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.

Use Cases
People who use Single Packet Authorization (SPA) or its security-challenged cousin Port Knocking (PK) usually access SSHD running on the same system where the SPA/PK software is deployed. That is, a firewall running on a host has a default-drop policy against all incoming SSH connections so that SSHD cannot be scanned, but a SPA daemon reconfigures the firewall to temporarily grant access to a passively authenticated SPA client:


"Basic SPA usage to access SSHD"
fwknop supports the above, but also goes much further and makes robust usage of NAT (for iptables/firewalld firewalls). After all, important firewalls are usually gateways between networks as opposed to just being deployed on standalone hosts. NAT is commonly used on such firewalls (at least for IPv4 communications) to provide Internet access to internal networks that are on RFC 1918 address space, and also to allow external hosts access to services hosted on internal systems.
Because fwknop integrates with NAT, SPA can be leveraged to access internal services through the firewall by users on the external Internet. Although this has plenty of applications on modern traditional networks, it also allows fwknop to support cloud computing environments such as Amazon's AWS:


"SPA usage on Amazon AWS cloud environments"

User Interface
The official cross-platform fwknop client user interface fwknop-gui (download, github) is developed by Jonathan Bennett. Most major client-side SPA modes are supported including NAT requests, HMAC and Rijndael keys (GnuPG is not yet supported), fwknoprc stanza saving, and more. Currently fwknop-gui runs on Linux, Mac OS X, and Windows - here is a screenshot from OS X:


  "fwknop-gui on Mac OS X" Similarly, an updated Android client is available as well.

Tutorial
A comprehensive tutorial on fwknop can be found here:
http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html

Features
The following is a complete list of features supported by the fwknop project:

• Implements Single Packet Authorization around iptables and firewalld firewalls on Linux, ipfw firewalls on *BSD and Mac OS X, and PF on OpenBSD.
• The fwknop client runs on Linux, Mac OS X, *BSD, and Windows under Cygwin. In addition, there is an Android app to generate SPA packets.
• Supports both Rijndael and GnuPG methods for the encryption/decryption of SPA packets.
• Supports HMAC authenticated encryption for both Rijndael and GnuPG. The order of operation is encrypt-then-authenticate to avoid various cryptanalytic problems.
• Replay attacks are detected and thwarted by SHA-256 digest comparison of valid incoming SPA packets. Other digest algorithms are also supported, but SHA-256 is the default.
• SPA packets are passively sniffed from the wire via libpcap. The fwknopd server can also acquire packet data from a file that is written to by a separate Ethernet sniffer (such as with tcpdump -w <file>), from the iptables ULOG pcap writer, or directly via a UDP socket in --udp-server mode.
• For iptables firewalls, ACCEPT rules added by fwknop are added and deleted (after a configurable timeout) from custom iptables chains so that fwknop does not interfere with any existing iptables policy that may already be loaded on the system.
• Supports inbound NAT connections for authenticated SPA communications (iptables firewalls only for now). This means fwknop can be configured to create DNAT rules so that you can reach a service (such as SSH) running on an internal system on an RFC 1918 IP address from the open Internet. SNAT rules are also supported which essentially turns fwknopd into a SPA-authenticating gateway to access the Internet from an internal network.
• Multiple users are supported by the fwknop server, and each user can be assigned their own symmetric or asymmetric encryption key via the /etc/fwknop/access.conf file.
• Automatic resolution of external IP address via https://www.cipherdyne.org/cgi-bin/myip (this is useful when the fwknop client is run from behind a NAT device). Because the external IP address is encrypted within each SPA packet in this mode, Man-in-the-Middle (MITM) attacks where an inline device intercepts an SPA packet and only forwards it from a different IP in an effort to gain access are thwarted.
• Port randomization is supported for the destination port of SPA packets as well as the port over which the follow-on connection is made via the iptables NAT capabilities. The later applies to forwarded connections to internal services and to access granted to local sockets on the system running fwknopd.
• Integration with Tor (as described in this DefCon 14 presentation). Note that because Tor uses TCP for transport, sending SPA packets through the Tor network requires that each SPA packet is sent over an established TCP connection, so technically this breaks the "single" aspect of "Single Packet Authorization". However, Tor provides anonymity benefits that can outweigh this consideration in some deployments.
• Implements a versioned protocol for SPA communications, so it is easy to extend the protocol to offer new SPA message types and maintain backwards compatibility with older fwknop clients at the same time.
• Supports the execution of shell commands on behalf of valid SPA packets.
• The fwknop server can be configured to place multiple restrictions on inbound SPA packets beyond those enforced by encryption keys and replay attack detection. Namely, packet age, source IP address, remote user, access to requested ports, and more.
• Bundled with fwknop is a comprehensive test suite that issues a series of tests designed to verify that both the client and server pieces of fwknop work properly. These tests involve sniffing SPA packets over the local loopback interface, building temporary firewall rules that are checked for the appropriate access based on the testing config, and parsing output from both the fwknop client and fwknopd server for expected markers for each test. Test suite output can easily be anonymized for communication to third parties for analysis.
• fwknop was the first program to integrate port knocking with passive OS fingerprinting. However, Single Packet Authorization offers many security benefits beyond port knocking, so the port knocking mode of operation is generally deprecated.

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-firewalld=/path/to/firewalld
                          Specify path to the firewalld executable
                          [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]

Examples:

./configure --disable-client --with-firewalld=/bin/firewall-cmd
./configure --disable-client --with-iptables=/sbin/iptables --with-firewalld=no