Network - netcat (nc, ncat)

netcat ¹⁾ is a command line tool that reads and writes data across network connections, using TCP or UDP protocol.

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

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

• 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

# 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";
}

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

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

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

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

tar -cz . | nc -l -p $PORT
# l for listen
# p to define the port

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

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

After a connection is established, 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:

• Linux

nc host port

• Windows

ncat host port

Netcat can also function as a server, by listening for inbound connections on arbitrary ports

• Linux

nc -l -p port

• Telnet has the “standard input EOF” problem. ie it will shut everything down and exit after an end-of-file on the standard input.
• 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.

• install nmap on Windows

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