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fwknop - Firewall Knock Operator


fwknop -A <'proto/ports'> -R|-a|-s -D <'host'> [options]


fwknop implements an authorization scheme known as Single Packet Authorization (SPA) for Linux systems running iptables. This mechanism requires only a single encrypted and non-replayed packet to communicate various pieces of information including desired access through an iptables or ipfw policy. The main application of this program is to use iptables in a default-drop stance 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.

An authorization server fwknopd passively monitors authorization packets via libpcap and hence there is no “server” to which to connect in the traditional sense. Any service protected by fwknop is inaccessible (by using iptables or ipfw to intercept packets within the kernel) before authenticating; anyone scanning for the service will not be able to detect that it is even listening. Single Packet Authorization offers many advantages over port knocking, including non-replayability of SPA packets, ability to use asymmetric ciphers (such as Elgamal), and SPA cannot be broken by simply spoofing packets to duplicate ports within the knock sequence on the server to break port knocking authentication.

SPA packets can easily be spoofed as well (this is a good thing in this context), and this makes it possible to make it appear as though, say, is trying to authenticate to a target system but in reality the actual connection will come from a seemingly unrelated IP.

Authorization packets are either encrypted with the Rijndael block cipher or via GnuPG and associated asymmetric ciphers. If the symmetric encryption method is chosen, then the encryption key is shared between the client and server (see the fwknopd access.conf file for details). If the GnuPG method is chosen, then the encryption keys are derived from GnuPG key rings. SPA packets generated by fwknop running as a client adhere to the following format (before they are encrypted):

    random number (16 bytes)
    software version
    mode (command mode (0) or access mode (1))
    if command mode => command to execute
    else access mode  => IP,proto,port
    message digest (SHA512 / SHA384 / SHA256 / SHA1 / MD5)

Each of the above fields are separated by a ":" character due to the variable length of several of the fields, and those that might contain ":" characters are base64 encoded. The message digest (SHA256 by default in all versions of fwknop greater than 1.9.1) allows the server to check message integrity after decryption, and the 16 bytes of random data ensures (with high probability) that no two messages are identical. This ensures that replay attacks are not possible against fwknop.

For each packet coming from an fwknop client, the fwknopd server can cache the digest calculated over the entire packet and compares against previous packet digests in order to detect attempted replay attacks. Syslog alerts are generated if a replay is detected.

By default, the fwknop client sends authorization packets over UDP port 62201, but this can be altered with the --server-port argument. The server must first be configured to acquire the SPA data on the changed protocol-port. Also, fwknop can send the SPA packet over a random port via the --rand-port argument. See fwknopd(8) for further details. See the EXAMPLES section for example invocations of the fwknop client.


These required arguments can be specified via command-line or from within the .fwknoprc file (see -n, --named-config option and the FWKNOPRC FILE section below.

-D, --destination=<IP-address>

Direct the fwknop client to authenticate with the fwknopd daemon/service at the specified destination hostname or IP address. The connection mode is discovered by the fwknopd daemon/service when it decrypts and parses the authentication packet.

-A, --access=<port list>

Provide a list of ports and protocols to access on a remote computer running fwknopd. The format of this list is “+<proto>/<port>…<proto>/<port>+”, e.g. “tcp/22,udp/53”. NOTE: The vast majority of usages for fwknop require the -A argument, but sending full commands with the --server-cmd argument via an SPA packet to be executed by fwknopd does not require this argument.


One of these options (see below) is required to tell the remote fwknopd daemon what IP should be let through the local firewall. It is recommend to use the -R or -a options instead of -s in order to harden SPA communications against possible Man-In-The-Middle (MITM) attacks.


-h, --help

Print a usage summary message and exit.

-B, --save-packet=<file>

Instruct the fwknop client to write a newly created SPA packet out to the specified file so that it can be examined off-line.

-G, --get-key=<file>

Load an encryption key/password from the specified file. The key file contains a line for each destination hostname or IP address, a colon (":"), optional space and the password, followed by a newline. Note that the last line has to have a terminating newline character. Also note: though this is a convenience, have a file on your system with cleartext passwords is not a good idea and is not recommended.

-l, --last-cmd

Execute fwknop with the command-line arguments from the previous invocation (if any). The previous arguments are parsed out of the ~/ file.

