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fwknopd - Firewall Knock Operator Daemon


fwknopd [options]


fwknopd is the server component for the FireWall Knock Operator, and is responsible for monitoring and processing Single Packet Authorization (SPA) packets that are generated by fwknop clients, modifying a firewall or ACL policy to allow the desired access after decrypting a valid SPA packet, and removing access after a configurable timeout.

The main application of this program is to protect services such as SSH with an additional layer of security in order to make the exploitation of vulnerabilities (both 0-day and unpatched code) much more difficult.

The main configuration for fwknopd is maintained within two files: fwknopd.conf and access.conf. The default location for these files is determined at package configuration (typically @sysconfdir@/fwknop)The configuration variables within these files are described below.


-a, --access-file=<access-file>

Specify the location of the access.conf file. If this option is not given, fwknopd will use the compile-time default location (typically @sysconfdir@/fwknop/access.conf).

-c, --config=<config-file>

Specify the location of the fwknopd.conf file. If this option is not given, fwknopd will use the default location (typically @sysconfdir@/fwknop/fwknopd.conf.

-C, --packet-limit=<n>

Specify the number of candidate SPA packets to process and exit when this limit is reached.

-d, --digest-file=<digest-file>

Specify the location of the digest.cache file. If this option is not given, fwknopd will use the compile-time default location (typically '@localstatedir@/run/fwknop/digest.cache).

-D, --Dump-config

Dump the configuration values that fwknopd derives from the fwknopd.conf (or override files) and access.conf on stderr.

-i, --interface=<interface>

Manually specify interface on which to sniff, e.g. “-i eth0”. This option is not usually needed because the “PCAP_INTF” keyword in the fwknopd.conf file defines the sniffing interface.

-f, --foreground

Run fwknopd in the foreground instead of becoming a daemon. When run in the foreground, message that would go to the log would instead be sent to stderr. This mode is usually used when testing and/or debugging.


List only firewall rules that any running fwknopd daemon has created and then exit.


List all firewall rules including those that have nothing to do with fwknopd.


Flush any firewall rules created by a running fwknopd process. This option allows the used to easily delete fwknopd firewall rules without having to wait for them to be timed out.

-K, --Kill

Kill the current fwknopd process. This provides a quick and easy way to stop fwknopd without having to look in the process table.

-l, --locale=<locale>

Set/override the system default locale setting.

-O, --Override-config=<file>

Override config variable values that are normally read from the fwknopd.conf file with values from the specified file. Multiple override config files can be given as a comma-separated list.

-p, --pid-file=<pid-file>

Specify the location of the file. If this option is not given, fwknopd will use the compile-time default location (typically '@localstatedir@/run/fwknop/

-P, --pcap-filter=<filter>

Specify a Berkeley packet filter statement on the fwknopd command line. This overrides the value of the PCAP_FILTER variable taken from the fwknopd.conf file.

-R, --Restart

Restart the currently running fwknopd processes. This option will preserve the command line options that were supplied to the original fwknopd process but will force fwknopd to re-read the fwknopd.conf and access.conf files. This will also force a flush of the current “FWKNOP” iptables chain(s).


Rotate the digest cache file by renaming it to “<name>-old”, and starting a new one. The digest cache file is typically found in @localstatedir@/run/fwknop/digest.cache.

-S, --Status

Display the status of any fwknopd processes that may or not be running.

-v, --verbose

Run fwknopd in verbose mode. This can option can be specified multiple times to increase the verbosity of the output to the system log file (or to the screen if running in the foreground).

-h, --help

Display usage information and exit.

-V, --Version

Display version information and exit.


fwknopd references the fwknopd.conf file for configuration variables that define its operational parameters (what network interface and port to sniff, what features to enable/disable, etc.). The fwknopd.conf file does not define any access control directives.

The access control directives are contained in the access.conf file. Access control directives define encryption keys and level of access that is granted to an fwknop client that has generated the appropriate encrypted SPA message.


