Provided by: fwknop-server_2.6.0-2.2_i386 bug


       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 authenticating
       and decrypting a valid SPA packet (in that order), and removing access
       after a configurable timeout.

       The main application of this program is to conceal 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.
       In addition, services that are concealed in this fashion naturally
       cannot be scanned for with Nmap.

       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 /etc/fwknop). The
       configuration variables within these files are described below.


       -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.

       -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 /etc/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

       -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 /var/run/fwknop/digest.cache).

       -D, --dump-config
           Dump the configuration values that fwknopd derives from the
           /etc/fwknop/fwknopd.conf' (or override files) and
           /etc/fwknop/access.conf on stderr.

           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
           /etc/fwknop/fwknopd.conf' file with values from the specified file.
           Multiple override config files can be given as a comma-separated

       -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 '/var/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 /etc/fwknop/fwknopd.conf' file.

           This option instructs fwknopd to read packet data from a pcap file
           instead of sniffing an interface directly. This mode is usually
           used for debugging purposes, and will disable SPA packet age
           checking unless it is manually enabled in the
           /etc/fwknop/fwknopd.conf file.

           Allow fwknopd to sniff SPA packets regardless of whether they are
           received on the sniffing interface or sent from the sniffing
           interface. In the later case, this can be useful to have fwknopd
           sniff SPA packets that are forwarded through a system and destined
           for a different network. If the sniffing interface is the egress
           interface for such packets (and hence SPA packets are sent by this
           interface instead of received), then this option will need to used
           in order for fwknopd to see them. The default is to only sniff
           packets that are received on the sniffing interface. Note that this
           setting is independent of promiscuous mode.

       -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 /etc/fwknop/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

       -S, --status
           Display the status of any fwknopd processes that may or not be
           running. If there is an existing fwknopd process then 0 is returned
           for the exit status and 1 is returned otherwise.

           Allow messages to be sent to syslog even if the foreground mode is

       -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 /etc/fwknop/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
       /etc/fwknop/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 /etc/fwknop/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.

       MAX_SPA_PACKET_AGE <seconds>
           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.

       ACCESS_EXPIRE_EPOCH <seconds>
           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

           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 /etc/fwknop/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.

       MAX_SNIFF_BYTES <bytes>
           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”.

       GPG_HOME_DIR <path>
           If GPG keys are used instead of a Rijndael symmetric key, this is
           the default GPG keys directory. Note that each access block in
           /etc/fwknop/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.

       PCAP_DISPATCH_COUNT <count>
           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

           Controls whether fwknopd is permitted to sniff SPA packets
           regardless of whether they are received on the sniffing interface
           or sent from the sniffing interface. In the later case, this can be
           useful to have fwknopd sniff SPA packets that are forwarded through
           a system and destined for a different network. If the sniffing
           interface is the egress interface for such packets, then this
           variable will need to be set to "Y" in order for fwknopd to see
           them. The default is "N" so that fwknopd only looks for SPA packets
           that are received on the sniffing interface (note that this is
           independent of promiscuous mode).

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

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

       SYSLOG_FACILITY <facility>
           Override syslog facility. The “SYSLOG_FACILITY” variable can be set

       This section describes the access control directives in the
       /etc/fwknop/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.

       SOURCE <IP,..,IP/NET,..,NET/ANY>
           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
           /etc/fwknop/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 <passphrase>
           Define the symmetric key used for decrypting an incoming SPA packet
           that is encrypted by the fwknop client with Rijndael. The actual
           encryption key that is used is derived from the standard PBKDF1
           algorithm. This variable is required for all SPA packets unless
           GnuPG is used instead (see the GPG variables below).

       KEY_BASE64 <base64 encoded passphrase>
           Same as the KEY option above, but specify the symmetric key as a
           base64 encoded string. This allows non-ascii characters to be
           included in the base64-decoded key.

       HMAC_KEY <key>
           Specify the HMAC key for authenticated encryption of SPA packets.
           This supports both Rijndael and GPG encryption modes, and is
           applied according to the encrypt-then-authenticate model.

