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\" Copyright (c) 1996 David Sacerdote
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Dd February 2, 2017
Dt NC 1
Os
Sh NAME
Nm nc
Nd arbitrary TCP and UDP connections and listens
Sh SYNOPSIS
Nm nc
Op Fl 46cDdFhklNnrStUuvz
Op Fl C Ar certfile
Op Fl e Ar name
Op Fl H Ar hash
Op Fl I Ar length
Op Fl i Ar interval
Op Fl K Ar keyfile
Op Fl M Ar ttl
Op Fl m Ar minttl
Op Fl O Ar length
Op Fl o Ar staplefile
Op Fl P Ar proxy_username
Op Fl p Ar source_port
Op Fl R Ar CAfile
Op Fl s Ar source
Op Fl T Ar keyword
\" .Op Fl V Ar rtable
Op Fl w Ar timeout
Op Fl X Ar proxy_protocol
Op Fl x Ar proxy_address Ns Op : Ns Ar port
Op Ar destination
Op Ar port
Sh DESCRIPTION
The
Nm
(or
Nm netcat )
utility is used for just about anything under the sun involving TCP,
UDP, or
Ux Ns -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
Xr telnet 1 ,
Nm
scripts nicely, and separates error messages onto standard error instead
of sending them to standard output, as
Xr telnet 1
does with some.
Pp
Common uses include:
Pp
Bl -bullet -offset indent -compact
It
simple TCP proxies
It
shell-script based HTTP clients and servers
It
network daemon testing
It
a SOCKS or HTTP ProxyCommand for
Xr ssh 1
It
and much, much more
El
Pp
The options are as follows:
Bl -tag -width Ds
It Fl 4
Forces
Nm
to use IPv4 addresses only.
It Fl 6
Forces
Nm
to use IPv6 addresses only.
It Fl C Ar certfile
Specifies the filename from which the public key part of the TLS
certificate is loaded, in PEM format.
May only be used with TLS.
It Fl c
If using a TCP socket to connect or listen, use TLS.
Illegal if not using TCP sockets.
It Fl D
Enable debugging on the socket.
It Fl d
Do not attempt to read from stdin.
It Fl e Ar name
Specify the name that must be present in the peer certificate when using TLS.
Illegal if not using TLS.
It Fl F
Pass the first connected socket using
Xr sendmsg 2
to stdout and exit.
This is useful in conjunction with
Fl X
to have
Nm
perform connection setup with a proxy but then leave the rest of the
connection to another program (e.g.\&
Xr ssh 1
using the
Xr ssh_config 5
Cm ProxyUseFdpass
option).
It Fl H Ar hash
Specifies the required hash string of the peer certificate when using TLS.
The string format required is that used by
Xr tls_peer_cert_hash 3 .
Illegal if not using TLS, and may not be used with -T noverify.
It Fl h
Prints out
Nm
help.
It Fl I Ar length
Specifies the size of the TCP receive buffer.
It Fl i Ar interval
Specifies a delay time interval between lines of text sent and received.
Also causes a delay time between connections to multiple ports.
It Fl K Ar keyfile
Specifies the filename from which the private key
is loaded in PEM format.
May only be used with TLS.
It Fl k
Forces
Nm
to stay listening for another connection after its current connection
is completed.
It is an error to use this option without the
Fl l
option.
When used together with the
Fl u
option, the server socket is not connected and it can receive UDP datagrams from
multiple hosts.
It Fl l
Used to specify that
Nm
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
Fl p ,
Fl s ,
or
Fl z
options.
Additionally, any timeouts specified with the
Fl w
option are ignored.
It Fl M Ar ttl
Set the TTL / hop limit of outgoing packets.
It Fl m Ar minttl
Ask the kernel to drop incoming packets whose TTL / hop limit is under
Ar minttl .
It Fl N
Xr shutdown 2
the network socket after EOF on the input.
Some servers require this to finish their work.
It Fl n
Do not do any DNS or service lookups on any specified addresses,
hostnames or ports.
It Fl O Ar length
Specifies the size of the TCP send buffer.
It Fl o Ar staplefile
Specifies the filename from which to load data to be stapled
during 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.
It Fl P Ar 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.
It Fl p Ar source_port
Specifies the source port
Nm
should use, subject to privilege restrictions and availability.
It is an error to use this option in conjunction with the
Fl l
option.
It Fl R Ar CAfile
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
Pa /etc/ssl/cert.pem .
It Fl 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.
It Fl S
Enables the RFC 2385 TCP MD5 signature option.
It Fl s Ar source
Specifies the IP of the interface which is used to send the packets.
For
Ux Ns -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
Fl l
option.
It Fl T Ar keyword
Change IPv4 TOS value or TLS options.
For TLS options
Ar keyword
may be one of
Ar tlsall ;
which allows the use of all supported TLS protocols and ciphers,
Ar noverify ;
which disables certificate verification;
Ar noname ,
which disables certificate name checking;
Ar clientcert ,
which requires a client certificate on incoming connections; or
Ar 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.
Pp
For IPv4 TOS value
Ar keyword
may be one of
Ar critical ,
Ar inetcontrol ,
Ar lowdelay ,
Ar netcontrol ,
Ar throughput ,
Ar reliability ,
or one of the DiffServ Code Points:
Ar ef ,
Ar af11 ... af43 ,
Ar cs0 ... cs7 ;
or a number in either hex or decimal.