This section list the more prominent configuration variables used by fwknopd. It is not a complete list. There are directives for the type of firewall used by fwknopd (i.e. iptables, ipfw, or pf). You will want to make sure to check these to make sure they have appropriate values. See the fwknopd.conf file for the full list and corresponding details.

PCAP_INTF <interface>

Specify the ethernet interface on which fwknopd will sniff packets.


By default fwknopd puts the pcap interface into promiscuous mode. Set this to “N” to disable that behavior (non-promiscuous).

PCAP_FILTER <pcap filter spec>

Define the filter used for PCAP modes; fwknopd defaults to UDP port 62201. However, if an fwknop client uses the --rand-port option to send the SPA packet over a random port, then this variable should be updated to something like “udp dst portrange 10000-65535”.


This instructs fwknopd to not honor SPA packets that have an old time stamp. The value for “old” is defined by the “MAX_SPA_PACKET_AGE” variable. If “ENABLE_SPA_PACKET_AGING” is set to “N”, fwknopd will not use the client time stamp at all.


Defines the maximum age (in seconds) that an SPA packet will be accepted. This requires that the client system is in relatively close time synchronization with the fwknopd server system (NTP is good). The default age is 120 seconds (two minutes).


Defines an expiration date for the access stanza in MM/DD/YYYY format. All SPA packets that match an expired stanza will be ignored. This parameter is optional.


Defines an expiration date for the access stanza as the epoch time, and is useful if a more accurate expiration time needs to be given than the day resolution offered by the ACCESS_EXPIRE variable above. All SPA packets that match an expired stanza will be ignored. This parameter is optional.


Track digest sums associated with previous SPA packets processed by fwknopd. This allows digest sums to remain persistent across executions of fwknopd. The default is “Y”. If set to “N”, fwknopd will not check incoming SPA packet data against any previously save digests. It is a good idea to leave this feature on to reduce the possibility of being vulnerable to a replay attack.


Allow SPA clients to request access to services through an iptables firewall instead of just to it (i.e. access through the FWKNOP_FORWARD chain instead of the INPUT chain).


Allow SPA clients to request access to a local socket via NAT. This still puts an ACCEPT rule into the FWKNOP_INPUT chain, but a different port is translated via DNAT rules to the real one. So, the user would do “ssh -p <port>” to access the local service (see the --NAT-local and --NAT-rand-port on the fwknop client command line).


Set this to “Y” to enable a corresponding SNAT rule. By default, if forwarding access is enabled (see the “ENABLE_IPT_FORWARDING” variable above), then fwknopd creates DNAT rules for incoming connections, but does not also complement these rules with SNAT rules at the same time. In some situations, internal systems may not have a route back out for the source address of the incoming connection, so it is necessary to also apply SNAT rules so that the internal systems see the IP of the internal interface where fwknopd is running.

SNAT_TRANSLATE_IP <ip_address>

Specify the IP address for SNAT. This functionality is only enabled when “ENABLE_IPT_SNAT” is set to “Y” and by default SNAT rules are built with the MASQUERADE target (since then the internal IP does not have to be defined here in the fwknopd.conf file), but if you want fwknopd to use the SNAT target, you must also define an IP address with the “SNAT_TRANSLATE_IP” variable.


Add ACCEPT rules to the FWKNOP_OUTPUT chain. This is usually only useful if there are no state tracking rules to allow connection responses out and the OUTPUT chain has a default-drop stance.


Specify the the maximum number of bytes to sniff per frame. 1500 is the default.


Flush all existing rules in the fwknop chains at fwknopd start time. The default is “Y”.


Flush all existing rules in the fwknop chains when fwknopd is stopped or otherwise exits cleanly. The default is “Y”.


If GPG keys are used instead of a Rijndael symmetric key, this is the default GPG keys directory. Note that each access block in access.conf can specify its own GPG directory to override this default. If not set here or in an access.conf stanza, then the $HOME/.gnupg directory of the user running fwknopd (most likely root).

