1NC(1) BSD General Commands Manual NC(1)
2
4 nc — arbitrary TCP and UDP connections and listens
5
7 nc [-46CDdhklnrStUuvz] [-I length] [-i interval] [-O length]
8 [-P proxy_username] [-p source_port] [-s source] [-T ToS] [-V rtable]
9 [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
10 [destination] [port]
11
13 The nc (or netcat) utility is used for just about anything under the sun
14 involving TCP, UDP, or UNIX-domain sockets. It can open TCP connections,
15 send UDP packets, listen on arbitrary TCP and UDP ports, do port scan‐
16 ning, and deal with both IPv4 and IPv6. Unlike telnet(1), nc scripts
17 nicely, and separates error messages onto standard error instead of send‐
18 ing them to standard output, as telnet(1) does with some.
19 Common uses include:
21 · simple TCP proxies
23 · shell-script based HTTP clients and servers
24 · network daemon testing
25 · a SOCKS or HTTP ProxyCommand for ssh(1)
26 · and much, much more
27 The options are as follows:
29 -4 Forces nc to use IPv4 addresses only.
31 -6 Forces nc to use IPv6 addresses only.
33 -C Send CRLF as line-ending.
35 -D Enable debugging on the socket.
37 -d Do not attempt to read from stdin.
39 -h Prints out nc help.
41 -I length
43 Specifies the size of the TCP receive buffer.
44 -i interval
46 Specifies a delay time interval between lines of text sent and
47 received. Also causes a delay time between connections to multi‐
48 ple ports.
49 -k Forces nc to stay listening for another connection after its cur‐
51 rent connection is completed. It is an error to use this option
52 without the -l option.
53 -l Used to specify that nc should listen for an incoming connection
55 rather than initiate a connection to a remote host. It is an
56 error to use this option in conjunction with the -p, -s, or -z
57 options. Additionally, any timeouts specified with the -w option
58 are ignored.
59 -n Do not do any DNS or service lookups on any specified addresses,
61 hostnames or ports.
62 -O length
64 Specifies the size of the TCP send buffer.
65 -P proxy_username
67 Specifies a username to present to a proxy server that requires
68 authentication. If no username is specified then authentication
69 will not be attempted. Proxy authentication is only supported
70 for HTTP CONNECT proxies at present.
71 -p source_port
73 Specifies the source port nc should use, subject to privilege
74 restrictions and availability. It is an error to use this option
75 in conjunction with the -l option.
76 -r Specifies that source and/or destination ports should be chosen
78 randomly instead of sequentially within a range or in the order
79 that the system assigns them.
80 -S Enables the RFC 2385 TCP MD5 signature option.
82 -s source
84 Specifies the IP of the interface which is used to send the pack‐
85 ets. For UNIX-domain datagram sockets, specifies the local tem‐
86 porary socket file to create and use so that datagrams can be
87 received. It is an error to use this option in conjunction with
88 the -l option.
89 -T ToS Specifies IP Type of Service (ToS) for the connection. Valid
91 values are the tokens “lowdelay”, “throughput”, “reliability”, or
92 an 8-bit hexadecimal value preceded by “0x”.
93 -t Causes nc to send RFC 854 DON'T and WON'T responses to RFC 854 DO
95 and WILL requests. This makes it possible to use nc to script
96 telnet sessions.
97 -U Specifies to use UNIX-domain sockets.
99 -u Use UDP instead of the default option of TCP. For UNIX-domain
101 sockets, use a datagram socket instead of a stream socket. If a
102 UNIX-domain socket is used, a temporary receiving socket is cre‐
103 ated in /tmp unless the -s flag is given.
104 -V rtable
106 Set the routing table to be used. The default is 0.
107 -v Have nc give more verbose output.
109 -w timeout
111 If a connection and stdin are idle for more than timeout seconds,
112 then the connection is silently closed. The -w flag has no
113 effect on the -l option, i.e. nc will listen forever for a con‐
114 nection, with or without the -w flag. The default is no timeout.
115 -X proxy_protocol
117 Requests that nc should use the specified protocol when talking
118 to the proxy server. Supported protocols are “4” (SOCKS v.4),
119 “5” (SOCKS v.5) and “connect” (HTTPS proxy). If the protocol is
120 not specified, SOCKS version 5 is used.
121 -x proxy_address[:port]
123 Requests that nc should connect to destination using a proxy at
124 proxy_address and port. If port is not specified, the well-known
125 port for the proxy protocol is used (1080 for SOCKS, 3128 for
126 HTTPS).
127 -z Specifies that nc should just scan for listening daemons, without
129 sending any data to them. It is an error to use this option in
130 conjunction with the -l option.
