pipe commands from a textfile to a telnet-server with netcat

nc $telnetserver 23 < $commandfile
sends commands specified in $commandfile to the telnet-server specified by $telnetserver. to have newlines in $commandfile interpreted as ENTER, save the file in CR+LF (aka "Windows-Textfile") format. if you want to save the output in a separate file, use: nc $telnetserver 23 < $commandfile > $resultfile

1
By: flokra
2009-08-07 21:32:38

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    Cowboy · 2009-08-14 17:01:57 0
  • This will keep trying to reconnect your netcat connection can be used with SSH or Telnet, just substitute nc for ssh or telnet very useful for troubleshooting VPNs were you want to send continuous packets towards a destination to trace.


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  • This command telnet and and looks for a line starting with "SSH" - works for OpenSSH since the SSH banner is something like "SSH-2.0-OpenSSH_6.0p1 Debian-4+deb7u3". Then it triggers an action accordingly. It can be packed as a script file to echo 0/1 indicating the SSH service availability: if [[ "$(sleep 1 | telnet -c <host> <port> 2>&1 | grep '^SSH')" == SSH* ]]; then echo 1; else echo 0; fi; Alternative uses: Trigger an action when server is UP (using &&): [[ "$(sleep 1 | telnet -c <host> <port> 2>&1 | grep '^SSH')" == SSH* ]] && <command when up> Trigger an action when server is DOWN (using ||): [[ "$(sleep 1 | telnet -c <host> <port> 2>&1 | grep '^SSH')" == SSH* ]] || <command when down>


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  • Ever needed to test firewalls but didn't have netcat, telnet or even FTP? Enter /dev/tcp, your new best friend. /dev/tcp/(hostname)/(port) is a bash builtin that bash can use to open connections to TCP and UDP ports. This one-liner opens a connection on a port to a server and lets you read and write to it from the terminal. How it works: First, exec sets up a redirect for /dev/tcp/$server/$port to file descriptor 5. Then, as per some excellent feedback from @flatcap, we launch a redirect from file descriptor 5 to STDOUT and send that to the background (which is what causes the PID to be printed when the commands are run), and then redirect STDIN to file descriptor 5 with the second cat. Finally, when the second cat dies (the connection is closed), we clean up the file descriptor with 'exec 5>&-'. It can be used to test FTP, HTTP, NTP, or can connect to netcat listening on a port (makes for a simple chat client!) Replace /tcp/ with /udp/ to use UDP instead.


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What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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