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51-612: Exim has been ported to most Unix-like systems, as well as to Microsoft Windows using the Cygwin emulation layer. Exim 4 is currently the default MTA on Debian Linux systems. Many Exim installations exist, especially within Internet service providers and universities in the United Kingdom. Exim is also widely used with the GNU Mailman mailing list manager, and cPanel . In March 2023

102-456: A PC without the need for actual porting (instead relying on the common porting of individual component libraries ). Porting arcade games to home systems with inferior hardware was difficult. The ported version of Pac-Man for the Atari 2600 omitted many of the visual features of the original game to compensate for the lack of ROM space and the hardware struggled when multiple ghosts appeared on

153-399: A central queue manager (i.e. an equivalent of qmail-send , qmgr , or scheduler ). There is thus no centralized load balancing of queue processing (leading to disproportionate amounts of time being spent on processing the same queue entries repeatedly). System-wide remote transport concurrency is unlimited by default (leading to a " thundering herd problem " when multiple messages addressed to

204-405: A cheaper solution). The number of significantly different CPUs and operating systems used on the desktop today is much smaller than in the past. The dominance of the x86 architecture means that most desktop software is never ported to a different CPU. In that same market, the choice of operating systems has effectively been reduced to three: Microsoft Windows , macOS , and Linux . However, in

255-399: A home console with the same specifications. This allowed arcade perfect games to be played at home. A "console port" is a game that was originally made for a console before an identical version is created which can be played on a personal computer . This term has been widely used by the gaming community. The process of porting a game from a console to a PC is often regarded negatively due to

306-447: A machine independent intermediate code in order to enhance portability of the compiler and minimize design efforts. The intermediate language defines a virtual machine that can execute all programs written in the intermediate language (a machine is defined by its language and vice versa). The intermediate code instructions are translated into equivalent machine code sequences by a code generator to create executable code . It

357-439: A policy was not always feasible; Bunten stated that "M.U.L.E. can't be done for an Apple", and that the non-Atari versions of The Seven Cities of Gold were inferior. Compute!'s Gazette wrote in 1986 that when porting from Atari to Commodore the original was usually superior. The latter's games' quality improved when developers began creating new software for it in late 1983, the magazine stated. In porting arcade games ,

408-467: A single domain are submitted at once) but can be limited by the configuration. In Philip Hazel's own words: In 1997, Hazel replaced Exim's POSIX regular expression library written by Henry Spencer with a new library he developed called PCRE ( Perl Compatible Regular Expressions). Perl regular expressions are much more powerful than POSIX and other common regular expressions, and PCRE has become popular in applications other than Exim. In 2021 (after

459-534: A spam relay, though modern spam delivery techniques are less influenced by bounce behavior. qmail was released to the public domain in November 2007. Until November 2007, qmail was license-free software , with permission granted for distribution in source form or in pre-compiled form (a "var-qmail package") only if certain restrictions (primarily involving compatibility) were met. This unusual licensing arrangement made qmail non-free according to some guidelines (such as

510-589: A study performed by E-Soft, Inc., approximated that 59% of the publicly reachable mail-servers on the Internet ran Exim. The first version of Exim was written in 1995 by Philip Hazel for use in the University of Cambridge Computing Service ’s e-mail systems. The name initially stood for EX perimental I nternet M ailer. It was originally based on an older MTA, Smail -3, but it has since diverged from Smail-3 in its design and philosophy. Exim, like Smail , still follows

561-449: A team of maintainers. Exim continues to be maintained actively, with frequent releases. Porting In software engineering , porting is the process of adapting software for the purpose of achieving some form of execution in a computing environment that is different from the one that a given program (meant for such execution) was originally designed for (e.g., different CPU , operating system, or third party library ). The term

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612-416: Is a practical attack, arguing that no real-world deployment of qmail would be susceptible. Configuration of resource limits for qmail components mitigates the vulnerability. On November 1, 2007, Bernstein raised the reward to US$ 1000. At a slide presentation the following day, Bernstein stated that there were 4 "known bugs" in the ten-year-old qmail-1.03, none of which were "security holes". He characterized

663-443: Is also possible to skip the generation of machine code by actually implementing an interpreter or JIT for the virtual machine. The use of intermediate code enhances portability of the compiler, because only the machine dependent code (the interpreter or the code generator) of the compiler itself needs to be ported to the target machine. The remainder of the compiler can be imported as intermediate code and then further processed by

