Provided by: knockd_0.8-2build2_amd64 
NAME
knockd - port-knock server
SYNOPSIS
knockd [options]
DESCRIPTION
knockd is a port-knock server. It listens to all traffic on an ethernet (or PPP) interface, looking for
special "knock" sequences of port-hits. A client makes these port-hits by sending a TCP (or UDP) packet
to a port on the server. This port need not be open -- since knockd listens at the link-layer level, it
sees all traffic even if it's destined for a closed port. When the server detects a specific sequence of
port-hits, it runs a command defined in its configuration file. This can be used to open up holes in a
firewall for quick access.
COMMANDLINE OPTIONS
-i, --interface <int>
Specify an interface to listen on. The default is eth0.
-d, --daemon
Become a daemon. This is usually desired for normal server-like operation.
-c, --config <file>
Specify an alternate location for the config file. Default is /etc/knockd.conf.
-D, --debug
Output debugging messages.
-l, --lookup
Lookup DNS names for log entries. This may be a security risk! See section SECURITY NOTES.
-4, --only-ip-v4
Ignore packets from IPv6 and handle only IPv4.
-v, --verbose
Output verbose status messages.
-V, --version
Display the version.
-h, --help
Syntax help.
CONFIGURATION
knockd reads all knock/event sets from a configuration file. Each knock/event begins with a title
marker, in the form [name], where name is the name of the event that will appear in the log. A special
marker, [options], is used to define global options.
Example #1:
This example uses two knocks. The first will allow the knocker to access port 22 (SSH), and the
second will close the port when the knocker is complete. As you can see, this could be useful if
you run a very restrictive (DENY policy) firewall and would like to access it discreetly.
[options]
logfile = /var/log/knockd.log
[openSSH]
sequence = 7000,8000,9000
seq_timeout = 10
tcpflags = syn
command = /sbin/iptables -A INPUT -s %IP% --dport 22 -j ACCEPT
[closeSSH]
sequence = 9000,8000,7000
seq_timeout = 10
tcpflags = syn
command = /sbin/iptables -D INPUT -s %IP% --dport 22 -j ACCEPT
Example #2:
This example uses a single knock to control access to port 22 (SSH). After receiving a successful
knock, the daemon will run the start_command, wait for the time specified in cmd_timeout, then
execute the stop_command. This is useful to automatically close the door behind a knocker. The
knock sequence uses both UDP and TCP ports.
[options]
logfile = /var/log/knockd.log
[opencloseSSH]
sequence = 2222:udp,3333:tcp,4444:udp
seq_timeout = 15
tcpflags = syn,ack
start_command = /usr/sbin/iptables -A INPUT -s %IP% -p tcp --syn -j ACCEPT
cmd_timeout = 5
stop_command = /usr/sbin/iptables -D INPUT -s %IP% -p tcp --syn -j ACCEPT
Example #3:
This example doesn't use a single, fixed knock sequence to trigger an event, but a set of
sequences taken from a sequence file (one time sequences), specified by the one_time_sequences
directive. After each successful knock, the used sequence will be invalidated and the next
sequence from the sequence file has to be used for a successful knock. This prevents an attacker
from doing a replay attack after having discovered a sequence (eg, while sniffing the network).
[options]
logfile = /var/log/knockd.log
[opencloseSMTP]
one_time_sequences = /etc/knockd/smtp_sequences
seq_timeout = 15
tcpflags = fin,!ack
start_command = /usr/sbin/iptables -A INPUT -s %IP% -p tcp --dport 25 -j ACCEPT
cmd_timeout = 5
stop_command = /usr/sbin/iptables -D INPUT -s %IP% -p tcp --dport 25 -j ACCEPT
Example #4:
Example to support IPv4 and IPv6. You can provide a dedicated command for each of the two
protocols.
[options]
logfile = /var/log/knockd.log
[opencloseSMTP]
one_time_sequences = /etc/knockd/smtp_sequences
seq_timeout = 15
tcpflags = fin,!ack
start_command = /usr/sbin/iptables -A INPUT -s %IP% -p tcp --dport 25 -j ACCEPT
start_command_6 = /usr/sbin/ip6tables -A INPUT -s %IP% -p tcp --dport 25 -j ACCEPT
cmd_timeout = 5
stop_command = /usr/sbin/iptables -D INPUT -s %IP% -p tcp --dport 25 -j ACCEPT
stop_command_6 = /usr/sbin/ip6tables -D INPUT -s %IP% -p tcp --dport 25 -j ACCEPT
CONFIGURATION: GLOBAL DIRECTIVES
UseSyslog
Log action messages through syslog(). This will insert log entries into your /var/log/messages or
equivalent.
