1NC(1)                     BSD General Commands Manual                    NC(1)
2

NAME

4     nc — arbitrary TCP and UDP connections and listens
5

SYNOPSIS

7     nc [-46cDdFhklNnrStUuvz] [-C certfile] [-e name] [-H hash] [-I length]
8        [-i interval] [-K keyfile] [-M ttl] [-m minttl] [-O length]
9        [-o staplefile] [-P proxy_username] [-p source_port] [-R CAfile]
10        [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit] [-w timeout]
11        [-X proxy_protocol] [-x proxy_address[:port]] [-Z peercertfile]
12        [destination] [port]
13

DESCRIPTION

15     The nc (or netcat) utility is used for just about anything under the sun
16     involving TCP, UDP, or UNIX-domain sockets.  It can open TCP connections,
17     send UDP packets, listen on arbitrary TCP and UDP ports, do port scan‐
18     ning, and deal with both IPv4 and IPv6.  Unlike telnet(1), nc scripts
19     nicely, and separates error messages onto standard error instead of send‐
20     ing them to standard output, as telnet(1) does with some.
21
22     Common uses include:
23
24           ·   simple TCP proxies
25           ·   shell-script based HTTP clients and servers
26           ·   network daemon testing
27           ·   a SOCKS or HTTP ProxyCommand for ssh(1)
28           ·   and much, much more
29
30     The options are as follows:
31
32     -4      Use IPv4 addresses only.
33
34     -6      Use IPv6 addresses only.
35
36     -C certfile
37             Load the public key part of the TLS peer certificate from
38             certfile, in PEM format.  Requires -c.
39
40     -c      Use TLS to connect or listen.  Cannot be used together with any
41             of the options -FuU.
42
43     -D      Enable debugging on the socket.
44
45     -d      Do not attempt to read from stdin.
46
47     -e name
48             Only accept the TLS peer certificate if it contains the name.
49             Requires -c.  If not specified, destination is used.
50
51     -F      Pass the first connected socket using sendmsg(2) to stdout and
52             exit.  This is useful in conjunction with -X to have nc perform
53             connection setup with a proxy but then leave the rest of the con‐
54             nection to another program (e.g. ssh(1) using the ssh_config(5)
55             ProxyUseFdpass option).  Cannot be used with -c or -U.
56
57     -H hash
58             Only accept the TLS peer certificate if its hash returned from
59             tls_peer_cert_hash(3) matches hash.  Requires -c and cannot be
60             used with -T noverify.
61
62     -h      Print out the nc help text and exit.
63
64     -I length
65             Specify the size of the TCP receive buffer.
66
67     -i interval
68             Sleep for interval seconds between lines of text sent and
69             received.  Also causes a delay time between connections to multi‐
70             ple ports.
71
72     -K keyfile
73             Load the TLS private key from keyfile, in PEM format.  Requires
74             -c.
75
76     -k      When a connection is completed, listen for another one.  Requires
77             -l.  When used together with the -u option, the server socket is
78             not connected and it can receive UDP datagrams from multiple
79             hosts.
80
81     -l      Listen for an incoming connection rather than initiating a con‐
82             nection to a remote host.  Cannot be used together with any of
83             the options -psxz.  Additionally, any timeouts specified with the
84             -w option are ignored.
85
86     -M ttl  Set the TTL / hop limit of outgoing packets.
87
88     -m minttl
89             Ask the kernel to drop incoming packets whose TTL / hop limit is
90             under minttl.
91
92     -N      shutdown(2) the network socket after EOF on the input.  Some
93             servers require this to finish their work.
94
95     -n      Do not perform domain name resolution.  If a name cannot be
96             resolved without DNS, an error will be reported.
97
98     -O length
99             Specify the size of the TCP send buffer.
100
101     -o staplefile
102             During the TLS handshake, load data to be stapled from
103             staplefile, which is expected to contain an OCSP response from an
104             OCSP server in DER format.  Requires -c and -C.
105
106     -P proxy_username
107             Specifies a username to present to a proxy server that requires
108             authentication.  If no username is specified then authentication
109             will not be attempted.  Proxy authentication is only supported
110             for HTTP CONNECT proxies at present.
111
112     -p source_port
113             Specify the source port nc should use, subject to privilege
114             restrictions and availability.
115
116     -R CAfile
