Michael Rash, Security Researcher

2011 Blog Archive    [Summary View]

Software Release - fwknop-2.0rc5

fwknop-2.0rc5 released The 2.0rc5 candidate release of fwknop is available for download. There may be a few tweaks to the code before the official 2.0 release is made, but this is pretty close as-is. Significant development work has gone into fwknop since the 2.0rc4 release, and adds some major new functionality as well as fixing a few bugs. Here is a summary of the changes:

iPhone fwknop client: Max Kastanas has contributed an iPhone port of the fwknop client. He had already contributed on Android client, so the iPhone was the next natural step! We're looking for a maintainer of the iPhone code so that eventually it can be made available through the App Store. If you have iPhone development experience and are interested in taking this on, please contact me.

PF firewall support on OpenBSD: For quite a while now fwknop has brought Single Packet Authorization support to iptables firewalls on Linux and ipfw firewalls on FreeBSD and Mac OS X systems. The 2.0rc5 release now introduces support for the PF firewall on OpenBSD systems. By interfacing with the pfctl command, fwknop creates new rules to access a protected service (such as SSHD) to a PF anchor. Rules are deleted out of a running anchor with pfctl -a <anchor> -f - with the expired rule(s) removed. There is support in the fwknop test suite (see the test/ directory in the fwknop-2.0rc5 sources) to validate fwknop operations on OpenBSD systems, and if there are any issues please let me know.

Expiring SPA keys: With large SPA deployments where many different encryption keys - either Rijndael or GPG keys - are used to service lots of external users, key management can become somewhat of a burden. This feature allows an expiration date to be set in the access.conf file on a per-key basis. Any SPA packet received for an expired key is ignored by fwknopd. This feature was suggested by ozmart from the fwknop mailing list.

FORCE_NAT mode: For iptables firewalls, a new FORCE_NAT mode has been implemented that works as follows: 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.

lsof launcher: The fwknop lsof launcher (extras/fwknop-launcher/ is a lightweight daemon that allows the user to not have to manually run the fwknop client when attempting to gain access to a service that is protected by via fwknopd. This is accomplished by checking the output of lsof to look for pending connections in the SYN_SENT state, which (usually) indicate that a remote firewall is blocking the attempted connection. At this point, the launcher executes the fwknop client with the --get-key arg (so the user must place the key in the local filesystem) to generate an SPA packet for the attempted connection. The remote fwknopd daemon will reconfigure the firewall to allow temporary access, and this usually happens fast enough that the original connection attempt will then succeed as TCP retries to establish the connection. The idea for this was originally for a pcap-based connection watcher by Sebastien Jeanquier.

Several other changes and small fixes have been made as well. The fwknop test suite supports running all tests through the excellent valgrind project, and this enabled several memory handling issues to be found and corrected.

fwknop is released under the GPL version 2, and the complete fwknop-2.0rc5 ChangeLog can be found here via the fwknop gitweb interface. Single Packet Authorization Proxy

Vasilis Mavroudis has developed a web proxy called that allows anyone to generate an fwknop SPA packet on their behalf with just a web browser. Although fwknop client portability has improved quite a bit in anticipation of the upcoming fwknop-2.0 release, it is a nice addition to the SPA world to not need the fwknop client installed at all. There are probably several platforms where the native client might not function but can run a web browser.

Using the proxy requires that the user provide the SPA key over SSL to, but this is a necessary step in exchange for not having to install the fwknop client. As of this writing, SPA via gpg keys is not yet supported, but there are plans to support this in the future. All requests to generate an SPA packet are protected by a captcha.

Behind the scenes, executes the fwknop client on behalf of users, and Vasilis informed me that he's using the latest client code (written in C) instead of the older perl client. This is good since all recent development is done on the C version of fwknop in order to make it as small and lightweight as possible.

The service is free, and will hopefully be open-sourced at some point as well. If there are any issues, please either email me or open a ticket on the fwknop github interface. Here is a screenshot of the current site: SPA proxy

fwknop in the OpenWrt and Pentoo Linux Distributions

fwknop in OpenWrt The C version fwknop has now made it into the OpenWrt Linux distribution for embedded devices. Jonathan Bennett made this possible by contributing a Makefile for OpenWrt, and it was picked up the OpenWrt maintainers. It is good to see progress made towards the integration of Single Packet Authorization into operating systems that are designed to function as secure gateway devices between multiple networks.

