Provided by: netcat6_1.0-8_amd64 
NAME
nc6 - network swiss army knife
SYNOPSIS
nc6 [options...] hostname port
nc6 -l -p port [-s addr] [options...] [hostname] [port]
DESCRIPTION
netcat6 is a simple unix utility which reads and writes data across network connections,
using the TCP or UDP protocol over IPv4 or IPv6 networks. It is designed to be a reliable
"back-end" tool that can be used directly or easily driven by other programs and scripts.
At the same time, it is a feature-rich network debugging and exploration tool, since it
can create almost any kind of connection you would need and has several interesting built-
in capabilities. netcat6 , or nc6 as the actual program is named, should have been
supplied long ago as another one of those cryptic but standard Unix tools.
In the simplest usage, " nc6 host port " creates a TCP connection to the given port on the
given target host. Your standard input is then sent to the host, and anything that comes
back across the connection is sent to your standard output. This continues indefinitely,
until the network side of the connection shuts down. Note that this behavior is different
from most other applications which shut everything down and exit after an end-of-file on
the standard input (though this style of operation can easily be enabled).
netcat6 can also function as a server, by listening for inbound connections on arbitrary
ports and then doing the same reading and writing. With minor limitations, netcat6
doesn't really care if it runs in "client" or "server" mode -- it still shovels data back
and forth until there isn't any more left. In this manual page, these two working modes
are referred as "connect mode" and "listen mode", respectively.
OPTIONS
-4 Forces the use of IPv4.
-6 Forces the use of IPv6 and inhibits the use IPV4-mapped addresses.
-b, --bluetooth
With this option set, netcat6 will use bluetooth to establish connections.
By default the L2CAP protocol will be used (also see '--sco').
--buffer-size=BYTES
Set the buffer size for the local and remote endpoints. netcat6 does all
reads into these buffers, so they should be large enough to minimize
excessive reads from the socket and in UDP mode it should be large enough to
receive an entire datagram (also see '--nru'). By default, the buffer size
is 8 kilobytes for TCP connections and 128 kilobytes for UDP.
--continuous Enable continuous accepting of connections in listen mode, like inetd. Must
be used with --exec to specify the command to run locally (try 'nc6
--continuous --exec cat -l -p <port>' to make a simple echo server).
--disable-nagle
Disable the use of the Nagle algorithm for TCP connections (see "NAGLE
ALGORITHM").
-e, --exec=CMD
Exec the listed CMD after a connect is established. All input from the
remote client will be available on stdin to the command, and all output from
the command will be sent back to the remote client.
-h, --help Display a brief help listing.
--half-close Properly handle (and send) TCP half closes for protocols that support them
(eg. TCP). See "HALF CLOSE".
-l, --listen Selects listen mode (for inbound connects).
--mtu=BYTES Set the Maximum Transmission Unit for the remote endpoint (network
transmits). This is only really useful for datagram protocols like UDP. For
TCP the MTU is determined by the kernel and this will only affect the amount
that is passed to the kernel at any one time. There is no default MTU for
TCP connections (and there should be no need to change this), and for UDP it
is 8 kilobytes.
-n Disables DNS queries - you'll have to use numeric IP address instead of
hostnames.
--no-reuseaddr
Disables the SO_REUSEADDR socket option (this is only useful in listen mode).
-T, --ttl=TTL
Set's the IP TTL of the packets leaving (this is only useful in connect
mode). This is mainly useful for multicast, otherwise packets will often
leave with the OS default of 1, making it rarely useful.
--nru=BYTES Set the miNimum Receive Unit for the remote endpoint (network receives).
Note that this does not mean that every network read will get the specified
number of bytes, only that nc6 MUST be able to receive at least that amount
of data when a read is done. This is only really useful for datagram
protocols like UDP where the entire datagram must be received in one call or
it is truncated. The default NRU for TCP connections is 1 byte, and 65536
bytes for UDP.
-p, --port=PORT
Sets the port number for the local endpoint of the connection.
-q, --hold-timeout=SEC1[:SEC2]
Sets the hold timeout(s) (see "TIMEOUTS"). Specifying just one value will
set the hold timeout on the local endpoint, specifying a second value will
set the hold timeout on the remote endpoint as well. Either value can be '0'
(for instant timeout) or '-' (for no hold timeout, eg. infinite hold).
-t, --idle-timeout=SEC
Sets the idle timeout (see "TIMEOUTS").