714-480: Is also used when software/hardware is changed to make them usable in different environments. Software is portable when the cost of porting it to a new platform is significantly less than the cost of writing it from scratch. The lower the cost of porting software relative to its implementation cost, the more portable it is said to be. The term "port" is derived from the Latin portāre , meaning "to carry". When code

765-461: Is appropriate to place in the run-time user; the latter was fixed in a security lockdown in revision 4.73, one of the very rare occasions when Exim has broken backwards compatibility with working configurations. Exim is highly configurable and therefore has features that are lacking in other MTAs. It has always had substantial facilities for mail policy controls, providing facilities for the administrator to control who may send or relay mail through

816-495: Is classed as a bug. The documentation consists of The Exim Specification and two ancillary files: the experimental specification for features that might disappear and "NewStuff", which tracks very recent changes that might not have been fully integrated into the main specification. The Exim Specification is available in multiple formats, including online in HTML and in plain-text for fast searching. The document preparation system ensures that

867-426: Is designed to have better performance than Simple Mail Transfer Protocol (SMTP), the de facto standard; and Quick Mail Queuing Protocol (QMQP), a network protocol designed to share e-mail queues between several hosts. qmail is nearly a completely modular system in which each major function is separated from the other major functions. It is easy to replace any part of the qmail system with a different module as long as

918-403: Is less complex and therefore easier to port than a code generator, because it is not able to do code optimizations due to its limited view of the program code (it only sees one instruction at a time, and users need a sequence to do optimization). Some interpreters are extremely easy to port, because they only make minimal assumptions about the instruction set of the underlying hardware. As a result,

969-479: Is not compatible with a particular operating system or architecture , the code must be "carried" to the new system. The term is not generally applied to the process of adapting software to run with less memory on the same CPU and operating system. Software developers often claim that the software they write is portable , meaning that little effort is needed to adapt it to a new environment. The amount of effort actually needed depends on several factors, including

1020-511: The Atari VCS became the console's killer app despite its differences, while the later Pac-Man port was notorious for its deviations from the arcade version. Arcade-accurate games became more prevalent starting in the 1990s as home consoles caught up to the power of arcade systems. Notably, the Neo Geo system from SNK , which was introduced as a multi-game arcade system, would also be offered as

1071-745: The GNU Compiler Collection , which provides consistent programming languages on different platforms, and Autotools , which automates the detection of minor variations in the environment and adapts the software accordingly before compilation. The compilers for some high-level programming languages (e.g. Eiffel , Esterel ) gain portability by outputting source code in another high level intermediate language (such as C ) for which compilers for many platforms are generally available. Two activities related to (but distinct from) porting are emulating and cross-compiling . Instead of translating directly into machine code , modern compilers translate to

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1122-605: The IETF in RFC 1894, meanwhile advanced to draft standard as RFC 3464, and recommended in the SMTP specification. Some qmail features have been criticized for introducing mail forwarding complications; for instance, qmail's "wildcard" delivery mechanism and security design prevents it from rejecting messages from forged or nonexistent senders during SMTP transactions. In the past, these differences may have made qmail behave differently when abused as

1173-502: The Sendmail design model , where a single binary controls all the facilities of the MTA. Exim has well-defined stages during which it gains or loses privileges . Exim's security has had a number of serious security problems diagnosed over the years. Since the redesigned version 4 was released there have been four remote code execution flaws and one conceptual flaw concerning how much trust it

1224-496: The embedded systems and mobile markets, portability remains a significant issue, with the ARM being a widely used alternative. International standards, such as those promulgated by the ISO , greatly facilitate porting by specifying details of the computing environment in a way that helps reduce differences between different standards-conforming platforms . Writing software that stays within

1275-428: The 4.95 release) Exim transitioned to PCRE2. Historically, Exim used a peculiar version numbering scheme where the first decimal digit is updated only whenever the main documentation is fully up to date; until that time, changes were accumulated in the file NewStuff. For this reason, a 0.01 version change can signify important changes, not necessarily fully documented. In 2005, changes to Exim's version numbering were on