117             Load the root CA bundle for TLS certificate verification from
118             CAfile, in PEM format, instead of /etc/ssl/cert.pem.  Requires
119             -c.
120
121     -r      Choose source and/or destination ports randomly instead of
122             sequentially within a range or in the order that the system
123             assigns them.
124
125     -S      Enable the RFC 2385 TCP MD5 signature option.
126
127     -s sourceaddr
128             Set the source address to send packets from, which is useful on
129             machines with multiple interfaces.  For UNIX-domain datagram
130             sockets, specifies the local temporary socket file to create and
131             use so that datagrams can be received.  Cannot be used together
132             with -l or -x.
133
134     -T keyword
135             Change the IPv4 TOS/IPv6 traffic class value or the TLS options.
136
137             For TLS options, keyword may be one of: noverify, which disables
138             certificate verification; noname, which disables certificate name
139             checking; clientcert, which requires a client certificate on
140             incoming connections; or muststaple, which requires the peer to
141             provide a valid stapled OCSP response with the handshake.  The
142             following TLS options specify a value in the form of a key=value
143             pair: ciphers, which allows the supported TLS ciphers to be spec‐
144             ified (see tls_config_set_ciphers(3) for further details);
145             protocols, which allows the supported TLS protocols to be speci‐
146             fied (see tls_config_parse_protocols(3) for further details).
147             Specifying TLS options requires -c.
148
149             For the IPv4 TOS/IPv6 traffic class value, keyword may be one of
150             critical, inetcontrol, lowcost, lowdelay, netcontrol, throughput,
151             reliability, or one of the DiffServ Code Points: ef, af11 ...
152             af43, cs0 ... cs7; or a number in either hex or decimal.
153
154     -t      Send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL
155             requests.  This makes it possible to use nc to script telnet ses‐
156             sions.
157
158     -U      Use UNIX-domain sockets.  Cannot be used together with any of the
159             options -cFx.
160
161     -u      Use UDP instead of TCP.  Cannot be used together with -c or -x.
162             For UNIX-domain sockets, use a datagram socket instead of a
163             stream socket.  If a UNIX-domain socket is used, a temporary
164             receiving socket is created in /tmp unless the -s flag is given.
165
166     -V rtable
167             Set the routing table to be used.
168
169     -v      Produce more verbose output.
170
171     -W recvlimit
172             Terminate after receiving recvlimit packets from the network.
173
174     -w timeout
175             Connections which cannot be established or are idle timeout after
176             timeout seconds.  The -w flag has no effect on the -l option,
177             i.e. nc will listen forever for a connection, with or without the
178             -w flag.  The default is no timeout.
179
180     -X proxy_protocol
181             Use proxy_protocol when talking to the proxy server.  Supported
182             protocols are 4 (SOCKS v.4), 5 (SOCKS v.5) and connect (HTTPS
183             proxy).  If the protocol is not specified, SOCKS version 5 is
184             used.
185
186     -x proxy_address[:port]
187             Connect to destination using a proxy at proxy_address and port.
188             If port is not specified, the well-known port for the proxy pro‐
189             tocol is used (1080 for SOCKS, 3128 for HTTPS).  An IPv6 address
190             can be specified unambiguously by enclosing proxy_address in
191             square brackets.  A proxy cannot be used with any of the options
192             -lsuU.
193
194     -Z peercertfile
195             Save the peer certificates to peercertfile, in PEM format.
196             Requires -c.
197
198     -z      Only scan for listening daemons, without sending any data to
199             them.  Cannot be used together with -l.
200
201     destination can be a numerical IP address or a symbolic hostname (unless
202     the -n option is given).  In general, a destination must be specified,
203     unless the -l option is given (in which case the local host is used).
204     For UNIX-domain sockets, a destination is required and is the socket path
205     to connect to (or listen on if the -l option is given).
206
207     port can be specified as a numeric port number or as a service name.
208     Port ranges may be specified as numeric port numbers of the form nn-mm.
209     In general, a destination port must be specified, unless the -U option is
210     given.
211