--rcvbuf-size=SIZE
Specify the size to be used for the kernel receive buffer for network
sockets.
--recv-only Only receive data, don't transmit. This also disables any hold timeouts.
-s, --address=ADDRESS
Sets the source address for the local endpoint of the connection.
--sco With this option set, netcat6 with use SCO over bluetooth (note that '-b' or
'--bluetooth' must also be specified).
--send-only Only send data, don't receive. This also disables any hold timeouts.
--sndbuf-size=SIZE
Specify the size to be used for the kernel send buffer for network sockets.
-u, --udp With this option set, netcat6 will use UDP as the transport protocol (TCP is
the default).
-v Enable verbose mode. This gives some basic information about what netcat6 is
doing. Use it twice for extra verbosity.
--version Print netcat6 version and exit.
-w, --timeout=SECONDS
Timeout for network connects and accepts (see "TIMEOUTS").
-C Send CRLF as line-ending
-x, --transfer
File transfer mode (see "FILE TRANSFER"). If listen mode is specified, this
is equivalent to "--recv-only --buffer-size=65536" otherwise it is equivalent
to "--send-only --buffer-size=65536". -X, --rev-transfer Reverse file
transfer mode (see "FILE TRANSFER"). This transfers in the opposite
direction to normal transfer. If listen mode is specified, this is
equivalent to "--send-only --buffer-size=65536" otherwise it is equivalent to
"--recv-only --buffer-size=65536".
-z Don't send any data (scanning mode).
UDP
UDP support in netcat6 works very well in both connect and in listen mode. When using UDP
in listen mode netcat6 accepts UDP packets from any source that matches the optional
address and/or port specified (if it was specified). However, once the first packet is
received, netcat6 will only receive packets from that client in future. This is done by
putting the UDP socket into "connected" state (see udp(4) and connect(2)). Packets from
other sources are discarded by the kernel and an ICMP unreachable response is sent.
When connecting to a remote host using UDP, nc6 will report that the connection is open
regardless of whether a remote server is listening. This is because UDP is a
connectionless protocol, and hence no connection establishment is actually required.
However, after sending the first packet of data, a server may reply with an ICMP
unreachable response causing nc6 to exit with a 'Connection refused' error message.
netcat6 allows for fine control over the buffer sizes, MTU's and NRU's for the connection,
which is especially useful for UDP connections. See the --buffer-size, --mtu and --nru
options.
TIMEOUTS
netcat6 currently implements a connect/accept timeout, and idle timeout, and hold timeouts
on both the remote and local endpoints.
• The connect/accept timeout is optional and is specified with the -w or --timeout
option. If a connection cannot be established within the specified time, nc6 will
exit with a non-zero exit status. If the option is not specified, nc6 will wait
forever when listening for new connections (in listen mode), and will use the standard
operating system timeout when trying to connect to remote systems. Note that the
connect timeout is essentially ignored when creating UDP connections to a remote
server, as UDP is a connectionless protocol.
• The idle timeout is optional and is specified with the -t or --idle-timeout option.
If no data is sent or received from the remote host in the specified time, nc6 will
exit with a non-zero exit status. The default is to not timeout idle connections.
• The hold timeouts specify how long the connection should be held open after a given
endpoint can no longer be read from (because EOF was received). When the hold timeout
occurs for a given endpoint, no more data is read from the opposite endpoint. After
data read from the timed out endpoint has been sent, then the connection is closed and
nc6 exits.
For example, setting the hold time of the local endpoint to 5 would cause reading from
the remote system to stop 5 seconds after stdin is closed (eg. by using ^D to send
EOF). All buffered data to be sent to the remote system is then sent, after which nc6
exits successfully (if no errors occurred).
By default, the hold timeout is disabled for the local endpoint (essentially an
'infinite' timeout), and the hold timeout is set to 0 for the remote endpoint (an
'instant' timeout). This means that nc6 effectively exits immediately when the read
from the remote server is closed.
In half close mode (see "HALF CLOSE") all hold timeouts are disabled by default.
FILE TRANSFER
netcat6 can be used to transfer files and data streams efficiently, using the ´-x' or
'--transfer' command line option (or the '-X' and '--rev-transfer' equivalents for
transfer in the opposite direction).
For example, to transfer a file from machine 'foo.example.com' to machine
´bar.example.com', one would use the following technique.
