1NC(1) BSD General Commands Manual NC(1)
2
4 nc — arbitrary TCP and UDP connections and listens
5
7 nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]
8 [-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_protocol] [-x
9 proxy_address[:port]] [hostname] [port[s]]
10
12 The nc (or netcat) utility is used for just about anything under the sun
13 involving TCP or UDP. It can open TCP connections, send UDP packets,
14 listen on arbitrary TCP and UDP ports, do port scanning, and deal with
15 both IPv4 and IPv6. Unlike telnet(1), nc scripts nicely, and separates
16 error messages onto standard error instead of sending them to standard
17 output, as telnet(1) does with some.
18 Common uses include:
20 · simple TCP proxies
22 · shell-script based HTTP clients and servers
23 · network daemon testing
24 · a SOCKS or HTTP ProxyCommand for ssh(1)
25 · and much, much more
26 The options are as follows:
28 -4 Forces nc to use IPv4 addresses only.
30 -6 Forces nc to use IPv6 addresses only.
32 -D Enable debugging on the socket.
34 -d Do not attempt to read from stdin.
36 -h Prints out nc help.
38 -i interval
40 Specifies a delay time interval between lines of text sent and
41 received. Also causes a delay time between connections to multi‐
42 ple ports.
43 -k Forces nc to stay listening for another connection after its cur‐
45 rent connection is completed. It is an error to use this option
46 without the -l option.
47 -l Used to specify that nc should listen for an incoming connection
49 rather than initiate a connection to a remote host. It is an
50 error to use this option in conjunction with the -p, -s, or -z
51 options. Additionally, any timeouts specified with the -w option
52 are ignored.
53 -n Do not do any DNS or service lookups on any specified addresses,
55 hostnames or ports.
56 -p source_port
58 Specifies the source port nc should use, subject to privilege
59 restrictions and availability. It is an error to use this option
60 in conjunction with the -l option.
61 -r Specifies that source and/or destination ports should be chosen
63 randomly instead of sequentially within a range or in the order
64 that the system assigns them.
65 -S Enables the RFC 2385 TCP MD5 signature option.
67 -s source_ip_address
69 Specifies the IP of the interface which is used to send the pack‐
70 ets. It is an error to use this option in conjunction with the
71 -l option.
72 -T ToS Specifies IP Type of Service (ToS) for the connection. Valid
74 values are the tokens “lowdelay”, “throughput”, “reliability”, or
75 an 8-bit hexadecimal value preceded by “0x”.
76 -C Send CRLF as line-ending
78 -t Causes nc to send RFC 854 DON'T and WON'T responses to RFC 854 DO
80 and WILL requests. This makes it possible to use nc to script
81 telnet sessions.
82 -U Specifies to use Unix Domain Sockets.
84 -u Use UDP instead of the default option of TCP.
86 -v Have nc give more verbose output.
88 -w timeout
90 If a connection and stdin are idle for more than timeout seconds,
91 then the connection is silently closed. The -w flag has no
92 effect on the -l option, i.e. nc will listen forever for a con‐
93 nection, with or without the -w flag. The default is no timeout.
94 -X proxy_version
96 Requests that nc should use the specified protocol when talking
97 to the proxy server. Supported protocols are “4” (SOCKS v.4),
98 “5” (SOCKS v.5) and “connect” (HTTPS proxy). If the protocol is
99 not specified, SOCKS version 5 is used.
100 -x proxy_address[:port]
102 Requests that nc should connect to hostname using a proxy at
103 proxy_address and port. If port is not specified, the well-known
104 port for the proxy protocol is used (1080 for SOCKS, 3128 for
105 HTTPS).
106 -z Specifies that nc should just scan for listening daemons, without
108 sending any data to them. It is an error to use this option in
109 conjunction with the -l option.
110 hostname can be a numerical IP address or a symbolic hostname (unless the
112 -n option is given). In general, a hostname must be specified, unless
113 the -l option is given (in which case the local host is used).
114 port[s] can be single integers or ranges. Ranges are in the form nn-mm.
