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 or Shodan.
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.
Additional information may be found in the tutorial “Single Packet Authorization: A Comprehensive Guide to Strong Service Concealment with fwknop” available online (see: http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html).
- -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).
Specify the location of the access.conf folder. If this option is given, rather than load a single access.conf file, all the .conf files in the given folders are processed.
- -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 /etc/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 /var/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.
Dump all possible fwknopd error codes to stdout and exit. This option is rarely needed in practice, and was added to assist with test coverage.
This option is only used for fault injection testing when fwknop is compiled to support the libfiu library (see: http://blitiri.com.ar/p/libfiu/). Under normal circumstances this option is not used, and any packaged version of fwknop will not have code compiled in so this capability is not enabled at run time. It is documented here for completeness. version of fwknop will not have code compiled in to enable this capability at run time. It is documented here for completeness.
- -A, --afl-fuzzing
Instruct fwknopd to acquire SPA packets directly from stdin in support of fuzzing operations from the American Fuzzy Lop (AFL) fuzzer written by Michal Zalewski. This requires that fwknop is compiled with the --enable-afl-fuzzing argument to the configure script as this allows encryption/digest short circuiting in a manner necessary for AFL to function properly. The benefit of this strategy is that AFL can fuzz the SPA packet decoding routines implemented by libfko.
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.
Parse config files (/etc/fwknop/fwknopd.conf, and /etc/fwknop/access.conf) and then exit. This provides a way to test whether the config files are properly structured without having to start processing network traffic.
Parse the digest cache file /var/fwknop/digest.cache and exit. This validates the structure of the digest cache file without having to start processing network traffic. Note that the standard configuration files are also parsed in this mode.
- -l, --locale=<locale>
Set/override the system default locale setting.
Disable the usage of the iptables -C option. This is not normally needed, and is only useful on older Linux distributions where iptables does not have -C support.
- -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 list.
Have fwknopd generate both Rijndael and HMAC keys that can be used for SPA packet encryption and authentication. These keys are derived from /dev/urandom and then base64 encoded before being printed to stdout, and are meant to be manually included in a stanza within the /etc/fwknop/access.conf file. Such keys are generally more secure than passphrases.
Write generated keys to the specified file. Note that the file is overwritten if it already exists. If this option is not given, then --key-gen writes the keys to stdout.
Specify the number of bytes for a generated Rijndael key. The maximum size is currently 128 bytes.
Specify the number of bytes for a generated HMAC key. The maximum size is currently 128 bytes.
- -p, --pid-file=<pid-file>
Specify the location of the fwknopd.pid file. If this option is not given, fwknopd will use the compile-time default location (typically /var/fwknop/fwknopd.pid).
- -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 /var/fwknop/digest.cache.
- -r, --run-dir=<path>
Specify the directory where fwknopd writes run time state files. The default is /var.
- -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 set.
- -t, --test
Run fwknopd in test mode. This instructs fwknopd to acquire and process SPA packets, but not manipulate firewall rules or execute commands that are provided by SPA clients. This option is mostly useful for the fuzzing tests in the test suite to ensure broad code coverage under adverse conditions.
- -U, --udp-server
Run fwknopd in UDP server mode so that SPA packets are acquired via a UDP socket directly without having to use libpcap. See the discussion of the “ENABLE_UDP_SERVER” configuration variable below for more information.
- -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 Config and Access Variables
fwknopd references the /etc/fwknop/fwknopd.conf file for configuration variables to define 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 or set any encryption or authentication keys.
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. You will want to make sure to check these to make sure they have appropriate values, but sensible defaults are provided for most systems. See the /etc/fwknop/fwknopd.conf file for additional details.
- PCAP_INTF <interface>
Specify the ethernet interface on which fwknopd will sniff packets.
- ENABLE_PCAP_PROMISC <Y/N>
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”.
- ENABLE_SPA_PACKET_AGING <Y/N>
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).
- ENABLE_DIGEST_PERSISTENCE <Y/N>
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.
