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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  dis-
       carded.   This  stops  potential  loops  caused by undeliverable bounce
       notifications.

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 implements exponential  backoff  by  doubling  the
              time between delivery attempts.

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

       hold   Messages  that  are  kept  "on hold" are kept here until someone
              sets them free.

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 maintained by the bounce(8) daemon.

       defer  Per-recipient  status  information  about  why  mail is delayed.
              These files are maintained 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 mes-
              sage  from  the  incoming 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 destina-
              tion.

       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 interval between delivery attempts  is  dou-
              bled after each attempt.

       destination status cache
              The  queue manager avoids unnecessary delivery attempts by main-
              taining a short-term, in-memory  list  of  unreachable  destina-
              tions.

       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 mes-
       sage.   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  out-
       side  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 postmas-
       ter is notified of bounces and of other trouble.

BUGS
       A single queue manager process has to compete for disk access with mul-
       tiple front-end processes such as smtpd. A sudden burst of inbound mail
       can negatively impact outbound delivery rates.

CONFIGURATION PARAMETERS
       The following main.cf parameters are especially relevant to  this  pro-
       gram.  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 '-'.

       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_clog_warn_time
              Minimal delay between warnings that a  specific  destination  is
              clogging up the active queue. Specify 0 to disable.

       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  trans-
              port.

       transport_recipient_limit
              Limit  on the number of in-memory recipients, for the named mes-
              sage 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 destination.

       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  destina-
              tion, 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 incremented  by  one  for  each
              transport_delivery_slot_cost  deliveries  handled  by transport.
              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  accumulate  at least transport_minimum_deliv-
              ery_slots available delivery slots is being delivered by  trans-
              port.

       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  deliv-
              ery  slots required is available, the preemption can happen when
              transport_delivery_slot_discount percent of the required  amount
              plus  transport_delivery_slot_loan  still  remains to be accumu-
              lated. Note that the full amount will still have to  be  accumu-
              lated 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
       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)