1326-502: The Apple rewrites?" the audience shouted "No!" Garriott responded, "[otherwise] the Apple version will never get done. From a publisher's point of view that's not money wise". Others worked differently. Ozark Softscape , for example, wrote M.U.L.E. for the Atari first because it preferred to develop for the most advanced computers, removing or altering features as necessary during porting. Such

1377-576: The Debian approach diverges significantly from the Exim one it is common to find a lack of support for the Debian approach on the regular Exim mailing-lists, with people advised to ask Debian questions on the Debian-managed mailing-list. The Ubuntu packaging still advises users to use the Debian mailing-list. Exim has extensive and exhaustive documentation; if a feature or some behaviour is not documented then this

1428-484: The Internet before execution can start on the target's Java virtual machine (JVM). Porting is also the term used when a video game designed to run on one platform, be it an arcade , video game console , or personal computer , is converted to run on a different platform, perhaps with some minor differences. From the beginning of video games through to the 1990s, "ports", at the time often known as "conversions", were often not true ports, but rather reworked versions of

1479-602: The bounds specified by these standards represents a practical although nontrivial effort. Porting such a program between two standards-compliant platforms (such as POSIX.1 ) can be just a matter of loading the source code and recompiling it on the new platform, but practitioners often find that various minor corrections are required, due to subtle platform differences. Most standards suffer from "gray areas" where differences in interpretation of standards lead to small variations from platform to platform. There also exists an ever-increasing number of tools to facilitate porting, such as

1530-522: The bug found by Guninski as a "potential overflow of an unchecked counter". "Fortunately, counter growth was limited by memory and thus by configuration, but this was pure luck." On May 19, 2020, a working exploit for Guninski's vulnerability was published by Qualys but exploit authors' state they were denied the reward because it contains additional environmental restrictions. The core qmail package has not been updated for many years. New features were initially provided by third-party patches, from which

1581-512: The configuration file is re-read at every exec, which happens post-fork for receiving inbound connections and at delivery. The second commonly encountered style is the Debian style which is designed to make it easier to have an installed application automatically provide mail integration support without having the administrator edit configuration files. There are a couple of variants of this and Debian provide documentation of their approach as part of

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1632-411: The extent to which the original environment (the source platform ) differs from the new environment (the target platform ), the experience of the original authors in knowing which programming language constructs and third party library calls are unlikely to be portable, and the amount of effort invested by the original authors in only using portable constructs (platform specific constructs often provide

1683-452: The games due to the limitations of different systems. For example, the 1982 game The Hobbit , a text adventure augmented with graphic images, has significantly different graphic styles across the range of personal computers that its ports were developed for. However, many 21st century video games are developed using software (often in C++ ) that can output code for one or more consoles as well as for

1734-406: The higher levels of performance that computers generally have being underutilized, partially due to console hardware being fixed throughout their run (with games being developed for console specs), while PCs become more powerful as hardware evolves, but also due to ported games sometimes being poorly optimized for PCs, or lazily ported. While broadly similar, architectural differences may exist such as

1785-408: The latter machines' sprites and other sophisticated features made porting from them to Apple "far more difficult, perhaps even impossible". Reviews complained of ports that suffered from "Apple conversionitis", retaining the Apple's "lousy sound and black-white-green-purple graphics"; after Garriott's statement, when Dan Bunten asked "Atari and Commodore people in the audience, are you happy with

1836-465: The main section with generic settings and variables, as well as the following optional sections: The configuration file permits inclusion of other files, which leads to two different configuration styles. There are two main schools of configuration style for Exim. The native school keeps the Exim configuration in one file and external files are only used as data sources; this is strongly influenced by Philip Hazel's preferences and notes on performance as

1887-461: The most important at the time were brought together in a single meta-patch called netqmail . qmail was not designed as a drop-in replacement for Sendmail , and does not behave exactly as Sendmail did in all situations. In some cases, these differences in behavior have become grounds for criticism. For instance, qmail's approach to bounce messages (a format called QSBMF) differs from the standard format of delivery status notifications specified by

1938-490: The new module retains the same interface as the original. In 1997, Bernstein offered a US$ 500 reward for the first person to publish a verifiable security hole in the latest software version. In 2005, security researcher Georgi Guninski found an integer overflow in qmail. On 64-bit platforms, in default configurations with sufficient virtual memory, the delivery of huge amounts of data to certain qmail components may allow remote code execution. Bernstein disputes that this

