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RedHat 9 (Linux i386) - man page for nqmgr (redhat section 8)

NQMGR(8)			     System Manager's Manual				 NQMGR(8)

       nqmgr - Postfix queue manager

       nqmgr [generic Postfix daemon options]

       The  nqmgr  daemon  awaits  the arrival of incoming mail and arranges for its delivery via
       Postfix delivery processes.  The actual mail routing strategy is delegated to the trivial-
       rewrite(8) daemon.  This program expects to be run from the master(8) process manager.

       Mail  addressed	to  the  local	double-bounce  address is silently discarded.  This stops
       potential loops caused by undeliverable bounce notifications.

       Mail addressed to a user listed in the optional relocated database is bounced with a "user
       has moved to new_location" message. See relocated(5) for a precise description.

       The nqmgr daemon maintains the following queues:

	      Inbound mail from the network, or mail picked up by the local pickup agent from the
	      maildrop directory.

       active Messages that the queue manager has opened for delivery. Only a limited  number  of
	      messages	is  allowed to enter the active queue (leaky bucket strategy, for a fixed
	      delivery rate).

	      Mail that could not be delivered upon the first attempt. The queue  manager  imple-
	      ments exponential backoff by doubling the time between delivery attempts.

	      Unreadable or damaged queue files are moved here for inspection.

       The  nqmgr  daemon  keeps  an  eye on per-message delivery status reports in the following
       directories. Each status report file has the same name as the corresponding message file:

       bounce Per-recipient status information about why mail is bounced.  These files are  main-
	      tained by the bounce(8) daemon.

       defer  Per-recipient  status information about why mail is delayed.  These files are main-
	      tained by the defer(8) daemon.

       The nqmgr daemon is responsible for asking the bounce(8) or defer(8) daemons to send  non-
       delivery reports.

       The  queue  manager  implements	a  variety  of	strategies for either opening queue files
       (input) or for message delivery (output).

       leaky bucket
	      This strategy limits the number of messages in the active queue  and  prevents  the
	      queue manager from running out of memory under heavy load.

	      When the active queue has room, the queue manager takes one message from the incom-
	      ing queue and one from the deferred queue. This prevents a large mail backlog  from
	      blocking the delivery of new mail.

       slow start
	      This  strategy eliminates "thundering herd" problems by slowly adjusting the number
	      of parallel deliveries to the same destination.

       round robin
	      The queue manager sorts delivery requests by  destination.   Round-robin	selection
	      prevents one destination from dominating deliveries to other destinations.

       exponential backoff
	      Mail  that cannot be delivered upon the first attempt is deferred.  The time inter-
	      val between delivery attempts is doubled after each attempt.

       destination status cache
	      The queue manager avoids unnecessary delivery attempts by maintaining a short-term,
	      in-memory list of unreachable destinations.

       preemptive message scheduling
	      The  queue manager attempts to minimize the average per-recipient delay while still
	      preserving the correct per-message delays, using a sophisticated preemptive message

       On  an idle system, the queue manager waits for the arrival of trigger events, or it waits
       for a timer to go off. A  trigger  is  a  one-byte  message.   Depending  on  the  message
       received, the queue manager performs one of the following actions (the message is followed
       by the symbolic constant used internally by the software):

	      Start a deferred queue scan.  If a deferred queue scan is already in progress, that
	      scan will be restarted as soon as it finishes.

	      Start  an  incoming  queue  scan. If an incoming queue scan is already in progress,
	      that scan will be restarted as soon as it finishes.

	      Ignore deferred queue file time stamps. The request affects the next deferred queue

	      Purge all information about dead transports and destinations.

	      Wakeup  call,  This is used by the master server to instantiate servers that should
	      not go away forever. The action is to start an incoming queue scan.

       The nqmgr daemon reads an entire buffer worth of  triggers.   Multiple  identical  trigger
       requests are collapsed into one, and trigger requests are sorted so that A and F precede D
       and I. Thus, in order to force a deferred queue run, one would request A F D; in order  to
       notify the queue manager of the arrival of new mail one would request I.

