Provided by: fwknop-client_2.6.10-20.2build3_amd64 
NAME
fwknop - Firewall Knock Operator
SYNOPSIS
fwknop -A <'proto/ports'> -R|-a|-s -D <'host'> [options]
DESCRIPTION
fwknop implements an authorization scheme known as Single Packet Authorization (SPA) for strong service
concealment. SPA requires only a single packet which is encrypted, non-replayable, and authenticated via
an HMAC in order to communicate desired access to a service that is hidden behind a firewall in a
default-drop filtering stance. The main application of SPA is to use a firewall to drop all attempts to
connect to services such as SSH in order to make the exploitation of vulnerabilities (both 0-day and
unpatched code) more difficult. Any service that is concealed by SPA naturally cannot be scanned for with
Nmap. The fwknop project natively supports four different firewalls: iptables, firewalld, PF, and ipfw
across Linux, OpenBSD, FreeBSD, and Mac OS X. There is also support for custom scripts so that fwknop can
be made to support other infrastructure such as ipset or nftables.
SPA is essentially next generation Port Knocking (PK), but solves many of the limitations exhibited by PK
while retaining its core benefits. PK limitations include a general difficulty in protecting against
replay attacks, asymmetric ciphers and HMAC schemes are not usually possible to reliably support, and it
is trivially easy to mount a DoS attack against a PK server just by spoofing an additional packet into a
PK sequence as it traverses the network (thereby convincing the PK server that the client doesn’t know
the proper sequence). All of these limitation are solved by SPA. At the same time, SPA hides services
behind a default-drop firewall policy, acquires SPA data passively (usually via libpcap or other means),
and implements standard cryptographic operations for SPA packet authentication and encryption/decryption.
This is the manual page for the fwknop client which is responsible for constructing SPA packets and
sending them over the network. The server side is implemented by the fwknopd daemon which sniffs the
network for SPA packets and interacts with the local firewall to allow SPA authenticated connections. It
is recommended to read the fwknopd(8) manual page as well. Further detailed information may be found in
the tutorial “Single Packet Authorization: A Comprehensive Guide to Strong Service Concealment with
fwknop” available online (see: http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html).
SPA packets generated by fwknop leverage HMAC for authenticated encryption in the
encrypt-then-authenticate model. Although the usage of an HMAC is currently optional (enabled via the
--use-hmac command line switch), it is highly recommended for three reasons: 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), and 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. Reason 3) is why an HMAC should still be used even when
SPA packets are encrypted with GnuPG due to the fact that SPA data is not sent through libgpgme functions
unless the HMAC checks out first. GnuPG and libgpgme are relatively complex bodies of code, and therefore
limiting the ability of a potential attacker to interact with this code through an HMAC operation helps
to maintain a stronger security stance. Generating an HMAC for SPA communications requires a dedicated
key in addition to the normal encryption key, and both can be generated with the --key-gen option.
fwknop encrypts SPA packets either with the Rijndael block cipher or via GnuPG and associated asymmetric
cipher. If the symmetric encryption method is chosen, then as usual the encryption key is shared between
the client and server (see the fwknopd /etc/fwknop/access.conf file for details). The actual encryption
key used for Rijndael encryption is generated via the standard PBKDF1 key derivation algorithm, and CBC
mode is set. If the GnuPG method is chosen, then the encryption keys are derived from GnuPG key rings.
SPA packets generated by fwknop running as a client adhere to the following format (before encryption and
the HMAC is applied):
random data (16 digits)
username
timestamp
software version
mode (command mode (0) or access mode (1))
if command mode => command to execute
else access mode => IP,proto,port
message digest (SHA512 / SHA384 / SHA256 / SHA1 / MD5 / SHA3_256 / SHA3_512)
Each of the above fields are separated by a ":" character due to the variable length of several of the
fields, and those that might contain ":" characters are base64 encoded. The message digest (SHA256 by
default) is part of the data to be encrypted and is independent of the HMAC which is appended to the SPA
packet data after encryption. The 16 digits of random data (about 53 bits) ensures that no two SPA
packets are identical, and this is in addition to and independent of using PBKDF1 for key derivation for
Rijndael in CBC mode (which uses an 8-byte random "salt" value). Because fwknopd tracks the SHA256 digest
of all incoming valid SPA packets and throws out duplicates, replay attacks are not feasible against
fwknop. Syslog alerts are generated if a replay is detected.
By default, the fwknop client sends authorization packets over UDP port 62201, but this can be altered
with the --server-port argument (this requires fwknopd to be configured to acquire SPA data over the
selected port). Also, fwknop can send the SPA packet over a random port via the --rand-port argument. See
fwknopd(8) for further details. See the EXAMPLES section for example invocations of the fwknop client.
The fwknop client is quite portable, and is known to run on various Linux distributions (all major
distros and embedded ones such as OpenWRT as well), FreeBSD, OpenBSD, Mac OS X, and Cygwin on Windows.
There is also a library libfko that both fwknop and fwknopd use for SPA packet encryption/decryption and
HMAC authentication operations. This library can be used to allow third party applications to use SPA
subject to the terms of the GNU General Public License (GPL v2+).
REQUIRED ARGUMENTS
These required arguments can be specified via command-line or from within the ~/.fwknoprc file (see -n,
--named-config option and the FWKNOPRC FILE section below).
-A, --access=<port list>
Provide a list of ports and protocols to access on a remote computer running fwknopd. The format of
this list is “+<proto>/<port>...<proto>/<port>+”, e.g. “tcp/22,udp/53”. NOTE: The vast majority of
usages for fwknop require the -A argument, but sending full commands with the --server-cmd argument
via an SPA packet to be executed by fwknopd does not require this argument.
