NC(1) General Commands Manual NC(1)
NAME
nc — arbitrary TCP and UDP connections and listens
SYNOPSIS
nc [-46bCDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
[-m minttl] [-O length] [-P proxy_username] [-p source_port]
[-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit]
[-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
[destination] [port]
DESCRIPTION
The nc (or netcat) utility is used for just about anything under the
sun involving TCP, UDP, or Unix-domain sockets. It can open TCP con-
nections, send UDP packets, listen on arbitrary TCP and UDP ports, do
port scanning, and deal with both IPv4 and IPv6. Unlike telnet(1), nc
scripts nicely, and separates error messages onto standard error in-
stead of sending them to standard output, as telnet(1) does with some.
Common uses include:
• simple TCP proxies
• shell-script based HTTP clients and servers
• network daemon testing
• a SOCKS or HTTP ProxyCommand for ssh(1)
• and much, much more
The options are as follows:
-4 Use IPv4 addresses only.
-6 Use IPv6 addresses only.
-b Allow broadcast.
-C Send CRLF as line-ending. Each line feed (LF) character from
the input data is translated into CR+LF before being written to
the socket. Line feed characters that are already preceded
with a carriage return (CR) are not translated. Received data
is not affected.
-D Enable debugging on the socket.
-d Do not attempt to read from stdin.
-F Pass the first connected socket using sendmsg(2) to stdout and
exit. This is useful in conjunction with -X to have nc perform
connection setup with a proxy but then leave the rest of the
connection to another program (e.g. ssh(1) using the
ssh_config(5) ProxyUseFdpass option). Cannot be used with -U.
-h Print out the nc help text and exit.
-I length
Specify the size of the TCP receive buffer.
-i interval
Sleep for interval seconds between lines of text sent and re-
ceived. Also causes a delay time between connections to multi-
ple ports.
-k When a connection is completed, listen for another one. Re-
quires -l. When used together with the -u option, the server
socket is not connected and it can receive UDP datagrams from
multiple hosts.
-l Listen for an incoming connection rather than initiating a con-
nection to a remote host. The destination and port to listen
on can be specified either as non-optional arguments, or with
options -s and -p respectively. Cannot be used together with
-x or -z. Additionally, any timeouts specified with the -w op-
tion are ignored.
-M ttl Set the TTL / hop limit of outgoing packets.
-m minttl
Ask the kernel to drop incoming packets whose TTL / hop limit
is under minttl.
-N shutdown(2) the network socket after EOF on the input. Some
servers require this to finish their work.
-n Do not perform domain name resolution. If a name cannot be re-
solved without DNS, an error will be reported.
-O length
Specify the size of the TCP send buffer.
-P proxy_username
Specifies a username to present to a proxy server that requires
authentication. If no username is specified then authentica-
tion will not be attempted. Proxy authentication is only sup-
ported for HTTP CONNECT proxies at present.
-p source_port
Specify the source port nc should use, subject to privilege re-
strictions and availability.
-q seconds
after EOF on stdin, wait the specified number of seconds and
then quit. If seconds is negative, wait forever (default).
Specifying a non-negative seconds implies -N.
-r Choose source and/or destination ports randomly instead of se-
quentially within a range or in the order that the system as-
signs them.
-S Enable the RFC 2385 TCP MD5 signature option.
-s sourceaddr
Set the source address to send packets from, which is useful on
machines with multiple interfaces. For Unix-domain datagram
sockets, specifies the local temporary socket file to create
and use so that datagrams can be received. Cannot be used to-
gether with -x.
-T keyword
Change the IPv4 TOS/IPv6 traffic class value. keyword may be
one of critical, inetcontrol, lowcost, lowdelay, netcontrol,
throughput, reliability, or one of the DiffServ Code Points:
ef, af11 ... af43, cs0 ... cs7; or a number in either hex or
decimal.
-t Send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL
requests. This makes it possible to use nc to script telnet
sessions.
-U Use Unix-domain sockets. Cannot be used together with -F or
-x. On Linux, if the name starts with an at symbol (`@') it is
read as an abstract namespace socket: the leading `@' is re-
placed with a NUL byte before binding or connecting. For de-
tails, see unix(7).
-u Use UDP instead of TCP. Cannot be used together with -x. For
Unix-domain sockets, use a datagram socket instead of a stream
socket. If a Unix-domain socket is used, a temporary receiving
socket is created in /tmp unless the -s flag is given.
-V rtable
Set the routing table to be used.
-v Produce more verbose output.
-W recvlimit
Terminate after receiving recvlimit packets from the network.
-w timeout
Connections which cannot be established or are idle timeout af-
ter timeout seconds. The -w flag has no effect on the -l op-
tion, i.e. nc will listen forever for a connection, with or
without the -w flag. The default is no timeout.
-X proxy_protocol
Use proxy_protocol when talking to the proxy server. Supported
protocols are 4 (SOCKS v.4), 5 (SOCKS v.5) and connect (HTTPS
proxy). If the protocol is not specified, SOCKS version 5 is
used.
-x proxy_address[:port]
Connect to destination using a proxy at proxy_address and port.
If port is not specified, the well-known port for the proxy
protocol is used (1080 for SOCKS, 3128 for HTTPS). An IPv6 ad-
dress can be specified unambiguously by enclosing proxy_address
in square brackets. A proxy cannot be used with any of the op-
tions -lsuU.
-Z DCCP mode.
-z Only scan for listening daemons, without sending any data to
them. Cannot be used together with -l.
destination can be a numerical IP address or a symbolic hostname (un-
less the -n option is given). In general, a destination must be speci-
fied, unless the -l option is given (in which case the local host is
used). For Unix-domain sockets, a destination is required and is the
socket path to connect to (or listen on if the -l option is given).
port can be specified as a numeric port number or as a service name.
