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annotate thesis/tex/3-MailTransferAgents.tex @ 317:3b7680af0ebe

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author meillo@marmaro.de
date Wed, 21 Jan 2009 17:26:18 +0100
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children 426ad56236ce
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meillo@89 1 \chapter{Mail transfer agents}
meillo@254 2 \label{chap:mail-transfer-agents}
meillo@89 3
meillo@217 4 After having analyzed the market for electronic mail and identified upcoming trends, in the last chapter; this chapter takes a look at \mta{}s---the intelligent nodes and thus the most important parts of the email infrastructure. The \MTA{}s will be grouped by similarities first. Then the four most popular \freesw\ \mta{}s, will be presented to the reader in a short overview and with the most important facts. At the end of this chapter these programs will be compared.
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meillo@120 9 \section{Types of MTAs}
meillo@217 10 ``Mail transfer agent'' is a term covering a variety of programs. One thing is common to them: they transfer email from one senders to recipients.
meillo@89 11
meillo@248 12 This is how \person{Bryan Costales} defines a \mta:
meillo@117 13 \begin{quote}
meillo@217 14 A mail transfer agent (\MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
meillo@218 15 \hfill\cite{costales97}
meillo@117 16 \end{quote}
meillo@217 17 \name{The Free Dictionary} is a bit more concrete on the term:
meillo@117 18 \begin{quote}
meillo@217 19 Message Transfer Agent - (\MTA, Mail Transfer Agent): Any program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another \MTA, [...] it [...] delivers it to any local addressees and/or forwards it to other remote \MTA{}s (routing) for delivery to remote recipients.
meillo@218 20 \hfill\citeweb{website:thefreedictionary}
meillo@117 21 \end{quote}
meillo@89 22
meillo@259 23 \person{Dent} and \person{Hafiz} agree \cite[page 19]{dent04} \cite[pages 3-5]{hafiz05}.
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meillo@259 25 Common to all \MTA{}s is the transport of mail; this is the actual job. Besides this similarity, \MTA{}s can be very different. Some of them have \NAME{POP3} and/or \NAME{IMAP} servers included. Some can fetch mails through these protocols. Others have have all features you can think of. And maybe there are some that do nothing else but transporting email.
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meillo@117 27 Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside.
meillo@117 28
meillo@117 29
meillo@120 30 \subsubsection*{Relay-only MTAs}
meillo@89 31 \label{subsec:relay-only}
meillo@217 32 Also called \name{forwarders}. This is the most simple kind of \MTA. It transfers mail only to defined \name{smart hosts}\footnote{\name{smart host}s are \MTA{}s that receives email and route it to the actual destination}. \name{Relay-only} \MTA{}s do not receive mail from outside the system, and they do not deliver locally. All they do is transfer mail to a specified smart host for further relay.
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meillo@89 34 Most \MTA{}s can be configured to act as such a \name{forwarder}. But this is usually an additional functionality.
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meillo@217 36 One uses this kind of \MTA\ to give a system the possibility to send mail, without the need to do lots of configuration. In a local network, usually the clients are set up with relay-only \MTA{}s, while there is one mail server that acts as a \name{smart host}. The ``dumb'' clients send mail to this \name{mail server} which does all further work.
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meillo@217 38 Example programs in that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
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meillo@89 40
meillo@117 41 \subsubsection*{Groupware}
meillo@217 42 Normally the term ``groupware'' does not mean one single program, but a suite of programs. They build a framework which is then populated with various modules that provide the actual functionality. Modules for mail transfer, file storage, calendars, resource management, instant messaging, and more, are commonly available.
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meillo@217 44 These program suites are used if the main work to do is providing integrated communication facilities and team working support for a group of people. Mail transfer is only one part of the problem to solve. The most common scenario are companies. They have \name{groupware} running to provide adequate services for their teams to work efficiently. But one may use \name{groupware} on the home server for his family members also.
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meillo@217 46 Examples for groupware are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org}, and \name{eGroupWare}.