-D, --destination=<hostname/IP-address>
Direct the fwknop client to authenticate with the fwknopd daemon/service at the specified destination
hostname or IP address. The connection mode is discovered by the fwknopd daemon/service when it
decrypts and parses the authentication packet.
-R|-a|-s
One of these options (see below) is required to tell the remote fwknopd daemon what IP should be
allowed through the firewall. It is recommend to use the -R or -a options instead of -s in order to
harden SPA communications against possible Man-In-The-Middle (MITM) attacks, and on the server side
set REQUIRE_SOURCE_ADDRESS variable in the /etc/fwknop/access.conf file. Note that the most secure
option is -a so that fwknop does not have to issue any HTTPS request to
https://www.cipherdyne.org/cgi-bin/myip in order to resolve the externally routable IP address. Using
-a requires that the user already knows what the external IP is for the network where fwknop is
running.
GENERAL OPTIONS
-h, --help
Print a usage summary message and exit.
-G, --get-key=<file>
Load an encryption key/password from the specified file. The key file contains a line for each
destination hostname or IP address, a colon (":"), optional space and the password, followed by a
newline. Note that the last line has to have a terminating newline character. Also note: though this
is a convenience, having a file on your system with clear text passwords is not a good idea and is
not recommended. Having the fwknop client prompt you for the key is generally more secure. Note also
that if a key is stored on disk, the fwknop rc file is a more powerful mechanism for specifying not
only the key but other options as well.
--stdin
Read the encryption key/password from stdin. This can be used to send the data via a pipe for
example. This command is similar to --fd 0.
--fd=<number>
Specify the file descriptor number to read the key/password from. This command avoids the user being
prompted for a password if none has been found in the user specific stanza, or none has been supplied
on the command line. A file descriptor set to 0 is similar to the stdin command.
--get-hmac-key=<file>
Load an HMAC key/password from the specified file. Similarly to the format for the --get-key option,
the HMAC key file contains a line for each destination hostname or IP address, a colon (":"),
optional space and the password, followed by a newline. Note that the last line has to have a
terminating newline character. Also note: though this is a convenience, having a file on your system
with clear text passwords is not a good idea and is not recommended. Having the fwknop client prompt
you for the HMAC key is generally more secure. Note also that if a key is stored on disk, the fwknop
rc file is a more powerful mechanism for specifying not only the HMAC key but other options as well.
--key-gen
Have fwknop generate both Rijndael and HMAC keys that can be used for SPA packet encryption and
authentication. These keys are derived from /dev/urandom and then base64 encoded before being printed
to stdout, and are meant to be included within the “$HOME/.fwknoprc” file (or the file referenced by
--get-key). Such keys are generally more secure than passphrases that are typed in from the command
line.
--key-gen-file=<file>
Write generated keys to the specified file. Note that the file is overwritten if it already exists.
If this option is not given, then --key-gen writes the keys to stdout.
--key-len=<length>
Specify the number of bytes for a generated Rijndael key. The maximum size is currently 128 bytes.
--hmac-key-len=<length>
Specify the number of bytes for a generated HMAC key. The maximum size is currently 128 bytes.
-l, --last-cmd
Execute fwknop with the command-line arguments from the previous invocation (if any). The previous
arguments are parsed out of the ~/.fwknop.run file.
-n, --named-config=<stanza name>
Specify the name of the configuration stanza in the “$HOME/.fwknoprc” file to pull configuration and
command directives. These named stanzas alleviate the need for remembering the various command-line
arguments for frequently used invocations of fwknop. See the section labeled, FWKNOPRC FILE below for
a list of the valid configuration directives in the .fwknoprc file.
--key-rijndael=<key>
Specify the Rijndael key on the command line. Since the key may be visible to utilities such as ps
under Unix, this form should only be used where security is not critical. Having the fwknop client
either prompt you for the key or acquire via the “$HOME/.fwknoprc” file is generally more secure.
--key-base64-rijndael=<key>
Specify the base64 encoded Rijndael key. Since the key may be visible to utilities such as ps under
Unix, this form should only be used where security is not critical. Having the fwknop client either
prompt you for the key or acquire via the “$HOME/.fwknoprc” file is generally more secure.
--key-base64-hmac=<key>
Specify the base64 encoded HMAC key. Since the key may be visible to utilities such as ps under Unix,
this form should only be used where security is not critical. Having the fwknop client either prompt
you for the key or acquire via the “$HOME/.fwknoprc” file is generally more secure.
--key-hmac=<key>
Specify the raw HMAC key (not base64 encoded). Since the key may be visible to utilities such as ps
under Unix, this form should only be used where security is not critical. Having the fwknop client
either prompt you for the key or acquire via the “$HOME/.fwknoprc” file is generally more secure.
--rc-file=<file>
Specify path to the fwknop rc file (default is “$HOME/.fwknoprc”).
--no-rc-file
Perform fwknop client operations without referencing the “$HOME/.fwknoprc” file.
--no-home-dir
Do not allow the fwknop client to look for the home directory associated with the user.
--save-rc-stanza=<stanza name>
Save command line arguments to the “$HOME/.fwknoprc” stanza specified with the -n option. If the -n
option is omitted, then the stanza name will default to the destination server value (hostname or IP)
given with the -D argument.
--force-stanza
Used with --save-rc-stanza to overwrite all of the variables for the specified stanza
--stanza-list
Dump a list of the stanzas found in “$HOME/.fwknoprc”.
--show-last
Display the last command-line arguments used by fwknop.
-E, --save-args-file=<file>
Save command line arguments to a specified file path. Without this option, and when --no-save-args is
not also specified, then the default save args path is ~/.fwknop.run.