-n, --named-config=<stanza name>

Specify the name of the configuration stanza in the “$HOME/.fwknoprc” file to pull configuration and command directives. These named stanzas alleviate the need for remembering the various command-line arguments for frequently used invocations of fwknop. See the section labeled, FWKNOPRC FILE below for a list of the valid configuration directives in the .fwknoprc file.


Display the last command-line arguments used by fwknop.

-T, --test

Test mode. Generate the SPA packet data, but do not send it. Instead, print a break-down of the SPA data fields, then run the data through the decryption and decoding process and print the break-down again. This is primarily a debugging feature.

-v, --verbose

Run the fwknop client in verbose mode. This causes fwknop to print some extra information about the current command and the resulting SPA data.

-V, --Version

Display version information and exit.


-a, --allow-ip=<IP-address>

Specify IP address that should be permitted through the destination fwknopd server firewall (this IP is encrypted within the SPA packet itself). This is useful to prevent a MTIM attack where a SPA packet can be intercepted en-route and sent from a different IP than the original. Hence, if the fwknopd server trusts the source address on the SPA packet IP header then the attacker gains access. The -a option puts the source address within the encrypted SPA packet, and so thwarts this attack. The -a option is also useful to specify the IP that will be granted access when the SPA packet itself is spoofed with the --spoof-src option. Another related option is -R (see below) which instructs the fwknop client to automatically resolve the externally routable IP address the local system is connected to by querying a website that returns the actual IP address it sees from the calling system.

-C, --server-cmd=<command to execute>

Instead of requesting access to a service with an SPA packet, the --server-cmd argument specifies a command that will be executed by the fwknopd server. The command is encrypted within the SPA packet and sniffed off the wire (as usual) by the fwknopd server.

-g, --gpg-encryption

Use GPG encryption on the SPA packet (default if not specified is Rijndael). Note: Use of this option will require the specification of a GPG recipient (see --gpg-recipient along with other GPG-related options below).

-H, --http-proxy=<proxy-host>[:port]

Specify an HTTP proxy that the fwknop client will use to send the SPA packet through. Using this option will automatically set the SPA packet transmission mode (usually set via the --server-proto argument) to "http". You can also specify the proxy port by adding ":<port>" to the proxy host name or ip.

-m, --digest-type=<digest>

Specify the message digest algorithm to use in the SPA data. Choices are: MD5, SHA1, SHA256 (the default), SHA384, and SHA512.

-N, --nat-access=<internalIP:forwardPort>

The fwknopd server offers the ability to provide SPA access through an iptables firewall to an internal service by interfacing with the iptables NAT capabilities. So, if the fwknopd server is protecting an internal network on an RFC-1918 address space, an external fwknop client can request that the server port forward an external port to an internal IP, i.e. “+--NAT-access,55000+”. In this case, access will be granted to via port 55000 to whatever service is requested via the --access argument (usually tcp/22). Hence, after sending such an SPA packet, one would then do “ssh -p 55000 user@host” and the connection would be forwarded on through to the internal system automatically. Note that the port “55000” can be randomly generated via the --nat-rand-port argument (described later).


On the fwknopd server, a NAT operation can apply to the local system instead of being forwarded through the system. That is, for iptables firewalls, a connection to, say, port 55,000 can be translated to port 22 on the local system. By making use of the --nat-local argument, the fwknop client can be made to request such access. This means that any external attacker would only see a connection over port 55,000 instead of the expected port 22 after the SPA packet is sent.


Usually fwknop is used to request access to a specific port such as tcp/22 on a system running fwknopd. However, by using the --nat-rand-port argument, it is possible to request access to a particular service (again, such as tcp/22), but have this access granted via a random translated port. That is, once the fwknop client has been executed in this mode and the random port selected by fwknop is displayed, the destination port used by the follow-on client must be changed to match this random port. For SSH, this is accomplished via the -p argument. See the --nat-local and --nat-access command line arguments to fwknop for additional details on gaining access to services via a NAT operation.

-p, --server-port=<port>

Specify the port number where fwknopd accepts packets via libpcap or ulogd pcap writer. By default fwknopd looks for authorization packets over UDP port 62201.

-P, --server-proto=<protocol>

Set the protocol (udp, tcp, http, tcpraw, or icmp) for the outgoing SPA packet. Note: The tcpraw and icmp modes use raw sockets and thus require root access to run. Also note: The tcp mode expects to establish a TCP connection to the server before sending the SPA packet. This is not normally done, but is useful for compatibility with the Tor for strong anonymity; see In this case, the fwknopd server will need to be configured to listen on the target TCP port (which is 62201 by default).

