Question

I am using netcat utility on linux to receive outputs from a program on a windows machine. My problem being that the program on the windows machine does not always give an output.

How can i check that either a connection has been made to netcat ?

What i am doing till now is “nc -l -v 9103 > output” then i check the size of output, the problem this poses is that netcat only write to a file after a certain buffer size has been reached or a new line char is encountered, so some cases evne though a connection has been made the file size is detected as zero.

How can i check if someone has made a connection with netcat.

I tried using

nc -l -v -e someprog.exe 9103 > output

but my netcat doesnt seem to support this

below are the options i have

    $ nc -h
usage: nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]
          [-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_version]
          [-x proxy_address[:port]] [hostname] [port[s]]
        Command Summary:
                -4              Use IPv4
                -6              Use IPv6
                -D              Enable the debug socket option
                -d              Detach from stdin
                -h              This help text
                -i secs         Delay interval for lines sent, ports scanned
                -k              Keep inbound sockets open for multiple connects
                -l              Listen mode, for inbound connects
                -n              Suppress name/port resolutions
                -p port         Specify local port for remote connects
                -r              Randomize remote ports
                -s addr         Local source address
                -T ToS          Set IP Type of Service
                -C              Send CRLF as line-ending
                -t              Answer TELNET negotiation
                -U              Use UNIX domain socket
                -u              UDP mode
                -v              Verbose
                -w secs         Timeout for connects and final net reads
                -X proto        Proxy protocol: "4", "5" (SOCKS) or "connect"
                -x addr[:port]  Specify proxy address and port
                -z              Zero-I/O mode [used for scanning]
        Port numbers can be individual or ranges: lo-hi [inclusive]

Answer 1

verbose mode will write connectivity to stderr, and you can redirect stderr to a file, the verbose log has something like
connect to [xxx] from [xxxx]

try

nc -l -v -p 9103 -k 1> output 2>connect.log

and monitor connect.log for connectivity

if you don’t use -k , netcat quits after 1st connection.