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PPoossttffiixx AArrcchhiitteeccttuurree OOvveerrvviieeww
-------------------------------------------------------------------------------
IInnttrroodduuccttiioonn
This document presents an overview of the Postfix architecture, and provides
pointers to descriptions of every Postfix command or server program. The text
gives the general context in which each command or server program is used, and
provides pointers to documents with specific usage examples and background
information.
Topics covered by this document:
* How Postfix receives mail
* How Postfix delivers mail
* Postfix behind the scenes
* Postfix support commands
HHooww PPoossttffiixx rreecceeiivveess mmaaiill
When a message enters the Postfix mail system, the first stop on the inside is
the incoming queue. The figure below shows the main processes that are involved
with new mail. Names followed by a number are Postfix commands or server
programs, while unnumbered names inside shaded areas represent Postfix queues.
trivial-
rewrite(8)
Network -> smtpd(8)
^ |
\ | v
Network -> qmqpd(8) -> cleanup(8) -> incoming
/
pickup(8) <- maildrop
^
|
Local -> sendmail(1) -> postdrop(1)
* Network mail enters Postfix via the smtpd(8) or qmqpd(8) servers. These
servers remove the SMTP or QMQP protocol encapsulation, enforce some sanity
checks to protect Postfix, and give the sender, recipients and message
content to the cleanup(8) server. The smtpd(8) server can be configured to
block unwanted mail, as described in the SMTPD_ACCESS_README document.
* Local submissions are received with the Postfix sendmail(1) compatibility
command, and are queued in the maildrop queue by the privileged postdrop(1)
command. This arrangement even works while the Postfix mail system is not
running. The local pickup(8) server picks up local submissions, enforces
some sanity checks to protect Postfix, and gives the sender, recipients and
message content to the cleanup(8) server.
* Mail from internal sources is given directly to the cleanup(8) server.
These sources are not shown in the figure, and include: mail that is
forwarded by the local(8) delivery agent (see next section), messages that
are returned to the sender by the bounce(8) server (see second-next
section), and postmaster notifications about problems with Postfix.
* The cleanup(8) server implements the final processing stage before mail is
queued. It adds missing From: and other message headers, and transforms
addresses as described in the ADDRESS_REWRITING_README document.
Optionally, the cleanup(8) server can be configured to do light-weight
content inspection with regular expressions as described in the
BUILTIN_FILTER_README document. The cleanup(8) server places the result as
a single file into the incoming queue, and notifies the queue manager (see
next section) of the arrival of new mail.
* The trivial-rewrite(8) server rewrites addresses to the standard
"user@fully.qualified.domain" form, as described in the
ADDRESS_REWRITING_README document. Postfix currently does not implement a
rewriting language, but a lot can be done via table lookups and, if need
be, regular expressions.
HHooww PPoossttffiixx ddeelliivveerrss mmaaiill
Once a message has reached the incoming queue the next step is to deliver it.
The figure shows the main components of the Postfix mail delivery apparatus.
Names followed by a number are Postfix commands or server programs, while
unnumbered names inside shaded areas represent Postfix queues.
trivial- smtp(8) -> Network
rewrite(8)
/
- lmtp(8) -> Network
^ |
| v /
incoming -> active -> qmgr(8) --- local(8) -> File, command
\
^ |
| v - virtual(8) -> File
deferred \
pipe(8) -> Command
* The queue manager (the qmgr(8) server process in the figure) is the heart
of Postfix mail delivery. It contacts the smtp(8), lmtp(8), local(8),
virtual(8), pipe(8), discard(8) or error(8) delivery agents, and sends a
delivery request for one or more recipient addresses. The discard(8) and
error(8) delivery agents are special: they discard or bounce all mail, and
are not shown in the figure above.
The queue manager maintains a limited active queue with the messages that
it has opened for delivery. The active queue acts as a limited window on
potentially large incoming or deferred queues. The limited active queue
prevents the queue manager from running out of memory under heavy load.
The queue manager maintains a separate deferred queue for mail that cannot
be delivered, so that a large mail backlog will not slow down normal queue
accesses. The queue manager's strategy for delayed mail delivery attempts
is described in the QSHAPE_README and TUNING_README documents.