So far, fwknop is available in the OpenWrt trunk packages feed, but will eventually become available via the opkg package manager too. Fortunately, OpenWrt makes everything available via git: $ git clone git:// openwrt_packages.git
Initialized empty Git repository in /home/mbr/src/openwrt_packages.git/.git/
remote: Counting objects: 56118, done.
remote: Compressing objects: 100% (21342/21342), done.
remote: Total 56118 (delta 29694), reused 54875 (delta 29054)
Receiving objects: 100% (56118/56118), 11.85 MiB | 2.57 MiB/s, done.
Resolving deltas: 100% (29694/29694), done.
$ cd openwrt_packages.git
$ git ls-files |grep fwknop
$ git log net/fwknop/Makefile
commit 89475e5d6136833fa3b59c3d47c4f2be02718c7a
Author: florian <florian@3c298f89-4303-0410-b956-a3cf2f4a3e73>
Date: Wed Aug 17 10:13:20 2011 +0000

[package] add fwknopd

Signed-off-by; Jonathan Bennett <[email removed]>

git-svn-id: svn:// 3c298f89-4303-0410-b956-a3cf2f4a3e73

In other news, both the perl and C versions of fwknop are also available in the Pentoo Linux distribution thanks to ozmart and the Pentoo maintainers. Pentoo is a live-cd distribution that is focused on security and derived from Gentoo. ozmart wrote a description of the use case for fwknop on Pentoo from a pentration testing perspective:

"...This is a useful script when combined with iptables and sshd. Configuration can accommodate pgp and replay attack checks. It allows the box to appear silent when running daemons if your box is deployed in say, a hostile environment.

It can also allow commands to be run without actually having to log into the box, say if you wanted to trigger something interesting from a remote location..."

Software Release - fwsnort-1.6

fwsnort-1.6 released The 1.6 release of fwsnort is available for download. This is a fairly significant release that adds support for the Snort fast_pattern keyword, makes enhancements to the --QUEUE and --NFQUEUE modes, supports the conntrack module for connection tracking, adds support for case-insensitive pattern matches using the --icase argument to the iptables string match extension, and several other things. The Snort fast_pattern keyword allows the rule author to influence the order in which Snort tries to match a pattern against network traffic. When multiple patterns are included in a rule, Snort usually tries to match the longest pattern first reasoning that the longest pattern is probably the least likely to trigger a match and therefore the remaining pattern searches would not have to be performed. But, there are times when the rule author would like to explicitly ask Snort to match on a particular pattern first, and the fast_pattern keyword is the mechanism that makes this possible. Because iptables matches are evaluated in order and a failing match short circuits a rule, fast_pattern support with the string match extension is possible through proper ordering in the iptables rule. Here is an example Snort rule from Emerging Threats with the fast_pattern keyword applied to the forth pattern: alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:"ET WEB_CLIENT Possible Internet Explorer srcElement Memory Corruption Attempt"; flow:established,to_client; file_data; content:"document.createEventObject"; distance:0; nocase; content:".innerHTML"; within:100; nocase; content:"window.setInterval"; distance:0; nocase; content:"srcElement"; fast_pattern; nocase; distance:0; classtype:attempted-user; reference:url,; reference:url,; reference:url,; reference:cve,2010-0249; reference:url,; reference:url,; sid:2010799; rev:5;) fwsnort translates this rule as follows in iptables-save format from the /etc/fwsnort/ file - the original iptables commands in non-save format are also available in the /etc/fwsnort/ script: -A FWSNORT_FORWARD_ESTAB -p tcp -m tcp --sport 80 -m string --string "srcElement" --algo bm --from 82 --icase -m string --string "document.createEventObject" --algo bm --from 64 --icase -m string --string ".innerHTML" --algo bm --to 190 --icase -m string --string "window.setInterval" --algo bm --from 74 --icase -m comment --comment "sid:2010799; msg:ET WEB_CLIENT Possible Internet Explorer srcElement Memory Corruption Attempt; classtype:attempted-user; reference:url,; rev:5; FWS:1.6;" -j LOG --log-ip-options --log-tcp-options --log-prefix "SID2010799 ESTAB " Note that the srcElement string is matched first in the iptables rule even though it is the last string in the original Snort rule. With fast_pattern support, fwsnort policies should be a bit faster at run time in the in the Linux kernel. On a final note, the iptables multiport match is also supported with the fwsnort-1.6 release, so Snort rules with lists of source or destination ports (like this: "alert tcp $HOME_NET [0:20,22:24,26:138,140:444,446:464,466:586,588:901,903:1432,1434:65535] -> any any") can be translated.

