How can I find out if a port is available on linux

silverburgh.meryl · External Since: Jul 05, 2007 Posts: 1

Hi,

How can I find out if a port is available on linux?
One thing i can do is to open() and then close() a port number.
I wonder if there is a more efficient way?

Thank you.

mike · External Since: Jul 06, 2007 Posts: 5

On Jul 5, 12:42 pm, "silverburgh.me...@gmail.com"
<silverburgh.me....RemoveThis@gmail.com> wrote:
> Hi,
>
> How can I find out if a port is available on linux?
> One thing i can do is to open() and then close() a port number.
> I wonder if there is a more efficient way?
>
> Thank you.

Run: netstat -atnp|grep LISTEN

It will show you the ports already in use. A quick way to tell if a
tcp port is free is to just 'telnet localhost XXXX' where XXXX is the
port. Connection refused means (usually) nothing is listening there.

Josef Moellers · External Since: Jul 07, 2005 Posts: 389

silverburgh.meryl DeleteThis @gmail.com wrote:
> Hi,
>
> How can I find out if a port is available on linux?
> One thing i can do is to open() and then close() a port number.
> I wonder if there is a more efficient way?

What "port"? Network port? You need to (try to) bind() to it, if that
fails, the port is in use. There is no use checking netstat because you
then have a classical race condition (netstat says: port is not in use,
someone else grabs that port, your bind() attempt fails).
Or do you mean serial port? You could look into /sys/class/tty for
entries "ttyS[0-9]", but on my notebook, I have ttyS0 through ttyS3, yet
no serial port is available on the outside Wink

Another way would be to
check dmesg output for lines like these:
ttyS00 at 0x03f8 (irq = 4) is a 16550A
ttyS01 at 0x02f8 (irq = 3) is a 16550A
(that's from my desktop which really has two serial ports).

Josef
--
These are my personal views and not those of Fujitsu Siemens Computers!
Josef Möllers (Pinguinpfleger bei FSC)
If failure had no penalty success would not be a prize (T. Pratchett)
Company Details: http://www.fujitsu-siemens.com/imprint.html