* The trivial-rewrite(8) server resolves each recipient address according to
its local or remote address class, as defined in the ADDRESS_CLASS_README
document. Additional routing information can be specified with the optional
transport(5) table. The trivial-rewrite(8) server optionally queries the
relocated(5) table for recipients whose address has changed; mail for such
recipients is returned to the sender with an explanation.
* The smtp(8) client looks up a list of SMTP servers for the destination(s)
in a delivery request, sorts the list by preference, and tries each server
in turn until it has delivered or bounced all recipients in the delivery
request. It encapsulates the sender, recipients and message content as
required by the SMTP protocol; this includes message body conversion from
8-bit MIME to 7-bit encoding, but does not include RFC 2047 header
encoding.
* The lmtp(8) client speaks a protocol similar to SMTP that is optimized for
delivery to mailbox servers such as Cyrus. The advantage of this setup is
that one Postfix machine can feed multiple mailbox servers over LMTP. The
opposite is true as well: one mailbox server can be fed over LMTP by
multiple Postfix machines.
* The local(8) delivery agent understands UNIX-style mailboxes, qmail-
compatible maildir files, Sendmail-style system-wide aliases(5) databases,
and Sendmail-style per-user .forward files. Multiple local delivery agents
can be run in parallel, but parallel delivery to the same user is usually
limited.
The local(8) delivery agent has hooks for alternative forms of local
delivery: you can configure it to deliver to mailbox files in user home
directories, you can configure it to delegate mailbox delivery to an
external command such as procmail, or you can delegate delivery to a
different Postfix delivery agent.
* The virtual(8) delivery agent is a bare-bones delivery agent that delivers
to UNIX-style mailbox or qmail-style maildir files only. This delivery
agent can deliver mail for multiple domains, which makes it especially
suitable for hosting lots of small domains on a single machine. This is
described in the VIRTUAL_README document.
* The pipe(8) mailer is the outbound interface to other mail processing
systems (the Postfix sendmail(1) command being the inbound interface). The
interface is UNIX compatible: the pipe(8) mailer provides information to a
child process command line, environment variables, and standard input
stream, and expects a child process exit status code as defined in
<sysexits.h>. Examples of delivery via the pipe(8) mailer are in the
FILTER_README, MAILDROP_README, and UUCP_README documents.
PPoossttffiixx bbeehhiinndd tthhee sscceenneess
The previous sections gave an overview of how Postfix server processes send and
receive mail. These server processes rely on other server processes that do
things behind the scenes. The text below attempts to visualize each service in
its own context. As before, names followed by a number are Postfix commands or
server programs, while unnumbered names inside shaded areas represent Postfix
queues.
* The resident master(8) server is the supervisor that keeps an eye on the
well-being of the Postfix mail system. It is typically started at system
boot time with the "postfix start" command, and keeps running until the
system goes down. The master(8) server is responsible for starting Postfix
server processes to receive and deliver mail, and for restarting servers
that terminate prematurely because of some problem. The master(8) server is
also responsible for enforcing the server process count limits as specified
in the mmaasstteerr..ccff configuration file. The picture below gives the program
hierarchy when Postfix is started up. Only some of the mail handling daemon
processes are shown.
postfix(1)
|
|
postfix-script(1)
/ | \
|
/ \
postsuper(1) master(8) postlog(1)
/ | \
|
/ \
smtpd(8) qmgr(8) local(8)
* The anvil(8) server implements client connection and request rate limiting
for all smtpd(8) servers. The TUNING_README document provides guidance for
dealing with mis-behaving SMTP clients. The anvil(8) service is available
in Postfix version 2.2 and later.
Network -> smtpd(8) <-> anvil(8)
* The bounce(8), defer(8) and trace(8) services each maintain their own queue
directory trees with per-message logfiles. Postfix uses this information
when sending "failed", "delayed" or "success" delivery status notifications
to the sender.
