The sys admin's daily grind: Single-packet authentication
Conventional, woodpecker-style port knocking is open to sniffing and brute force knocking attacks. Sending an encrypted packet with an access request to the server is safer and more modern. Learn more about Firewall Knock Operator, a.k.a. Fwknop.
Conventional port knocking, which I described last month , protects you against attackers who routinely scan whole networks looking for "low-hanging fruit." A cracker who takes more time and logs communications can also identify knocking signals because the sequences will repeat.
In theory, you might consider using lists of one-off knocking signals that become obsolete after use. Unfortunately, this is really complex. Besides, if the administrator is not creative enough, an attacker could just try out popular knocking sequences (port 7000, 8000, 9000, …) to gain access.
Single-Packet Authentication (SPA) is one possible solution. The knocking system sends a single packet containing the encrypted authentication credentials – typically a pass phrase – and the client request to open a specific port. An SPA implementation that works really well is Firewall Knock Operator, or Fwknop . Besides the normal build tools, the installation requires Perl, the libpcap-dev package, and the CPAN Net::Pcap module. After installing all of these resources, installing Fwknop is a breeze thanks to the Perl-based installer.
Fwknop comprises the fwknopd server and the fwknop client. By editing two files below /etc/fwknop/, you can configure the server; fwknop.conf contains the basic configuration. Initially, you will just need to change a couple of parameters, which are tagged __CHANGEME__.
The other knobs you could tweak here have very sensible defaults. Note that you need to synchronize the time between the server and the client because if the difference is too big, fwknopd will ignore the knocking client.
The entries in /etc/fwknop/access.conf define how fwknopd responds to a client knocking. The secret key that the client uses to identify itself is stored here. The SOURCE line can be used to restrict the networks from which the daemon accepts knocking. To set the port that the system opens on successful knocking – for example, tcp/22 for SSH – you can use OPEN_PORTS. Figure 1 shows a successful attempt. The fwknop client picks up the key from its own /etc/fwknop/access.conf.
If the SSH connection doesn't open quickly enough, the FW_ACCESS_TIMEOUT on the server triggers. This time is normally set to 30 seconds, but I went for twice that – never rush an admin on the job!
- "Knock-Knock" by Charly Kühnast, Linux Magazine, September 2008, http://www.linux-magazine.com/issues/2008/94/knock_knock
- Fwknop: http://www.cipherdyne.org/fwknop/
Buy this article as PDF
Mozilla’s product think tank sinks silently into history.
TODO group will focus on open source tools in large-scale environments.
New tool will look like GParted but support a wider range of storage technologies.
New public key pinning feature will help prevent man-in-the-middle attacks.
Carnegie Mellon researchers say 3 million pages could fall down the phishing hole in the next year.
The US government rolls new best-practice rules for protecting SSH.
Klaus Knopper announces the latest version of his iconic Live Linux system.
All websites that use these popular CMS tools could be vulnerable to denial of service attacks if users don't install the updates.
According to a report, many potential victims of the Heartbleed attack have patched their systems, but few have cleaned up the crime scene to protect themselves from the effects of a previous intrusion.
DARPA and NICTA release the code for the ultra-secure microkernel system used in aerial drones.