LogFile = /path/to/file
Log actions directly to a file, usually /var/log/knockd.log.
PidFile = /path/to/file
Pidfile to use when in daemon mode, default: /var/run/knockd.pid.
Interface = <interface_name>
Network interface to listen on. Only its name has to be given, not the path to the device (eg,
"eth0" and not "/dev/eth0"). Default: eth0.
CONFIGURATION: KNOCK/EVENT DIRECTIVES
Sequence = <port1>[:<tcp|udp>],<port2>[:<tcp|udp>][,<port3>[:<tcp|udp>] ...]
Specify the sequence of ports in the special knock. If a wrong port with the same flags is
received, the knock is discarded. Optionally, you can define the protocol to be used on a per-
port basis (default is TCP).
One_Time_Sequences = /path/to/one_time_sequences_file
File containing the one time sequences to be used. Instead of using a fixed sequence, knockd will
read the sequence to be used from that file. After each successful knock attempt this sequence
will be disabled by writing a '#' character at the first position of the line containing the used
sequence. That used sequence will then be replaced by the next valid sequence from the file.
Because the first character is replaced by a '#', it is recommended that you leave a space at the
beginning of each line. Otherwise the first digit in your knock sequence will be overwritten with
a '#' after it has been used.
Each line in the one time sequences file contains exactly one sequence and has the same format as
the one for the Sequence directive. Lines beginning with a '#' character will be ignored.
Note: Do not edit the file while knockd is running!
Seq_Timeout = <timeout>
Time to wait for a sequence to complete in seconds. If the time elapses before the knock is
complete, it is discarded.
TCPFlags = fin|syn|rst|psh|ack|urg
Only pay attention to packets that have this flag set. When using TCP flags, knockd will IGNORE
tcp packets that don't match the flags. This is different than the normal behavior, where an
incorrect packet would invalidate the entire knock, forcing the client to start over. Using
"TCPFlags = syn" is useful if you are testing over an SSH connection, as the SSH traffic will
usually interfere with (and thus invalidate) the knock.
Separate multiple flags with commas (eg, TCPFlags = syn,ack,urg). Flags can be explicitly
excluded by a "!" (eg, TCPFlags = syn,!ack).
Target = <ip-address>
Use the specified IP address instead of the address determined for the Interface when matching the
Sequence. This is useful if knockd is running on a router and you want to do something in
response to an actual connection attempt to a routed host - e.g., invoking etherwake to send the
host a WOL packet.
Start_Command = <command>
Specify the command to be executed when a client makes the correct port-knock with IPv4. All
instances of %IP% will be replaced with the knocker's IP address. The Command directive is an
alias for Start_Command.
Start_Command_6 = <command>
Specify the command to be executed when a client makes the correct port-knock with IPv6. All
instances of %IP% will be replaced with the knocker's IP address. The Command_6 directive is an
alias for Start_Command_6. If not present it will automatically fallback onto the same IPV4
Start_Command value. You can use empty value to force doing nothing.
Cmd_Timeout = <timeout>
Time to wait (in seconds) between Start_Command and Stop_Command. This directive is optional,
only required if Stop_Command is used.
Stop_Command = <command>
Specify the command to be executed when Cmd_Timeout seconds have passed since Start_Command has
been executed. All instances of %IP% will be replaced with the knocker's IP address. This
directive is optional.
Stop_Command_6 = <command>
Specify the command to be executed when Cmd_Timeout seconds have passed since Start_Command_6 has
been executed. All instances of %IP% will be replaced with the knocker's IP address. This
directive is optional. If not present it will automatically fallback onto the same IPV4
Stop_Command value. You can use empty value to force doing nothing.
SECURITY NOTES
Using the -l or --lookup commandline option to resolve DNS names for log entries may be a security risk!
An attacker may find out the first port of a sequence if he can monitor the DNS traffic of the host
running knockd. Also a host supposed to be stealth (eg, dropping packets to closed TCP ports instead of
replying with an ACK+RST packet) may give itself away by resolving a DNS name if an attacker manages to
hit the first (unknown) port of a sequence.
SEE ALSO
knock is the accompanying port-knock client, though telnet or netcat could be used for simple TCP knocks
instead. For more advanced knocks, see hping, sendip or packit.
AUTHOR
Judd Vinet <jvinet@zeroflux.org>
knockd 0.8 April 22, 2021 knockd(1)