NC(1) | 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¶
Thenc
(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
-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 usenc
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.
-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 usingnc
. 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
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
$ nc host.example.com
1234
$ (shell prompt from
host.example.com)
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 usingnc
to listen on a specific
port, with output captured into a file:
$ nc -l 1234 >
filename.out
nc
process, feeding it the file which is to
be transferred:
$ nc host.example.com 1234 <
filename.in
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
$ 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!
$ 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!
-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
$ nc -u host.example.com
53
$ nc -s 10.1.2.3 host.example.com
42
$ nc -lU
/var/tmp/dsocket
ProxyCommand
directive in
ssh_config(5) for more information.
$ nc -x10.2.3.4:8080 -Xconnect
host.example.com 42
$ 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⟩.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.February 7, 2012 | Debian |