fwknop - Single Packet Authorization

Introduction

fwknop implements an authorization scheme known as Single Packet Authorization (SPA) for strong service concealment. SPA requires only a single packet which is encrypted, non-replayable, and authenticated via an HMAC in order to communicate desired access to a service that is hidden behind a firewall in a default-drop filtering stance. The main application of SPA is to use a firewall to drop all attempts to connect to services such as SSH in order to make the exploitation of vulnerabilities (both 0-day and unpatched code) more difficult. Because there are no open ports, any service that is concealed by SPA naturally cannot be scanned for with Nmap. The fwknop project supports three different firewalls: iptables on Linux systems, pf on OpenBSD, and ipfw on FreeBSD and Mac OS X.

SPA is essentially next generation Port Knocking (PK), but solves many of the limitations exhibited by PK while retaining its core benefits. PK limitations include a general difficulty in protecting against replay attacks, asymmetric ciphers and HMAC schemes are not usually possible to reliably support, and it is trivially easy to mount a DoS attack against a PK server just by spoofing an additional packet into a PK sequence as it traverses the network (thereby convincing the PK server that the client doesn't know the proper sequence). All of these limitation are solved by SPA. At the same time, SPA hides services behind a default-drop firewall policy, acquires SPA data passively (usually via libpcap or other means), and implements standard cryptographic operations for SPA packet authentication and encryption/decryption.

SPA packets generated by fwknop leverage HMAC for authenticated encryption in the encrypt-then-authenticate model. Although the usage of an HMAC is currently optional (enabled via the --use-hmac command line switch), it is highly recommended for three reasons:

1. Without an HMAC, cryptographically strong authentication is not possible with fwknop unless GnuPG is used, but even then an HMAC should still be applied.
2. An HMAC applied after encryption protects against cryptanalytic CBC-mode padding oracle attacks such as the Vaudenay attack and related trickery (like the more recent "Lucky 13" attack against SSL).
3. The code required by the fwknopd daemon to verify an HMAC is much more simplistic than the code required to decrypt an SPA packet, so an SPA packet without a proper HMAC isn't even sent through the decryption routines.

The final reason above is why an HMAC should still be used even when SPA packets are encrypted with GnuPG due to the fact that SPA data is not sent through libgpgme functions unless the HMAC checks out first. GnuPG and libgpgme are relatively complex bodies of code, and therefore limiting the ability of a potential attacker to interact with this code through an HMAC operation helps to maintain a stronger security stance. Generating an HMAC for SPA communications requires a dedicated key in addition to the normal encryption key, and both can be generated with the --key-gen option.

fwknop encrypts SPA packets either with the Rijndael block cipher or via GnuPG and associated asymmetric cipher. If the symmetric encryption method is chosen, then as usual the encryption key is shared between the client and server (see the /etc/fwknop/access.conf file for details). The actual encryption key used for Rijndael encryption is generated via the standard PBKDF1 key derivation algorithm, and CBC mode is set. If the GnuPG method is chosen, then the encryption keys are derived from GnuPG key rings.

A comprehensive tutorial on fwknop can be found here:

http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html

License

The fwknop project is released as open source software under the terms of the GNU General Public License (GPL v2). The latest release can be found at http://www.cipherdyne.org/fwknop/

Current State

This README file describes the present state of the fwknop project as of the 2.5 release made in July, 2013. At present, we have an implementation of the Firewall Knock Operator library; libfko, as well as the fwknop client and server applications. The library provides the API and back-end functionality for managing the Single Packet Authorization (SPA) data that the other fwknop components employ. It also can be used by other programs that need SPA functionality (see the perl directory for the FKO perl module as an example, and there are python bindings as well in the python directory).

Upgrading

If you are upgrading from an older version of fwknop (and this includes the original perl implementation as well), then you will want to read the following link to ensure a smooth transition to fwknop-2.5:

http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html#backwards-compatibility

Building fwknop

This distribution uses GNU autoconf for setting up the build. Please see the INSTALL file for the general basics on using autoconf.

There are some "configure" options that are specific to fwknop. They are
(extracted from ./configure --help):

  --disable-client        Do not build the fwknop client component. The
                          default is to build the client.
  --disable-server        Do not build the fwknop server component. The
                          default is to build the server.
  --with-gpgme            support for gpg encryption using libgpgme
                          [default=check]
  --with-gpgme-prefix=PFX prefix where GPGME is installed (optional)
  --with-gpg=/path/to/gpg Specify path to the gpg executable that gpgme will
                          use [default=check path]
  --with-iptables=/path/to/iptables
                          Specify path to the iptables executable
                          [default=check path]
  --with-ipfw=/path/to/ipfw
                          Specify path to the ipfw executable [default=check
                          path]
  --with-pf=/path/to/pfctl
                          Specify path to the pf executable [default=check
                          path]
  --with-ipf=/path/to/ipf Specify path to the ipf executable [default=check
                          path]

Notes

Migrating from the Perl version of fwknop

For those of you who are currently using the Perl version and plan to migrate to this version, there are some things to be aware of:

- Not all of the features and functionality of the Perl-based
  fwknop were ported to this implementation. We felt it important
  to keep the C version as lean and lightweight as possible. Most
  of the omitted feature/functions (like email alerts) can be
  accomplished through other means (i.e. use an external script
  to monitor log files and alert based on appropriate log messages).

- There are some differences in the fwknop configuration and access
  file directives and values. Some of these are fairly subtle. You
  should pay careful attention to the documentation and comments in
  those files.

For fwknop developers

If you are pulling this distribution from git, you should run the autogen.sh script to generate the autoconf files. If you get errors about missing directories or files, try running autogen.sh again. After that you can run the autoreconf -i when you want to regenerate the configuration. If, for some reason, autoreconf does not work for you, the autogen.sh script should suffice.

The fwknop and fwknopd man page nroff sources are included in their respective directories (client and server). These nroff files are derived from the asciidoc sources in the 'docs' directory. See the README in docs for details.