The complete fwsnort-1.6 ChangeLog can be found here via the fwsnort gitweb interface.

Switched from subversion to git

switched to git After using subversion for several years, I've switched to git for all projects. Subversion has certainly served its purpose, but it is hard to look at git and not feel a compelling draw. Further, with easy to set up web interfaces to git repositories such as gitweb and free hosting services such as github, providing a public git repository is trivial. Git itself can allow repositories to be cloned directly over HTTP without needing infrastructure like WebDAV, and here are links for the projects (github and gitweb links too):

The trac interface will remain active for a little while to see the legacy svn repositories, but the git repositories were all converted from these in order to preserve the history so trac is no longer important. If you are interested in the latest code changes in, say, fwsnort then just clone the repository and then you can make your own changes: $ git clone
Initialized empty Git repository in /home/mbr/tmp/git/fwsnort/.git/
$ cd fwsnort
$ git status
# On branch master
nothing to commit (working directory clean)
$ git show --summary
commit 00c4379a69975097948ed9e5ba356eeba69c0c93
Author: Michael Rash <>
Date: Mon Jun 20 21:00:57 2011 -0400

Added the --Conntrack-state argument

Added the --Conntrack-state argument to specify a conntrack state in place of
the "established" state that commonly accompanies the Snort "flow" keyword.
By default, fwsnort uses the conntrack state of "ESTABLISHED" for this. In
certain corner cases, it might be useful to use "ESTABLISHED,RELATED" instead
to apply application layer inspection to things like ICMP port unreachable
messages that are responses to real attempted communications. (Need to add
UDP tracking for the _ESTAB chains for this too - coming soon.)

fwknop in BackTrack 5

fwknop has made it into BackTrack Linux distribution. Well, it didn't make it into the default image, but it is in the BackTrack software repository so installing it is a snap with apt-get or aptitude. Below is a screenshot of the BackTrack 5 VM installed under VMware Player running on Ubuntu, and running a search shows both the fwknop-client and fwknop-server packages that are maintained by Franck Joncourt for the Debian distribution. fwknop in BackTrack 5
Update 06/24/2011: Thanks to Sebastien Jeanquier for calling this to my attention and helping to support the inclusion of fwknop in BackTrack Linux.

Software Release - fwsnort-1.5

fwsnort-1.5 released The 1.5 release of fwsnort is available for download. This is a major release that moves to using the iptables-save format for instantiating the fwsnort policy, and this allows the run time for adding the fwsnort policy to the kernel to be drastically reduced. fwsnort now splices in the translated Snort rules into the iptables policy in the running kernel at the time of translation. So, any updates to the iptables policy that are made after fwsnort is executed and before is run would be lost. Hence, it is advisable to execute soon after running fwsnort. This is a reasonable tradeoff though considering the performance benefit as seen below - which gives an example of how long it takes to add an fwsnort iptables policy via the old strategy of executing one iptables command at a time vs. implementing the same policy with iptables-restore. First, fwsnort is used to translate the Snort web-misc.rules, web-cgi.rules, backdoor.rules files like so: [root@minastirith /etc/fwsnort]# fwsnort --snort-rfile web-misc.rules,web-cgi.rules,backdoor.rules --no-ipt-sync

[+] Generated iptables rules for 713 out of 754 signatures: 94.56%

[+] Logfile: /var/log/fwsnort/fwsnort.log
[+] iptables script (individual commands): /etc/fwsnort/

Main fwsnort iptables-save file: /etc/fwsnort/

You can instantiate the fwsnort policy with the following command:

/sbin/iptables-restore < /etc/fwsnort/

Or just execute: /etc/fwsnort/
The output above illustrates the changes for the fwsnort-1.5 release. All of the previous behavior in fwsnort has been preserved in the /etc/fwsnort/ script. That is, each individual iptables command to add every fwsnort rule one by one is implemented in this script - this is analogous to how the script was built by older versions of fwsnort. But, with the 1.5 release the script now just executes the iptables-restore command against the new file.

