NQMGR(8) 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 triv-
ial-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 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 sta-
tus 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 message 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 deliver-
ies to the same destination.
round robin
The queue manager sorts delivery requests by desti-
nation. Round-robin selection prevents one desti-
nation from dominating deliveries to other destina-
tions.
exponential backoff
Mail that cannot be delivered upon the first
attempt is deferred. The time interval 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 cor-
rect per-message delays, using a sophisticated pre-
emptive 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 for-
ever. 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 pre-
cede 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 sin-
gle-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 environ-
ment.
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 trou-
ble.
BUGS
A single queue manager process has to compete for disk
access with multiple front-end processes such as smtpd. A
sudden burst of inbound mail can negatively impact out-
bound delivery 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
'-'.
queue_directory
Top-level directory of the Postfix queue.
Active queue controls
In the text below, transport is the first field in a mas-
ter.cf entry.
qmgr_clog_warn_time
Minimal delay between warnings that a specific des-
tination 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 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 bro-
ken delivery transport.
Concurrency controls
initial_destination_concurrency
Initial per-destination concurrency level for par-
allel 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 destination, for delivery via the named mes-
sage transport.
Recipient controls
default_destination_recipient_limit
Default limit on the number of recipients per mes-
sage 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 mes-
sage delivered by transport can be preempted by
another message. An internal per-message/transport
counter is incremented by one for each trans-
port_delivery_slot_cost deliveries handled by
transport. This counter represents the number of
"available delivery slots" for use by other mes-
sages. 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_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 preemption can happen when trans-
port_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 parame-
ters 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)