nc - netcat

establish arbitrary TCP and UDP connections and listens

nc [-46DCdhklnrtUuvz] [-b if]⌘ [-s source_ip_address][-p source_port] [-i interval] [-w timeout]
[-X proxy_protocol] [-x proxy_address[:port]]
          [host] [port[s]]

open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6.
scripts nicely, and separates error messages to standard error

    Common uses include:
  1. simple TCP proxies
  2. shell-script based HTTP clients and servers
  3. network daemon testing
  4. a SOCKS or HTTP ProxyCommand for ssh(1)
-l listen for an incoming connection rather than initiate a connection to a remote host.

Do not use with -z, -s, -p or -w

-k keep listening for another connection after its current connection is completed. used with -l
-z scan for listening daemons, without sending any data to them.
-s source_ipsource IP of the interface
-p source_port source port.
-w seconds If a connection and stdin are idle for more than timeout seconds, the connection is closed.
default is no timeout.
-d Do not to read from stdin.
-i interval delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports.
-t send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. to script telnet sessions.
-n no DNS host or service lookups
-U use Unix Domain Sockets.
-X 4|5|connect protocol when talking to the proxy server.
SOCKS v.4, connect(HTTPS proxy) SOCKS v.5(default).
-x proxy_address[:port] connect 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).
-b if interface to bind the socket to.
-4 IPv4 addresses only.
-6 IPv6 addresses only.
-r randomly choose source and/or destination ports instead of sequentially within a range or in the order that the system assigns them.
-D Enable debugging on the socket.
-C Do not to use cellular data context.
-u UDP otherwise default TCP.
-v verbose
-h help.
host a numerical IP address or a symbolic hostname (except with -n ).
A host must be specified, unless -l (listen) is used, in which case the local host is used).
port[s] single integers or ranges. Ranges are in the form nn-mm.
Destination port must be specified, except with -U when a socket must be specified.

Simple syslog example:

 > nc -u -v dalogger.dyndns.org 514
found 0 associations
found 1 connections:
     1: flags=82<CONNECTED,PREFERRED>
    outif (null)
    src 192.168.1.10 port 52278
    dst 74.105.211.198 port 514
    rank info not available

Connection to dalogger.dyndns.org port 514 [udp/syslog] succeeded!
<8>testing from nc
produces 
2017-01-17T17:07:44.534168-05:00 testing from nc
dapie doesn't seem to recognize the timestamp (he adds his own) and a null message creates entry with rtr as host!
colon's are in the original message (i.e. not part of the format)

< facility*8+level> doesn't seem to be required but is used for destination file

Client/Serve Model

On one console, start nc listening on a specific port (1234) for a connection.
For example: 
$ nc -l 1234
On a second console (or a second machine), connect to the machine and port being listened on: 
$ nc 127.0.0.1 1234
There is now 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 there is no concept of `server' or `client'. The connection is terminated with an EOF (`^D').

Data Transfer

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.

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
This displays the headers sent by the web server. They can be filtered, using sed(1).

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: 
           RCPT TO: 
           DATA
           Body of email.
           .
           QUIT
           EOF

port scanning

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

Examples

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

See

cat(1), ssh(1) CAVEATS UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.