- RULES_CHECK_THRESHOLD <count>
Defines the number of times firewall rule expiration times must be checked before a "deep" check is run. This allows fwknopd to remove rules that contain a proper exp<time> even if a third party program added them instead of fwknopd. The default value for this variable is 20, and this typically results in this check being run every two seconds or so. To disable this type of checking altogether, set this variable to zero.
- ENABLE_IPT_FORWARDING <Y/N>
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).
- ENABLE_IPT_LOCAL_NAT <Y/N>
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).
- ENABLE_IPT_SNAT <Y/N>
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. Note that this variable is generally deprecated in favor of the “FORCE_SNAT” variable in the /etc/fwknop/access.conf file which enables per-stanza control over the SNAT IP.
- ENABLE_IPT_OUTPUT <Y/N>
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_IPT_AT_INIT <Y/N>
Flush all existing rules in the fwknop chains at fwknopd start time. The default is “Y”.
- FLUSH_IPT_AT_EXIT <Y/N>
Flush all existing rules in the fwknop chains when fwknopd is stopped or otherwise exits cleanly. The default is “Y”.
- EXIT_AT_INTF_DOWN <Y/N>
When fwknopd is sniffing an interface, if the interface is administratively downed or unplugged, fwknopd will cleanly exit and an assumption is made that any process monitoring infrastructure like systemd or upstart will restart it. However, if fwknopd is not being monitored by systemd, upstart, or anything else, this behavior can be disabled with the “EXIT_AT_INTF_DOWN” variable. If disabled, fwknopd will try to recover when a downed interface comes back up.
- ENABLE_RULE_PREPEND <Y/N>
For systems running iptables or firewalld, have fwknopd insert new SPA rules at the beginning of the relevant chain (such as “FWKNOP_INPUT”) instead of appending them to the end of the chain. This causes newly created rules to have precedence over older ones.
- ENABLE_NAT_DNS <Y/N>
Allow fwknopd to resolve hostnames in NAT access messages.
- 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 stanza 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).
- GPG_EXE <path>
Specify the path to GPG, and defaults to /usr/bin/gpg if not set.
- LOCALE <locale>
Set the locale (via the LC_ALL variable). This can be set to override the default system locale.
- ENABLE_SPA_OVER_HTTP <Y/N>
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_X_FORWARDED_FOR <Y/N>
Allows fwknopd to use the X-Forwarded-for header from a captured SPA packet over HTTP as the source IP. This can happen when using SPA through an HTTP proxy.
- ENABLE_TCP_SERVER <Y/N>
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.
- 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”.
- ENABLE_UDP_SERVER <Y/N>
Enable the fwknopd UDP server. This instructs fwknopd to acquire SPA packets via a UDP socket directly without having to use libpcap. When this mode is enabled, fwknop should be compiled with --enable-udp-server (passed to the configure script) so that libpcap can be removed as a dependency. As one would expect, when the UDP server is used, no incoming packets are ever acknowledged by fwknopd and therefore collecting SPA packets in this mode is a good alternative to sniffing the wire directly.
- UDPSERV_PORT <port>
Set the port number that the UDP server listens on. This server is only spawned when “ENABLE_UDP_SERVER” is set to “Y”.
- 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 100000, or 1/10th of a second.
- ENABLE_PCAP_ANY_DIRECTION <Y/N>
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).
- 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 to
- ENABLE_DESTINATION_RULE <Y/N>
Controls whether fwknopd will set the destination field on the firewall rule to the destination address specified on the incoming SPA packet. This is useful for interfaces with multiple IP addresses hosting separate services. If “ENABLE_IPT_OUTPUT” is set to “Y”, the source field of the firewall rule is set. FORWARD and SNAT rules are not affected however, DNAT rules will also have their destination field set. The default is “N”, which sets the destination field to 0.0.0.0/0 (any).
- FWKNOP_RUN_DIR <path>
Specify the directory where fwknopd writes run time state files. The default is /var.
