Network - netcat (nc, ncat)

1 - About

netcat is command line tool that reads and writes data across network connections, using TCP or UDP protocol.

  • You can see it as the equivalent of telnet
  • It emprunts its name from the cat command

3 - Example

3.1 - Testing a port

An utility function to control that the service is up by controlling that we can make a tcp connection to the service port

3.1.1 - Simple if statement


TOMCAT_PORT=6006
if [[ $(nc -z localhost ${TOMCAT_PORT}) -eq 0 ]]; then
	echo "Tomcat is up"
else
	echo "Tomcat is shutdown"
fi

3.1.2 - While Statement

  • while <note important>A port may be not open but still locked by a process. See the second statement</note>

while [[ $(nc -z localhost ${TOMCAT_PORT}) -eq 1 ]]; do
        echo "Waiting for the tomcat port ${INFA_TOMCAT_PORT} to be closed before starting"
	sleep 5 # wait 5 second before check again
done
###############################################################
# It seems that a port may returns 0 even if a process is still bound to a port
###############################################################
while [[ ! $(netstat -t|grep ${TOMCAT_PORT} | wc -l) -eq 0 ]]; 
do
	echo "Waiting for the tomcat port ${INFA_TOMCAT_PORT} to be closed before starting"
	sleep 5 # wait 5 second before check again
done

3.1.3 - Wait function with timeout


# usage
# wait_for_service service_name port
wait_for_service() {
  local SERVICE_NAME=$1
  local PORT=$2
  SERVICE_WAIT_TIMEOUT_SEC=20
  echo "Waiting for $SERVICE_NAME to start..."
  local CURRENT_WAIT_TIME=0

  while [[ $(echo | nc -w1 localhost $PORT >/dev/null 2>&1 ;echo $?) -ne 0 ]]; do
      printf '.'
      sleep 1
      if [ $((++CURRENT_WAIT_TIME)) -eq $SERVICE_WAIT_TIMEOUT_SEC ]; then
        printf "\nError: timed out while waiting for $SERVICE_NAME to start.\n"
        exit 1
      fi
  done
  printf '\n'
  echo "$SERVICE_NAME has started";
}

3.1.4 - Wait for it

For a full functional example, see wait-for-it.sh

3.2 - Send a HTTP request to a process

After having created a tcp connection, an application:

  • send a standard input (on a io level) to the host via the connection (ie a request)
  • receive the result as a standard output.

As netcat creates a tcp connection, you can for instance send HTTP get request. Example:


GET / HTTP/1.1
Host: gerardnico.com

we can send it as standard input with echo to a connection created with netcat

For Https, you need to use another utility such as openssl because netcat does not support it.

echo -e "GET / HTTP/1.1\nHost: gerardnico.com\n" | nc gerardnico.com 80

and you should get a redirection response telling you that you need to go to https://gerardnico.com/


HTTP/1.1 301 Moved Permanently
Date: Mon, 13 Apr 2020 12:26:26 GMT
Transfer-Encoding: chunked
Connection: keep-alive
Cache-Control: max-age=3600
Expires: Mon, 13 Apr 2020 13:26:26 GMT
Location: https://gerardnico.com/
Server: cloudflare
CF-RAY: 58352ceb1ba9bf78-AMS

3.3 - Send a directory between two computers

  • On the source server, pipe the result of a tar (archive) to a connection created witn nc

tar -cz . | nc -l -p $PORT

  • On the remote server, read the connection and untar.

nc -w 10 $REMOTE_HOST $PORT | tar -xz

4 - Syntax

After having created a connection, the standard input is sent to the host, and anything that comes back across the connection is sent to your standard output.

To create a TCP connection:

4.1 - Linux


nc host port

4.2 - Windows


ncat host port

5 - Why not telnet

  • Telnet has the “standard input EOF” problem
  • Telnet will not transfer arbitrary binary data, because certain characters are interpreted as telnet options
  • Telnet also emits some of its diagnostic messages to standard output, where netcat keeps such things separated from its *output*
  • Telnet is incapable of listening for inbound connections, or using UDP instead.

6 - Installation

6.1 - Windows

  • install nmap on Windows

6.2 - Linux


apt-get install -y netcat
# centos, Redhat
yum install nmap


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