Read and write data across networks – arbitrary TCP and UDP connections and listens.
Syntax nc [-46AcCdDhklnrStuUvz] [-b boundif] [-i interval] [-p source_port] [-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]] [hostname] [port[s]] Options -4 Use IPv4 addresses only. -6 Use IPv6 addresses only. -A Set SO_RECV_ANYIF on socket. ** -b boundif The interface to bind the socket to. -c Send CRLF as line-ending. ** -C Force nc not to use cellular data context. ** -d Do not attempt to read from stdin. This will prevent any text being sent from a client to the server. -D Enable debugging on the socket. -h Print help. -i interval Specifies a delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports. -G conntimeout TCP connection timeout in seconds. -H keepidle Initial TCP keep alive timeout in seconds. -I keepintvl Interval for repeating TCP keep alive timeouts in seconds. -J keepcnt Number of times to repeat TCP keep alive packets. -k Force 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 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 Number of probes to send to the peer before declaring that the peer is not reachable and generating an adpative timeout read/write event. -n Skip all DNS or service lookups on addresses, hostnames or ports. -p source_port Specify 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 Select the source and/or destination ports randomly instead of sequentially within a range or in the order that the system assigns them. -s source_ip_address 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. -S Enables the RFC 2385 TCP MD5 signature option. ** -t 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. -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'. ** -U Use Unix Domain Sockets. -u Use UDP instead of the default option of TCP. -v 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 con- nection, 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] 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 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.
** = These options are not available in all versions of Netcat, check your local man pages.
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).
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:
- Simple TCP proxies
- Shell-script based HTTP clients and servers
- Network daemon testing
- A SOCKS or HTTP ProxyCommand for ssh(1)
Client / Server Model
Netcat can perform basic client/server functions that allow ‘chat’ between machines.
On one console, start nc listening on a specific port for a connection:
# On a computer A with IP 10.10.10.10 we listen on port 64
$ nc -l -p 64
nc is now listening on port 64 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 64
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 64 > 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 64 < filename.in
$ cat filename.in | nc host.example.com -p 64
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: $ 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, 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: <email@example.com> RCPT TO: <firstname.lastname@example.org> 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
UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.
For more advanced port scanning, use nmap.
If you need to, netcat can be installed using most package managers:
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; see the ProxyCommand directive in ssh_config(5) for more information.
$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
“The more connections you and your lover make, not just between your bodies, but between your minds, your hearts, and your souls, the more you will strengthen the fabric of your relationship, and the more real moments you will experience together” ~ Barbara De Angelis
Related linux commands:
cat – Concatenate and print (display) the content of files
ncat – Read and write data across networks (a rewrite of netcat as part of the larger nmap toolset)
ssh – OpenSSH SSH client (remote login program)