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

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

NAME
       nqmgr - Postfix queue manager

SYNOPSIS
       nqmgr [generic Postfix daemon options]

DESCRIPTION
       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.

MAIL QUEUES
       The nqmgr daemon maintains the following queues:

       incoming
	      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).

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

       corrupt
	      Unreadable or damaged queue files are moved here for inspection.

DELIVERY STATUS REPORTS
       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.

STRATEGIES
       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.

       fairness
	      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
	      scheduling.

TRIGGERS
       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):

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

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

       A (QMGR_REQ_SCAN_ALL)
	      Ignore deferred queue file time stamps. The request affects the next deferred queue
	      scan.

       F (QMGR_REQ_FLUSH_DEAD)
	      Purge all information about dead transports and destinations.

       W (TRIGGER_REQ_WAKEUP)
	      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.

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

SECURITY
       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.

DIAGNOSTICS
       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.

BUGS
       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.

CONFIGURATION PARAMETERS
       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.

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

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

       queue_directory
	      Top-level directory of the Postfix queue.

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

       qmgr_message_active_limit
	      Limit the number of messages in the active queue.

       qmgr_message_recipient_limit
	      Limit the number of in-memory recipients.

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

       qmgr_message_recipient_minimum
	      Per message minimum of in-memory recipients.

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

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

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

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

Timing controls
       minimal_backoff_time
	      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_backoff_time
	      Maximal time in seconds between delivery attempts of a deferred message.

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

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

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

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

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

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

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

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

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.

       transport_minimum_delivery_slots
	      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)

       transport_delivery_slot_loan
	      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_delivery_slot_cost

       default_minimum_delivery_slots

       default_delivery_slot_discount

       default_delivery_slot_loan
	      Default values for the transport specific parameters described above.

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

LICENSE
       The Secure Mailer license must be distributed with this software.

AUTHOR(S)
       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

											 NQMGR(8)


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