netcat Man page

NC(1) BSD General Commands Manual NC(1)

NAME

nc — arbitrary TCP and UDP connections and listens

SYNOPSIS

nc [-46bCDdhklnrStUuvZz] [-I length] [-i interval] [-O length] [-P proxy_username] [-p source_port] [-q seconds] [-s source] [-T toskeyword] [-V rtable] [-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 connections,
send UDP packets, listen on arbitrary TCP and UDP ports, do port scan‐
ning, and deal with both IPv4 and IPv6. Unlike telnet, nc scripts
nicely, and separates error messages onto standard error instead of send‐
ing them to standard output, as telnet 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
· and much, much more

The options are as follows:

-4 Forces nc to use IPv4 addresses only.

-6 Forces nc to use IPv6 addresses only.

-b Allow broadcast.

-C Send CRLF as line-ending.

-D Enable debugging on the socket.

-d Do not attempt to read from stdin.

-h Prints out nc help.

-I length
Specifies the size of the TCP receive buffer.

-i interval
Specifies a delay time interval between lines of text sent and
received. Also causes a delay time between connections to multi‐
ple ports.

-k Forces nc to stay listening for another connection after its cur‐
rent connection is completed. It is an error to use this option
without the -l option.

-l Used to specify that nc should listen for an incoming connection
rather than initiate a connection to a remote host. It is an
error to use this option in conjunction with the -p, -s, or -z
options. Additionally, any timeouts specified with the -w option
are ignored.

-n Do not do any DNS or service lookups on any specified addresses,
hostnames or ports.

-O length
Specifies 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 authentication
will not be attempted. Proxy authentication is only supported
for HTTP CONNECT proxies at present.

-p source_port
Specifies the source port nc should use, subject to privilege
restrictions and availability.

-q seconds
after EOF on stdin, wait the specified number of seconds and then
quit. If seconds is negative, wait forever.

-r Specifies that source and/or destination ports should be chosen
randomly instead of sequentially within a range or in the order
that the system assigns them.

-S Enables the RFC 2385 TCP MD5 signature option.

-s source
Specifies the IP of the interface which is used to send the pack‐
ets. For UNIX-domain datagram sockets, specifies the local tem‐
porary socket file to create and use so that datagrams can be
received. It is an error to use this option in conjunction with
the -l option.

-T toskeyword
Change IPv4 TOS value. toskeyword 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 Causes nc to 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 Specifies to use UNIX-domain sockets.

-u Use UDP instead of the default option of TCP. 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 cre‐
ated in /tmp unless the -s flag is given.

-V rtable
Set the routing table to be used. The default is 0.

-v Have nc give more verbose output.

-w timeout
Connections which cannot be established or are idle timeout after
timeout seconds. The -w flag has no effect on the -l option,
i.e. nc will listen forever for a connection, with or without the
-w flag. The default is no timeout.

-X proxy_protocol
Requests that nc should use the specified 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] Requests that nc should 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).

-Z DCCP mode.

-z Specifies that nc should just scan for listening daemons, without
sending any data to them. It is an error to use this option in
conjunction with the -l option.

destination can be a numerical IP address or a symbolic hostname (unless
the -n option is given). In general, a destination must be specified,
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 a single integer or a range of ports. Ranges are in 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 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’).

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 connection
will be output to the other end, and input and output can be easily cap‐
tured 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 host.example.com 1234 < filename.in After the file has been transferred, the connection will close automati‐ cally. 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 neces‐ sary 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, if necessary.

More complicated examples can be built up when the user knows the format
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:
RCPT TO:
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.

You can also specify a list of ports to scan, for example:

$ nc -zv host.example.com 80 20 22
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!

The ports are scanned by the order you given.

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; 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, ssh

AUTHORS
Original implementation by *Hobbit* ⟨hobbit@avian.org⟩.
Rewritten with IPv6 support by Eric Jackson .
Modified for Debian port by Aron Xu ⟨aron@debian.org⟩.

CAVEATS
UDP port scans using the -uz combination of flags will always report suc‐
cess irrespective of the target machine’s state. However, in conjunction
with a traffic sniffer either on the target machine or an intermediary
device, the -uz combination could be useful for communications diagnos‐
tics. Note that the amount of UDP traffic generated may be limited
either due to hardware resources and/or configuration settings.

BSD February 7, 2012 BSD