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 notifi-
cations.
Mail addressed to a user listed in the optional relocated database is bounced with a "user has moved to new_location" message. See relo-
cated(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 implements 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 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 sym-
bolic 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 '-'.
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 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.
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_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 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)