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词条 Postfix (software)
释义

  1. Typical deployment

  2. Operating systems

  3. Architecture

  4. Implementation

  5. Robustness

  6. Performance

  7. Base configuration

  8. Release history

  9. See also

  10. References

  11. Further reading

  12. External links

{{About|the Mail Transfer Agent||Postfix (disambiguation)}}{{Infobox software
| name = Postfix
| logo =
| screenshot =
| caption =
| developer = Wietse Venema and many others (Google, formerly: IBM)
| released = {{start date and age|1998|12|14}}
| latest release version = 3.4.5
| latest release date = {{start date and age|2019|03|30}}
| latest preview version = 3.5-20190330
| latest preview date = {{release date|2019|03|30}}
| operating system = Cross-platform
| programming language = C[1]
| genre = Mail transfer agent
| license = IBM Public License or Eclipse Public License
| website = www.postfix.org
}}

Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail.

It is released under the IBM Public License 1.0 which is a free software license. Alternatively, starting with version 3.2.5, it is available under the Eclipse Public License 2.0 at the user's option.[2]

Originally written in 1997 by Wietse Venema at the IBM Thomas J. Watson Research Center in New York, and first released in December 1998[3], Postfix continues {{as of| 2019 |lc= on}} to be actively developed by its creator and other contributors. The software is also known by its former names VMailer and IBM Secure Mailer.

In a December 2017 study performed by E-Soft, Inc.,[4] approximately 34% of the publicly reachable mail-servers on the Internet ran Postfix making it the second most popular mail server behind Exim

Typical deployment

As an SMTP server, Postfix implements a first layer of defense against spambots and malware. Administrators can combine Postfix with other software that provides spam/virus filtering (e.g., Amavisd-new), message-store access (e.g., Dovecot), or complex SMTP-level access-policies (e.g., postfwd, policyd-weight or greylisting).

As an SMTP client, Postfix implements a high-performance parallelized mail-delivery engine. Postfix is often combined with mailing-list software (such as Mailman).

Operating systems

Postfix runs (or has run) on AIX, BSD, HP-UX, Linux, macOS, Solaris and, generally speaking, on every Unix-like operating system that ships with a C compiler and delivers a standard POSIX development environment. It is the default MTA for the

macOS, NetBSD[5]

and Ubuntu operating systems.[6]

Architecture

Postfix consists of a combination of server programs that run in the background, and client programs that are invoked by user programs or by system administrators.

The Postfix core consists of several dozen server programs that run in the background, each handling one specific aspect of email delivery. Examples are the SMTP server, the scheduler, the address rewriter, and the local delivery server. For damage-control purposes, most server programs run with fixed reduced privileges, and terminate voluntarily after processing a limited number of requests. To conserve system resources, most server programs terminate when they become idle.

Client programs run outside the Postfix core. They interact with Postfix server programs through mail delivery instructions in the user's ~/.forward file, and through small "gate" programs to submit mail or to request queue status information.

Other programs provide administrative support to start or stop Postfix, query status information, manipulate the queue, or to examine or update its configuration files.

Yellow ellipses
One of Postfix' many daemons serving exactly one purpose. This split-up into many smaller pieces of software is considered one of the reasons why Postfix is secure and stable.
Blue boxes
The blue boxes represent so-called lookup tables. A lookup table consists of two columns (key and value) containing information used for access control, e-mail routing etc.
Yellow boxes
The yellow boxes are either mail queues or files. In either case, e-mails are stored on persistent media (e.g., a hard disk).
White clouds
The clouds stand for points at which e-mails enter or leave Postfix. For example, smtpd receives mail from other mail servers or users whereas smtp relays mail to other MTAs.

Implementation

The Postfix implementation uses safe subsets of the C language and of the POSIX system API. These subsets are buried under an abstraction layer that contains about 50% of all Postfix source code, and that provides the foundation on which all Postfix programs are built. For example, the "vstring" primitive makes Postfix code resistant to buffer overflow[7]

attacks, and the "safe open" primitive makes Postfix code resistant to race condition attacks on systems that implement the POSIX file system

API. This abstraction layer does not affect the attack resistance of non-Postfix code, such as code in system libraries or in third-party libraries.

