🖥️nc

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# To 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

# To open a UDP connection to port 53 of host.example.com:
nc -u host.example.com 53

# To 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

# To create and listen on a UNIX-domain stream socket:
nc -lU /var/tmp/dsocket

# To 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

# The same example again, this time enabling proxy authentication with username "ruser" if the proxy requires it:
nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

# To choose the source IP for the testing using the -s option
nc -zv -s source_IP target_IP Port

#==============================#
# CMD NC - netcat
#==============================##==============================#
nc -C imap.example.org 143

nc -zv localhost 80
# Check if port 80 is open or not. We can replace ‘80‘ with any other port number to check if it is opened or closed

nc -q1 -lvp 1234 < file.txt
# poor man his file serve. Use nc serverhost 1234 > output.txt to retrieve file from remote host. NAT bugs this.

nc -C localhost 80
    GET / HTTP/1.0
    Host: localhost

nc -C localhost 3128
    GET http://www.example.org/ HTTP/1.0
    Host: www.example.org

nc pop3.example.org 110
    +OK Dovecot ready.
    user $user
    +OK
    pass $pass
    +OK Logged in.
    list
    +OK 1

nc -C smtp 25
    220 smtp ESMTP Postfix (Debian/GNU)
    helo $myhostname
    250 smtp
    mail from: $sender
    250 2.1.0 Ok
    rcpt to: $recipient
    250 2.1.5 Ok
    data
    354 End data with <CR><LF>.<CR><LF>
    Subject: Test

    Nicht mehr als ein Test.
    .
    250 2.0.0 Ok: queued as 82C3D440A5
    quit
    221 2.0.0 Bye

### Portscanning
nc -z -v smtp.example.net 22-25

netcat -l 1234 > file.out
#

netcat host.example.net 1234 < file.in
#

netcat -C mail.example.net 25 <<EOT
HELO host.example.net
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Testmail
Body of email.
.
QUIT
EOT

nc -q1 -lvp 1234 < file.txt 
# poor man's file serve. Use nc serverhost 1234 > output.txt to retrieve file from remote host. NAT bugs this.

#-----------------------------------------#
# Bash/nc: netcat as a simple telnet server
#-------------------------------------------------------------------------------------#

## CLIENT/SERVER MODEL
# It is quite simple to build a very basic client/server model using nc.  On one console, start nc listening on a specific port for a connection.  For example:

nc -l 1234
#  nc is now listening on port 1234 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 1234
# 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’).

# There is no -c or -e option in this netcat, but you still can execute a command after connection being established by redirecting file descriptors. Be cautious here because opening a port and let anyone connected execute arbitrary command on your site isDANGEROUS. If you really need to do this, here is an example:

## -> On ‘server’ side:
rm -f /tmp/f; mkfifo /tmp/f
cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

## -> On ‘client’ side:
nc host.example.com 1234
$ (shell prompt from host.example.com)

# By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side.

# When connection is terminated, nc quits as well. Use -k if you want it keep listening, but if the command quits this option won't restart it or keep nc running. Also don't forget to remove the file descriptor once you do not need it anymore:
rm -f /tmp/f
#-------------------------------------------------------------------------------------#

#---------------------------------------#
# Bash/nc: netcat as a simple ftp server
#-------------------------------------------------------------------------------------#

## 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.
#-------------------------------------------------------------------------------------#

#----------------------------------------#
# Bash/nc: netcat as a simple port scanner
#-------------------------------------------------------------------------------------#

## 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. Usually it's useful to turn on verbose output to stderr by use this option in conjunction with -v option.

# For example:
nc -zv 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, and is scanned by increasing order.

# You can also specify a list of ports to scan, for example:
nc -zv host.example.com 80 20 22
           nc: connect to host.example.com 80 (tcp) failed: Connection refused
           nc: connect to host.example.com 20 (tcp) failed: Connection refused
           Connection to host.example.com port [tcp/ssh] succeeded!
	   # The ports are scanned by the order you given.

# 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
#-------------------------------------------------------------------------------------#

#--------------------------------------------#
# Bash/nc: netcat as a simple http/smtp client 
#-------------------------------------------------------------------------------------#

## 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:
printf "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(1), 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 [-C] localhost 25 << EOF
           HELO host.example.com
           MAIL FROM:<[email protected]>
           RCPT TO:<[email protected]>
           DATA
           Body of email.
           .
           QUIT
           EOF
#-------------------------------------------------------------------------------------#
	   
	   
	   

nc -l 8762 < video.mpg  Client: nc server 8762 | mplayer -cache 1000 -
# Server: Poor man his video streaming service at video bitrate.

nc -zv 192.168.1.15 22
# Using netcat, you can check if a single or multiple or a range of open ports as follows. The command above will help us see if the port 22 is open on the host 192.168.56.10
# In the command above, the flag:
#    -z – sets nc to simply scan for listening daemons, without actually sending any data to them.
#    -v – enables verbose mode.

nc -zv 192.168.56.10 80 22 21
# The next command will check if ports 80, 22 and 21 are open on the remote host 192.168.5.10 (we can use the hostname as well)

nc -zv 192.168.56.10 20-80
# It is also possible to specify a range of ports to be scanned

nc -k -l 4444 
# Starts a WebSocket server and prints all requests to stdout 

nc -u -p 123 myrtle 56789 <<<"turtle" 
# Test a firewall rule on host 'myrtle' that is configured for traffic from udp source port 123.

# Fast portscanner via xargs
xargs -i -P 1200 nc -zvn {} 22 < textfile-with-hosts.txt

# Fast portscanner via Parallel
parallel -j200% -n1 -a textfile-with-hosts.txt nc -vz {} ::: 22

#==============================##==============================#
# CMD NC - netcat
#==============================##==============================#

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