--no-save-args
Do not save the command line arguments given when fwknop is executed.
-T, --test
Test mode. Generate the SPA packet data, but do not send it. Instead, print a break-down of the SPA
data fields, then run the data through the decryption and decoding process and print the break-down
again. This is primarily a debugging feature.
-B, --save-packet=<file>
Instruct the fwknop client to write a newly created SPA packet out to the specified file so that it
can be examined off-line.
-b, --save-packet-append
Append the generated packet data to the file specified with the -B option.
--fault-injection-tag=<tag>
This option is only used for fault injection testing when fwknop is compiled to support the libfiu
library (see: http://blitiri.com.ar/p/libfiu/). Under normal circumstances this option is not used,
and any packaged version of fwknop will not have code compiled in so this capability is not enabled
at run time. It is documented here for completeness.
-v, --verbose
Run the fwknop client in verbose mode. This causes fwknop to print some extra information about the
current command and the resulting SPA data.
-V, --version
Display version information and exit.
SPA OPTIONS
--use-hmac
Set HMAC mode for authenticated encryption of SPA communications. As of fwknop 2.5, this is an
optional feature, but this will become the default in a future release.
-a, --allow-ip=<IP-address>
Specify IP address that should be permitted through the destination fwknopd server firewall (this IP
is encrypted within the SPA packet itself). This is useful to prevent a MITM attack where a SPA
packet can be intercepted en-route and sent from a different IP than the original. Hence, if the
fwknopd server trusts the source address on the SPA packet IP header then the attacker gains access.
The -a option puts the source address within the encrypted SPA packet, and so thwarts this attack.
The -a option is also useful to specify the IP that will be granted access when the SPA packet itself
is spoofed with the --spoof-src option. Another related option is -R (see below) which instructs the
fwknop client to automatically resolve the externally routable IP address the local system is
connected to by querying https://www.cipherdyne.org/cgi-bin/myip. This returns the actual IP address
it sees from the calling system.
-g, --gpg-encryption
Use GPG encryption on the SPA packet (default if not specified is Rijndael). Note: Use of this
option will also require a GPG recipient (see --gpg-recipient along with other GPG-related options
below).
--hmac-digest-type=<digest>
Set the HMAC digest algorithm for authenticated encryption of SPA packets. Choices are: MD5, SHA1,
SHA256 (the default), SHA384, SHA512, SHA3_256, and SHA3_512.
-N, --nat-access=<internalIP:forwardPort>
The fwknopd server offers the ability to provide SPA access through an iptables firewall to an
internal service by interfacing with the iptables NAT capabilities. So, if the fwknopd server is
protecting an internal network on an RFC-1918 address space, an external fwknop client can request
that the server port forward an external port to an internal IP, i.e. “+--NAT-access
192.168.10.2,55000+”. In this case, access will be granted to 192.168.10.2 via port 55000 to whatever
service is requested via the --access argument (usually tcp/22). Hence, after sending such an SPA
packet, one would then do “ssh -p 55000 user@host” and the connection would be forwarded on through
to the internal 192.168.10.2 system automatically. Note that the port “55000” can be randomly
generated via the --nat-rand-port argument (described later).
--nat-local
On the fwknopd server, a NAT operation can apply to the local system instead of being forwarded
through the system. That is, for iptables firewalls, a connection to, say, port 55,000 can be
translated to port 22 on the local system. By making use of the --nat-local argument, the fwknop
client can be made to request such access. This means that any external attacker would only see a
connection over port 55,000 instead of the expected port 22 after the SPA packet is sent.
--nat-port
Usually fwknop is used to request access to a specific port such as tcp/22 on a system running
fwknopd. However, by using the --nat-port argument, it is possible to request access to a (again,
such as tcp/22), but have this access granted via the specified port (so, the -p argument would then
be used on the SSH client command line). See the --nat-local and --nat-access command line arguments
to fwknop for additional details on gaining access to services via a NAT operation.
--nat-rand-port
Usually fwknop is used to request access to a specific port such as tcp/22 on a system running
fwknopd. However, by using the --nat-rand-port argument, it is possible to request access to a
particular service (again, such as tcp/22), but have this access granted via a random translated
port. That is, once the fwknop client has been executed in this mode and the random port selected by
fwknop is displayed, the destination port used by the follow-on client must be changed to match this
random port. For SSH, this is accomplished via the -p argument. See the --nat-local and --nat-access
command line arguments to fwknop for additional details on gaining access to services via a NAT
operation.
-p, --server-port=<port>
Specify the port number where fwknopd accepts packets via libpcap or ulogd pcap writer. By default
fwknopd looks for authorization packets over UDP port 62201.
-P, --server-proto=<protocol>
Set the protocol (udp, tcp, http, udpraw, tcpraw, or icmp) for the outgoing SPA packet. Note: The
udpraw, tcpraw, and icmp modes use raw sockets and thus require root access to run. Also note: The
tcp mode expects to establish a TCP connection to the server before sending the SPA packet. This is
not normally done, but is useful for compatibility with the Tor for strong anonymity; see
http://tor.eff.org/. In this case, the fwknopd server will need to be configured to listen on the
target TCP port (which is 62201 by default).
-Q, --spoof-src=<IP>
Spoof the source address from which the fwknop client sends SPA packets. This requires root on the
client side access since a raw socket is required to accomplish this. Note that the --spoof-user
argument can be given in this mode in order to pass any REQUIRE_USERNAME keyword that might be
specified in /etc/fwknop/access.conf.
-r, --rand-port
Instruct the fwknop client to send an SPA packet over a random destination port between 10,000 and
65535. The fwknopd server must use a PCAP_FILTER variable that is configured to accept such packets.
For example, the PCAP_FILTER variable could be set to: “+udp dst portrange 10000-65535+”.