This section describes the access control directives in the /etc/fwknop/access.conf file. Theses directives define encryption and authentication keys, and the level of access that is granted to fwknop clients that have generated an appropriate encrypted and authenticated SPA packet.
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. “192.168.10.0/24”), 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. “192.168.10.0/24,10.1.1.123”)
- DESTINATION <IP,..,IP/NET,..,NET/ANY>
This defines the destination address for which the SPA packet will be accepted. The string “ANY” is also accepted if a valid SPA packet should be honored to any destination IP. Networks should be specified in CIDR notation (e.g. “192.168.10.0/24”), 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. “192.168.10.0/24,10.1.1.123”)
- 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.
- %include <file>
Have fwknopd import an additional access.conf file. This allows more access stanzas to be defined in other locations in the filesystem, and this can be advantageous in some scenarios by letting non-privileged users define their own encryption and authentication keys for SPA operations. This way, users do not need write access to the main /etc/fwknop/access.conf file to change keys around or define new ones.
- %include_folder <directory>
Similarly to the %include option above, the %include_folder directive has fwknopd import all .conf files from the specified directory. There is also command line support for this via the access-folder option.
- 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, SHA512, SHA3_256, or SHA3_512. This is an optional field, and if not specified then fwknopd defaults to using SHA256 if the access stanza requires an HMAC.
- ACCESS_EXPIRE <MM/DD/YYYY>
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.
- ENABLE_CMD_EXEC <Y/N>
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.
- ENABLE_CMD_SUDO_EXEC <Y/N>
sudo provides a powerful means of restricting the sets of commands that users can execute via the “sudoers” file. By enabling this feature (and in “ENABLE_CMD_EXEC” mode), all incoming commands from valid SPA packets will be prefixed by “/path/to/sudo -u <user> -g <group>” where the path to sudo is set by the “SUDO_EXE” variable, “<user>” is set by the “CMD_SUDO_EXEC_USER” variable (default is “root” if not set), and “<group>” is set by “CMD_SUDO_EXEC_GROUP” (default is also “root” if not set).
- CMD_EXEC_USER <username>
Specify the user (via setuid) that will execute a command contained within a SPA packet. If this variable is not given, fwknopd will execute the command as the user it is running as (most likely root). Setting this to a non-root user such as “nobody” is highly recommended if elevated permissions are not needed.
- CMD_SUDO_EXEC_USER <username>
Specify the user (via “sudo -u <user>”) that will execute a command contained within a SPA packet. If this variable is not given, fwknopd will assume the command should be executed as root.
- CMD_EXEC_GROUP <groupname>
Specify the group (via setgid) that will execute a command contained within a SPA packet. If this variable is not given, fwknopd will execute the command as the user it is running as (most likely root). Setting this to a non-root user such as “nobody” is highly recommended if elevated permissions are not needed.
- CMD_SUDO_EXEC_GROUP <groupname>
Specify the group (via “sudo -g <group>”) that will execute a command contained within a SPA packet. If this variable is not given, fwknopd will assume the command should be executed as root.
- CMD_CYCLE_OPEN <command>
Specify a command open/close cycle to be executed upon receipt of a valid SPA packet. This directive sets the initial command, and is meant to be used in conjunction with the “CMD_CYCLE_CLOSE” variable below. The main application of this feature is to allow fwknopd to interact with firewall or ACL’s that are not natively supported, and facilitate the same access model as for the main supported firewalls such as iptables. That is, a command is executed to open the firewall or ACL, and then a corresponding close command is executed after a timer expires. Both the “CMD_CYCLE_OPEN” and “CMD_CYCLE_CLOSE” variables support special substitution strings to allow values to be taken from the SPA payload and used on the command line of the executed command. These strings begin with a “$” character, and include “$IP” (the allow IP decrypted from the SPA payload), “$SRC” (synonym for “$IP”) , “$PKT_SRC” (the source IP in the network layer header of the SPA packet), “$DST” (the destination IP), “$PORT” (the allow port), and “$PROTO” (the allow protocol), “$TIMEOUT” (set the client timeout if specified).
