nc (netcat) on Mac OS X 10.8.4 gets stuck

Question

I encountered a little issue while using the nc utility on Mac OS X, a utility i often use as a quick and dirty solution to check if a port is open and what version the daemon is running.

We deployed a new set of computers the other day and i wanted to verify what version of sshd they were running, without having to leave my chair.

This is the command i ran and the resulting output:

$ for i in {183..200}; do echo "hello" | nc -n -w 2 -v 10.120.113.$i 22; done
Connection to 10.120.113.183 22 port [tcp/*] succeeded!
SSH-2.0-OpenSSH_5.9
Protocol mismatch.
nc: connect to 10.120.113.184 port 22 (tcp) failed: Connection refused
^C
$

It finds the first machine on 183 and returns the daemon version, it dont look like the sshd is running on 184, but when it hits 185 it simply stops and i have to kill it with ctrl+c.

As i have understood the man page for nc it should time out when using the '-w' switch, but it don't. Its the same issue from multiple machines.

Is this simply the case of me misunderstanding the man page? Is there any other way to make nc time out after X seconds if don´t receive any response? Is there any other way to do this using the builtin tools in Mac OS X?

I´ve also tried running nc with only the '-z' switch with the same results. The machines are placed in our production so i´m not allowed to install any 3rd party applications like nmap.

Platform: Mac OS X 10.8.4
Executable: /usr/bin/nc

Sorry if this question has been answered, i searched but could not find any solution to this.

Answer 1

I believe you are looking for the -G option. From the man pages:

-G conntimeout TCP connection timeout in seconds.

-w is used to set the timeout after connection. -G option is used to set the timeout before connection. This should give you what you want

nc -n -G 2 -v xxx.xxx.xxx.xxx 22