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* Make a FIFO file named replypipe
* listen on 1234
* pass the request to unix socket
* unix socket will reply to replypipe
* replypipe will write reply to the client
A TCP server that keeps the same socket open, sending the contents of "file" repeatedly.
Using netcat (nc)
25 can be replaced with the friendly value (smtp). Check error code for result or use -v option to echo output
nc -z localhost smtp && echo open || echo closed
nc -zv host protocol
netstat will list all open ports on the system, unix sockets, tcp sockets and udp sockets. the t flag limits to tcp ports the l flag limits to listening ports and the n flag disables the translation of port to service ( ie :25 displayed instead of :smtp ). then grep for the port you are interested in preceeded by a colon.
Check trough unix sockets if tcp port is open or close
Simple way to test if a port is available to the public. Run this command on the "server" and run a `telnet host-ip port-number` on the client. Test by sending strings to the server, which will be displayed in the server terminal.
you can use a pair of commands to test firewalls.
1st launch this command at destination machine
ncat -l [-u] [port] | cat
then use this command at source machine to test remote port
echo foo | ncat [-u] [ip address] [port]
First command will listen at specified port.
It will listen TCP. If you use -u option will listen UDP.
Second command will send "foo" through ncat and will reach defined IP and port.
Very handy way to perform a host scan if you don't have nmap,ncat,nc ...or other tools installed locally.
When executing a command on a /dev/tcp/$host/$port pseudo-device file, Bash opens a TCP connection to the associated socket and UDP connection when using /dev/udp/$host/$port.A simlpe way to get servers banner is to run this command "cat < /dev/tcp/localhost/25" , here you will get mail server's banner.
NOTE: Bash, as packaged for Debian, does not support using the /dev/tcp and /dev/udp pseudo-device it's not enabled by default Because bash in Debian is compiled with ?disable-net-redirections.
on multihomed hosts, connected to several networks, could be usefull to know the source address (local ip address) used to reach the target host, this command does not require root priviledges.
The command use a TCP socket, if there is any error the command return an empty string, elsewhere return a valid ip address.
Using netcat, usuallly installed on debian/ubuntu.
Also to test against a sample server the following two commands may help
echo got milk? | netcat -l -p 25
python -c "import SocketServer; SocketServer.BaseRequestHandler.handle = lambda self: self.request.send('got milk?\n'); SocketServer.TCPServer(('0.0.0.0', 25), SocketServer.BaseRequestHandler).serve_forever()"
this command will send a message to the socket 25 on host 192.168.1.2 in tcp.
works on udp and icmp
understand only IP address, not hostname.
on the other side (192.168.1.2), you can listen to this socket and test if you receive the message.
easy to diagnose a firewall problem or not.