIDR range allowed to use this stanza. A single address with a /32 mask is narrower than an entire office network. OPEN_PORTS tcp/22 limits the authorization to SSH rather than exposing unrelated services. DATA_COLLECT_MODE PCAP tells the stanza to use packet capture, and FW_ACCESS_TIMEOUT 30 makes the resulting rule expire after 30 seconds.
Choosing the source value
Use the public source address that the server will see, not necessarily the workstation’s private LAN address. If the client is behind NAT, the apparent source can change or be shared. The manpage permits SOURCE ANY, but that accepts a valid SPA packet from any source address and should be used only when the deployment genuinely requires it. A narrowly scoped address or CIDR range reduces the number of locations from which a stolen or leaked authorization credential can be used.
Adding GnuPG-based authentication
For a GnuPG setup, add the server key-ring path and the server decryption and remote-signing identifiers required by your key arrangement. The Ubuntu guide documents directives such as:
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GPG_HOME_DIR /var/lib/fwknop/.gnupg;
GPG_DECRYPT_ID 0xA1B2C3D4;
GPG_DECRYPT_PW <server-decryption-secret>;
GPG_REMOTE_ID 0xE5F6A7B8;The key IDs above are examples only. Use the actual public-key IDs from your controlled keyring, and replace the secret marker with the method appropriate to your installation. Never publish a private key, passphrase, or production secret in a tutorial, shell history, ticket, or shared configuration archive. Protect the server’s keyring and distribute matching client/server material through a secure channel.
If you use symmetric credentials instead of GnuPG, configure the matching client and server key material according to the installed fwknop documentation. The important operational rule is that both ends must use the same authentication scheme and compatible credentials; a packet encrypted for a different key cannot be authorized.
Start the daemon and inspect its output
Use the service unit installed by your Ubuntu package. First discover the exact unit name on the target release:
systemctl list-unit-files | grep -i fwknopThen check and start the unit shown by that command:
sudo systemctl status <fwknop-unit-name>
sudo systemctl start <fwknop-unit-name>Enable it for boot only after you have confirmed that the configuration is valid:
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sudo systemctl enable <fwknop-unit-name>For troubleshooting, stop the service before running the daemon in the foreground. The Ubuntu manpage documents -f and --foreground, which send diagnostics to standard error:
sudo fwknopd --foregroundWatch for messages showing the selected interface, packet collection, authentication or decryption results, source address, and firewall-rule creation. If the foreground process reports that the interface cannot be opened, correct PCAP_INTF or the permissions before testing from the client.
Send an SPA packet for SSH
Basic client command
From the machine running fwknop-client, request TCP port 22 on the server:
fwknop -A tcp/22 -D 203.0.113.10 -sIn this example, -A tcp/22 is the requested access, -D 203.0.113.10 is the fwknopd host, and -s tells the server to authorize the packet’s observed source address. Replace the documentation address with the protected host’s real address.
Explicit and NAT-aware source selection
Use -a ADDRESS when you need to place an explicit address in the request. Use -R to resolve the client’s external address when the workstation is behind NAT and the public address is not known locally. The address that the client advertises must still agree with the source policy and the route through which the packet arrives.
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GnuPG command pattern
When GnuPG signing and encryption are configured, the Ubuntu guide’s command pattern is:
fwknop -A tcp/22 --gpg-recip 0xA1B2C3D4 --gpg-sign 0xE5F6A7B8 -s -k 203.0.113.10Substitute the server recipient key, client signing key, and server address that belong to your key setup. The exact prompts and key-name syntax depend on the keyring and trust configuration. Do not copy key IDs from this example into a real deployment.
Unless you changed the configuration, fwknop sends SPA over UDP port 62201. Changing the client port without changing the daemon’s capture setting produces a silent-looking failure: the packet can leave the workstation, but fwknopd never sees it.
Connect during the authorization window
- Send the SPA packet and wait for the client to report that it has transmitted the request.
- Immediately initiate SSH to the same server address:
ssh [email protected] - Authenticate with your normal SSH credentials. SPA does not replace SSH host-key checking, user authentication, or authorization.
- After the configured timeout, attempt a new connection from the same source. With a 30-second timeout, the temporary allow rule should have been removed and the connection should no longer be admitted by the firewall.