       HMAC_KEY_BASE64 <base64 encoded key>
           Specify the HMAC key as a base64 encoded string. This allows
           non-ascii characters to be included in the base64-decoded key.

       FW_ACCESS_TIMEOUT <seconds>
           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.

       ENCRYPTION_MODE <mode>
           Specify the encryption mode when AES is used. The default is CBC
           mode, but other modes can be selected such as OFB and CFB. In
           general, it is recommended to not use this variable and leave it as
           the default. Note that the string “legacy” can be specified in
           order to generate SPA packets with the old initialization vector
           strategy used by versions of fwknop before 2.5. With the 2.5
           release, fwknop uses PBKDF1 for key derivation.

       HMAC_DIGEST_TYPE <digest algorithm>
           Specify the digest algorithm for incoming SPA packet
           authentication. Must be one of MD5, SHA1, SHA256, SHA384, or
           SHA512. This is an optional field, and if not specified then
           fwknopd defaults to using SHA256 if the access stanza requires an

           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

       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_USERNAME <username>
           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 and set it
           via the -a argument.

       FORCE_NAT <IP> <PORT>
           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.

       FORCE_SNAT <IP>
           For any valid SPA packet, add an SNAT rule in addition to any DNAT
           rule created with a corresponding (required) FORCE_NAT variable.
           This is analogous to “SNAT_TRANSLATE_IP” from the
           /etc/fwknop/fwknopd.conf file except that it is per access stanza
           and overrides any value set with “SNAT_TRANSLATE_IP”. This is
           useful for situations where an incoming NAT’d connection may be
           otherwise unanswerable due to routing constraints (i.e. the system
           receiving the SPA authenticated connection has a default route to a
           different device than the SPA system itself).

           This is similar to the “FORCE_SNAT” variable, except that it is not
           necessary to also specify an IP address for SNAT rules because the
           MASQUERADE target is used instead.

       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
           /etc/fwknop/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.

       GPG_DECRYPT_ID <keyID>
           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 /etc/fwknop/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

       GPG_ALLOW_NO_PW <Y/N>
           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.


       fwknopd requires libfko which is normally included with both source and
       binary distributions, and is a dedicated library developed by the
       fwknop project.

       For packet sniffing, fwknopd currently requires libpcap, but future
       versions still remove this as a dependency.

       For GPG functionality, GnuPG must also be correctly installed and
       configured along with the libgpgme library.

       To take advantage of all of the authentication and access management
       features of the fwknopd daemon/service a functioning iptables, ipfw, or
       pf firewall is required on the underlying operating system.


       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.

       The most comprehensive way to gain diagnostic information on fwknopd is
       to run the test suite script located in the test/
       directory in the fwknop sources. The test suite runs sends fwknop
       through a large number of run time tests, has valgrind support,
       validates both SPA encryption and HMAC results against OpenSSL, and
       even has its own built in fuzzer for SPA communications.


       fwknopd(8), iptables(8), pf(4), pfctl(8), ipfw(8), gpg(1), libfko

       More information on Single Packet Authorization can be found in the
       paper “Single Packet Authorization with fwknop” available at A comprehensive
       tutorial on fwknop operations and theory can be found at This
       tutorial also includes information about the design of fwknop that may
       be worth reading for those interested in why fwknop is different from
       other SPA implementations.

       fwknop uses the git versioning system as its source code repository
       along with Github for tracking of issues and milestones:

               $ git clone fwknop.git

       Additional commentary on Single Packet Authorization can be found via
       Michael Rash’s Twitter feed:,


       Damien Stuart <>, Michael Rash <>


       This “C” version of fwknop 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. A few contributors deserve to be
       singled out including: Franck Joncourt, Max Kastanas, Vlad Glagolev,
       Sean Greven, Hank Leininger, Fernando Arnaboldi, and Erik Gomez.

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


       Send bug reports to or, or open
       a new issue on Github (see
       Suggestions and/or comments are always welcome as well. Additional
       information may be found in the fwknop mailing list archives (see:


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