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 daem

Answer 1

I tried this on my Mac running 10.8.4 and after about 6 minutes it was looking like:

andys-MacBook-Pro:EquipDB uw$ for i in {183..200}; do echo "hello" | nc -n -w 2 -v 10.120.113.$i 22; done
nc: connect to 10.120.113.183 port 22 (tcp) failed: Operation timed out
nc: connect to 10.120.113.184 port 22 (tcp) failed: Operation timed out
nc: connect to 10.120.113.185 port 22 (tcp) failed: Operation timed out
nc: connect to 10.120.113.185 port 22 (tcp) failed: Operation timed out

So it is timing out on mine, but just takes quite a while.

Hmmm... was thinking my test would be pointless because I will never actually connect to something since I'm not on your network. But I did see this:

-w # => Timeout after # seconds

Note that -w also sets the network inactivity timeout. This does not have any effect until standard input closes, but then if nothing further arrives from the network in the next seconds, netcat tries to read the net once more for good measure, and then closes and exits. There are a lot of network services now that accept a small amount of input and return a large amount of output, such as Gopher and Web servers, which is the main reason netcat was written to ``block'' on the network staying open rather than standard input. Handling the timeout this way gives uniform behavior with network servers that don't close by themselves until told to.

This works for final net reads but not for connections.

If I'm understanding this right it only times out once it connects, since mine never connects it just has a built in timeout that has nothing to do with -w? found here, helpful?

Answer 2

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