Robustness

Conceptually, Postfix manages pipelines of processes that pass the responsibility for message delivery and error notification from one process to the next. All message and notification "state" information is persisted in the file system. The processes in a pipeline operate mostly without centralized control; this relative autonomy simplifies error recovery. When a process fails before completing its part of a file or protocol transaction, its predecessor in the pipeline backs off and retries the request later, and its successor in the pipeline discards unfinished work. Many Postfix daemons can simply "die" when they run into a problem; they are automatically restarted when the next service request arrives. This approach makes Postfix highly resilient, as long as the operating system or hardware don't fail catastrophically.

Performance

One single Postfix instance has been clocked at ~300 message deliveries/second[8]

across the Internet, running on commodity hardware (a vintage-2003 Dell 1850 system with battery-backed MegaRAID controller and two SCSI

disks). This delivery rate is an order of magnitude below the "intrinsic" limit of 2500 message deliveries/second[8]

that was achieved with the mail queue on a RAM disk while delivering to the "discard" transport (with a dual-core Opteron system in 2007).

Mail systems such as Postfix and Qmail achieve high performance by delivering mail in parallel sessions. With mail systems such as Sendmail and Exim that make one connection at a time, high performance can be achieved by submitting limited batches of mail in parallel, so that each batch is delivered by a different process. Postfix and Qmail require parallel submission into different MTA instances once they reach their intrinsic performance limit, or the performance limits of the hardware or operating system.

It should be noted that the delivery rates cited above are largely theoretical. With bulk mail delivery, the true delivery rate is primarily determined by the receiver's mail receiving policies and by the sender's reputation.

Base configuration

The main.cf file stores site-specific Postfix configuration parameters while master.cf defines daemon processes.[9] The Postfix Basic Configuration tutorial covers the core settings that each site needs to consider, and the Postfix Standard Configuration Examples document discusses configuration settings for a few common environments. The Postfix Address Rewriting document covers address rewriting and mail routing. The full documentation collection is at Postfix Documentation

More complex Postfix implementations may include: integration with other applications such as SpamAssassin; support for multiple virtual domain names - and use databases such as MySQL to control complex configurations.