The trace(8) service also implements support for the Postfix "sendmail -bv"
and "sendmail -v" commands which produce reports about how Postfix delivers
mail, and is available with Postfix version 2.1 and later. See DEBUG_README
for examples.
qmgr(8) Delivery
cleanup(8) -> Postfix -> agents
queue
^ | |
| v v
(Non-) bounce(8) Queue id,
delivery <- defer(8) <- recipient,
notice trace(8) status
^ |
| v
Per-
message
logfiles
* The flush(8) servers maintain per-destination logs and implement "sendmail
-qRsite", "sendmail -qIqueueid" "postqueue -s site", "postqueue -
i queueid", and ETRN as described in the ETRN_README document. This moves
selected queue files from the deferred queue back to the incoming queue and
requests their delivery. The flush(8) service is available with Postfix
version 1.0 and later.
incoming
^
deferred
^
|
smtpd(8) Destination Deferred Delivery
sendmail(1) - to flush -> flush(8) <- destination, - agents,
postqueue(1) queue id qmgr(8)
^ |
| v
Per-dest-
ination
logs
* The proxymap(8) servers provide read-only and read-write table lookup
service to Postfix processes. This overcomes chroot restrictions, reduces
the number of open lookup tables by sharing one open table among multiple
processes, and implements single-updater tables.
* The scache(8) server maintains the connection cache for the Postfix smtp(8)
client. When connection caching is enabled for selected destinations, the
smtp(8) client does not disconnect immediately after a mail transaction,
but gives the connection to the connection cache server which keeps the
connection open for a limited amount of time. The smtp(8) client continues
with some other mail delivery request. Meanwhile, any smtp(8) process can
ask the scache(8) server for that cached connection and reuse it for mail
delivery. As a safety measure, Postfix limits the number of times that a
connection may be reused.
When delivering mail to a destination with multiple mail servers,
connection caching can help to skip over a non-responding server, and thus
dramatically speed up delivery. SMTP connection caching is available in
Postfix version 2.2 and later. More information about this feature is in
the CONNECTION_CACHE_README document.
/-- smtp(8) --> Internet
qmgr(8)
|
\-- | smtp(8)
|
| ^
v |
scache(8)
A Postfix smtp(8) client can reuse a TLS-encrypted connection (with
"smtp_tls_connection_reuse = yes"). This can greatly reduce the overhead of
connection setup and improves message delivery rates. After a Postfix smtp
(8) client connects to a remote SMTP server and sends plaintext EHLO and
STARTTLS commands, the smtp(8) client inserts a tlsproxy(8) process into
the connection as shown in the top of the figure below.
/-- smtp(8) --> tlsproxy(8) --> Internet
qmgr(8)
|
\-- | smtp(8)
|
| ^
v |
scache(8)
After the mail transaction completes, the Postfix smtp(8) client gives the
smtp(8)-to-tlsproxy(8) connection to the scache(8) server, which keeps the
connection open for a limited amount of time. The smtp(8) client continues
with some other mail delivery request. Meanwhile, any Postfix smtp(8)
client can ask the scache(8) server for that cached connection and reuse it
for mail delivery.
* The showq(8) servers list the Postfix queue status. This is the queue
listing service that does the work for the mailq(1) and postqueue(1)
commands.
mailq(1) Postfix
Output <- post- <- showq(8) <- queue
queue(1)
* The spawn(8) servers run non-Postfix commands on request, with the client
connected via socket or FIFO to the command's standard input, output and
error streams. You can find examples of its use in the SMTPD_POLICY_README
document.
* The tlsmgr(8) server runs when TLS (Transport Layer Security, formerly
known as SSL) is turned on in the Postfix smtp(8) client or smtpd(8)
server. This process has two duties:
o Maintain the pseudo-random number generator (PRNG) that is used to seed
the TLS engines in Postfix smtp(8) client or smtpd(8) server processes.
The state of this PRNG is periodically saved to a file, and is read
when tlsmgr(8) starts up.
o Maintain the optional Postfix smtp(8) client or smtpd(8) server caches
with TLS session keys. Saved keys can improve performance by reducing
the amount of computation at the start of a TLS session.
TLS support is available in Postfix version 2.2 and later. Information
about the Postfix TLS implementation is in the TLS_README document.
<---seed--- ---seed--->
Network-> smtpd(8) tlsmgr(8) smtp(8) ->Network
<-session-> <-session->
/ | \
|
/ \
smtpd PRNG smtp
session state session
cache file cache
* The verify(8) server verifies that a sender or recipient address is
deliverable before the smtpd(8) server accepts it. The verify(8) server
queries a cache with address verification results. If a result is not
found, the verify(8) server injects a probe message into the Postfix queue
and processes the status update from a delivery agent or queue manager.
