.TH NQMGR 8 .ad .fi .SH NAME nqmgr \- Postfix queue manager .SH SYNOPSIS .na .nf \fBnqmgr\fR [generic Postfix daemon options] .SH DESCRIPTION .ad .fi The \fBnqmgr\fR 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 \fBtrivial-rewrite\fR(8) daemon. This program expects to be run from the \fBmaster\fR(8) process manager. Mail addressed to the local \fBdouble-bounce\fR address is silently discarded. This stops potential loops caused by undeliverable bounce notifications. .SH MAIL QUEUES .na .nf .ad .fi The \fBnqmgr\fR daemon maintains the following queues: .IP \fBincoming\fR Inbound mail from the network, or mail picked up by the local \fBpickup\fR agent from the \fBmaildrop\fR directory. .IP \fBactive\fR Messages that the queue manager has opened for delivery. Only a limited number of messages is allowed to enter the \fBactive\fR queue (leaky bucket strategy, for a fixed delivery rate). .IP \fBdeferred\fR Mail that could not be delivered upon the first attempt. The queue manager implements exponential backoff by doubling the time between delivery attempts. .IP \fBcorrupt\fR Unreadable or damaged queue files are moved here for inspection. .IP \fBhold\fR Messages that are kept "on hold" are kept here until someone sets them free. .SH DELIVERY STATUS REPORTS .na .nf .ad .fi The \fBnqmgr\fR 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: .IP \fBbounce\fR Per-recipient status information about why mail is bounced. These files are maintained by the \fBbounce\fR(8) daemon. .IP \fBdefer\fR Per-recipient status information about why mail is delayed. These files are maintained by the \fBdefer\fR(8) daemon. .PP The \fBnqmgr\fR daemon is responsible for asking the \fBbounce\fR(8) or \fBdefer\fR(8) daemons to send non-delivery reports. .SH STRATEGIES .na .nf .ad .fi The queue manager implements a variety of strategies for either opening queue files (input) or for message delivery (output). .IP "\fBleaky bucket\fR" This strategy limits the number of messages in the \fBactive\fR queue and prevents the queue manager from running out of memory under heavy load. .IP \fBfairness\fR When the \fBactive\fR queue has room, the queue manager takes one message from the \fBincoming\fR queue and one from the \fBdeferred\fR queue. This prevents a large mail backlog from blocking the delivery of new mail. .IP "\fBslow start\fR" This strategy eliminates "thundering herd" problems by slowly adjusting the number of parallel deliveries to the same destination. .IP "\fBround robin\fR The queue manager sorts delivery requests by destination. Round-robin selection prevents one destination from dominating deliveries to other destinations. .IP "\fBexponential backoff\fR" Mail that cannot be delivered upon the first attempt is deferred. The time interval between delivery attempts is doubled after each attempt. .IP "\fBdestination status cache\fR" The queue manager avoids unnecessary delivery attempts by maintaining a short-term, in-memory list of unreachable destinations. .IP "\fBpreemptive message scheduling\fR" 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. .SH TRIGGERS .na .nf .ad .fi 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): .IP "\fBD (QMGR_REQ_SCAN_DEFERRED)\fR" Start a deferred queue scan. If a deferred queue scan is already in progress, that scan will be restarted as soon as it finishes. .IP "\fBI (QMGR_REQ_SCAN_INCOMING)\fR" Start an incoming queue scan. If an incoming queue scan is already in progress, that scan will be restarted as soon as it finishes. .IP "\fBA (QMGR_REQ_SCAN_ALL)\fR" Ignore deferred queue file time stamps. The request affects the next deferred queue scan. .IP "\fBF (QMGR_REQ_FLUSH_DEAD)\fR" Purge all information about dead transports and destinations. .IP "\fBW (TRIGGER_REQ_WAKEUP)\fR" 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. .PP The \fBnqmgr\fR daemon reads an entire buffer worth of triggers. Multiple identical trigger requests are collapsed into one, and trigger requests are sorted so that \fBA\fR and \fBF\fR precede \fBD\fR and \fBI\fR. Thus, in order to force a deferred queue run, one would request \fBA F D\fR; in order to notify the queue manager of the arrival of new mail one would request \fBI\fR. .SH STANDARDS .na .nf .ad .fi None. The \fBnqmgr\fR daemon does not interact with the outside world. .SH SECURITY .na .nf .ad .fi The \fBnqmgr\fR 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 \fBnqmgr\fR daemon does not talk to the outside world, and it can be run at fixed low privilege in a chrooted environment. .SH DIAGNOSTICS .ad .fi Problems and transactions are logged to the syslog daemon. Corrupted message files are saved to the \fBcorrupt\fR queue for further inspection. Depending on the setting of the \fBnotify_classes\fR parameter, the postmaster is notified of bounces and of other trouble. .SH BUGS .ad .fi A single queue manager process has to compete for disk access with