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:man source: Fwknop Client
:man manual: Fwknop Client
FWKNOP(8)
=========
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 passive service protection. SPA requires only a single
non-replayed encrypted packet together with an HMAC in order to communicate
various pieces of information including desired access to a service that is
otherwise blocked by a firewall. The main application of SPA is to use a
firewall in a default-drop stance to protect services such as 'SSH' with an
additional layer of security in order to make the exploitation of
vulnerabilities (both 0-day and unpatched code) more difficult. In
addition, services that are protected in this fashion naturally cannot be
scanned for with 'Nmap'.
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 supported, 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 lightweight 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 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, it is highly recommended for three reasons: '1)' without
an HMAC, cryptographically strong authentication is not possible with *fwknop*
unless GnuPG is used, '2)' an HMAC applied after encryption protects against
CBC-mode padding oracle attacks such as the Vaudenay attack and 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. Generating an HMAC for SPA
communications requires a dedicated key in addition to the normal encryption
key.
*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* 'access.conf' file for details). 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 bytes)
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 bytes of random data 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. 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 as well such as OpenWRT),
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.
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*='<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.
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.
*--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/random and then base64
encoded before being printed to stdout, and are meant to be included within
the ``$HOME/.fwknoprc'' file (or the file referenced by *--get-key*).
*-l, --last-cmd*::
Execute *fwknop* with the command-line arguments from the previous
invocation (if any). The previous arguments are parsed out of the
'~/.fwknop.run' file.
*-n, --named-config*='<stanza name>'::
Specify the name of the configuration stanza in the ``$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.
*--force-stanza*::
Used with --save-rc-stanza to overwrite all of the variables for the
specified stanza
*--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 require the specification of
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-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.
*--resolve-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.
*-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 'access.conf' file on the
server side via the ENCRYPTION_MODE variable. In general, it is
recommended to not use this argument and just use the default (CBC).
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-RELATED 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
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. 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.
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*::
Specify the IP or hostname of the destination (*fwknopd*) server
('-D, --destination').
*ALLOW_IP*::
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*::
Set the one or more protocol/ports to open on the firewall ('-A, --access').
*SPA_SERVER_PORT*::
Set the server port to use for sending the SPA packet ('-p, --server-port').
*SPA_SERVER_PROTO*::
Set the protocol to use for sending the SPA packet ('-P, --server-proto').
*KEY*::
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*::
Specify the encryption passphrase as a base64 encoded string. This allows
non-ascii characters to be included.
*USE_HMAC*::
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*::
Specify the HMAC key for authenticated encryption of SPA packets.
*HMAC_KEY_BASE64*::
Specify the HMAC key as a base64 encoded string. This allows non-ascii
characters to be included.
*HMAC_DIGEST_TYPE*::
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*::
Set the source port to use for sending the SPA packet ('-S, --source-port').
*FW_TIMEOUT*::
Set the firewall rule timeout value ('-f, --fw-timeout').
*RESOLVE_URL*::
Set to a URL that will be used for resolving the source IP address
(--resolve-url).
*TIME_OFFSET*::
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*::
Specify the encryption mode when AES is used. This variable is a synonym
for the '--encryption-mode' command line argument.
*DIGEST_TYPE*::
Set the SPA message digest type ('-m, --digest-type').
*USE_GPG*::
Set to 'Y' to specify the use of GPG for encryption ('--gpg-encryption').
*USE_GPG*::
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_SIGNER*::
Specify the GPG key name or ID for signing the GPG-encrypted SPA data
('--gpg-signer-key').
*GPG_RECIPIENT*::
Specify the GPG key name or ID for the recipient of the GPG-encrypted SPA
data ('--gpg-recipient-key').
*GPG_HOMEDIR*::
Specify the GPG home directory ('--gpg-home-dir').
*SPOOF_USER*::
Set the username in the SPA data to the specified value ('-U,
--spoof-user').
*SPOOF_SOURCE_IP*::
Set the source IP of the outgoing SPA packet to the specified value
('-Q, --spoof-source').
*RAND_PORT*::
Send the SPA packet over a randomly assigned port ('-r, --rand-port').
*KEY_FILE*::
Load an encryption key/password from a file ('-G, --get-key').
*HTTP_USER_AGENT*::
Set the HTTP User-Agent for resolving the external IP via -R, or for
sending SPA packets over HTTP ('-u, --user-agent').
*NAT_ACCESS*::
Gain NAT access to an internal service protected by the fwknop server
('-N, --nat-access').
*NAT_LOCAL*::
Access a local service via a forwarded port on the fwknopd server
system ('--nat-local').
*NAT_PORT*::
Specify the port to forward to access a service via NAT ('--nat-port').
*NAT_RAND_PORT*::
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 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:
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 '2.2.2.2' (note that the
'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):
..........................
$ fwknop -A tcp/22 --use-hmac -a 1.1.1.1 -D 2.2.2.2
Enter encryption key:
Enter HMAC key:
$ ssh -l user 2.2.2.2
user@2.2.2.2'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 2.2.2.2 --verbose
Enter encryption key:
Enter HMAC key:
Random Value: 1916307060193417
Username: mbr
Timestamp: 1368498909
FKO Version: 2.5.0
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 '2.2.2.2':
..........................
$ fwknop -A "tcp/22,tcp/1194" --use-hmac -a 1.1.1.1 -D 2.2.2.2
..........................
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 2.2.2.2
..........................
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 2.2.2.2
..........................
Instruct the fwknop server running at 2.2.2.2 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 2.2.2.2
..........................
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 '2.2.2.2'. In this case, the 'SSH' connection made to '2.2.2.2' is
translated into the '192.168.10.2' system automatically:
..........................
$ fwknop -A tcp/22 -N 192.168.10.2:22 -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.
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 runs 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
..........................
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. Suggestions
and/or comments are always welcome as well.
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'.
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