Confirm both sides of the behavior. A successful SSH session during the window proves that the rule was created; a refused or filtered new connection after expiration proves that cleanup occurred. Review the daemon’s service log or foreground output for the packet digest, source, authentication/decryption result, rule creation, and expiration.
Troubleshoot a packet that does not open SSH
| Symptom | Likely cause | Checks and correction |
|---|---|---|
| No daemon message at all | Wrong interface, UDP port, route, or a network filter drops UDP 62201 | Verify PCAP_INTF, confirm both sides use UDP 62201, inspect the route, and run fwknopd --foreground while sending a new packet. |
| Packet is seen but source is rejected | SOURCE does not match the address the server observes |
Check the public NAT address, use -R when appropriate, or update the CIDR deliberately. Do not broaden to ANY as a first response. |
| Decryption or authentication fails | Client and server use different keys, IDs, passphrases, or GnuPG home directories | Verify the keyring path and identifiers in access.conf, confirm the client selected the intended keys, and redistribute matching material securely. |
| Authentication succeeds but no firewall rule appears | Firewall backend setting does not match the host, or the daemon lacks the required privileges | Check the configured firewall type, confirm the Ubuntu backend is installed and active, and inspect foreground diagnostics for rule-creation errors. |
| First packet works but a retransmission is rejected | Replay protection detected the same SPA digest | Generate a fresh packet rather than repeatedly sending a captured or identical packet. Duplicate rejection is expected security behavior. |
| SSH still fails during the window | Wrong destination address, SSH is bound to another port, or another firewall layer blocks the path | Confirm that OPEN_PORTS matches the actual SSH listener, connect to the address authorized by the packet, and inspect host and upstream firewall policies separately. |
| Access disappears sooner or later than expected | The configured timeout is different from the value being tested, or the test was started outside the window | Read FW_ACCESS_TIMEOUT, send a fresh packet, and time the connection attempt from successful authorization rather than from an earlier command. |
Choose the deployment controls deliberately
There is no universal best combination of key management, source policy, capture mode, and firewall backend. Select each control according to the network and operational constraints of the host.
| Decision | Option | When it fits | Trade-off |
|---|---|---|---|
| Credential model | Symmetric key | Small environments where a protected shared secret can be distributed to every authorized client | Every client that receives the shared secret must be trusted; rotating one client’s access can require broader key changes. |
| Credential model | GnuPG keys | Deployments that want separate client identities and public-key distribution | Keyring, trust, signing, decryption, and passphrase management are more involved. |
| Source policy | Fixed address or narrow CIDR | Administrators connect from a stable office, VPN egress, or known host | Address changes require a policy update or a different client path. |
| Source policy | NAT-aware resolution with -R |
Client’s public address is assigned dynamically or hidden behind NAT | The resolved address must still agree with the route and policy; shared NAT can reduce source specificity. |
| Packet collection | PCAP | The host and access stanza are configured for packet capture on the selected interface | An incorrect interface or capture filter makes valid packets invisible. |
| Firewall integration | iptables/Netfilter integration | The target Ubuntu release provides and uses the matching backend | A backend mismatch can prevent rule creation even when packet authentication succeeds. |
Security checklist before production use
- Leave the host’s default-drop policy in place; fwknop should add a narrow, temporary exception rather than replace the firewall.
- Allow only the required service and port. An SSH stanza should request
tcp/22, not a broad range, unless a documented operational need exists. - Prefer a specific
SOURCEaddress or CIDR range. TreatSOURCE ANYas an exceptional choice. - Protect symmetric secrets, GnuPG private keys, keyrings, and passphrases like login credentials.
- Verify the capture interface, UDP port, and firewall backend on the actual Ubuntu host, not on a reference system.
- Test authorization, SSH connectivity, and expiration from the real administration network before closing your recovery session.
- Keep an independent recovery path, such as a provider console or local console, for repairing a bad interface, key, or firewall setting.
- Regenerate a fresh SPA packet for each authorization attempt; do not reuse packet captures.
Expected final behavior
A correct installation has three observable properties: ordinary scans cannot use SSH while the default-drop policy is active; a newly generated, correctly authenticated SPA packet creates an allow rule only for the permitted source and port; and that rule disappears after the configured timeout. If any one of these properties is missing, use foreground diagnostics and the interface, port, source, key, and backend checks above before widening the policy.
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