multiple front-end processes such as \fBsmtpd\fR. A sudden burst of inbound mail can negatively impact outbound delivery rates. .SH CONFIGURATION PARAMETERS .na .nf .ad .fi The following \fBmain.cf\fR parameters are especially relevant to this program. See the Postfix \fBmain.cf\fR file for syntax details and for default values. Use the \fBpostfix reload\fR command after a configuration change. .SH Miscellaneous .ad .fi .IP \fBallow_min_user\fR Do not bounce recipient addresses that begin with '-'. .IP \fBqueue_directory\fR Top-level directory of the Postfix queue. .SH "Active queue controls" .ad .fi In the text below, \fItransport\fR is the first field in a \fBmaster.cf\fR entry. .IP \fBqmgr_clog_warn_time\fR Minimal delay between warnings that a specific destination is clogging up the active queue. Specify 0 to disable. .IP \fBqmgr_message_active_limit\fR Limit the number of messages in the active queue. .IP \fBqmgr_message_recipient_limit\fR Limit the number of in-memory recipients. .sp This parameter also limits the size of the short-term, in-memory destination cache. .IP \fBqmgr_message_recipient_minimum\fR Per message minimum of in-memory recipients. .IP \fBdefault_recipient_limit\fR Default limit on the number of in-memory recipients per transport. .IP \fItransport\fB_recipient_limit\fR Limit on the number of in-memory recipients, for the named message \fItransport\fR. .IP \fBdefault_extra_recipient_limit\fR Default limit on the total number of per transport in-memory recipients that the preempting messages can have. .IP \fItransport\fB_extra_recipient_limit\fR Limit on the number of in-memory recipients which all preempting messages delivered by the transport \fItransport\fR can have. .SH "Timing controls" .ad .fi .IP \fBminimal_backoff_time\fR Minimal time in seconds between delivery attempts of a deferred message. .sp This parameter also limits the time an unreachable destination is kept in the short-term, in-memory destination status cache. .IP \fBmaximal_backoff_time\fR Maximal time in seconds between delivery attempts of a deferred message. .IP \fBmaximal_queue_lifetime\fR Maximal time in days a message is queued before it is sent back as undeliverable. .IP \fBqueue_run_delay\fR Time in seconds between deferred queue scans. Queue scans do not overlap. .IP \fBtransport_retry_time\fR Time in seconds between attempts to contact a broken delivery transport. .SH "Concurrency controls" .ad .fi .IP \fBinitial_destination_concurrency\fR Initial per-destination concurrency level for parallel delivery to the same destination. .IP \fBdefault_destination_concurrency_limit\fR Default limit on the number of parallel deliveries to the same destination. .IP \fItransport\fB_destination_concurrency_limit\fR Limit on the number of parallel deliveries to the same destination, for delivery via the named message \fItransport\fR. .SH "Recipient controls" .ad .fi .IP \fBdefault_destination_recipient_limit\fR Default limit on the number of recipients per message transfer. .IP \fItransport\fB_destination_recipient_limit\fR Limit on the number of recipients per message transfer, for the named message \fItransport\fR. .SH "Message scheduling" .ad .fi .IP "\fItransport\fB_delivery_slot_cost\fR (valid range: 0,2,3...) This parameter basically controls how often a message delivered by \fItransport\fR can be preempted by another message. An internal per-message/transport counter is incremented by one for each \fItransport\fB_delivery_slot_cost\fR deliveries handled by \fItransport\fR. 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 \fItransport\fR agents than the value of the "available delivery slots" counter. .sp Value equal to 0 disables the message preemption for \fItransport\fR. .IP \fItransport\fB_minimum_delivery_slots\fR Message preemption is not attempted at all whenever a message that can't ever accumulate at least \fItransport\fB_minimum_delivery_slots\fR available delivery slots is being delivered by \fItransport\fR. .IP "\fItransport\fB_delivery_slot_discount\fR (valid range: 0..100)" .IP \fItransport\fB_delivery_slot_loan\fR 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 \fItransport\fB_delivery_slot_discount\fR percent of the required amount plus \fItransport\fB_delivery_slot_loan\fR still remains to be accumulated. Note that the full amount will still have to be accumulated before another preemption can take place later. .IP \fBdefault_delivery_slot_cost\fR .IP \fBdefault_minimum_delivery_slots\fR .IP \fBdefault_delivery_slot_discount\fR .IP \fBdefault_delivery_slot_loan\fR Default values for the transport specific parameters described above. .SH SEE ALSO .na .nf master(8), process manager syslogd(8) system logging trivial-rewrite(8), address routing .SH LICENSE .na .nf .ad .fi The Secure Mailer license must be distributed with this software. .SH AUTHOR(S) .na .nf 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