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nqmgr (8)
>> nqmgr (8) ( Linux man: Команды системного администрирования )
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 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
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 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 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 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 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 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
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 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 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 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.
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_delivery_slots
available delivery slots is being delivered by transport.
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
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.
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
