Over a million developers have joined DZone.

Telnet/netcat: Waiting for a Port to be Open

· DevOps Zone

Get complete visibility into and control of deployments across all your teams with GoCD - an open source continuous delivery tool. 

On Friday Nathan and I were setting up a new virtual machine and we needed a firewall rule to be created to allow us to connect to another machine which had some JAR files we wanted to download.

We wanted to know when it had been done by one of our operations team and I initially thought we might be able to do that using telnet:

$ telnet 8081
telnet: connect to address Operation timed out
telnet: Unable to connect to remote host

We wanted to put a watch on the command so that it would be repeated every few seconds and indicate when we’d could connect to the port. However, as as far as I can tell there’s no way to reduce the length of the telnet timeout so Nathan suggested using netcat instead.

We ended up with the following command…

$ nc -v -w 1 8081
nc: connect to port 8081 (tcp) failed: Connection refused

…which we can then wire up with watch like so:

$ watch "nc -v -w 1 8081"
Every 2.0s: nc -v -w 1 8081                         Sun Jan 20 15:48:05 2013
nc: connect to port 8081 (tcp) timed out: Operation now in progress

And then when it works:

Every 2.0s: nc -v -w 1 8081                         Sun Jan 20 15:49:53 2013
Connection to 8081 port [tcp] succeeded!

Gain visibility into deployments across your organization while giving teams the power to model and visualize complex workflows with GoCD - an open source continuous delivery tool. 


Published at DZone with permission of Mark Needham, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

The best of DZone straight to your inbox.

Please provide a valid email address.

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}