-R, --resolve-ip-https
This is an important option, and instructs the fwknop client to issue an HTTPS request to a script
running on cipherdyne.org that returns the client’s IP address (as seen by the web server). In some
cases, this is needed to determine the IP address that should be allowed through the firewall policy
at the remote fwknopd server side. This option is useful if the fwknop client is being used on a
system that is behind an obscure NAT address, and the external Internet facing IP is not known to the
user. The full resolution URL is: https://www.cipherdyne.org/cgi-bin/myip, and is accessed by fwknop
via wget in --secure-protocol mode. Note that it is generally more secure to use the -a option if the
externally routable IP address for the client is already known to the user since this eliminates the
need for fwknop to issue any sort of HTTPS request.
--resolve-url <url>
Override the default URL used for resolving the source IP address. For best results, the URL
specified here should point to a web service that provides just an IP address in the body of the HTTP
response.
--resolve-http-only
This option forces the fwknop client to resolve the external IP via HTTP instead of HTTPS. There are
some circumstances where this might be necessary such as when wget is not available (or hasn’t been
compiled with SSL support), but generally this is not recommended since it opens the possibility of a
MITM attack through manipulation of the IP resolution HTTP response. Either specify the IP manually
with -a, or use -R and omit this option.
-w, --wget-cmd=<wget full path>
Manually set the full path to the wget command. Normally the configure script finds the wget command,
but this option can be used to specify the path if it is located in a non-standard place.
-s, --source-ip
Instruct the fwknop client to form an SPA packet that contains the special-case IP address
“+0.0.0.0+” which will inform the destination fwknopd SPA server to use the source IP address from
which the SPA packet originates as the IP that will be allowed through upon modification of the
firewall ruleset. This option is useful if the fwknop client is deployed on a machine that is behind
a NAT device and the external IP is not known. However, usage of this option is not recommended, and
either the -a or -R options should be used instead. The permit-address options -s, -R and -a are
mutually exclusive.
-S, --source-port=<port>
Set the source port for outgoing SPA packet.
--server-resolve-ipv4
This option forces the fwknop client to only accept an IPv4 address from DNS when a hostname is used
for the SPA server. This is necessary in some cases where DNS may return both IPv6 and IPv4
addresses.
-f, --fw-timeout=<seconds>
Specify the length of time (seconds) that the remote firewall rule that grants access to a service is
to remain active. The default maintained by fwknopd is 30 seconds, but any established connection can
be kept open after the initial accept rule is deleted through the use of a connection tracking
mechanism that may be offered by the firewall.
-C, --server-cmd=<command to execute>
Instead of requesting access to a service with an SPA packet, the --server-cmd argument specifies a
command that will be executed by the fwknopd server. The command is encrypted within the SPA packet
and sniffed off the wire (as usual) by the fwknopd server.
-H, --http-proxy=<proxy-host>[:port]
Specify an HTTP proxy that the fwknop client will use to send the SPA packet through. Using this
option will automatically set the SPA packet transmission mode (usually set via the --server-proto
argument) to "http". You can also specify the proxy port by adding ":<port>" to the proxy host name
or ip.
-m, --digest-type=<digest>
Specify the message digest algorithm to use in the SPA data. Choices are: MD5, SHA1, SHA256 (the
default), SHA384, and SHA512, SHA3_256, and SHA3_512.
-M, --encryption-mode=<mode>
Specify the encryption mode when AES is used for encrypting SPA packets. The default is CBC mode, but
others can be chosen such as CFB or OFB as long as this is also specified in the
/etc/fwknop/access.conf file on the server side via the ENCRYPTION_MODE variable. In general, it is
recommended to not include this argument and let the default (CBC) apply. Note that the string
“legacy” can be specified in order to generate SPA packets with the old initialization vector
strategy used by versions of fwknop prior to 2.5. With the 2.5 release, fwknop generates
initialization vectors in a manner that is compatible with OpenSSL via the PBKDF1 algorithm.
--time-offset-plus=<time>
By default, the fwknopd daemon on the server side enforces time synchronization between the clocks
running on client and server systems. The fwknop client places the local time within each SPA packet
as a time stamp to be validated by the fwknopd server after decryption. However, in some
circumstances, if the clocks are out of sync and the user on the client system does not have the
required access to change the local clock setting, it can be difficult to construct and SPA packet
with a time stamp the server will accept. In this situation, the --time-offset-plus option can allow
the user to specify an offset (e.g. “60sec” “60min” “2days” etc.) that is added to the local time.
--time-offset-minus=<time>
This is similar to the --time-offset-plus option (see above), but subtracts the specified time offset
instead of adding it to the local time stamp.
-u, --user-agent=<user-agent-string>
Set the HTTP User-Agent for resolving the external IP via -R, or for sending SPA packets over HTTP.
--use-wget-user-agent
By default when the fwknop client resolves the external IP with wget via SSL, it sets the User-Agent
to “Fwknop/<version>” unless it was already manually specified with the --user-agent option mentioned
above. However, the --user-wget-user-agent option lets the default wget User-Agent string apply
without influence from fwknop.
-U, --spoof-user=<user>
Specify the username that is included within SPA packet. This allows the fwknop client to satisfy any
non-root REQUIRE_USERNAME keyword on the fwknopd server (--spoof-src mode requires that the fwknop
client is executed as root).
--icmp-type=<type>
In -P icmp mode, specify the ICMP type value that will be set in the SPA packet ICMP header. The
default is echo reply.
--icmp-code=<code>
In -P icmp mode, specify the ICMP code value that will be set in the SPA packet ICMP header. The
default is zero.