-Q, --spoof-src=<IP>

Spoof the source address from which the fwknop client sends SPA packets. This requires root on the client side access since a raw socket is required to accomplish this. Note that the --spoof-user argument can be given in this mode in order to pass any REQUIRE_USERNAME keyword that might be specified in /etc/fwknop/access.conf.

-r, --rand-port

Instruct the fwknop client to send an SPA packet over a random destination port between 10,000 and 65535. The fwknopd server must use a PCAP_FILTER variable that is configured to accept such packets. For example, the PCAP_FILTER variable could be set to: “+udp dst portrange 10000-65535+”.

-R, --resolve-ip-http

This is an important option, and instructs the fwknop client and the fwknopd daemon/service to query a web server that returns the caller’s IP address (as seen by the web server). In some cases, this is needed to determine the IP address that should be allowed through the iptables policy at the remote fwknopd server side. This is useful if the fwknop client is being used on a system that is behind an obscure NAT address. Presently, fwknop uses the URL: to resolve the caller IP.


Override the default URL used for resolving the source IP address. For best results, the URL specified here should point to a web service that provides just an IP address in the body of the HTTP response.

-s, --source-ip

Instruct the fwknop client to form an SPA packet that contains the special-case IP address “+” which will inform the destination fwknopd SPA server to use the source IP address from which the SPA packet originates as the IP that will be allowed through upon modification of the firewall ruleset. This option is useful if the fwknop client is deployed on a machine that is behind a NAT device. The permit-address options -s, -R and -a are mutually exclusive.


By default, the fwknopd daemon on the server side enforces time synchronization between the clocks running on client and server systems. The fwknop client places the local time within each SPA packet as a time stamp to be validated by the fwknopd server after decryption. However, in some circumstances, if the clocks are out of sync and the user on the client system does not have the required access to change the local clock setting, it can be difficult to construct and SPA packet with a time stamp the server will accept. In this situation, the --time-offset-plus option can allow the user to specify an offset (e.g. “60sec” “60min” “2days” etc.) that is added to the local time.


This is similar to the --time-offset-plus option (see above), but subtracts the specified time offset instead of adding it to the local time stamp.

-u, --user-agent=<user-agent-string>

Set the HTTP User-Agent for resolving the external IP via -R, or for sending SPA packets over HTTP.

-U, --spoof-user=<user>

Specify the username that is included within SPA packet. This allows the fwknop client to satisfy any non-root REQUIRE_USERNAME keyword on the fwknopd server (--spoof-src mode requires that the fwknop client is executed as root).


Instruct fwknop to acquire GnuPG key password from a running gpg-agent instance (if available).


Specify the path to the GnuPG directory; normally this path is derived from the home directory of the user that is running the fwknop client. This is useful when a “root” user wishes to log into a remote machine whose sshd daemon/service does not permit root login.

--gpg-recipient=<key ID or Name>

Specify the GnuPG key ID, e.g. “+1234ABCD+” (see the output of "gpg—list-keys") or the key name (associated email address) of the recipient of the Single Packet Authorization message. This key is imported by the fwknopd server and the associated private key is used to decrypt the SPA packet. The recipient’s key must first be imported into the client GnuPG key ring.

--gpg-signer-key=<key ID or Name>

Specify the GnuPG key ID, e.g. “+ABCD1234+” (see the output of "gpg --list-keys") or the key name to use when signing the SPA message. The user is prompted for the associated GnuPG password to create the signature. This adds a cryptographically strong mechanism to allow the fwknopd daemon on the remote server to authenticate who created the SPA message.


The .fwknoprc file is used to set various parameters to override default program parameters at runtime. It also allows for additional named configuration stanzas for setting program parameters for a particular invocation.

The fwknop client will create this file if it does not exist in the user’s home directory. This initial version has some sample directives that are commented out. It is up to the user to edit this file to meet their needs.

The .fwknoprc file contains a default configuration area or stanza which holds global configuration directives that override the program defaults. You can edit this file and create additional named stanzas that can be specified with the -n or --named-config option. Parameters defined in the named stanzas will override any matching default stanza directives. Note that command-line options will still override any corresponding .fwknoprc directives.

