Michael Rash, Security Researcher

Software Release: fwknop-2.6.4

fwknop-2.6.4 software release The 2.6.4 release of fwknop is available for download. New functionality has been developed for 2.6.4, including a new UDP listener mode to remove libpcap as a dependency for fwknopd, support for firewalld on recent versions of Fedora, RHEL, and Centos (contributed by Gerry Reno), and support for Michal Zalewski's 'American Fuzzy Lop' fuzzer. Further, on systems where execvpe() is available, all system() and popen() calls have been replaced so that the shell is not invoked and no environment is used. As usual, fwknop has a Coverity Scan score of zero, and the code coverage report achieved by the 2.6.4 test suite is available here.

Here is the complete ChangeLog for fwknop-2.6.4:

  • [server] Added a UDP server mode so that SPA packets can be acquired via UDP directly without having to use libpcap. This is an optional feature since it opens a UDP port (and therefore requires the local firewall be opened for communications to this port), but fwknopd is careful to never send anything back to a client that sends data to this port. So, from the perspective of an attacker or scanner, fwknopd remains invisible. This feature is enabled in fwknopd either with a new command line argument --udp-server or in the fwknopd.conf file with the ENABLE_UDP_SERVER variable. When deployed in this mode, it is advisable to recompile fwknop beforehand with './configure --enable-udp-server' so that fwknopd does not link against libpcap.
  • [server] Replaced all popen() and system() calls with execvpe() with no usage of the environment. This is a defensive measure to not make use of the shell for firewall command execution, and is supported on systems where execvpe() is available.
  • (Gerry Reno) Added support for firewalld to the fwknopd daemon on RHEL 7 and CentOS 7. This is implemented using the current firewalld '--direct --passthrough' capability which accepts raw iptables commands. More information on firewalld can be found here:
  • [server] Added support for the 'American Fuzzy Lop' (AFL) fuzzer from 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. See the test/afl/ directory for some automation around AFL fuzzing.
  • (Bill Stubbs) submitted a patch to fix a bug where fwknopd could not handle Ethernet frames that include the Frame Check Sequence (FCS) header. This header is four bytes long, and is placed at the end of each Ethernet frame. Normally the FCS header is not visible to libpcap, but some card/driver combinations result in it being included. Bill noticed this on the following platform: BeagleBone Black rev C running 3.8.13-bone50 #1 SMP Tue May 13 13:24:52 UTC 2014 armv7l GNU/Linux
  • [client] Bug fix to ensure that a User-Agent string can be specified when the fwknop client uses wget via SSL to resolve the external IP address. This closes issue #134 on github reported by Barry Allard. The fwknop client now uses the wget '-U' option to specify the User-Agent string with a default of "Fwknop/<version>". In addition, a new command line argument "--use-wget-user-agent" to allow the default wget User-Agent string to apply instead.
  • [python module] When an HMAC key is passed to spa_data_final() then default to HMAC SHA256 if no HMAC mode was specified.