Michael Rash, Security Researcher

Software Release - fwknop-1.8.3

fwknop-1.8.3 release The 1.8.3 release of fwknop is ready for download. This release reinstates the legacy port knocking operation mode (for those that really want to use it instead of Single Packet Authorization). A few bugs have also been fixed, particularly for the auto-resolution of external NAT addresses via (and a backup resolution URL exists now as well that you can hit with the --URL option on the fwknop client command line).

Below is an illustration of the old port knocking mode in action. The fwknopd server running on reconfigures the iptables policy to allow an SSH connection from the client system after receiving the encrypted port knock sequence: $ fwknop -A tcp/22 -a -D --Server-mode knock
[+] Starting fwknop client (encrypted port knocking mode)...
[+] Enter an encryption key. This key must match a key in the file
/etc/fwknop/access.conf on the remote system.

Encryption Key:
[+] Clear-text sequence (11 bytes): 192 168 10 2 0 22 6 28 109 98 114
[+] Cipher-text sequence (32 bytes): 83 97 108 116 101 100 95 95 110 133 220 202 45 184 129 230 175 166 62 162 104 46 183 22 193 82 17 126 174 38 76 222
[+] Sending port knocking sequence to knock server:
   -> tcp/61083 (packet: 0)
   -> tcp/61097 (packet: 1)
   -> tcp/61108 (packet: 2)
   -> tcp/61116 (packet: 3)
   -> tcp/61101 (packet: 4)
   -> tcp/61100 (packet: 5)
   -> tcp/61095 (packet: 6)
   -> tcp/61095 (packet: 7)
   -> tcp/61110 (packet: 8)
   -> tcp/61133 (packet: 9)
   -> tcp/61220 (packet: 10)
   -> tcp/61202 (packet: 11)
   -> tcp/61045 (packet: 12)
   -> tcp/61184 (packet: 13)
   -> tcp/61129 (packet: 14)
   -> tcp/61230 (packet: 15)
   -> tcp/61175 (packet: 16)
   -> tcp/61166 (packet: 17)
   -> tcp/61062 (packet: 18)
   -> tcp/61162 (packet: 19)
   -> tcp/61104 (packet: 20)
   -> tcp/61046 (packet: 21)
   -> tcp/61183 (packet: 22)
   -> tcp/61022 (packet: 23)
   -> tcp/61193 (packet: 24)
   -> tcp/61082 (packet: 25)
   -> tcp/61017 (packet: 26)
   -> tcp/61126 (packet: 27)
   -> tcp/61174 (packet: 28)
   -> tcp/61038 (packet: 29)
   -> tcp/61076 (packet: 30)
   -> tcp/61222 (packet: 31)
[+] Finished knock sequence.
$ ssh -l mbr
On the fwknopd server, the following messages are written to syslog that show an iptables ACCEPT rule being added for the client system for 30 seconds and then removed. The SSH connection from the client remains open by using the Netfilter connection tracking subsystem to allow packets in the ESTABLISHED state through, but once the ACCEPT rule is removed no new SSH connections can be established: Nov 17 10:34:47 isengard fwknopd: successful knock decrypt for (SOURCE block: 1)
Nov 17 10:34:47 isengard fwknopd: adding iptables FWKNOP_INPUT ACCEPT rule for -> tcp/22 (30 seconds)
Nov 17 10:35:19 isengard fwknopd: removed iptables FWKNOP_INPUT ACCEPT rule for -> tcp/22, 30 second timeout exceeded
Port knocking sequences do not necessarily have to be encrypted, and fwknop supports shared sequences. This can be useful to allow systems where perl is not installed to take advantage of some port knocking capabilities without requiring the fwknop client. In the screenshot below, the fwknopd server (in the right hand terminal) has been configured to accept a sequence that consists of the two TCP ports 1234 followed 5001. The client (in the left hand terminal) just needs to use any program such as netcat or telnet to hit these two ports, which generates iptables log messages at the fwknopd server where the shared sequence is parsed and validated. Once the correct sequence is seen, fwknopd opens port 22 for 30 seconds (this timeout is configured in the /etc/fwknop/access.conf file):
fwknop-1.8.3 release
For those interested in the changes in the fwknop-1.8.3 release, here is the complete ChangeLog:
  • Updated external IP resolution to point to, and added as a backup site for fwknop IP resolution.
  • Added storage of source IP along with SPA MD5 sum. This allows the user to infer which networks are more hostile if an SPA packet is replayed.
  • Added SPA packet hex dumps in 'fwknopd --debug' mode so that the integration of third-party encryption algorithms is easier to troubleshoot. Sean Greven contributed a patch for this.
  • Reinstated the legacy port knocking mode. It appears that all encrypted output from the updated Crypt::Rijndael module is at least 32 bytes long, so port knocking sequences are now 32 bytes long as well (they were previously 16 bytes long in old versions of fwknop).
  • Bugfix to ensure the key length is at least 8 chars in --get-key mode.
  • Minor update to remove init message on OS X install.
  • Updated to set the LANG environmental variable to "en_US.UTF-8". This should fix the problem where the output of ifconfig was not interpreted correctly if the locale LANG setting is not English.
  • Implemented verbose email alerting by setting the ALERTING_METHODS variable to "verbose". This instructs fwknopd to generate a new email message for each message that it normally logs vis syslog (this feature is not the default, and must be manually enabled).