1989-408: The packages. In these approaches, a debconf configuration file is used to build the Exim configuration file, together with templates and directories with configuration fragments. The meta-config is tuned with macros which have names starting DC_ . When the supervisor for exim is invoked it re-processes the configuration files producing a single-file configuration that the exim binary uses. Because

2040-457: The plain-text format is highly usable. Exim has been deployed in busy environments, often handling thousands of emails per hour efficiently. Exim is designed to deliver email immediately, without queueing. However, its queue processing performance is comparatively poor when queues are large (which happens rarely on typical low-traffic sites but can happen regularly on high-traffic sites). Unlike qmail , Postfix , and ZMailer , Exim does not have

2091-404: The ported code generator or interpreter, thus producing the compiler software or directly executing the intermediate code on the interpreter. The machine independent part can be developed and tested on another machine (the host machine ). This greatly reduces design efforts, because the machine independent part needs to be developed only once to create portable intermediate code. An interpreter

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2142-448: The queue manager or the SMTP sender. qmail was also implemented with a security-aware replacement to the C standard library and, as a result, has not been vulnerable to stack and heap overflows, format string attacks or temporary file race conditions . When it was released, qmail was significantly faster than Sendmail, particularly for bulk mail tasks such as mailing list servers. qmail

2193-563: The screen creating a flickering effect. The poor performance of the Atari 2600 Pac-Man is cited by some scholars as a cause of the video game crash of 1983 . Many early ports suffered significant gameplay quality issues because computers greatly differed. Richard Garriott stated in 1984 at Origins Game Fair that Origin Systems developed video games for the Apple II first then ported them to Commodore 64 and Atari 8-bit computers , because

2244-456: The system. In version 4.x this has matured to an Access Control List based system allowing very detailed and flexible controls. The integration of a framework for content scanning, which allowed for easier integration of anti-virus and anti-spam measures, happened in the 4.x releases. This made Exim very suitable for enforcing diverse mail policies. The configuration is done through a (typically single) configuration file, which must include

2295-458: The table of discussion. In more recent times, the document preparation system for Exim has been overhauled and changes are much more likely to just go immediately into The Exim Specification. The 4.70 release just followed on naturally from 4.69 and the 4.6x releases had up-to-date documentation. Philip Hazel retired from the University of Cambridge in 2007 and maintenance of Exim transitioned to

2346-573: The target. According to the designers of the BCPL language, interpreted code (in the BCPL case) is more compact than machine code, typically by a factor of two to one. Interpreted code however runs about ten times slower than compiled code on the same machine. The designers of the Java programming language try to take advantage of the compactness of interpreted code, because a Java program may need to be transmitted over

2397-405: The terms "arcade perfect" or "arcade accurate" were often used to describe how closely the gameplay, graphics, and other assets on the ported version matched the arcade version. Many arcade ports in the early 1980s were far from arcade perfect as home consoles and computers lacked the sophisticated hardware in arcade games, but games could still approximate the gameplay. Notably, Space Invaders on

2448-400: The use of unified memory on a console. Qmail qmail is a mail transfer agent (MTA) that runs on Unix . It was written, starting December 1995, by Daniel J. Bernstein as a more secure alternative to the popular Sendmail program. Originally license-free software , qmail's source code was later dedicated to the public domain by the author. When first published, qmail

2499-402: The virtual machine is even simpler than the target CPU. Writing the compiler sources entirely in the programming language the compiler is supposed to translate, makes the following approach, better known as compiler bootstrapping , feasible on the target machine: The difficult part of coding the optimization routines is done using the high-level language instead of the assembly language of

2550-475: Was originally designed as a way to manage large mailing lists. At the time of qmail's introduction, Sendmail configuration was notoriously complex, while qmail was simple to configure and deploy. qmail encourages the use of several innovations in mail (some originated by Bernstein, others not): qmail also introduces the Quick Mail Transport Protocol (QMTP), an e-mail transmission protocol that

2601-458: Was the first security-aware mail transport agent; since then, other security-aware MTAs have been published. The most popular predecessor to qmail, Sendmail , was not designed with security as a goal and, as a result, has been a perennial target for attackers. In contrast to sendmail, qmail has a modular architecture composed of mutually untrusting components; for instance, the SMTP listener component of qmail runs with different credentials from

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