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meillo@89 48
meillo@120 49 \subsubsection*{``Real'' MTAs}
meillo@217 50 There is a third type of \mta{}s in between the minimalistic \name{relay-only} \MTA{}s and the feature loaded \name{groupware}. Those programs may be named ``real \MTA{}s'', or ``proper \MTA{}s'', though there is no common name. They are what is meant with the term ``\mta''---programs that transfer mail between hosts.
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meillo@224 52 Common to them is their focus on transferring email, while being able to act as \name{smart host}s. Their variety ranges from ones mostly restricted to mail transfer (e.g.\ \qmail) to others having interfaces for adding further mail processing modules (e.g.\ \postfix). This group covers everything in between the other two groups.
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meillo@265 54 ``Real \MTA{}s'' include \sendmail, \exim, \qmail, and \postfix.
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meillo@89 56
meillo@117 57 \subsubsection*{Other segmenting}
meillo@124 58 \name{Mail transfer agents} can also be split in other ways.
meillo@308 59
meillo@308 60 Due to \sendmail's significance in the early times of email, compatibility interfaces for \sendmail\ are important for \unix\ \MTA{}s. The reason is that many mail applications simply the \sendmail\ \MTA\ to be installed on the system. Being not \emph{sendmail-compatible} may not matter for some fields of action, but makes the program ineligible for serving as a general purpose \MTA\ on \unix\ systems. Hence being sendmail-compatible is a major property of a \mta. %todo: how many MTAs are sendmail-compatible?
meillo@124 61 \MTA{}s not having a \emph{sendmail-compatible} interface or not offering it as a compatibility add-on, will not be covered here. One example for such a program is \name{Apache James}. %FIXME: check if correct
meillo@89 62
meillo@217 63 Another separation can be done between \freesw\ \MTA{}s and proprietary ones. Many of the \MTA{}s for \unix\ systems are \freesw. Only these are regarded in the following sections, because comparing \freesw\ with proprietary or commercial software is not what typical users of programs like \masqmail\ do. %fixme: what are typical users?
meillo@217 64 Comparison with non-free programs may be a point for large \freesw\ projects, trying to step into the business world. Small projects, mostly used by individuals at home, %fixme: is this the right target field? see chap02
meillo@217 65 need to be compared against other projects of similar shape. The document is seen from \masqmail's point of view---an \MTA\ for \unix\ systems on home servers and workstations---so non-free software is out of the way.
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meillo@265 70
meillo@265 71
meillo@265 72 \subsubsection*{\masqmail's position}
meillo@265 73
meillo@265 74 Now, where does \masqmail\ fit in? It is not groupware nor a simple forwarder, thus it belongs to the ``real \MTA{}s''. Additionally it is Free Software and is intended to be sendmail-compatible. This makes it similar to \sendmail, \exim, \qmail, and \postfix. \masqmail\ is intended to be a replacement for those \MTA{}s.
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meillo@265 76 But: It was not designed to be used as a general replacement for them (see: section \ref{sec:masqmail-target-field}). In fact, \masqmail\ is only a replacement \emph{in some situations}. This primary excludes working in an untrusted environment.
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meillo@120 87 \section{Popular MTAs}
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meillo@308 89 This section introduces a selection of popular \MTA{}s; they are the most likely substitutes for \masqmail. All are sendmail-compatible ``smart'' \freesw\ \MTA{}s that focus on mail transfer, as is \masqmail.
meillo@89 90
meillo@217 91 The programs chosen to be compared, with each other and with \masqmail, are: \sendmail, \exim, \qmail, and \postfix. They are the most important representatives of the regarded group.
meillo@117 92
meillo@145 93
meillo@145 94 \subsection{Market share analysis}
meillo@145 95
meillo@217 96 \MTA\ statistics are rare, differ, and good data is hard to collect. These points are bad if one wants good statistics. Thus it is obvious there are only few available.
meillo@217 97
meillo@248 98 Table \ref{tab:mta-market-share} shows the most used \MTA{}s determined by three different statistics. The first was done by \person{Daniel~J.\ Bernstein} (the author of \qmail) in 2001 \cite{bernstein01}. The second is by \person{Simpson} and \person{Bekman} in 2007 and was published on \name{O'ReillyNet} \cite{simpson07}. And the third is from \name{MailRadar.com} with unknown date\footnote{The footer of the website shows ``Copyright 2007'' but more likely does this refer to the whole website.} \citeweb{mailradar:mta-stats}.
