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Damien Stuart b95d222d3c More tweaks, clean-up and documentation tweaks for the first release. Made client http-proxy option allow case insensitive match and to take an option :port as part of the argument. 
git-svn-id: file:///home/mbr/svn/fwknop/trunk@225 510a4753-2344-4c79-9c09-4d669213fbeb
2010-06-27 21:07:56 +00:00

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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 Linux systems running iptables, and for Mac OS X
and FreeBSD systems running ipfw. This mechanism requires only a single
encrypted and non-replayed packet to communicate various pieces of
information including desired access through an iptables or ipfw policy.
The main application of this program is to use iptables or ipfw 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) much more difficult.
An authorization server *fwknopd* passively monitors authorization packets
via 'libpcap' and hence there is no "server" to which to connect in the
traditional sense. Any service protected by *fwknop* is inaccessible (by
using 'iptables' or 'ipfw' to intercept packets within the kernel) before
authenticating; anyone scanning for the service will not be able to detect
that it is even listening. Single Packet Authorization offers many
advantages over port knocking, including non-replayability of SPA packets,
ability to use asymmetric ciphers (such as Elgamal), and SPA cannot be
broken by simply spoofing packets to duplicate ports within the knock
sequence on the server to break port knocking authentication.
SPA packets can easily be spoofed as well (this is a good thing in this
context), and 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.
Authorization packets are either encrypted with the 'Rijndael' block cipher
or via 'GnuPG' and associated asymmetric ciphers. If the symmetric encryption
method is chosen, then 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 they are encrypted):
..........................
random number (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 in all versions of *fwknop* greater than 1.9.1) allows the server
to check message integrity after decryption, and the 16 bytes of random data
ensures (with high probability) that no two messages are identical. This
ensures that replay attacks are not possible against *fwknop*.
For each packet coming from an *fwknop* client, the *fwknopd* server can
cache the digest calculated over the entire packet and compares against
previous packet digests in order to detect attempted replay attacks. 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. The server
must first be configured to acquire the SPA data on the changed protocol-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.
REQUIRED ARGUMENTS
------------------
*-D, --destination*='<IP-address>'::
Direct the *fwknop* client to authenticate with the *fwknopd*
daemon/service at the destination address <IP>. The connection mode
is discovered by the *fwknopd* daemon/service when it decrypts and
parses the authentication packet.
*-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.
*-R|-a|-s*::
One of these options (see below) is required to tell the remote
*fwknopd* daemon what IP should be let 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.
*-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.
*-G, --get-key*='<file>'::
Load an encryption key/password from the specified file.
*-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.
*--show-last*::
Display the last command-line arguments used by *fwknop*.
*-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.
*-v, --verbose*::
Run the *fwknop* client in verbose mode.
*-V, --Version*::
Display version information and exit.
SPA OPTIONS
-----------
*-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 'Man-In-The-Middle' (MTIM) attack
where an SPA packet can be intercepted enroute 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 the 'http://www.whatismyip.com'
website.
*-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.
*-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).
*-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*.
*-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, tcpraw, icmp) for the outgoing SPA packet.
Note: The *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 uses the *fwknop_serv* daemon to listen on a TCP
port (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 'http://www.whatismyip.com' 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.
*-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.
The permit-address options *-s* (default), *-R* and *-a* are mutually
exclusive.
*--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).
GPG-RELATED OPTIONS
-------------------
*--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.
ENVIRONMENT
-----------
*GPG_AGENT_INFO* (only used in *--gpg-agent* mode).
EXAMPLES
--------
The following examples illustrate the command line arguments that could
be supplied to the fwknop client in a few situations:
Access mode examples
~~~~~~~~~~~~~~~~~~~~
Packet contents printed to stdout at the fwknop client when creating
an ``access mode'' SPA packet:
..........................
Random data: 6565240948266426
Username: mbr
Timestamp: 1203863233
Version: 1.9.2
Type: 1 (access mode)
Access: 127.0.0.2,tcp/22
SHA256 sum: gngquSL8AuM7r27XsR4qPmJhuBo9pG2PYwII06AaJHw
..........................
Use the Single Packet Authorization mode to gain access to
tcp/22 (ssh) and udp/53 running on the system 10.0.0.123 from
the IP 192.168.10.4:
..........................
$ fwknop -A "tcp/22,udp/53" -a 192.168.10.4 -D 10.0.0.123
..........................
Same as above example, but gain access from whatever source IP
is seen by the fwknop server (useful if the fwknop client is
behind a NAT device):
..........................
$ fwknop -A "tcp/22,udp/53" -s -D 10.0.0.123
..........................
Same as above example, but use the IP identification website
'http://www.whatismyip.com' to derive the client IP address. This
is a safer method of acquiring the client IP address than using the
*-s* option 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:
..........................
$ fwknop -A "tcp/22,udp/53" -R -D 10.0.0.123
..........................
Use the Single Packet Authorization mode to gain access to tcp/22
(ssh) and udp/53 running on the system 10.0.0.123, and use GnuPG keys
to encrypt and decrypt:
..........................
$ fwknop -A "tcp/22,udp/53" --gpg-sign ABCD1234 --gpg--recipient
1234ABCD -R -D 10.0.0.123
..........................
Instruct the fwknop server running at 10.0.0.123 to allow 172.16.5.4
to connect to TCP/22, but spoof the authorization packet from an IP
associated with www.yahoo.com:
..........................
# fwknop --Spoof-src ’www.yahoo.com’ -A tcp/22 -a 172.16.5.4 -D
10.0.0.123
..........................
DEPENDENCIES
------------
*fwknop* requires 'libfko' (which is normally included with both source and
binary distributions).
For GPG functionality, GnuPG must also be correctly installed and configured.
To take advantage of all of the authentication and access management
features of the *fwknopd* daemon/service a functioning iptables firewall
is required on the underlying operating system.
DIAGNOSTICS
-----------
fwknop can be run 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.
SEE ALSO
--------
fwknopd(8), iptables(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'.
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.
The phrase ``Single Packet Authorization'' was coined by MadHat and Simple
Nomad at the BlackHat Briefings of 2005 (see: 'http://www.nmrc.org').
BUGS
----
Send bug reports to dstuart@dstuart.org. Suggestions and/or comments
are always welcome as well.
DISTRIBUTION
------------
*fwknop* is distributed under the GNU General Public License (GPL), and
the latest version may be downloaded from 'http://www.cipherdyne.org'.
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