CLIENT/SERVER MODEL

213     It is quite simple to build a very basic client/server model using nc.
214     On one console, start nc listening on a specific port for a connection.
215     For example:
216
217           $ nc -l 1234
218
219     nc is now listening on port 1234 for a connection.  On a second console
220     (or a second machine), connect to the machine and port being listened on:
221
222           $ nc 127.0.0.1 1234
223
224     There should now be a connection between the ports.  Anything typed at
225     the second console will be concatenated to the first, and vice-versa.
226     After the connection has been set up, nc does not really care which side
227     is being used as a ‘server’ and which side is being used as a ‘client’.
228     The connection may be terminated using an EOF (‘^D’).
229

DATA TRANSFER

231     The example in the previous section can be expanded to build a basic data
232     transfer model.  Any information input into one end of the connection
233     will be output to the other end, and input and output can be easily cap‐
234     tured in order to emulate file transfer.
235
236     Start by using nc to listen on a specific port, with output captured into
237     a file:
238
239           $ nc -l 1234 > filename.out
240
241     Using a second machine, connect to the listening nc process, feeding it
242     the file which is to be transferred:
243
244           $ nc -N host.example.com 1234 < filename.in
245
246     After the file has been transferred, the connection will close automati‐
247     cally.
248

TALKING TO SERVERS

250     It is sometimes useful to talk to servers “by hand” rather than through a
251     user interface.  It can aid in troubleshooting, when it might be neces‐
252     sary to verify what data a server is sending in response to commands
253     issued by the client.  For example, to retrieve the home page of a web
254     site:
255
256           $ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
257
258     Note that this also displays the headers sent by the web server.  They
259     can be filtered, using a tool such as sed(1), if necessary.
260
261     More complicated examples can be built up when the user knows the format
262     of requests required by the server.  As another example, an email may be
263     submitted to an SMTP server using:
264
265           $ nc localhost 25 << EOF
266           HELO host.example.com
267           MAIL FROM:<user@host.example.com>
268           RCPT TO:<user2@host.example.com>
269           DATA
270           Body of email.
271           .
272           QUIT
273           EOF
274

PORT SCANNING

276     It may be useful to know which ports are open and running services on a
277     target machine.  The -z flag can be used to tell nc to report open ports,
278     rather than initiate a connection.  For example:
279
280           $ nc -z host.example.com 20-30
281           Connection to host.example.com 22 port [tcp/ssh] succeeded!
282           Connection to host.example.com 25 port [tcp/smtp] succeeded!
283
284     The port range was specified to limit the search to ports 20 - 30.
285
286     Alternatively, it might be useful to know which server software is run‐
287     ning, and which versions.  This information is often contained within the
288     greeting banners.  In order to retrieve these, it is necessary to first
289     make a connection, and then break the connection when the banner has been
290     retrieved.  This can be accomplished by specifying a small timeout with
291     the -w flag, or perhaps by issuing a "QUIT" command to the server:
292
293           $ echo "QUIT" | nc host.example.com 20-30
294           SSH-1.99-OpenSSH_3.6.1p2
295           Protocol mismatch.
296           220 host.example.com IMS SMTP Receiver Version 0.84 Ready
297

EXAMPLES

299     Open a TCP connection to port 42 of host.example.com, using port 31337 as
300     the source port, with a timeout of 5 seconds:
301
302           $ nc -p 31337 -w 5 host.example.com 42
303
304     Open a TCP connection to port 443 of www.example.com, and negotiate TLS
305     with any supported TLS protocol version and "compat" ciphers:
306
307           $ nc -cv -T protocols=all -T ciphers=compat www.example.com 443
308
309     Open a TCP connection to port 443 of www.google.ca, and negotiate TLS.
310     Check for a different name in the certificate for validation:
311
312           $ nc -cv -e adsf.au.doubleclick.net www.google.ca 443
313
314     Open a UDP connection to port 53 of host.example.com:
315
316           $ nc -u host.example.com 53
317
318     Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as
319     the IP for the local end of the connection:
320
321           $ nc -s 10.1.2.3 host.example.com 42
322
323     Create and listen on a UNIX-domain stream socket:
324
325           $ nc -lU /var/tmp/dsocket
326
327     Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
328     port 8080.  This example could also be used by ssh(1); see the
329     ProxyCommand directive in ssh_config(5) for more information.
330
331           $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
332
333     The same example again, this time enabling proxy authentication with
334     username “ruser” if the proxy requires it:
335
336           $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
337

SEE ALSO

339     cat(1), ssh(1)
340

AUTHORS

342     Original implementation by *Hobbit* <hobbit@avian.org>.
343     Rewritten with IPv6 support by
344     Eric Jackson <ericj@monkey.org>.
345

CAVEATS

347     UDP port scans using the -uz combination of flags will always report suc‐
348     cess irrespective of the target machine's state.  However, in conjunction
349     with a traffic sniffer either on the target machine or an intermediary
350     device, the -uz combination could be useful for communications diagnos‐
351     tics.  Note that the amount of UDP traffic generated may be limited
352     either due to hardware resources and/or configuration settings.
353
354BSD                            February 12, 2020                           BSD
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