This process is described in the ADDRESS_VERIFICATION_README document. The
verify(8) service is available with Postfix version 2.1 and later.
probe Postfix
message -> mail
Network -> smtpd(8) <-> verify(8) -> queue
|
v
<- probe Postfix -> Local
status <- delivery -> Network
^ agents
|
v
Address
verification
cache
* The postscreen(8) server can be put "in front" of Postfix smtpd(8)
processes. Its purpose is to accept connections from the network and to
decide what SMTP clients are allowed to talk to Postfix. According to the
2008 MessageLabs annual report, 81% of all email was spam, and 90% of that
was sent by botnets; by 2010, those numbers were 92% and 95%, respectively.
While postscreen(8) keeps the zombies away, more smtpd(8) processes remain
available for legitimate clients.
postscreen(8) maintains a temporary allowlist for clients that pass its
tests; by allowing allowlisted clients to skip tests, postscreen(8)
minimizes its impact on legitimate email traffic.
The postscreen(8) server is available with Postfix 2.8 and later. To keep
the implementation simple, postscreen(8) delegates DNS allow/denylist
lookups to dnsblog(8) server processes, and delegates TLS encryption/
decryption to tlsproxy(8) server processes. This delegation is invisible to
the remote SMTP client.
zombie
\
zombie - tlsproxy(8) - - smtpd(8)
\ /
other --- postscreen(8)
/ \
other - - smtpd(8)
/
zombie
* The postlogd(8) server provides an alternative to syslog logging, which
remains the default. This feature is available with Postfix version 3.4 or
later, and supports the following modes:
o Logging to file, which addresses a usability problem with MacOS, and
eliminates information loss caused by systemd rate limits.
commands -> postlogd(8) -> /path/to/file
or daemons
o Logging to stdout, which eliminates a syslog dependency when Postfix
runs inside a container.
commands -> postlogd(8) -> stdout inherited
or daemons from "postfix start-fg"
See MAILLOG_README for details and limitations.
PPoossttffiixx ssuuppppoorrtt ccoommmmaannddss
The Postfix architecture overview ends with a summary of command-line utilities
for day-to-day use of the Postfix mail system. Besides the Sendmail-compatible
sendmail(1), mailq(1), and newaliases(1) commands, the Postfix system comes
with it own collection of command-line utilities. For consistency, these are
all named postsomething.
* The postfix(1) command controls the operation of the mail system. It is the
interface for starting, stopping, and restarting the mail system, as well
as for some other administrative operations. This command is reserved to
the super-user.
* The postalias(1) command maintains Postfix aliases(5) type databases. This
is the program that does the work for the newaliases(1) command.
* The postcat(1) command displays the contents of Postfix queue files. This
is a limited, preliminary utility. This program is likely to be superseded
by something more powerful that can also edit Postfix queue files.
* The postconf(1) command displays or updates Postfix main.cf parameters and
displays system dependent information about the supported file locking
methods, and the supported types of lookup tables.
* The postdrop(1) command is the mail posting utility that is run by the
Postfix sendmail(1) command in order to deposit mail into the maildrop
queue directory.
* The postkick(1) command makes some Postfix internal communication channels
available for use in, for example, shell scripts.
* The postlock(1) command provides Postfix-compatible mailbox locking for use
in, for example, shell scripts.
* The postlog(1) command provides Postfix-compatible logging for shell
scripts.
* The postmap(1) command maintains Postfix lookup tables such as canonical
(5), virtual(5) and others. It is a cousin of the UNIX makemap command.
* The postmulti(1) command repeats the "postfix start" etc. command for each
Postfix instance, and supports creation, deletion etc. of Postfix
instances. For a tutorial, see MULTI_INSTANCE_README.
* The postqueue(1) command is the privileged command that is run by Postfix
sendmail(1) and mailq(1) in order to flush or list the mail queue.
* The postsuper(1) command maintains the Postfix queue. It removes old
temporary files, and moves queue files into the right directory after a
change in the hashing depth of queue directories. This command is run at
mail system startup time and when Postfix is restarted.
|
