The
nc
(or
netcat
utility is used for just about anything under the sun involving TCP
or UDP.
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.
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.
-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.
-n
Do not do any DNS or service lookups on any specified addresses,
hostnames or ports.
-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_ip_address
Specifies the IP of the interface which is used to send the packets.
It is an error to use this option in conjunction with the
-l
option.
-T ToS
Specifies IP Type of Service (ToS) for the connection.
Valid values are the tokens
``lowdelay''
``throughput''
``reliability''
or an 8-bit hexadecimal value preceded by
``0x''
-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.
-v
Have
nc
give more verbose output.
-w timeout
If a connection and stdin are idle for more than
timeout
seconds, then the connection is silently closed.
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_version
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
hostname
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.
hostname
can be a numerical IP address or a symbolic hostname
(unless the
-n
option is given).
In general, a hostname must be specified,
unless the
-l
option is given
(in which case the local host is used).
port [s
]
can be single integers or ranges.
Ranges are in the form nn-mm.
In general,
a destination port must be specified,
unless the
-U
option is given
(in which case a socket must be specified).
CLIENT/SERVER MODEL
It is quite simple to build a very basic client/server model using
.
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 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:
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:
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
Qq Dv QUIT
command to the server:
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
Create and listen on a Unix Domain 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_config5
for more information.