GPG OPTIONS
Note that the usage of GPG for SPA encryption/decryption can and should involve GPG keys that are signed
by each side (client and server). The basic procedure for this involves the following steps after the
client key has been transferred to the server and vice-versa:
[spaserver]# gpg --import client.asc
[spaserver]# gpg --edit-key 1234ABCD
Command> sign
[spaclient]$ gpg --import server.asc
[spaclient]$ gpg --edit-key ABCD1234
Command> sign
More comprehensive information on this can be found here:
http://www.cipherdyne.org/fwknop/docs/gpghowto.html.
--gpg-agent
Instruct fwknop to acquire GnuPG key password from a running gpg-agent instance (if available).
--gpg-home-dir=<dir>
Specify the path to the GnuPG directory; normally this path is derived from the home directory of the
user that is running the fwknop client (so the default is ~/.gnupg). This is useful when a “root”
user wishes to log into a remote machine whose sshd daemon/service does not permit root login.
--gpg-recipient=<key ID or Name>
Specify the GnuPG key ID, e.g. “+1234ABCD+” (see the output of "gpg—list-keys") or the key name
(associated email address) of the recipient of the Single Packet Authorization message. This key is
imported by the fwknopd server and the associated private key is used to decrypt the SPA packet. The
recipient’s key must first be imported into the client GnuPG key ring.
--gpg-signer-key=<key ID or Name>
Specify the GnuPG key ID, e.g. “+ABCD1234+” (see the output of "gpg --list-keys") or the key name to
use when signing the SPA message. The user is prompted for the associated GnuPG password to create
the signature. This adds a cryptographically strong mechanism to allow the fwknopd daemon on the
remote server to authenticate who created the SPA message.
--gpg-no-signing-pw
Instruct fwknop to not acquire a passphrase for usage of GnuPG signing key. This option is provided
to make SPA packet construction easier for client-side operations in automated environments where the
passphrase for the signing key has been removed from the GnuPG key ring. However, it is usually
better to leverage gpg-agent instead of using this option.
FWKNOPRC FILE
The .fwknoprc file is used to set various parameters to override default program parameters at runtime.
It also allows for additional named configuration stanzas for setting program parameters for a particular
invocation.
The fwknop client will create this file if it does not exist in the user’s home directory. This initial
version has some sample directives that are commented out. It is up to the user to edit this file to meet
their needs.
The .fwknoprc file contains a default configuration area or stanza which holds global configuration
directives that override the program defaults. You can edit this file and create additional named stanzas
that can be specified with the -n or --named-config option. Parameters defined in the named stanzas will
override any matching default stanza directives. Note that command-line options will still override any
corresponding .fwknoprc directives.
There are directives to match most of the command-line parameters fwknop supports. Here is the current
list of each directive along with a brief description and its matching command-line option(s):
SPA_SERVER <hostname/IP-address>
Specify the hostname or IP of the destination (fwknopd) server (-D, --destination).
ALLOW_IP <IP-address>
Specify the address to allow within the SPA data. Note: This parameter covers the -a, -s, and -R
command-line options. You can specify a hostname or IP address (the -a option), specify the word
"source" to tell the fwknopd server to accept the source IP of the packet as the IP to allow (the -s
option), or use the word "resolve" to have fwknop resolve the external network IP via HTTP request
(the -R option).
ACCESS <port list>
Set the one or more protocol/ports to open on the firewall (-A, --access). The format of this list is
“+<proto>/<port>...<proto>/<port>+”, e.g. “tcp/22,udp/53”.
SPA_SERVER_PORT <port>
Set the server port to use for sending the SPA packet (-p, --server-port).
SPA_SERVER_PROTO <protocol>
Set the protocol to use for sending the SPA packet (-P, --server-proto).
KEY <passphrase>
This is the passphrase that is used for SPA packet encryption and applies to both Rijndael or GPG
encryption modes. The actual encryption key that is used for Rijndael is derived from the PBKDF1
algorithm, and the GPG key is derived from the specified GPG key ring.
KEY_BASE64 <base64 encoded passphrase>
Specify the encryption passphrase as a base64 encoded string. This allows non-ascii characters to be
included in the base64-decoded key.
USE_HMAC <Y/N>
Set HMAC mode for authenticated encryption of SPA packets. This will have fwknop prompt the user for
a dedicated HMAC key that is independent of the encryption key. Alternatively, the HMAC key can be
specified with the HMAC_KEY or HMAC_KEY_BASE64 directives (see below).
HMAC_KEY <key>
Specify the HMAC key for authenticated encryption of SPA packets. This supports both Rijndael and GPG
encryption modes, and is applied according to the encrypt-then-authenticate model.
HMAC_KEY_BASE64 <base64 encoded key>
Specify the HMAC key as a base64 encoded string. This allows non-ascii characters to be included in
the base64-decoded key.
HMAC_DIGEST_TYPE <digest algorithm>
Set the HMAC digest algorithm used for authenticated encryption of SPA packets. Choices are: MD5,
SHA1, SHA256 (the default), SHA384, SHA512, SHA3_256, and SHA3_512.
SPA_SOURCE_PORT <port>
Set the source port to use for sending the SPA packet (-S, --source-port).
FW_TIMEOUT <seconds>
Set the firewall rule timeout value (-f, --fw-timeout).
RESOLVE_IP_HTTPS <Y/N>
Set to Y to automatically resolve the externally routable IP associated with the fwknop client. This
is done over SSL via wget in --secure-protocol mode against the IP resolution service available at
https://www.cipherdyne.org/cgi-bin/myip.