131 destination can be a numerical IP address or a symbolic hostname (unless
133 the -n option is given). In general, a destination must be specified,
134 unless the -l option is given (in which case the local host is used).
135 For UNIX-domain sockets, a destination is required and is the socket path
136 to connect to (or listen on if the -l option is given).
137 port can be one or more comma-separated integers or ranges of ports.
139 Ranges are in the form nn-mm. In general, a destination port must be
140 specified, unless the -U option is given.
141
143 It is quite simple to build a very basic client/server model using nc.
144 On one console, start nc listening on a specific port for a connection.
145 For example:
146 $ nc -l 1234
148 nc is now listening on port 1234 for a connection. On a second console
150 (or a second machine), connect to the machine and port being listened on:
151 $ nc 127.0.0.1 1234
153 There should now be a connection between the ports. Anything typed at
155 the second console will be concatenated to the first, and vice-versa.
156 After the connection has been set up, nc does not really care which side
157 is being used as a ‘server’ and which side is being used as a ‘client’.
158 The connection may be terminated using an EOF (‘^D’).
159
161 The example in the previous section can be expanded to build a basic data
162 transfer model. Any information input into one end of the connection
163 will be output to the other end, and input and output can be easily cap‐
164 tured in order to emulate file transfer.
165 Start by using nc to listen on a specific port, with output captured into
167 a file:
168 $ nc -l 1234 > filename.out
170 Using a second machine, connect to the listening nc process, feeding it
172 the file which is to be transferred:
173 $ nc host.example.com 1234 < filename.in
175 After the file has been transferred, the connection will close automati‐
177 cally.
178
180 It is sometimes useful to talk to servers “by hand” rather than through a
181 user interface. It can aid in troubleshooting, when it might be neces‐
182 sary to verify what data a server is sending in response to commands
183 issued by the client. For example, to retrieve the home page of a web
184 site:
185 $ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
187 Note that this also displays the headers sent by the web server. They
189 can be filtered, using a tool such as sed(1), if necessary.
190 More complicated examples can be built up when the user knows the format
192 of requests required by the server. As another example, an email may be
193 submitted to an SMTP server using:
194 $ nc localhost 25 << EOF
196 HELO host.example.com
197 MAIL FROM:<user@host.example.com>
198 RCPT TO:<user2@host.example.com>
199 DATA
200 Body of email.
201 .
202 QUIT
203 EOF
204
206 It may be useful to know which ports are open and running services on a
207 target machine. The -z flag can be used to tell nc to report open ports,
208 rather than initiate a connection. For example:
209 $ nc -z host.example.com 20-30
211 Connection to host.example.com 22 port [tcp/ssh] succeeded!
212 Connection to host.example.com 25 port [tcp/smtp] succeeded!
213 The port range was specified to limit the search to ports 20 - 30.
215 Alternatively, it might be useful to know which server software is run‐
217 ning, and which versions. This information is often contained within the
218 greeting banners. In order to retrieve these, it is necessary to first
219 make a connection, and then break the connection when the banner has been
220 retrieved. This can be accomplished by specifying a small timeout with
221 the -w flag, or perhaps by issuing a "QUIT" command to the server:
222 $ echo "QUIT" | nc host.example.com 20-30
224 SSH-1.99-OpenSSH_3.6.1p2
225 Protocol mismatch.
226 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
227
229 Open a TCP connection to port 42 of host.example.com, using port 31337 as
230 the source port, with a timeout of 5 seconds:
231 $ nc -p 31337 -w 5 host.example.com 42
233 Open a UDP connection to port 53 of host.example.com:
235 $ nc -u host.example.com 53
237 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as
239 the IP for the local end of the connection:
240 $ nc -s 10.1.2.3 host.example.com 42
242 Create and listen on a UNIX-domain stream socket:
244 $ nc -lU /var/tmp/dsocket
246 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
248 port 8080. This example could also be used by ssh(1); see the
249 ProxyCommand directive in ssh_config(5) for more information.
250 $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
252 The same example again, this time enabling proxy authentication with
254 username “ruser” if the proxy requires it:
255 $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
257
259 cat(1), ssh(1)
260
262 Original implementation by *Hobbit* ⟨hobbit@avian.org⟩.
263 Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>.
264
266 UDP port scans will always succeed (i.e. report the port as open), ren‐
267 dering the -uz combination of flags relatively useless.
268BSD June 22, 2019 BSD