       None. The nqmgr daemon does not interact with the outside world.

       The  nqmgr  daemon  is  not  security  sensitive.  It reads single-character messages from
       untrusted local users, and thus may be susceptible to denial of service attacks. The nqmgr
       daemon  does  not talk to the outside world, and it can be run at fixed low privilege in a
       chrooted environment.

       Problems and transactions are logged to the syslog daemon.  Corrupted  message  files  are
       saved to the corrupt queue for further inspection.

       Depending  on  the  setting of the notify_classes parameter, the postmaster is notified of
       bounces and of other trouble.

       A single queue manager process has to compete for disk access with multiple front-end pro-
       cesses such as smtpd. A sudden burst of inbound mail can negatively impact outbound deliv-
       ery rates.

       The following main.cf parameters are especially relevant to this program. See the  Postfix
       main.cf	file  for  syntax  details and for default values. Use the postfix reload command
       after a configuration change.

	      Do not bounce recipient addresses that begin with '-'.

	      Tables with contact information for users, hosts or domains that no  longer  exist.
	      See relocated(5).

	      Top-level directory of the Postfix queue.

Active queue controls
       In the text below, transport is the first field in a master.cf entry.

	      Limit the number of messages in the active queue.

	      Limit the number of in-memory recipients.

	      This parameter also limits the size of the short-term, in-memory destination cache.

	      Per message minimum of in-memory recipients.

	      Default limit on the number of in-memory recipients per transport.

	      Limit on the number of in-memory recipients, for the named message transport.

	      Default  limit  on  the total number of per transport in-memory recipients that the
	      preempting messages can have.

	      Limit on the number of in-memory recipients which all preempting messages delivered
	      by the transport transport can have.

Timing controls
	      Minimal time in seconds between delivery attempts of a deferred message.

	      This  parameter  also  limits  the  time	an unreachable destination is kept in the
	      short-term, in-memory destination status cache.

	      Maximal time in seconds between delivery attempts of a deferred message.

	      Maximal time in days a message is queued before it is sent back as undeliverable.

	      Time in seconds between deferred queue scans. Queue scans do not overlap.

	      Time in seconds between attempts to contact a broken delivery transport.

Concurrency controls
	      Initial per-destination concurrency level for parallel delivery to the same  desti-

	      Default limit on the number of parallel deliveries to the same destination.

	      Limit  on  the  number of parallel deliveries to the same destination, for delivery
	      via the named message transport.

Recipient controls
	      Default limit on the number of recipients per message transfer.

	      Limit on the number of recipients per  message  transfer,  for  the  named  message

Message scheduling
       transport_delivery_slot_cost (valid range: 0,2,3...)
	      This parameter basically controls how often a message delivered by transport can be
	      preempted by another message.  An internal per-message/transport counter is  incre-
	      mented  by  one  for each transport_delivery_slot_cost deliveries handled by trans-
	      port. This counter represents the number of "available delivery slots" for  use  by
	      other  messages.	 Current  message  can	be preempted by another message when that
	      other message can be delivered using less transport agents than the  value  of  the
	      "available delivery slots" counter.

	      Value equal to 0 disables the message preemption for transport.

	      Message preemption is not attempted at all whenever a message that can't ever accu-
	      mulate at least transport_minimum_delivery_slots available delivery slots is  being
	      delivered by transport.

       transport_delivery_slot_discount (valid range: 0..100)

	      These parameters speed up the moment when a message preemption can happen.  Instead
	      of waiting until the full amount of delivery slots required is available, the  pre-
	      emption  can  happen  when transport_delivery_slot_discount percent of the required
	      amount plus transport_delivery_slot_loan still remains to be accumulated. Note that
	      the  full  amount  will  still have to be accumulated before another preemption can
	      take place later.




	      Default values for the transport specific parameters described above.

       master(8), process manager
       relocated(5), format of the "user has moved" table
       syslogd(8) system logging
       trivial-rewrite(8), address routing

       The Secure Mailer license must be distributed with this software.

       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA

       Scheduler enhancements:
       Patrik Rak
       Modra 6
       155 00, Prague, Czech Republic


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