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 scanning, and deal
with both IPv4 and IPv6. Unlike
telnet(1),
nc scripts nicely, and separates error messages onto
standard error instead 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
- 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 multiple
ports.
- -k
- Forces nc to stay listening for another
connection after its current 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
packets. For UNIX-domain datagram sockets,
specifies the local temporary 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 created 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 descriptors. 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 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 host.example.com 1234 <
filename.in
After the file has been transferred, the connection will close automatically.
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
necessary 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 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:<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.
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 running, 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
conjunction with a traffic sniffer either on the target machine or an
intermediary device, the
-uz combination could be useful for
communications diagnostics. Note that the amount of UDP traffic generated may
be limited either due to hardware resources and/or configuration
settings.