RESOLVE_HTTP_ONLY <Y/N>
When the fwknop client is instructed to resolve the external client IP, this option can be used to
force an HTTP connection instead of an HTTPS connection when set to Y. This option is useful when
wget is not installed on the local OS, or when it is not compiled against an SSL library.
RESOLVE_URL <url>
Set to a URL that will be used for resolving the source IP address (--resolve-url).
WGET_CMD <wget full path>
Set the full path to the wget command (used for client IP resolution).
TIME_OFFSET <time>
Set a value to apply to the timestamp in the SPA packet. This can be either a positive or negative
value (--time-offset-plus/minus).
ENCRYPTION_MODE <mode>
Specify the encryption mode when AES is used. This variable is a synonym for the -M,
--encryption-mode command line argument. In general, it is recommended to not include this argument
and let the default (CBC) apply. Note that the string “legacy” can be specified in order to generate
SPA packets with the old initialization vector strategy used by versions of fwknop prior to 2.5.
DIGEST_TYPE <digest algorithm>
Set the SPA message digest type (-m, --digest-type). Choices are: MD5, SHA1, SHA256 (the default),
SHA384, SHA512, SHA3_256, and SHA3_512.
USE_GPG <Y/N>
Set to Y to specify the use of GPG for encryption (--gpg-encryption).
USE_GPG_AGENT <Y/N>
Set to Y to have fwknop interface with a GPG agent instance for the GPG key password (--gpg-agent).
Agent information itself is specified with the GPG_AGENT_INFO environmental variable.
GPG_SIGNING_PW <passphrase>
This is the passphrase that is used for signing SPA packet data in GPG encryption mode, and is a
synonym for the KEY variable (i.e. the signing passphrase can be specified with the KEY variable
instead). The SPA packet is encrypted with the remote server key and signed with the local client
key.
GPG_SIGNING_PW_BASE64 <base64 encoded passphrase>
Specify the GPG signing passphrase as a base64 encoded string. This allows non-ascii characters to be
included in the base64-decoded key.
GPG_SIGNER <key ID or Name>
Specify the GPG key name or ID for signing the GPG-encrypted SPA data (--gpg-signer-key).
GPG_RECIPIENT <key ID or Name>
Specify the GPG key name or ID for the recipient of the GPG-encrypted SPA data (--gpg-recipient-key).
GPG_HOMEDIR <dir>
Specify the GPG home directory (--gpg-home-dir). Defaults to ~/.gnupg.
GPG_EXE <path>
Specify the path to GPG (--gpg-exe). Defaults to /usr/bin/gpg.
SPOOF_USER <user>
Set the username in the SPA data to the specified value (-U, --spoof-user).
SPOOF_SOURCE_IP <IP>
Set the source IP of the outgoing SPA packet to the specified value (-Q, --spoof-source).
RAND_PORT <Y/N>
Send the SPA packet over a randomly assigned port (-r, --rand-port).
KEY_FILE <file>
Load an encryption key/password from a file (-G, --get-key).
HTTP_USER_AGENT <agent string>
Set the HTTP User-Agent for resolving the external IP via -R, or for sending SPA packets over HTTP
(-u, --user-agent).
USE_WGET_USER_AGENT <Y/N>
Allow default wget User-Agent string to be used when resolving the external IP instead of a
User-Agent supplied by the fwknop client.
NAT_ACCESS <internalIP:forwardPort>
Gain NAT access to an internal service protected by the fwknop server (-N, --nat-access).
NAT_LOCAL <Y/N>
Access a local service via a forwarded port on the fwknopd server system (--nat-local).
NAT_PORT <port>
Specify the port to forward to access a service via NAT (--nat-port).
NAT_RAND_PORT <Y/N>
Have the fwknop client assign a random port for NAT access (--nat-rand-port).
ENVIRONMENT
SPOOF_USER, GPG_AGENT_INFO (only used in --gpg-agent mode).
SPA PACKET SPOOFING
Because fwknop places the IP to be allowed through the firewall within the encrypted SPA payload (unless
-s is used which is not recommended and can be prohibited in the fwknopd server configuration), SPA
packets can easily be spoofed, and this is a good thing in this context. That is, the source IP of an SPA
packet is ignored by the fwknopd daemon (when the REQUIRE_SOURCE_ADDRESS variable is set in the
/etc/fwknop/access.conf file) and only the IP that is contained within an authenticated and properly
decrypted SPA packet is granted access through the firewall. This makes it possible to make it appear as
though, say, www.yahoo.com is trying to authenticate to a target system but in reality the actual
connection will come from a seemingly unrelated IP.
EXAMPLES
The following examples illustrate the command line arguments that could be supplied to the fwknop client
in a few situations:
Quick start
The most effective and easiest way to use fwknop is to have the client generate both an encryption key
and an HMAC key, and then save them to the “$HOME/.fwknoprc” file along with access request specifics.
The keys will also need to be transferred to the system where fwknopd is running. The also client
supports a separate set of encryption and HMAC keys for each SPA destination if multiple fwknopd servers
are running on different systems.