On 'bar', set up a listening nc6 on any local port:
bar$ nc6 -x -l -p 7676 > targetfile
Then on 'foo', use nc6 to send the file to the listening nc6:
foo$ nc6 -x bar.example.com 7676 < sourcefile
You can also use file transfer mode to pipe the output of programs over networks. For
example, you can use tar with nc6 to copy entire directories:
bar$ nc6 -x -l -p 7676 | tar xpzf -
foo$ tar cpzf - /home | nc6 -x bar.example.com 7676
The '-x' or '--transfer' option is just an alias for various other options which set the
direction of transfer and increase the buffer sizes used. In listen mode, it is
equivalent to "--recv-only --buffer-size=65536" otherwise it is equivalent to "--send-only
--buffer-size=65536".
The normal file transfer mode options assume the file will be sent TO the netcat6 process
that is in listen mode, FROM the netcat6 process that is connecting to it. To transfer
the other way use the -X or --rev-transfer options.
HALF CLOSE
Stream based data transfers, such as TCP, have 2 separate streams - the receive and the
transmit stream. Contrary to common belief, the streams can be closed independently of
each other - in TCP this is called a half-close.
In standard mode, netcat6 closes the entire remote connection when it detects that the
remote receive stream has been closed. This is a common operation for most TCP
applications, because it is unusual that a server will keep accepting data after it has
indicated that it will no longer send any. netcat6 also doesn't close it's remote
transmit when it has no more data to send (eg. when EOF has been received on stdin), so as
not to confuse remote servers by making them think that netcat6 isn't listening for data
any more.
However, some servers do make active use of TCP half-closes, and the default operation of
netcat6 doesn't work well with these services. For example, when using netcat6 to
simulate a HTTP server, some HTTP clients close their transmit stream after they send
their request. In default mode, netcat6 will close the entire connection before sending
the response back to the client.
However, by specifying the "--half-close" option, netcat6 will keep trying to send data to
the remote server after the receive stream has closed. It will also close the remote
transmit stream when there is no more data to send (EOF has been received on stdin).
NAGLE ALGORITHM
Named after its creator, John Nagle, the Nagle algorithm is used to combine together small
writes to a TCP stream and thus decrease the number of packets that must be sent out on
the network.
This was designed as a means for dealing with what Nagle called the small packet problem,
created when an application generates data one byte at a time, causing the network to be
overloaded with packets. A single character originating from a keyboard could result in
the an entire packet (the single byte plus packet headers) being sent onto the network.
The algorithm can, under some situations, add a small delay before the data is sent out on
the wire - so as to allow for this concatenation to occur. This is done in such a way
that it is generally unnoticeable for normal applications, including interactive
applications.
There are some situations, such as real-time protocols, that can be adversely affected by
this small delay, which is why it is possible to disable the Nagle algorithm on most
systems, using the TCP_NODELAY socket option.
netcat6 provides the '--disable-nagle' command line option to disable the Nagle algorithm
for the connection.
ERRORS
netcat6 attempts to exit cleanly (exit value 0) when all data was sent or received
successfully, and to exit with an error value otherwise. However, the specific definition
of successfully varies depending on the way nc6 is run.
In normal mode, nc6 exits successfully when all data from the remote machine has been
received (up to and including an EOF), and the data has been output to the stdout without
any errors. It DOES NOT indicate whether all data read from stdin has been successfully
sent to the remote machine.
In --half-close mode, nc6 exists successfully when an EOF has been received from both the
remote machine AND from stdin, and all data has been successfully sent. Thus the exit
status indicates that all data was transfered successfully.
If hold timeouts (see "TIMEOUTS") have been set for either the remote or local endpoint,
nc6 will exit successfully if all buffered data from the endpoint that timed out is
successfully sent. It DOES NOT indicate whether all data from the opposite endpoint was
sent (or even read).
For connectionless protocols like UDP there is no way of determining whether the remote
system has stopped sending data, or if it will accept further data sent. So in UDP mode,
nc6 will either never exit (until interrupted via ctrl-C) or will exit after an error when
sending data to the remote system, in which case the exit code is a failure (exit value
1).
VERSION
This man page is current for version 1.0 of netcat6
COPYRIGHT
GPL.
AUTHORS
Mauro Tortonesi <mauro@deepspace6.net>
Chris Leishman <chris@leishman.org>
The original netcat was written by a guy we know as the Hobbit <hobbit@avian.org>.
NC6(1)