Release history

VersionRelease date [10]Significant changes
MajorMinor
o |2.0}}
o |2.1}}
o |2.2}}
o |2.3}}
o |2.4}}
o |2.5}} 2.5.0 24 January 2008
2.5.8 28 August 2009 Withdrawn release.
2.5.17 6 February 2012 EOL 2.5
o |2.6}}2.6.0 12 May 2009
2.6.3 2 August 2009 Unannounced release.
2.6.4 26 August 2009 Withdrawn release.
2.6.19 4 February 2013 EOL 2.6
o |2.7}} 2.7.0 13 February 2010
2.7.16 16 January 2014 EOL 2.7[11]
o |2.8}}2.8.0 20 January 2011 Introduction of postscreen daemon that sits before the main postfix and acts as a Zombie blocker. Also: DNS whitelisting, SQLite support[12]
2.8.1 22 February 2011
2.8.2 21 March 2011
2.8.3 9 May 2011
2.8.4 7 July 2011
2.8.5 3 September 2011
2.8.6 24 October 2011
2.8.7 7 November 2011
2.8.8 1 February 2012
2.8.9 5 March 2012
2.8.10 24 April 2012
2.8.11 20 May 2012
2.8.12 1 August 2012
2.8.13 13 December 2012
2.8.14 4 February 2013
2.8.15 22 June 2013
2.8.16 5 September 2013
2.8.17 16 January 2014
2.8.18 13 October 2014
2.8.19 19 October 2014
2.8.20 8 February 2015 EOL 2.8[13]
o |2.9}}2.9.0 1 February 2012 Memcache support, gradual degradation[14]
2.9.1 18 February 2012
2.9.2 24 April 2012
2.9.3 20 May 2012
2.9.4 1 August 2012
2.9.5 13 December 2012
2.9.6 4 February 2013
2.9.7 22 June 2013
2.9.8 5 September 2013
2.9.9 16 January 2014
2.9.10 13 October 2014
2.9.11 19 October 2014
2.9.12 8 February 2015
2.9.13 12 April 2015
2.9.14 20 July 2015
2.9.15 10 October 2015 EOL 2.9[15]
o |2.10}}2.10.0 11 February 2013 Support for TLSv1.1 and TLSv1.2[16]
2.10.1 22 June 2013
2.10.2 5 September 2013
2.10.3 16 January 2014
2.10.4 13 October 2014
2.10.5 19 October 2014
2.10.6 8 February 2015
2.10.7 12 April 2015
2.10.8 20 July 2015
2.10.9 10 October 2015
2.10.10 15 May 2016 EOL 2.10[17]
co |2.11}}2.11.0 15 January 2014 Support for DANE[18][19]
2.11.1 7 May 2014
2.11.2 13 October 2014
2.11.3 19 October 2014
2.11.4 8 February 2015
2.11.5 12 April 2015
2.11.6 20 July 2015
2.11.7 10 October 2015
2.11.8 15 May 2016
2.11.9 1 January 2017
2.11.10 13 June 2017
2.11.11 27 January 2018
co |3.0}}3.0.0 8 February 2015 Support for internationalized domain names and address localparts as defined in RFC 6530 and related documents, Postfix dynamically-linked libraries and database plugins, operations on multiple lookup tables, pseudo-tables that make simple things easy to implement, and more.[20]
3.0.1 12 April 2015
3.0.2 20 July 2015
3.0.3 10 October 2015
3.0.4 21 February 2016
3.0.5 15 May 2016
3.0.6 28 August 2016
3.0.7 1 October 2016
3.0.8 1 January 2017
3.0.10 13 June 2017
3.0.11 28 October 2017
3.0.12 27 January 2018
co |3.1}}3.1.0 24 February 2016 Simplified setup of opportunistic TLS and SMTP server key/certificate management, positive and negative DNS reply TTL support in postscreen, SASL AUTH rate limit in the SMTP server, safety limit on the number of address verify requests, JSON queue listing, destination-independent delivery rate delay, inter alia.[21]
3.1.1 15 May 2016
3.1.2 28 August 2016
3.1.3 1 October 2016
3.1.4 1 January 2017
3.1.6 13 June 2017
3.1.7 28 October 2017
3.1.8 27 January 2018
c |3.2}}3.2.0 28 February 2017 Elliptic curve negotiation with OpenSSL ≥ 1.0.2, stored-procedure support for MySQL databases, cidr: table support for if/endif and negation, support for per-client Milter configuration, "PASS" and "STRIP" actions in header/body_checks, and more.[22]
3.2.2 13 June 2017
3.2.3 24 September 2017
3.2.4 28 October 2017
3.2.5 27 January 2018 {{Version |l |show=111111}}

See also

{{Portal|Free and open-source software}}
  • List of mail servers
  • Comparison of mail servers
  • Email filtering