So, assuming that the IP 2.2.2.2 is the system where fwknopd is deployed and SSH is protected by the
firewall on that system in a default-drop stance, run the client like so to generate encryption and HMAC
keys:
$ fwknop -A tcp/22 --use-hmac -R -D 2.2.2.2 --key-gen --save-rc-stanza --verbose
[+] Wrote Rijndael and HMAC keys to rc file: /home/user/.fwknoprc
With the access request arguments and encryption and HMAC keys generated and saved in “$HOME/.fwknoprc”,
the keys themselves need to be transferred to the 2.2.2.2 system where fwknopd is running. As always,
this should be done via some secure means such as SSH before SPA is enabled and SSHD is blocked by the
firewall. Here is what the new 2.2.2.2 stanza looks like in the ~/.fwknoprc file:
$ tail -n 8 /home/user/.fwknoprc
[2.2.2.2]
ACCESS tcp/22
SPA_SERVER 2.2.2.2
KEY_BASE64 HvUtIOramehLGKimD4ECXOzinaH4h3U8H1WXum7b54Q=
HMAC_KEY_BASE64 DLeLf93a3yBT2vhEpM+dWlirGta5GU+jdyG5uXp4461HgOtbqMem4gX0Bp2PJGzYZlbbcavcOM00UPm+0GqkXA==
USE_HMAC Y
VERBOSE Y
RESOLVE_IP_HTTPS Y
The keys are base64 encoded blobs of random data, and both the KEY_BASE64 and HMAC_KEY_BASE64 lines
should be copied verbatim and placed within the /etc/fwknop/access.conf file on 2.2.2.2. Once this is
done, fwknopd can be started on that system, a default-drop policy against SSH connections can be put in
place, and then access to SSH is managed via fwknop. To access SSH, just use the -n argument to reference
the 2.2.2.2 stanza out of the .fwknoprc file (some --verbose output is included for illustration):
$ fwknop -n 2.2.2.2
FKO Field Values:
=================
Random Value: 8950423288486978
Username: mbr
Timestamp: 1370194770
FKO Version: 2.5
Message Type: 1 (Access msg)
Message String: 1.1.1.1,tcp/22
Nat Access: <NULL>
Server Auth: <NULL>
Client Timeout: 0 (seconds)
Digest Type: 3 (SHA256)
HMAC Type: 3 (SHA256)
Encryption Type: 1 (Rijndael)
Encryption Mode: 2 (CBC)
...
$ ssh -l user 2.2.2.2
user@2.2.2.2's password:
Access mode examples
The most common usage of fwknop is to gain access to SSH running on a remote system that has the fwknopd
daemon deployed along with a default-drop firewall policy. The following command illustrates this where
IP 1.1.1.1 is the IP to be allowed through the firewall running on 3.3.3.3 (note that the
/etc/fwknop/access.conf file consumed by fwknopd will need to have matching encryption and HMAC keys, and
configuration specifics can be found in the fwknopd(8) manual page). Also, note the examples below prompt
the user to supply the encryption and HMAC keys via stdin instead of writing them to disk as in the case
of using the “$HOME/.fwknoprc” file in the example above. However, all of the following examples can be
converted to using the ~/.fwknoprc file just by adding the --save-rc-stanza argument:
$ fwknop -A tcp/22 --use-hmac -a 1.1.1.1 -D 3.3.3.3
Enter encryption key:
Enter HMAC key:
$ ssh -l user 3.3.3.3
user@3.3.3.3's password:
If the --verbose flag is added to the command line, then some SPA packet specifics are printed to stdout
(not all output is shown for brevity):
$ fwknop -A tcp/22 --use-hmac -a 1.1.1.1 -D 3.3.3.3 --verbose
Enter encryption key:
Enter HMAC key:
Random Value: 1916307060193417
Username: mbr
Timestamp: 1368498909
FKO Version: 2.5
Message Type: 1 (Access msg)
Message String: 1.1.1.1,tcp/22
Nat Access: <NULL>
Server Auth: <NULL>
Client Timeout: 0 (seconds)
Digest Type: 3 (SHA256)
HMAC Type: 3 (SHA256)
Encryption Type: 1 (Rijndael)
Encryption Mode: 2 (CBC)
Simultaneous access to multiple services is also supported, and here is an example of requesting access
to both SSH and OpenVPN on 3.3.3.3:
$ fwknop -A "tcp/22,tcp/1194" --use-hmac -a 1.1.1.1 -D 3.3.3.3
There are many cases where an fwknop client is deployed on a network behind a NAT device and the
externally routable IP is not known to the user. In this case, use the IP resolution service available at
https://www.cipherdyne.org/cgi-bin/myip via the -R command line switch in order to derive the external
client IP address. This is a safer method of acquiring the client IP address than using the -s option
mentioned earlier in this manual page because the source IP is put within the encrypted packet instead of
having the fwknopd daemon grant the requested access from whatever IP address the SPA packet originates
(i.e. using -s opens the possibility of a MITM attack):
$ fwknop -A tcp/22 --use-hmac -R -D 3.3.3.3
Use the Single Packet Authorization mode to gain access to SSH and this time use GnuPG keys to encrypt
and decrypt:
$ fwknop -A tcp/22 --use-hmac --gpg-sign ABCD1234 --gpg--recipient 1234ABCD -R -D 3.3.3.3
Instruct the fwknop server running at 3.3.3.3 to allow 1.1.1.1 to connect to SSH, but spoof the
authorization packet from an IP associated with www.yahoo.com (requires root on the fwknop client OS):
# fwknop --spoof-src "www.yahoo.com" -A tcp/22 --use-hmac -a 1.1.1.1 -D 3.3.3.3
When fwknopd is running on an iptables firewall with systems deployed behind it, it is possible to take
advantage of the NAT capabilities offered by iptables in order to transparently reach systems behind the
firewall via SPA. Here is an example where the fwknop client is used to gain access to SSH running on the
non-routable IP 192.168.10.23 that is deployed on the network behind 3.3.3.3. In this case, the SSH
connection made to 3.3.3.3 is translated via NAT to the 192.168.10.2 system automatically:
$ fwknop -A tcp/22 -N 192.168.10.2:22 -R -D 3.3.3.3
BACKWARDS COMPATIBILITY
With the 2.5 release, fwknop underwent significant changes in its usage of cryptography including the
addition of support for HMAC authenticated encryption for both Rijndael and GnuPG modes, ensuring the
proper usage of PBKDF1 for key derivation when SPA packets are encrypted with Rijndael, and several bugs
were fixed from previous versions of fwknop. In general, this implies that when Rijndael is used, SPA
packets produced by the 2.5 release are incompatible with previous versions of fwknop. The GnuPG
encryption mode is unaffected by these updates. However, even with Rijndael is used, backwards
compatibility is supported through setting the legacy encryption mode with -M on the fwknop client
command line and/or the ENCRYPTION_MODE variable in the /etc/fwknop/access.conf file. This way, a pre-2.5
server can decrypt SPA packets produced by a 2.5 and later client (set -M legacy), and a 2.5 and later
server can decrypt SPA packets produced by pre-2.5 clients (set ENCRYPTION_MODE legacy in the access.conf
file). Note that HMAC is only supported as of 2.5 and is an optional feature, so backwards compatibility
is only for configurations that don’t use an HMAC on either side. It is strongly recommended to upgrade
all fwknop clients and servers to 2.5 and use the new HMAC mode for properly authenticated SPA
communications. The backwards compatibility support is used to make it easier to upgrade clients and
servers with a phased approach.
