- NetBSD Manual Pages
NC(1) NetBSD General Commands Manual NC(1)
Powered by man-cgi (2021-06-01).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
nc -- arbitrary TCP and UDP connections and listens
nc [-46cDdFhklNnrStUuvz] [-C certfile] [-e name] [-H hash] [-I length]
[-i interval] [-K keyfile] [-M ttl] [-m minttl] [-O length]
[-o staplefile] [-P proxy_username] [-p source_port] [-R CAfile]
[-s source] [-T keyword] [-w timeout] [-X proxy_protocol]
[-x proxy_address[:port]] [destination] [port]
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(1), nc scripts
nicely, and separates error messages onto standard error instead of send-
ing 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.
Specifies the filename from which the public key part of the TLS
certificate is loaded, in PEM format. May only be used with TLS.
-c If using a TCP socket to connect or listen, use TLS. Illegal if
not using TCP sockets.
-D Enable debugging on the socket.
-d Do not attempt to read from stdin.
Specify the name that must be present in the peer certificate
when using TLS. Illegal if not using TLS.
-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 con-
nection to another program (e.g. ssh(1) using the ssh_config(5)
Specifies the required hash string of the peer certificate when
using TLS. The string format required is that used by
tls_peer_cert_hash(3). Illegal if not using TLS, and may not be
used with -T noverify.
-h Prints out nc help.
Specifies the size of the TCP receive buffer.
Specifies a delay time interval between lines of text sent and
received. Also causes a delay time between connections to multi-
Specifies the filename from which the private key is loaded in
PEM format. May only be used with TLS.
-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. When used together with the -u option,
the server socket is not connected and it can receive UDP data-
grams 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
-M ttl Set the TTL / hop limit of outgoing packets.
Ask the kernel to drop incoming packets whose TTL / hop limit is
-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.
Specifies the size of the TCP send buffer.
Specifies the filename from which to load data to be stapled dur-
ing the TLS handshake. The file is expected to contain an OCSP
response from an OCSP server in DER format. May only be used
with TLS and when a certificate is being used.
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.
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.
Specifies the filename from which the root CA bundle for certifi-
cate verification is loaded, in PEM format. Illegal if not using
TLS. The default is /etc/ssl/cert.pem.
-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.
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.
Change IPv4 TOS value or TLS options. For TLS options keyword
may be one of tlsall; which allows the use of all supported TLS
protocols and ciphers, noverify; which disables certificate veri-
fication; noname, which disables certificate name checking;
clientcert, which requires a client certificate on incoming con-
nections; or muststaple, which requires the peer to provide a
valid stapled OCSP response with the handshake. It is illegal to
specify TLS options if not using TLS.
For IPv4 TOS value keyword 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
-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 Have nc give more verbose output.
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.
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.
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
-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 specified as a numeric port number, or as a service name.
Ports may be specified in a range of the form nn-mm. In general, a des-
tination port must be specified, unless the -U option is given.
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.
$ 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').
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
$ 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 automati-
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
$ 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
Body of email.
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 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
220 host.example.com IMS SMTP Receiver Version 0.84 Ready
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 TCP connection to port 443 of www.google.ca, and negotiate TLS.
Check for a different name in the certificate for validation.
$ nc -v -c -e adsf.au.doubleclick.net www.google.ca 443
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
Original implementation by *Hobbit* <firstname.lastname@example.org>.
Rewritten with IPv6 support by Eric Jackson <email@example.com>.
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.
NetBSD 9.2 February 2, 2017 NetBSD 9.2