If we execute the script and time its execution, we get the following on my desktop system: [root@minastirith /etc/fwsnort]# time /etc/fwsnort/
[+] Adding backdoor rules:
Rules added: 122
[+] Adding web-cgi rules:
Rules added: 696
[+] Adding web-misc rules:
Rules added: 600
[+] Finished.

real 0m24.391s
user 0m1.560s
sys 0m11.500s
[root@minastirith /etc/fwsnort]# iptables -v -nL |wc -l
So, the fwsnort policy together with the running iptables policy is about 1500 rules long, and it took over 24 seconds to add to the running kernel. Now, let's time the script instead (which is just a wrapper around the iptables-restore command): [root@minastirith /etc/fwsnort]# time /etc/fwsnort/

[+] Splicing fwsnort rules into the iptables policy...

real 0m0.121s
user 0m0.060s
sys 0m0.040s
[root@minastirith /etc/fwsnort]# iptables -v -nL |wc -l
Ok, over 24 seconds to instantiate the fwsnort policy for the old strategy, and about a 10th of a second for the new strategy for a speed up of 240 times! This gets even better for an fwsnort policy with thousands of rules. Note that the number of iptables rules is the same between the two executions.

The complete ChangeLog entries are displayed below:

  • Major update to use the iptables-save format instead of the older strategy of always just executing iptables commands directly (which was very flow for large fwsnort policies). The /etc/fwsnort/ script now just executes:

    /sbin/iptables-restore < /etc/fwsnort/

    All fwsnort rules are now placed in the /etc/fwsnort/ file, but the older output (for the individual commands version) is still available at /etc/fwsnort/ This functionality extends to ip6tables policies as well. The fwsnort man page explain this in better detail:

    "As of fwsnort-1.5 all iptables rules built by fwsnort are written out to the /etc/fwsnort/ file in iptables-save format. This allows a long fwsnort policy (which may contain thousands of iptables rules translated from a large Snort signature set) to be quickly instantiated via the "iptables-restore" command. A wrapper script /etc/fwsnort/ is also written out to make this easy. Hence, the typical work flow for fwsnort is to: 1) run fwsnort, 2) note the Snort rules that fwsnort was able to successfully translate (the number of such rules is printed to stdout), and then 3) execute the /etc/fwsnort/ wrapper script to instantiate the policy in the running kernel."
  • Added the --rules-url argument so that the URL for updating the Emerging Threats rule set can be specified from the command line. The default is:
  • Updated to automatically check for the maximum length string that the string match supports, and this is used to through out any Snort rules with content matches longer than this length.
  • Updated the iptables capabilities testing routines to add and delete testing rules to/from the custom chain 'FWS_CAP_TEST'. This maintains a a cleaner separation between fwsnort and any existing iptables policy even during the capabilities testing phase.
  • Added the --ipt-check-capabilities argument to have fwsnort test the capabilities of the local iptables firewall and exit.
  • Added the --string-match-alg argument to allow the string matching algorithm used by fwsnort to be specified from the command line. The default algorithm is 'bm' for 'Boyer-Moore', but 'kmp' may also be specified (short for the 'Knuth-Morris-Pratt' algorithm).
  • Updated to the latest complete rule set from Emerging Threats (see

Single Packet Authorization on Android

fwknop on Android Kicking off 2011 with a bang is easy this year - Max Kastanas has ported the fwknop client to the Android mobile operating sytem. This brings Single Packet Authorization to Google's smart phones, and was accomplished using Damien Stuart's libfko implementation. All of the Android code can be found in the Cipherdyne Trac respository. A couple of screenshots of using the SPA app from an Android phone appear below - after the SPA packet is sent and verified passively by the fwknop daemon on the remote system, the fwknop Android app automatically launches Connectbot to access SSHD:

fwknop Android app fwknop Android app