For emphasis, if the fwknopd server is upgraded to 2.5 (or later), but older clients cannot be upgraded
at the same time, then for each SOURCE stanza in the /etc/fwknop/access.conf file, add the following
line:
ENCRYPTION_MODE legacy
In addition, if the KEY variable has an encryption key longer than 16 bytes, it will need to be truncated
to 16 characters in the access.conf file in order for pre-2.5 clients to work properly. This limitation
is fixed in 2.5, and provides additional motivation for upgrading all clients and servers to 2.5 or
later.
Now, flipping the scenario around, if the fwknop clients are upgraded but the fwknopd server is still at
a pre-2.5 version, then add the -M legacy argument to the fwknop command line:
$ fwknop -A tcp/22 -M legacy -R -D 2.2.2.2
DEPENDENCIES
The fwknop client requires libfko which is normally included with both source and binary distributions,
and is a dedicated library developed by the fwknop project. Whenever the fwknopd server is used, libpcap
is a required dependency. However, the upcoming 2.6 release will offer a UDP listener mode along with
privilege separation support and will not require libpcap in this mode. In UDP listener mode, even though
fwknopd binds to a UDP port, SPA packets are never acknowledged so from an attacker’s perspective there
is no difference between fwknopd sniffing the wire passively vs. listening on a UDP socket in terms of
what can be scanned for.
For GPG functionality, GnuPG must also be correctly installed and configured along with the libgpgme
library.
To take advantage of all of the authentication and access management features of the fwknopd
daemon/service a functioning iptables, ipfw, or pf firewall is required on the underlying operating
system.
DIAGNOSTICS
The most comprehensive way to gain diagnostic information on fwknop is to run the test suite
test-fwknop.pl script located in the test/ directory in the fwknop sources. The test suite sends fwknop
through a large number of run time tests, has valgrind support, validates both SPA encryption and HMAC
results against OpenSSL, and even has its own built in fuzzer for SPA communications (and fwknop in
version 2.6.4 supports the American Fuzzy Lop (AFL) from Michal Zalewski as well). For more basic
diagnostic information, fwknop can be executed with the -T (or --test) command line option. This will
have fwknop simply create and print the SPA packet information, then run it through a decrypt/decode
cycle and print it again. In addition, the --verbose command line switch is useful to see various SPA
packet specifics printed to stdout.
SEE ALSO
fwknopd(8), iptables(8), pf(4), pfctl(8), ipfw(8), gpg(1), libfko documentation.
More information on Single Packet Authorization can be found in the paper “Single Packet Authorization
with fwknop” available at http://www.cipherdyne.org/fwknop/docs/SPA.html. A comprehensive tutorial on
fwknop operations and theory can be found at http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html.
This tutorial also includes information about the design of fwknop that may be worth reading for those
interested in why fwknop is different from other SPA implementations.
fwknop uses the git versioning system as its source code repository along with Github for tracking of
issues and milestones:
$ git clone https://github.com/mrash/fwknop.git fwknop.git
Additional commentary on Single Packet Authorization can be found via Michael Rash’s Twitter feed:
http://twitter.com/michaelrash, @michaelrash
AUTHORS
The primary developers of fwknop are Michael Rash (project creator) <mbr@cipherdyne.org>, Damien Stuart
<dstuart@dstuart.org>, and Jonathan Bennett <jbennett@incomsystems.biz>.
CONTRIBUTORS
This “C” version of fwknop was derived from the original Perl-based version on which many people who are
active in the open source community have contributed. See the CREDITS file in the fwknop sources, or
visit https://github.com/mrash/fwknop/blob/master/CREDITS to view the online list of contributors. A few
contributors deserve to be singled out including: Franck Joncourt, Max Kastanas, Vlad Glagolev, Sean
Greven, Hank Leininger, Fernando Arnaboldi, and Erik Gomez.
The phrase “Single Packet Authorization” was coined by MadHat and Simple Nomad at the BlackHat Briefings
of 2005.
BUGS
Send bug reports to dstuart@dstuart.org or mbr@cipherdyne.org, or open a new issue on Github (see
https://github.com/mrash/fwknop.git). Suggestions and/or comments are always welcome as well. Additional
information may be found in the fwknop mailing list archives (see:
https://lists.sourceforge.net/lists/listinfo/fwknop-discuss).
DISTRIBUTION
fwknop is distributed under the GNU General Public License (GPL v2+), and the latest version may be
downloaded from http://www.cipherdyne.org.
Fwknop Client 08/06/2018 FWKNOP(8)