LOCALE <locale>

Set the locale (via the LC_ALL variable). This can be set to override the default system locale.


Allow fwknopd to acquire SPA data from HTTP requests (generated with the fwknop client in --HTTP mode). Note that when this is enabled, the “PCAP_FILTER” variable would need to be updated to sniff traffic over TCP/80 connections and a web server should be running on the same server as fwknopd.


Enable the fwknopd TCP server. This is a "dummy" TCP server that will accept TCP connection requests on the specified TCPSERV_PORT. If set to "Y", fwknopd will fork off a child process to listen for, and accept incoming TCP request. This server only accepts the request. It does not otherwise communicate. This is only to allow the incoming SPA over TCP packet which is detected via PCAP. The connection is closed after 1 second regardless. Note that fwknopd still only gets its data via pcap, so the filter defined by PCAP_FILTER needs to be updated to include this TCP port.


Sets the number of packets that are processed when the pcap_dispatch() call is made. The default is zero, since this allows fwknopd to process as many packets as possible in the corresponding callback where the SPA handling routine is called for packets that pass a set of prerequisite checks. However, if fwknopd is running on a platform with an old version of libpcap, it may be necessary to change this value to a positive non-zero integer. More information can be found in the pcap_dispatch(3) man page.

PCAP_LOOP_SLEEP <microseconds

Sets the number of microseconds to passed as an argument to usleep() in the pcap loop. The default is 10000, or 1/10th of a second.


Set the port number that the “dummy” TCP server listens on. This server is only spawned when “ENABLE_TCP_SERVER” is set to “Y”.


Override syslog identity on message logged by fwknopd. The defaults are usually ok.


Override syslog facility. The “SYSLOG_FACILITY” variable can be set to one of “LOG_LOCAL{0-7}” or “LOG_DAEMON” (the default).


This section describes the access control directives in the access.conf file. Theses directives define encryption keys and level of access that is granted to fwknop clients that have generated the appropriate encrypted message.

The access.conf variables described below provide the access directives for the SPA packets with a source (or embedded request) IP that matches an address or network range defined by the “SOURCE” variable. All variables following “SOURCE” apply to the source stanza. Each “SOURCE” directive starts a new stanza.


This defines the source address from which the SPA packet will be accepted. The string “ANY” is also accepted if a valid SPA packet should be honored from any source IP. Every authorization stanza in access.conf definition must start with the “SOURCE” keyword. Networks should be specified in CIDR notation (e.g. “”), and individual IP addresses can be specified as well. Also, multiple IP’s and/or networks can be defined as a comma separated list (e.g. “,”)

OPEN_PORTS: <proto/port>,…​,<proto/port>

Define a set of ports and protocols (tcp or udp) that will be opened if a valid knock sequence is seen. If this entry is not set, fwknopd will attempt to honor any proto/port request specified in the SPA data (unless of it matches any “RESTRICT_PORTS” entries). Multiple entries are comma-separated.

RESTRICT_PORTS: <proto/port>,…​,<proto/port>

Define a set of ports and protocols (tcp or udp) that are explicitly not allowed regardless of the validity of the incoming SPA packet. Multiple entries are comma-separated.

KEY: <password>

Define the key used for decrypting an incoming SPA packet that is using its built-in (Rijndael) encryption. This variable is required for all non-GPG-encrypted SPA packets.


Define the length of time access will be granted by fwknopd through the firewall after a valid knock sequence from a source IP address. If “FW_ACCESS_TIMEOUT” is not set then the default timeout of 30 seconds will automatically be set.


This instructs fwknopd to accept complete commands that are contained within an authorization packet. Any such command will be executed on the fwknopd server as the user specified by the “CMD_EXEC_USER” or as the user that started fwknopd if that is not set.

CMD_EXEC_USER: <username>

This specifies the user that will execute commands contained within a SPA packet. If not specified, fwknopd will execute it as the user it is running as (most likely root). Setting this to a non-root user is highly recommended.