It Fl t
Causes
Nm
to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
This makes it possible to use
Nm
to script telnet sessions.
It Fl U
Specifies to use
Ux Ns -domain
sockets.
It Fl u
Use UDP instead of the default option of TCP.
For
Ux Ns -domain
sockets, use a datagram socket instead of a stream socket.
If a
Ux Ns -domain
socket is used, a temporary receiving socket is created in
Pa /tmp
unless the
Fl s
flag is given.
\" .It Fl V Ar rtable
\" Set the routing table to be used.
It Fl v
Have
Nm
give more verbose output.
It Fl w Ar timeout
Connections which cannot be established or are idle timeout after
Ar timeout
seconds.
The
Fl w
flag has no effect on the
Fl l
option, i.e.\&
Nm
will listen forever for a connection, with or without the
Fl w
flag.
The default is no timeout.
It Fl X Ar proxy_protocol
Requests that
Nm
should use the specified protocol when talking to the proxy server.
Supported protocols are
Dq 4
(SOCKS v.4),
Dq 5
(SOCKS v.5)
and
Dq connect
(HTTPS proxy).
If the protocol is not specified, SOCKS version 5 is used.
It Fl x Ar proxy_address Ns Op : Ns Ar port
Requests that
Nm
should connect to
Ar destination
using a proxy at
Ar proxy_address
and
Ar port .
If
Ar port
is not specified, the well-known port for the proxy protocol is used (1080
for SOCKS, 3128 for HTTPS).
It Fl z
Specifies that
Nm
should just scan for listening daemons, without sending any data to them.
It is an error to use this option in conjunction with the
Fl l
option.
El
Pp
Ar destination
can be a numerical IP address or a symbolic hostname
(unless the
Fl n
option is given).
In general, a destination must be specified,
unless the
Fl l
option is given
(in which case the local host is used).
For
Ux Ns -domain
sockets, a destination is required and is the socket path to connect to
(or listen on if the
Fl l
option is given).
Pp
Ar 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 destination port must be specified,
unless the
Fl U
option is given.
Sh CLIENT/SERVER MODEL
It is quite simple to build a very basic client/server model using
Nm .
On one console, start
Nm
listening on a specific port for a connection.
For example:
Pp
Dl $ nc -l 1234
Pp
Nm
is now listening on port 1234 for a connection.
On a second console
Pq or a second machine ,
connect to the machine and port being listened on:
Pp
Dl $ nc 127.0.0.1 1234
Pp
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,
Nm
does not really care which side is being used as a
Sq server
and which side is being used as a
Sq client .
The connection may be terminated using an
Dv EOF
Pq Sq ^D .
Sh 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.
Pp
Start by using
Nm
to listen on a specific port, with output captured into a file:
Pp
Dl $ nc -l 1234 \*(Gt filename.out
Pp
Using a second machine, connect to the listening
Nm
process, feeding it the file which is to be transferred:
Pp
Dl $ nc -N host.example.com 1234 \*(Lt filename.in
Pp
After the file has been transferred, the connection will close automatically.
Sh TALKING TO SERVERS
It is sometimes useful to talk to servers
Dq 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:
Bd -literal -offset indent
$ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
Ed
Pp
Note that this also displays the headers sent by the web server.
They can be filtered, using a tool such as
Xr sed 1 ,
if necessary.
Pp
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:
Bd -literal -offset indent
$ nc localhost 25 \*(Lt\*(Lt EOF
HELO host.example.com
MAIL FROM:\*([email protected]\*(Gt
RCPT TO:\*([email protected]\*(Gt
DATA
Body of email.
\&.
QUIT
EOF
Ed
Sh PORT SCANNING
It may be useful to know which ports are open and running services on
a target machine.
The
Fl z
flag can be used to tell
Nm
to report open ports,
rather than initiate a connection.
For example:
Bd -literal -offset indent
$ 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!
Ed
Pp
The port range was specified to limit the search to ports 20 \- 30.
Pp
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
Fl w
flag, or perhaps by issuing a
Qq Dv QUIT
command to the server:
Bd -literal -offset indent
$ 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
Ed
Sh 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:
Pp
Dl $ nc -p 31337 -w 5 host.example.com 42
Pp
Open a TCP connection to port 443 of www.google.ca, and negotiate TLS.
Check for a different name in the certificate for validation.
Pp
Dl $  nc -v -c -e adsf.au.doubleclick.net www.google.ca 443
Pp
Open a UDP connection to port 53 of host.example.com:
Pp
Dl $ nc -u host.example.com 53
Pp
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:
Pp
Dl $ nc -s 10.1.2.3 host.example.com 42
Pp
Create and listen on a
Ux Ns -domain
stream socket:
Pp
Dl $ nc -lU /var/tmp/dsocket
Pp
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
Xr ssh 1 ;
see the
Cm ProxyCommand
directive in
Xr ssh_config 5
for more information.
Pp
Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
Pp
The same example again, this time enabling proxy authentication with username
Dq ruser
if the proxy requires it:
Pp
Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
Sh SEE ALSO
Xr cat 1 ,
Xr ssh 1
Sh AUTHORS
Original implementation by *Hobbit*
Aq Mt [email protected] .
br
Rewritten with IPv6 support by
An Eric Jackson Aq Mt [email protected] .
Sh CAVEATS
UDP port scans using the
Fl 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
Fl 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.

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