There are directives to match most of the command-line parameters fwknop supports. Here is the current list of each directive along with a brief description and its matching command-line option(s):


Set the SPA message digest type (-m, --digest-type).


Set the protocol to use for sending the SPA packet (-P, --server-proto).


Specify the IP or hostname of the destination (fwknopd) server ('-D, --destination).


Set the server port to use for sending the SPA packet (-p, --server-port).


Set the source port to use for sending the SPA packet (-S, --source-port).


Set the firewall rule timeout value (-f, --fw-timeout).


Specify the address to allow within the SPA data. Note: This parameter covers the -a, -s, and -R command-line options. You can specify a hostname or IP address (the -a option), specify the word "source" to tell the fwknopd server to accept the source IP of the packet as the IP to allow (the -s option), or use the word "resolve" to have fwknop resolve the external network IP via HTTP request (the -R option).


Set to a URL that will be used for resolving the source IP address (--resolve-url).


Set a value to apply to the timestamp in the SPA packet. This can be either a positive or negative value (--time-offset-plus/minus).


Set to Y to specify the use of GPG for encryption (--gpg-encryption).


Specify the GPG key name or ID for signing the GPG-encrypted SPA data (--gpg-signer-key).


Specify the GPG key name or ID for the recipient of the GPG-encrypted SPA data (--gpg-recipient-key).


Specify the GPG home directory (--gpg-home-dir).


Set the username in the SPA data to the specified value (-U, --spoof-user).


Set the source IP of the outgoing SPA packet to the specified value (-Q, --spoof-source).


Set the one or more protocol/ports to open on the firewall (-A, --access).


Send the SPA packet over a randomly assigned port (-r, --rand-port).


Load an encryption key/password from a file (-G, --get-key).


Set the HTTP User-Agent for resolving the external IP via -R, or for sending SPA packets over HTTP (-u, --user-agent).


Gain NAT access to an internal service protected by the fwknop server (-N, --nat-access).


Access a local service via a forwarded port on the fwknopd server system (--nat-local).


Specify the port to forward to access a service via NAT (--nat-port).


Have the fwknop client assign a random port for NAT access (--nat-rand-port).


SPOOF_USER, GPG_AGENT_INFO (only used in --gpg-agent mode).


The following examples illustrate the command line arguments that could be supplied to the fwknop client in a few situations:

Access mode examples

Packet contents printed to stdout at the fwknop client when creating an “access mode” SPA packet:

    Random data:    6565240948266426
    Username:       mbr
    Timestamp:      1203863233
    Version:        1.9.2
    Type:           1 (access mode)
    SHA256 sum:     gngquSL8AuM7r27XsR4qPmJhuBo9pG2PYwII06AaJHw

Use the Single Packet Authorization mode to gain access to tcp/22 (ssh) and udp/53 running on the system from the IP

    $ fwknop -A "tcp/22,udp/53" -a -D

Same as above example, but gain access from whatever source IP is seen by the fwknop server (useful if the fwknop client is behind a NAT device):

    $ fwknop -A "tcp/22,udp/53" -s -D

Same as above example, but use an IP identification website to derive the client IP address. This is a safer method of acquiring the client IP address than using the -s option because the source IP is put within the encrypted packet instead of having the fwknopd daemon grant the requested access from whatever IP address the SPA packet originates:

    $ fwknop -A "tcp/22,udp/53" -R -D

Use the Single Packet Authorization mode to gain access to tcp/22 (ssh) and udp/53 running on the system, and use GnuPG keys to encrypt and decrypt:

    $ fwknop -A "tcp/22,udp/53" --gpg-sign ABCD1234 --gpg--recipient
    1234ABCD -R -D

Instruct the fwknop server running at to allow to connect to TCP/22, but spoof the authorization packet from an IP associated with

    # fwknop --Spoof-src ’’ -A tcp/22 -a -D


fwknop requires libfko (which is normally included with both source and binary distributions).

For GPG functionality, GnuPG must also be correctly installed and configured.

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


fwknop can be run with the -T (or --test) command line option. This will have fwknop simply create and print the SPA packet information, then run it through a decrypt/decode cycle and print it again.


fwknopd(8), iptables(8), gpg(1), libfko documentation.

More information on Single Packet Authorization can be found in the paper “Single Packet Authorization with fwknop” available at


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.

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.


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

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