References

1. ^{{cite web|url=http://www.lextrait.com/Vincent/implementations.html|title=The Programming Languages Beacon, v10.3|first=Vincent|last=Lextrait|date=July 2010|accessdate=5 September 2010}}
2. ^{{cite web|url=http://www.postfix.org/announcements/postfix-3.2.5.html|title=Postfix stable release 3.2.5, and legacy releases 3.1.8, 3.0.12, and 2.11.11|date=January 2018|accessdate=30 January 2018}}
3. ^{{cite web|url=https://www.nytimes.com/library/tech/98/12/biztech/articles/14blue.html|title=Sharing Software, IBM to Release Mail Program Blueprint|first=John|last=Markoff|date=December 1998|accessdate=17 September 2017}}
4. ^{{Cite web|url=http://www.securityspace.com/s_survey/data/man.201711/mxsurvey.html|title=E-Soft MX survey |website=securityspace.com |publisher=E-Soft Inc. |date={{date|2017-12-01}} |accessdate={{date|2017-12-24}}}}
5. ^{{cite web | url = http://netbsd.org/docs/guide/en/chap-mail.html | title = The NetBSD Guide. Chapter 27. Mail and news. | accessdate = 2010-05-10}}
6. ^{{cite web |url=https://help.ubuntu.com/community/Postfix |title=Postfix |publisher=Community Documentation, Ubuntu Wiki |accessdate=2012-03-09}}
7. ^{{cite book| last1 = Hontañón| first1 = Ramón J| title = Linux Security| publisher = Sybex| date = July 10, 2001| location = San Francisco| page = 166| url = https://books.google.com/books?id=wYBbM8ol2McC&pg=PA166&lpg=PA166&dq=postfix+resilient+against+buffer+overflow&source=bl&ots=PQLUXJcGfA&sig=IeKTayHKaZHj_8j4vF-_wD0x3w4&hl=en&ei=Pq7lTNuQKYaglAfY5IzfCw&sa=X&oi=book_result&ct=result&resnum=8&ved=0CE0Q6AEwBw| accessdate = 2011-09-21| isbn = 0-7821-2741-X}}
8. ^1 {{cite web | url = https://groups.google.com/forum/?fromgroups=#!topic/mailing.postfix.users/pPcRJFJmdeA | title = Bulk Mailing Performance | accessdate=2012-09-09}}
9. ^Postfix-Tutorial.com: Postfix, Courier/POP, SASL & Spamassassin – with MySQL admin
10. ^Postfix Announcements
11. ^{{cite web |url=http://www.postfix.org/announcements/postfix-2.10.3.html |title=Postfix legacy releases 2.10.3, 2.9.9, 2.8.17, and 2.7.16| work=Postfix.org| accessdate=2014-01-24}}
12. ^{{cite web |url=http://www.postfix.org/announcements/postfix-2.8.0.html |title=Postfix stable release 2.8.0 |work=Postfix.org |accessdate=2013-09-19}}
13. ^{{cite web |url=http://www.postfix.org/announcements/postfix-2.11.4.html |title=Postfix legacy releases 2.11.4, 2.10.6, 2.9.12, and 2.8.20 |work=Postfix.org| accessdate=2015-04-29}}
14. ^{{cite web |url=http://www.postfix.org/announcements/postfix-2.9.0.html |title=Postfix stable release 2.9.0 |work=Postfix.org| accessdate=2013-09-19}}
15. ^{{cite web |url=http://www.postfix.org/announcements/postfix-3.1.0.html |title=Postfix stable release 3.1.0 |work=Postfix.org| accessdate=2017-11-21}}
16. ^{{cite web |url=http://www.postfix.org/announcements/postfix-2.10.0.html |title=Postfix stable release 2.10.0| work=Postfix.org|accessdate= 2013-09-19}}
17. ^{{cite web |url=http://www.postfix.org/announcements/postfix-3.1.4.html |title=Postfix stable release 3.1.4 and legacy releases 3.0.8, 2.11.9 |work=Postfix.org| accessdate=2017-11-20}}
18. ^{{cite web |url=http://www.ietf.org/mail-archive/web/dane/current/msg06294.html |title=Postfix 2.11.0-RC2 available with feature-complete DANE support |work=IETF.org |accessdate=2015-04-29}}
19. ^{{cite web |url=http://www.postfix.org/announcements/postfix-2.11.0.html |title=Postfix stable release 2.11.0| work=Postfix.org|accessdate=2014-01-24}}
20. ^{{cite web |url=http://www.postfix.org/announcements/postfix-3.0.0.html |title=Postfix stable release 3.0.0| work=Postfix.org|accessdate= 2015-04-29}}
21. ^{{cite web |url=http://www.postfix.org/announcements/postfix-3.1.0.html |title=Postfix stable release 3.1.0| work=Postfix.org|accessdate=2016-03-29}}
22. ^{{cite web |url=http://www.postfix.org/announcements/postfix-3.2.0.html |title=Postfix stable release 3.2.0| work=Postfix.org|accessdate=2017-11-21}}

Further reading

  • {{cite book|author=Kyle D. Dent|title=Postfix: The Definitive Guide|publisher=O'Reilly Media|year=2003|isbn=0-596-00212-2}}
  • {{cite book|author=Ralf Hildebrandt and Patrick Koetter|title=The book of Postfix: state-of-the-art message transport|publisher=No Starch Press|year=2005|isbn=1-59327-001-1}}

External links

  • Official website
  • [https://calomel.org/postfix.html Postfix "how to" with configuration examples and explanation]
  • Postfix policy delegation server
  • [https://www.policyd.org/ Postfix policy delegation server]
  • [https://web.archive.org/web/20111005051354/http://www.360is.com/06-postfix.htm Postfix introduction and analysis for secure environments (archived version)]
  • {{freenode|postfix}} on freenode
  • Postfix Architecture Overview
{{Email servers}}{{Authority control}}{{DEFAULTSORT:Postfix (Software)}}

6 : Message transfer agents|Free email server software|IBM software|Unix network-related software|1997 software|Email server software for Linux
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