116 In general, a destination port must be specified, unless the -U option is
117 given (in which case a socket must be specified).
118
120 It is quite simple to build a very basic client/server model using nc.
121 On one console, start nc listening on a specific port for a connection.
122 For example:
123 $ nc -l 1234
125 nc is now listening on port 1234 for a connection. On a second console
127 (or a second machine), connect to the machine and port being listened on:
128 $ nc 127.0.0.1 1234
130 There should now be a connection between the ports. Anything typed at
132 the second console will be concatenated to the first, and vice-versa.
133 After the connection has been set up, nc does not really care which side
134 is being used as a ‘server’ and which side is being used as a ‘client’.
135 The connection may be terminated using an EOF (‘^D’).
136
138 The example in the previous section can be expanded to build a basic data
139 transfer model. Any information input into one end of the connection
140 will be output to the other end, and input and output can be easily cap‐
141 tured in order to emulate file transfer.
142 Start by using nc to listen on a specific port, with output captured into
144 a file:
145 $ nc -l 1234 > filename.out
147 Using a second machine, connect to the listening nc process, feeding it
149 the file which is to be transferred:
150 $ nc host.example.com 1234 < filename.in
152 After the file has been transferred, the connection will close automati‐
154 cally.
155
157 It is sometimes useful to talk to servers “by hand” rather than through a
158 user interface. It can aid in troubleshooting, when it might be neces‐
159 sary to verify what data a server is sending in response to commands
160 issued by the client. For example, to retrieve the home page of a web
161 site:
162 $ echo -n "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
164 Note that this also displays the headers sent by the web server. They
166 can be filtered, using a tool such as sed(1), if necessary.
167 More complicated examples can be built up when the user knows the format
169 of requests required by the server. As another example, an email may be
170 submitted to an SMTP server using:
171 $ nc [-C] localhost 25 << EOF
173 HELO host.example.com
174 MAIL FROM: <user@host.example.com>
175 RCPT TO: <user2@host.example.com>
176 DATA
177 Body of email.
178 .
179 QUIT
180 EOF
181
183 It may be useful to know which ports are open and running services on a
184 target machine. The -z flag can be used to tell nc to report open ports,
185 rather than initiate a connection. For example:
186 $ nc -z host.example.com 20-30
188 Connection to host.example.com 22 port [tcp/ssh] succeeded!
189 Connection to host.example.com 25 port [tcp/smtp] succeeded!
190 The port range was specified to limit the search to ports 20 - 30.
192 Alternatively, it might be useful to know which server software is run‐
194 ning, and which versions. This information is often contained within the
195 greeting banners. In order to retrieve these, it is necessary to first
196 make a connection, and then break the connection when the banner has been
197 retrieved. This can be accomplished by specifying a small timeout with
198 the -w flag, or perhaps by issuing a "QUIT" command to the server:
199 $ echo "QUIT" | nc host.example.com 20-30
201 SSH-1.99-OpenSSH_3.6.1p2
202 Protocol mismatch.
203 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
204
206 Open a TCP connection to port 42 of host.example.com, using port 31337 as
207 the source port, with a timeout of 5 seconds:
208 $ nc -p 31337 -w 5 host.example.com 42
210 Open a UDP connection to port 53 of host.example.com:
212 $ nc -u host.example.com 53
214 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as
216 the IP for the local end of the connection:
217 $ nc -s 10.1.2.3 host.example.com 42
219 Create and listen on a Unix Domain Socket:
221 $ nc -lU /var/tmp/dsocket
223 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
225 port 8080. This example could also be used by ssh(1); see the
226 ProxyCommand directive in ssh_config(5) for more information.
227 $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
229
231 cat(1), ssh(1)
232
234 Original implementation by *Hobbit* ⟨hobbit@avian.org⟩.
235 Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>.
236
238 UDP port scans will always succeed (i.e. report the port as open), ren‐
239 dering the -uz combination of flags relatively useless.
240BSD August 22, 2006 BSD