NC(1) | General Commands Manual | NC(1) |
nc
— arbitrary TCP
and UDP connections and listens
nc |
[-46AcDCdhklnrtUuvz ]
[-b boundif]
[-i interval]
[-p source_port]
[-s source_ip_address]
[-w timeout]
[-X proxy_protocol]
[-x
proxy_address[:port]]
[--apple-delegate-pid pid]
[--apple-delegate-uuid uuid]
[--apple-ext-bk-idle ]
[--apple-nowakefromsleep n]
[--apple-ecn mode]
[hostname]
[port[s]] |
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.
Common uses include:
The options are as follows:
-4
nc
to use IPv4 addresses only.-6
nc
to use IPv6 addresses only.-A
-b
boundif-c
-D
-C
nc
not to use cellular data context.-d
-h
nc
help.-i
interval-G
conntimeout-H
keepidle-I
keepintvl-J
keepcnt-k
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
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.-L
num_probes-n
-p
source_portnc
should use, subject
to privilege restrictions and availability. It is an error to use this
option in conjunction with the -l
option.-r
-s
source_ip_address-l
option.-t
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
-u
-v
nc
give more verbose output.-w
timeout-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_versionnc
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]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
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.--apple-delegate-pid
nc
should delegate the socket for
the specified PID. It is an error to use this option in conjunction with
the --apple-delegate-uuid
option.--apple-delegate-uuid
nc
should delegate the socket for
the specified UUID. It is an error to use this option in conjunction with
the --apple-delegate-pid
option.--apple-ext-bk-idle
nc
marks its socket for extended
background idle time when the process becomes suspended.--apple-nowakefromsleep
nc
marks its socket to exclude the local port
from the list of opened ports that is queried by drivers when the system
goes to sleep. When n is greater than 1, set the
socket option that generates the KEV_SOCKET_CLOSED kernel event when the
socket gets closed.--apple-ecn
nc
marks to use the socket option
TCP_ECN_MODE to set the ECN mode (default, enable, disable)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).
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. 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’).
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.
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:
$ echo -n "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: <user@host.example.com> RCPT TO: <user2@host.example.com> 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 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_config(5) for more
information.
$ nc -x10.2.3.4:8080 -Xconnect
host.example.com 42
Original implementation by *Hobbit*
⟨hobbit@avian.org⟩.
Rewritten with IPv6 support by Eric Jackson
⟨ericj@monkey.org⟩.
UDP port scans will always succeed (i.e. report the port as open),
rendering the -uz
combination of flags relatively
useless.
June 25, 2001 | macOS 14.6 |