arbitrary TCP and UDP connections and
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
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:
ncto use IPv4 addresses only.
ncto 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.
- If using a TCP socket to connect or listen, use TLS. Illegal if not using TCP sockets.
- Enable debugging on the socket.
- 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.
- Pass the first connected socket using
sendmsg(2) to stdout and exit. This is useful in conjunction with
ncperform 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)
- 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.
- Prints out
- 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 multiple ports.
- Specifies the filename from which the private key is loaded in PEM format. May only be used with TLS.
ncto stay listening for another connection after its current connection is completed. It is an error to use this option without the
-loption. When used together with the
-uoption, the server socket is not connected and it can receive UDP datagrams from multiple hosts.
- Used to specify that
ncshould 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
-zoptions. Additionally, any timeouts specified with the
-woption are ignored.
- shutdown(2) the network socket after EOF on the input. Some servers require this to finish their work.
- Do not do any DNS or service lookups on any specified addresses, hostnames or ports.
- Specifies the size of the TCP send buffer.
- 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
ncshould use, subject to privilege restrictions and availability. It is an error to use this option in conjunction with the
- Specifies the filename from which the root CA bundle for certificate verification is loaded, in PEM format. Illegal if not using TLS. The default is /etc/ssl/cert.pem.
- 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.
- Enables the RFC 2385 TCP MD5 signature option.
- 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
- Change IPv4 TOS value or TLS options. For TLS options
keyword may be one of
tlslegacy, which allows legacy TLS protocols;
noverify, which disables certificate verification;
noname, which disables certificate name checking; or
clientcert, which requires a client certificate on
incoming connections. It is illegal to specify TLS options if not using
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.
ncto send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. This makes it possible to use
ncto script telnet sessions.
- Specifies to use UNIX-domain sockets.
- 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
-sflag is given.
- Set the routing table to be used.
ncgive more verbose output.
- Connections which cannot be established or are idle timeout after
timeout seconds. The
-wflag has no effect on the
ncwill listen forever for a connection, with or without the
-wflag. The default is no timeout.
- Requests that
ncshould 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
ncshould 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).
- Specifies that
ncshould just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunction with the
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.
It is quite simple to build a very basic client/server model using
nc. On one console, start
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
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
$ 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:<email@example.com> RCPT TO:<firstname.lastname@example.org> DATA Body of email. . QUIT EOF
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
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*
Rewritten with IPv6 support by Eric Jackson <email@example.com>.
UDP port scans using the
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
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.