Detect port on remote machine on local network [SOLVED]
lazarus / 38-Networking and Web Programming

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new2linux · 2014-07-14 18:27

I have a function (below) to probe another machine on the local
network. The problem is that when the port is closed, it times out
after a (much too) long time. How to do better?

function IsPortOpen(
  ip: string; port: integer): boolean;
begin
  result := false;
  with {synapse}blcksocket.TTCPBlockSocket.create do try
    Connect(ip, IntToStr(port));
    result := LastError = 0;
  finally
    Free;
  end;
end;

*) TTCPBlockSocket is from the Synapse library.
**) Maybe this isn't a good way to probe for a port.

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marcov · 2014-07-15 09:15

Port scanners usually have two modes, the socket level (like you use)
for compatibility, but all you can do is wait and tweak the timeout of
the component.

The other mode uses raw socket access (SYN?) and thus usually requires
administrative permissions. On Windows it might even need a packet
filter driver installed. That is not readily prepared in TCP/IP socket
suites, so you will have to code your own

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rvk · 2014-07-15 10:03

Quote from: marcov on July 15, 2014, 11:15:04 am... but all you can do
is wait and tweak the timeout of the component.Yep. According to this
"Timeout for Connect is hardcoded in communication stack of your
operating system".

So using non-blocking mode would be a possible way to go.

But according this it should already be implemented in Synapse.
(at least in the SVN version, see here for the ConnectionTimeout
property in the connect-function).

Did you try the SVN-version?
In that case you could just add a "ConnectionTimeout := xxx;" just
before the Connect-call.

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new2linux · 2014-07-15 12:33

Two high quality replies, thank you!

My goal for today was to investigate non-blocking mode and otherwise
use a dozen of threads. I'm glad that was shot down.

Indeed I found the new Connect() code and it was already in Synapse.
My function becomes:


function IsPortOpen(
  ip: string; port: integer): boolean;
begin
  result := false;
  with {synapse}blcksocket.TTCPBlockSocket.create do try
    //-----------insert -----------
    ConnectionTimeout := 500;
    //-----------------------------
    Connect(ip, IntToStr(port));
    result := LastError = 0;
  finally
    Free;
  end;
end;

My app saved

*) Synapse versions? The most recent download says "Release 39,
2009-10-08", which doesn't inspire much confidence. They'd better do a
dummy release ; )

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BigChimp · 2014-07-15 12:50

Quote from: new2linux on July 15, 2014, 02:33:21 pm*) Synapse
versions? The most recent download says "Release 39, 2009-10-08",
which doesn't inspire much confidence. They'd better do a dummy
release ; )
Strange. Where did you see that download? Have a look at
1. The Synapse homepage:
http://synapse.ararat.cz/doku.php
Quote2012-04-24 New Synapse release 40!2. As mentioned, you can also
try the svn trunk version. In my (limited) experience very stable and
useful fixes added:
http://synapse.ararat.cz/doku.php/download

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new2linux · 2014-07-15 22:46

Quote from: BigChimp on July 15, 2014, 02:50:47 pmQuote from:
new2linux on July 15, 2014, 02:33:21 pm*) Synapse versions? The most
recent download says "Release 39, 2009-10-08", which doesn't inspire
much confidence. They'd better do a dummy release ; )
Strange. Where did you see that download? Have a look at
1. The Synapse homepage:
http://synapse.ararat.cz/doku.php
Quote2012-04-24 New Synapse release 40!2. As mentioned, you can also
try the svn trunk version. In my (limited) experience very stable and
useful fixes added:
http://synapse.ararat.cz/doku.php/download

Indeed. I searched to get the latest Synapse and found:

  http://sourceforge.net/projects/synalist/

Which is version 39 and does NOT have the fix.

It turns out I already had the fix in my libs because I downloaded the
AWS package by Marcos Douglas which included Synapse:

  https://github.com/mdbs99/aws

As you say, the latest Synapse is to be found at:

  http://synapse.ararat.cz/doku.php/download

What a mess.

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rvk · 2014-07-16 07:43

Stable release #40 from page
http://synapse.ararat.cz/doku.php/download
is from April 2014 which does not contain the ConnectionTimeout .

You'll need at least SVN version #160 (from June 2012) or higher for
that.

Yeah... the stable version is old...

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BigChimp · 2014-07-17 09:49

Quote from: new2linux on July 16, 2014, 12:46:23 amWhat a mess.
I'm sure a friendly note to Lukas (the maintainer) in the Synapse
mailing list may help to address this oversight. At least it's a
better chance than posting this on an unrelated forum