NAME
nc
—
arbitrary TCP and UDP connections and
listens
SYNOPSIS
nc |
[-46DdFhklNnrStUuvz ]
[-I length]
[-i interval]
[-O length]
[-P proxy_username]
[-p source_port]
[-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. -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 havenc
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). -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. When used together with the-u
option, the server socket is not connected and it can receive UDP datagrams from multiple hosts. -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
- shutdown(2) the network socket after EOF on the input. Some servers require this to finish their work.
-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. It is an error to use this option in conjunction with the-l
option. -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, 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.
-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
- 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’).
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 -N 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 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. For example:
$ nc -z 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.
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
AUTHORS
Original implementation by *Hobbit*
<hobbit@avian.org>.
Rewritten with IPv6 support by Eric Jackson
<ericj@monkey.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.