Provided by: fwknop-client_2.6.0-2_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 supports three different firewalls: iptables on Linux
systems, pf on OpenBSD, and ipfw on FreeBSD and Mac OS X.
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.
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)
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, 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).
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 local 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 an HTTP request to
http://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. These keys are derived from /dev/urandom and then base64 encoded before
being printed to stdout, and are meant to be included within the “$HOME/.fwknoprc”
file (or the file referenced by --get-key). Such keys are generally more secure than
passphrases that are typed in from the command line.
--key-len=<length>
Specify the number of bytes for a generated Rijndael key. The maximum size is
currently 128 bytes.
--hmac-key-len=<length>
Specify the number of bytes for a generated HMAC key. The maximum size is currently
128 bytes.
-l, --last-cmd
Execute fwknop with the command-line arguments from the previous invocation (if any).
The previous arguments are parsed out of the ~/.fwknop.run file.
-n, --named-config=<stanza name>
Specify the name of the configuration stanza in the “$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).
--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.
-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 a website that
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, and SHA512.
-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-http
This is an important option, and instructs the fwknop client and the fwknopd
daemon/service to query a web server that returns the caller’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 iptables policy at the remote fwknopd server side. This is
useful if the fwknop client is being used on a system that is behind an obscure NAT
address. Presently, fwknop uses the URL: http://www.cipherdyne.org/cgi-bin/myip to
resolve the caller IP. 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 elminates the need for fwknop to issue an HTTP 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.
-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.
-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.
-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.
-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, and SHA512.
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_URL <url>
Set to a URL that will be used for resolving the source IP address (--resolve-url).
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, and SHA512.
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.
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).
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_HTTP 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 http://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 motiviation 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. 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
Damien Stuart <dstuart@dstuart.org>, Michael Rash <mbr@cipherdyne.org>
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 http://www.cipherdyne.org/fwknop/docs/contributors.html 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) version 2, and the latest
version may be downloaded from http://www.cipherdyne.org.