- CMD_CYCLE_CLOSE <command>
Specify the close command that corresponds to the open command set by the “CMD_CYCLE_OPEN” variable described above. The same string substitutions such as “$IP”, “$PORT”, and “$PROTO” are supported. In addition, the special value “NONE” can be set to allow no close command to be executed after the open command. This might be handy in certain situations where, say, indefinite access is desired and allowed.
- CMD_CYCLE_TIMER <seconds>
Set the number of seconds after which the close command set in “CMD_CYCLE_CLOSE” will be executed. This defines the open/close timer interval.
- SUDO_EXE <path>
Define the path to the sudo binary. Default is “/usr/bin/sudo”.
- 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.
- REQUIRE_SOURCE_ADDRESS <Y/N>
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.
- REQUIRE_SOURCE_ADDRESS <Y/N>
Synonym for “REQUIRE_SOURCE_ADDRESS”.
- 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).
- FORCE_MASQUERADE <Y/N>
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.
- FORWARD_ALL <Y/N>
In NAT scenarios, control whether all traffic is forwarded through the fwknopd system as opposed to just forwarding connections to specific services as requested by the fwknop client.
- DISABLE_DNAT <Y/N>
Control whether DNAT rules are created in FORCE_NAT scenarios. This is mainly used in conjunction with the FORWARD_ALL variable to allow fwknopd to act essentially as an SPA gateway. I.e., the fwknop client is used to gain access via SPA to the broader Internet after being granted an IP via DHCP, but prior to sending the SPA packet all traffic is blocked by default to the Internet.
- 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: “http://www.cipherdyne.org/fwknop/docs/gpghowto.html”.
- 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.
- 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: “http://www.gnupg.org/faq/GnuPG-FAQ.html#how-can-i-use-gnupg-in-an-automated-environment”. 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.
- GPG_REQUIRE_SIG <Y/N>
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 Y.
- GPG_DISABLE_SIG <Y/N>
Disable signature verification for incoming SPA messages. This is not a recommended setting, and the default is N.
- GPG_IGNORE_SIG_VERIFY_ERROR <Y/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. Signature verification is enabled by default, and can only be disabled if “GPG_DISABLE_SIG” is set to Y (not a recommended setting). Separate multiple entries with a comma.
- GPG_FINGERPRINT_ID <keyID,...,keyID>
Specify a set of full-length GnuPG key fingerprints instead of the shorter key identifiers set with the “GPG_REMOTE_ID” variable. Here is an example fingerprint for one of the fwknop test suite keys: 00CC95F05BC146B6AC4038C9E36F443C6A3FAD56.
- 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_EXE <path>
Define the path to the GnuPG executable. If this keyword is not specified within /etc/fwknop/access.conf then fwknopd will default to using /usr/bin/gpg.
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 will (optionally) 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 test-fwknop.pl 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 documentation.
More information on Single Packet Authorization can be found in the paper “Single Packet Authorization with fwknop” available at http://www.cipherdyne.org/fwknop/docs/SPA.html. A comprehensive tutorial on fwknop operations and theory can be found at http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html. 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 https://github.com/mrash/fwknop.git fwknop.git
Additional commentary on Single Packet Authorization can be found via Michael Rash’s Twitter feed: http://twitter.com/michaelrash, @michaelrash
The primary developers of fwknop are Michael Rash (project creator) <email@example.com>, Damien Stuart <firstname.lastname@example.org>, and Jonathan Bennett <email@example.com>.
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 https://github.com/mrash/fwknop/blob/master/CREDITS 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 firstname.lastname@example.org or email@example.com, or open a new issue on Github (see https://github.com/mrash/fwknop.git). Suggestions and/or comments are always welcome as well. Additional information may be found in the fwknop mailing list archives (see: https://lists.sourceforge.net/lists/listinfo/fwknop-discuss).
fwknopd is distributed under the GNU General Public License (GPL v2+), and the latest version may be downloaded from http://www.cipherdyne.org.