nc.1

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.\" Copyright (c) 1996 David Sacerdote
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.Dd $Mdocdate: December 27 2018 $
.Dt NC 1
.Os
.Sh NAME
.Nm nc
.Nd arbitrary TCP and UDP connections and listens
.Sh SYNOPSIS
.Nm nc
.Op Fl 46bCDdFhklNnrStUuvZz
.Op Fl I Ar length
.Op Fl i Ar interval
.Op Fl M Ar ttl
.Op Fl m Ar minttl
.Op Fl O Ar length
.Op Fl P Ar proxy_username
.Op Fl p Ar source_port
.Op Fl q Ar seconds
.Op Fl s Ar source
.Op Fl T Ar keyword
.Op Fl V Ar rtable
.Op Fl W Ar recvlimit
.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
Use IPv4 addresses only.
.It Fl 6
Use IPv6 addresses only.
.It Fl b
Allow broadcast.
.It Fl C
Send CRLF as line-ending.  Each line feed (LF) character from the input
data is translated into CR+LF before being written to the socket.  Line
feed characters that are already preceded with a carriage return (CR)
are not translated.  Received data is not affected.
.It Fl D
Enable debugging on the socket.
.It Fl d
Do not attempt to read from stdin.
.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).
Cannot be used with
.Fl U .
.It Fl h
Print out the
.Nm
help text and exit.
.It Fl I Ar length
Specify the size of the TCP receive buffer.
.It Fl i Ar interval
Sleep for
.Ar interval
seconds between lines of text sent and received.
Also causes a delay time between connections to multiple ports.
.It Fl k
When a connection is completed, listen for another one.
Requires
.Fl l .
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
Listen for an incoming connection rather than initiating a
connection to a remote host.
The
.Ar destination
and
.Ar port
to listen on can be specified either as non-optional arguments, or with
options
.Fl s
and
.Fl p
respectively.
Cannot be used together with
.Fl x
or
.Fl z .
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
Specify the size of the TCP send buffer.
.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
Specify the source port
.Nm
should use, subject to privilege restrictions and availability.
.It Fl q Ar seconds
after EOF on stdin, wait the specified number of
.Ar seconds
and then quit. If
.Ar seconds
is negative, wait forever (default).  Specifying a non-negative
.Ar seconds
implies
.Fl N .
.It Fl r
Choose source and/or destination ports randomly
instead of sequentially within a range or in the order that the system
assigns them.
.It Fl S
Enable the RFC 2385 TCP MD5 signature option.
.It Fl s Ar source
Send packets from the interface with the
.Ar source
IP address.
For
.Ux Ns -domain
datagram sockets, specifies the local temporary socket file
to create and use so that datagrams can be received.
Cannot be used together with
.Fl x .
.It Fl T Ar keyword
Change the IPv4 TOS/IPv6 traffic class value.
.Ar keyword
may be one of
.Cm critical ,
.Cm inetcontrol ,
.Cm lowcost ,
.Cm lowdelay ,
.Cm netcontrol ,
.Cm throughput ,
.Cm reliability ,
or one of the DiffServ Code Points:
.Cm ef ,
.Cm af11 No ... Cm af43 ,
.Cm cs0 No ... Cm cs7 ;
or a number in either hex or decimal.
.It Fl t
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
Use
.Ux Ns -domain
sockets.
Cannot be used together with
.Fl F
or
.Fl x .
.It Fl u
Use UDP instead of TCP.
Cannot be used together with
.Fl x .
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
Produce more verbose output.
.It Fl W Ar recvlimit
Terminate after receiving
.Ar recvlimit
packets from the network.
.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
Use
.Ar proxy_protocol
when talking to the proxy server.
Supported protocols are
.Cm 4
(SOCKS v.4),
.Cm 5
(SOCKS v.5)
and
.Cm 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
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).
An IPv6 address can be specified unambiguously by enclosing
.Ar proxy_address
in square brackets.
A proxy cannot be used with any of the options
.Fl lsuU .
.It Fl Z
DCCP mode.
.It Fl z
Only scan for listening daemons, without sending any data to them.
Cannot be used together with
.Fl l .
.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 specified as a numeric port number or as a service name.
Port ranges may be specified as numeric port numbers of the form
.Ar nn Ns - Ns Ar 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 .
.Pp
There is no
.Fl c
or
.Fl e
option in this netcat, but you still can execute a command after connection
being established by redirecting file descriptors. Be cautious here because
opening a port and let anyone connected execute arbitrary command on your
site is DANGEROUS. If you really need to do this, here is an example:
.Pp
On
.Sq server
side:
.Pp
.Dl $ rm -f /tmp/f; mkfifo /tmp/f
.Dl $ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f
.Pp
On
.Sq client
side:
.Pp
.Dl $ nc host.example.com 1234
.Dl $ (shell prompt from host.example.com)
.Pp
By doing this, you create a fifo at /tmp/f and make nc listen at port 1234
of address 127.0.0.1 on
.Sq server
side, when a
.Sq client
establishes a connection successfully to that port, /bin/sh gets executed
on
.Sq server
side and the shell prompt is given to
.Sq client
side.
.Pp
When connection is terminated,
.Nm
quits as well. Use
.Fl k
if you want it keep listening, but if the command quits this option won't
restart it or keep
.Nm
running. Also don't forget to remove the file descriptor once you don't need
it anymore:
.Pp
.Dl $ rm -f /tmp/f
.Pp
.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 [\-C] localhost 25 \*(Lt\*(Lt EOF
HELO host.example.com
MAIL FROM:\*(Ltuser@host.example.com\*(Gt
RCPT TO:\*(Ltuser2@host.example.com\*(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. Usually it's useful to turn on verbose
output to stderr by use this option in conjunction with
.Fl v
option.
.Pp
For example:
.Bd -literal -offset indent
$ nc \-zv 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, and is
scanned by increasing order (unless the
.Fl r
flag is set).
.Pp
You can also specify a list of ports to scan, for example:
.Bd -literal -offset indent
$ nc \-zv host.example.com http 20 22-23
nc: connect to host.example.com 80 (tcp) failed: Connection refused
nc: connect to host.example.com 20 (tcp) failed: Connection refused
Connection to host.example.com port [tcp/ssh] succeeded!
nc: connect to host.example.com 23 (tcp) failed: Connection refused
.Ed
.Pp
The ports are scanned by the order you given (unless the
.Fl r
flag is set).
.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 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
.An *Hobbit* Aq Mt hobbit@avian.org .
.br
Rewritten with IPv6 support by
.An Eric Jackson Aq Mt ericj@monkey.org .
.br
Modified for Debian port by Aron Xu
.Aq aron@debian.org .
.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.