Require a specific username from the client system as encoded in the SPA data. This variable is optional and if not specified, the username data in the SPA data is ignored.


Force all SPA packets to contain a real IP address within the encrypted data. This makes it impossible to use the -s command line argument on the fwknop client command line, so either -R has to be used to automatically resolve the external address (if the client behind a NAT) or the client must know the external IP.


For any valid SPA packet, force the requested connection to be NAT’d through to the specified (usually internal) IP and port value. This is useful if there are multiple internal systems running a service such as SSHD, and you want to give transparent access to only one internal system for each stanza in the access.conf file. This way, multiple external users can each directly access only one internal system per SPA key.

GPG_HOME_DIR: <path>

Define the path to the GnuPG directory to be used by the fwknopd server. If this keyword is not specified within access.conf then fwknopd will default to using the /root/.gnupg directory for the server key(s) for incoming SPA packets handled by the matching access.conf stanza.


Define a GnuPG key ID to use for decrypting SPA messages that have been encrypted by an fwknop client. This keyword is required for authentication that is based on GPG keys. The GPG key ring on the client must have imported and signed the fwknopd server key, and vice versa. It is ok to use a sensitive personal GPG key on the client, but each fwknopd server should have its own GPG key that is generated specifically for fwknop communications. The reason for this is that the decryption password for the server key must be placed within the access.conf file for fwknopd to function (it has to be able to decrypt SPA messages that have been encrypted with the server’s public key). For more information on using fwknop with GnuPG keys, see the following link: “”;.

GPG DECRYPT_PW: <decrypt password>

Specify the decryption password for the gpg key defined by the “GPG_DECRYPT_ID” above. This is a required field for gpg-based authentication.


Allow fwknopd to leverage a GnuPG key pair that does not have an associated password. While this may sound like a controversial deployment mode, in automated environments it makes sense because "there is usually no way to store a password more securely than on the secret keyring itself" according to: “”;. Using this feature and removing the passphrase from a GnuPG key pair is useful in some environments where libgpgme is forced to use gpg-agent and/or pinentry to collect a passphrase.


With this setting set to Y, fwknopd check all GPG-encrypted SPA messages for a signature (signed by the sender’s key). If the incoming message is not signed, the decryption process will fail. If not set, the default is N.


Setting this will allow fwknopd to accept incoming GPG-encrypted packets that are signed, but the signature did not pass verification (i.e. the signer key was expired, etc.). This setting only applies if the GPG_REQUIRE_SIG is also set to Y.

GPG_REMOTE_ID: <keyID,…​,keyID>

Define a list of gpg key ID’s that are required to have signed any incoming SPA message that has been encrypted with the fwknopd server key. This ensures that the verification of the remote user is accomplished via a strong cryptographic mechanism. This setting only applies if the “GPG_REQUIRE_SIG” is set to Y. Separate multiple entries with a comma.



The main configuration file for fwknop.


Defines all knock sequences and access control directives.


The fwknopd daemon requires a functioning firewall on the underlying operating system. Supported firewalls as of the fwknop-2.0 release are iptables, ipfw, and pf.


fwknopd can be run in debug mode by combining the -f, --foreground and the -v, --verbose command line options. This will disable daemon mode execution, and print verbose information to the screen on stderr as packets are received.


fwknop(8), iptables(8), libfko documentation.


Damien Stuart <>, Michael Rash <>


This “C” version of fwknopd was derived from the original Perl-based version on which many people who are active in the open source community have contributed. See the CREDITS file in the fwknop sources, or visit to view the online list of contributors.

The phrase “Single Packet Authorization” was coined by MadHat and Simple Nomad at the BlackHat Briefings of 2005 (see:


Send bug reports to Suggestions and/or comments are always welcome as well.


fwknopd is distributed under the GNU General Public License (GPL), and the latest version may be downloaded from

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