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Full Discussion: Process running but not listening
Operating Systems AIX Process running but not listening Post 302704731 by abohmeed on Sunday 23rd of September 2012 12:53:40 AM
Old 09-23-2012
Quote:
Originally Posted by bakunin
Such a thing is impossible by design of the IP-stack. Let me elaborate:

First, what is a port: a port is an address on layer 4 of the network stack - just like the IP-address is an address on layer 3 (and the MAC address on layer 2). Other than the IP address it identifies a service rather than a host. There are 2 bytes identifying the port and therefore there are 65535 possible ports. Think of it like appartments to a house: the IP address identifies the house itself, the port number identifies the appartment. if you ring at different doors, different people will open, even if you are still in the same house. Some might not open at all, because the apartment is empty.

Communication now works in this way: behind any port(-number) a daemon can listen and offer some service - or not. If a daemon indeed listens it will pick up what comes its way and somehow react - by answering, by doing something, establishing a connection, whatever. Some ports, especially the ones up to # 1023, are for offering standard services: telnet, ftp, http, ssh, lpd, .... and so on. These are called "well known services" and only root can start a process using these ports. Usually a symbolic name is defined for these ports - see "/etc/services" - but this is not necessary.

You can even try this with a printer: if you have a network printer it sure runs a "lpd" daemon. This listens at port 515 per default. Using your telnet client (it allows to configure the port used) try:

Code:
telnet printer.yournetwork.com 515

Of course you will not get any telnet connection - the lpd speaks lpd and not telnet - but you will probably get some banner, identifying the printer. Something like "HP JetDirect Line Printer daemon v1.23 .....", maybe listing some EPROM software revision or so before it closes the connection.

If no daemon is listening, there is simply no answer. If you attempt to initiate a communication with a remote port where no daemon listens you get a "connection refused" answer usually, but this comes from your system, not the remote one - the remote one is simply not saying anything at all.

You see, there is no "closed" ports because ports can neither be closed nor opened. They are just listened at - or not. The daemon listening itself can of course terminate a certain connection and send your system the message "connection closed" - but this does not "close" any port, it just terminates the status that to a certain port someone is listening - if using some specific port was part of the session setup. Some daemons are contacted at a common port, then set up connections at different ports (above 1024) and listen again at the common port for new sessions.

I hope this helps.

bakunin
Thanks kindly for your time and effort and i really appreciate your elaboration,
 

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LEARN ABOUT CENTOS

cups-lpd

cups-lpd(8)							    Apple Inc.							       cups-lpd(8)

NAME

       cups-lpd - receive print jobs and report printer status to lpd clients

SYNOPSIS

       cups-lpd [ -h hostname[:port] ] [ -n ] [ -o option=value ]

DESCRIPTION

       cups-lpd  is  the CUPS Line Printer Daemon ("LPD") mini-server that supports legacy client systems that use the LPD protocol. cups-lpd does
       not act as a standalone network daemon but instead operates as a socket-activatable systemd(1) service.

OPTIONS

       -h hostname[:port]
	    Sets the CUPS server (and port) to use.

       -n
	    Disables reverse address lookups; normally cups-lpd will try to discover the hostname of the client via a reverse DNS lookup.

       -o name=value
	    Inserts options for all print queues. Most often this is used to disable the "l" filter so that remote  print  jobs  are  filtered	as
	    needed  for printing; the examples in the previous section set the "document-format" option to "application/octet-stream" which forces
	    autodetection of the print file format.

PERFORMANCE

       cups-lpd performs well with small numbers of clients and printers. However, since a new process is created for each  connection	and  since
       each  process  must  query  the printing system before each job submission, it does not scale to larger configurations. We highly recommend
       that large configurations use the native IPP support provided by CUPS instead.

SECURITY

       cups-lpd currently does not perform any access control based on the settings in cupsd.conf(5) or in  the  hosts.allow(5)  or  hosts.deny(5)
       files  used  by	TCP  wrappers.	Therefore, running cups-lpd on your server will allow any computer on your network (and perhaps the entire
       Internet) to print to your server.

       You should use configure the firewall to limit TCP port 515 access to only those computers that	should	be  able  to  print  through  your
       server.

       cups-lpd  is  not  enabled  by the standard CUPS distribution.  Please consult with your operating system vendor to determine whether it is
       enabled on your system.

COMPATIBILITY

       cups-lpd does not enforce the restricted source port number specified in RFC 1179, as using restricted ports does not  prevent  users  from
       submitting  print  jobs.  While	this  behavior is different than standard Berkeley LPD implementations, it should not affect normal client
       operations.

       The output of the status requests follows RFC 2569, Mapping between LPD and IPP Protocols. Since many LPD implementations stray	from  this
       definition, remote status reporting to LPD clients may be unreliable.

SEE ALSO

       cups(1), cupsd(8), systemd(1), http://localhost:631/help

COPYRIGHT

       Copyright 2007-2013 by Apple Inc.

4 August 2008							       CUPS							       cups-lpd(8)
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