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, www.yahoo.com 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)
    username
    timestamp
    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.

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):

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:

    Random data:    6565240948266426
    Username:       mbr
    Timestamp:      1203863233
    Version:        1.9.2
    Type:           1 (access mode)
    Access:         127.0.0.2,tcp/22
    SHA256 sum:     gngquSL8AuM7r27XsR4qPmJhuBo9pG2PYwII06AaJHw

    $ fwknop -A "tcp/22,udp/53" -a 192.168.10.4 -D 10.0.0.123

    $ fwknop -A "tcp/22,udp/53" -s -D 10.0.0.123

    $ fwknop -A "tcp/22,udp/53" -R -D 10.0.0.123

    $ fwknop -A "tcp/22,udp/53" --gpg-sign ABCD1234 --gpg--recipient
    1234ABCD -R -D 10.0.0.123

    # fwknop --Spoof-src ’www.yahoo.com’ -A tcp/22 -a 172.16.5.4 -D
    10.0.0.123

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 http://www.cipherdyne.org/fwknop/docs/SPA.html.

Damien Stuart <dstuart@dstuart.org>, Michael Rash <mbr@cipherdyne.org>

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 http://www.cipherdyne.org/fwknop/docs/contributors.html 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: http://www.nmrc.org).

Send bug reports to dstuart@dstuart.org or mbr@cipherdyne.org. 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 http://www.cipherdyne.org.