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 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)
Mac OS X 10.3 - Generated Sat Jun 7 14:51:12 CDT 2008