Port ranges may be specified as numeric port numbers of the form nn-mm.
In general, a destination port must be specified, unless the -U option
is given.
CLIENT/SERVER MODEL
It is quite simple to build a very basic client/server model using nc.
On one console, start nc listening on a specific port for a connection.
For example:
$ nc -l 1234
nc is now listening on port 1234 for a connection. On a second console
(or a second machine), connect to the machine and port being listened
on:
$ nc -N 127.0.0.1 1234
There should now be a connection between the ports. Anything typed at
the second console will be concatenated to the first, and vice-versa.
After the connection has been set up, nc does not really care which
side is being used as a ‘server’ and which side is being used as a
‘client’. The connection may be terminated using an EOF (‘^D’), as the
-N flag was given.
There is no -c or -e option in this netcat, but you still can execute a
command after connection being established by redirecting file descrip-
tors. Be cautious here because opening a port and let anyone connected
execute arbitrary command on your site is DANGEROUS. If you really need
to do this, here is an example:
On ‘server’ side:
$ rm -f /tmp/f; mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f
On ‘client’ side:
$ nc host.example.com 1234
$ (shell prompt from host.example.com)
By doing this, you create a fifo at /tmp/f and make nc listen at port
1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes
a connection successfully to that port, /bin/sh gets executed on
‘server’ side and the shell prompt is given to ‘client’ side.
When connection is terminated, nc quits as well. Use -k if you want it
keep listening, but if the command quits this option won't restart it
or keep nc running. Also don't forget to remove the file descriptor
once you don't need it anymore:
$ rm -f /tmp/f
DATA TRANSFER
The example in the previous section can be expanded to build a basic
data transfer model. Any information input into one end of the connec-
tion will be output to the other end, and input and output can be eas-
ily captured in order to emulate file transfer.
Start by using nc to listen on a specific port, with output captured
into a file:
$ nc -l 1234 > filename.out
Using a second machine, connect to the listening nc process, feeding it
the file which is to be transferred:
$ nc -N host.example.com 1234 < filename.in
After the file has been transferred, the connection will close automat-
ically.
TALKING TO SERVERS
It is sometimes useful to talk to servers “by hand” rather than through
a user interface. It can aid in troubleshooting, when it might be nec-
essary to verify what data a server is sending in response to commands
issued by the client. For example, to retrieve the home page of a web
site:
$ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
Note that this also displays the headers sent by the web server. They
can be filtered, using a tool such as sed(1), if necessary.
More complicated examples can be built up when the user knows the for-
mat of requests required by the server. As another example, an email
may be submitted to an SMTP server using:
$ nc [-C] localhost 25 << EOF
HELO host.example.com
MAIL FROM:<user@host.example.com>
RCPT TO:<user2@host.example.com>
DATA
Body of email.
.
QUIT
EOF
PORT SCANNING
It may be useful to know which ports are open and running services on a
target machine. The -z flag can be used to tell nc to report open
ports, rather than initiate a connection. Usually it's useful to turn
on verbose output to stderr by use this option in conjunction with -v
option.
For example:
$ nc -zv host.example.com 20-30
Connection to host.example.com 22 port [tcp/ssh] succeeded!
Connection to host.example.com 25 port [tcp/smtp] succeeded!
The port range was specified to limit the search to ports 20 - 30, and
is scanned by increasing order (unless the -r flag is set).
You can also specify a list of ports to scan, for example:
$ nc -zv host.example.com http 20 22-23
nc: connect to host.example.com 80 (tcp) failed: Connection refused
nc: connect to host.example.com 20 (tcp) failed: Connection refused
Connection to host.example.com port [tcp/ssh] succeeded!
nc: connect to host.example.com 23 (tcp) failed: Connection refused
The ports are scanned by the order you given (unless the -r flag is
set).
Alternatively, it might be useful to know which server software is run-
ning, and which versions. This information is often contained within
the greeting banners. In order to retrieve these, it is necessary to
first make a connection, and then break the connection when the banner
has been retrieved. This can be accomplished by specifying a small
timeout with the -w flag, or perhaps by issuing a "QUIT" command to the
server:
$ echo "QUIT" | nc host.example.com 20-30
SSH-1.99-OpenSSH_3.6.1p2
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready
EXAMPLES
Open a TCP connection to port 42 of host.example.com, using port 31337
as the source port, with a timeout of 5 seconds:
$ nc -p 31337 -w 5 host.example.com 42
Open a UDP connection to port 53 of host.example.com:
$ nc -u host.example.com 53
Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as
the IP for the local end of the connection:
$ nc -s 10.1.2.3 host.example.com 42
Create and listen on a Unix-domain stream socket:
$ nc -lU /var/tmp/dsocket
Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
port 8080. This example could also be used by ssh(1); see the
ProxyCommand directive in ssh_config(5) for more information.
$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
The same example again, this time enabling proxy authentication with
username “ruser” if the proxy requires it:
$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
SEE ALSO
cat(1), ssh(1)
AUTHORS
Original implementation by *Hobbit* <hobbit@avian.org>.
Rewritten with IPv6 support by
Eric Jackson <ericj@monkey.org>.
Modified for Debian port by Aron Xu ⟨aron@debian.org⟩.
CAVEATS
UDP port scans using the -uz combination of flags will always report
success irrespective of the target machine's state. However, in con-
junction with a traffic sniffer either on the target machine or an in-
termediary device, the -uz combination could be useful for communica-
tions diagnostics. Note that the amount of UDP traffic generated may
be limited either due to hardware resources and/or configuration set-
tings.
Debian September 11, 2022 NC(1)
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