NC(1) FreeBSD General Commands Manual NC(1)
NAME
nc - arbitrary TCP and UDP connections and listens
SYNOPSIS
nc [-46DdEFhklMNnrStUuvz] [-e IPsec_policy] [-I length] [-i interval]
[--no-tcpopt] [--sctp] [-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.
-E Shortcut for "-e 'in ipsec esp/transport//require' -e 'out ipsec
esp/transport//require'", which enables IPsec ESP transport mode
in both directions.
-e If IPsec support is available, then one can specify the IPsec
policies to be used using the syntax described in
ipsec_set_policy(3). This flag can be specified up to two times,
as typically one policy for each direction is needed.
-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).
-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.
-M Collect per-connection TCP statistics using the stats(3)
framework and print them in JSON format to stderr(4) after the
connection is closed.
-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.
--no-tcpopt
Disables the use of TCP options on the socket, by setting the
boolean TCP_NOOPT socket option.
--sctp Use SCTP instead of the default option of TCP.
-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 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 ("FIB") 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
Open a TCP connection to port 42 of host.example.com using IPsec ESP for
incoming and outgoing traffic.
$ nc -E host.example.com 42
Open a TCP connection to port 42 of host.example.com using IPsec ESP for
outgoing traffic only.
$ nc -e 'out ipsec esp/transport//require' 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
EXIT STATUS
The nc utility exits 0 on success, and >0 if an error occurs.
SEE ALSO
cat(1), setfib(1), ssh(1), tcp(4)
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.
FreeBSD 13.1-RELEASE-p6 July 10, 2020 FreeBSD 13.1-RELEASE-p6
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