[PD] TCP weirdness ? ...

Roman Haefeli reduzent at gmail.com
Wed Jan 30 09:02:26 CET 2019

On Wed, 2019-01-30 at 01:02 +0100, Christof Ressi wrote:
> > situation and try to avoid to connect to ports, that aren't open ;-
> > )
> or ping them first in the command line :-D

ping doesn't help much with testing for open ports. nc (netcat) can be
used, but it behaves the same: If it receives no response, it waits for
TIMEOUT seconds.

BTW: It depends on the other end's firewall whether a connection
attempt fails immediately or only after a while. Firewalls can be
configured to REJECT connections to closed ports, which means a
notification (ICMP message, probably, I'm not sure) is sent to the
initiating party, or the can be configured to DROP connections to
closed ports -> no notification is sent. Without notification, the
initiating party cannot know whether the receiving end is still
processing the request or if it was silently dropped. That's why the
default timeouts are so large.

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