meillo@117 99
meillo@130 100 \begin{table}
meillo@130 101 \begin{center}
meillo@271 102 \input{tbl/mta-market-share.tbl}
meillo@130 103 \end{center}
meillo@130 104 \caption{Market share of \MTA{}s}
meillo@130 105 \label{tab:mta-market-share}
meillo@130 106 \end{table}
meillo@89 107
meillo@316 108 All surveys show high market shares for the four \MTA{}s: \sendmail, \exim, \qmail, and \postfix. Only the \name{Microsoft} mail server software and \name{IMail} have comparable large shares. Other \freesw\ \mta{}s (\name{smail}, \name{zmailer}, \NAME{MMDF}, \name{courier-mta}) are less important and seldom used.
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meillo@217 110 The three surveys base on different data. \person{Bernstein} took 1\,000\,000 randomly chosen \NAME{IP} addresses, containing 39\,206 valid hosts; 958 of them accepted \NAME{SMTP} connections. The \person{Simpson} and \person{Bekman} survey used only domains owned by companies; in total 400\,000 hosts. \name{MailRadar} scanned 2\,818\,895 servers, leading to 59\,209 accepted connections.
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meillo@225 112 All surveys show \sendmail\ to be the most popular \MTA. \postfix, \qmail, and \exim\ are among the best seven in each. \exim\ has slightly smaller shares than the other two. The four together share more than half of the market according to \person{Bernstein} and the \name{MailRadar} statistics. \person{Simpson} and \person{Bekman} have their share to be somewhere between a third and the half. This uncertainty comes from the large amount of unidentifiable \MTA{}s.
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meillo@225 114 The 22 percent of \name{mail security layers} in the \name{O'Reilly} survey is remarkable. Mail security layers are software guards between the network and the \mta\ that filter unwanted mail before it reaches the \MTA. This increases security by filtering malicious content and by blocking attacks against the \MTA. This large share may be a result of only regarding business mail servers. The problem concerning the survey is the disguise of the \mta\ working behind the security layer. It seems wrong to assume equal shares for the \MTA{}s behind the guards as for the unguarded \MTA{}s, because mail security layers will be more often used to guard weak \MTA{}s, as strong ones do not need them so much. This needs to be kept in mind when using the \name{O'Reilly} survey.
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meillo@225 116 The date of the \name{Mailradar} statistics is not mentioned with it; a mail to \name{Mailradar} asking for information was not replied, unfortunately. However, it seems quite sure that the statistics were published after 2001, caused by the \sendmail\ and \postfix\ shares. But to decide whether before or after the one from \name{O'Reilly} would be just guessing.
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meillo@145 118
meillo@145 119 \subsection{The four major Free Software MTAs}
meillo@143 120
meillo@248 121 Now follows a small introduction to the four programs chosen for comparison. \masqmail\ is not presented here, as it was already introduced in chapter \ref{chap:introduction}. Longer introductions, including analysis and comparison, were written by \person{Jonathan de Boyne Pollard} \cite{jdebp}.
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meillo@117 124
meillo@120 125 \subsubsection*{sendmail}
meillo@89 126 \label{sec:sendmail}
meillo@217 127 \sendmail\ is the best known \mta, since it was one of the first and surely the one that made \MTA{}s popular. It also was shipped as default \MTA{}s by many vendors of \unix\ systems. %fixme: ref
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meillo@248 129 The program was written by \person{Eric Allman} as the successor of his program \name{delivermail}. \person{Allman} was not the only one working on the program. Other people developed own versions of it and a variety of flavors came up, especially in the late eighties when Allman was inactive. %fixme: ref
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meillo@224 131 \sendmail\ designed to transfer mails between different protocols and networks, this lead to a very flexible, though complex, configuration.
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meillo@312 133 It was first released with \NAME{BSD} 4.1c in 1983.
meillo@312 134 %todo: write about its importance and about sendmail-compat
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meillo@312 136 The latest version is 8.14.3 from May 2008. The program is distributed under the \name{Sendmail License} as both, \freesw\ and proprietary software.
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meillo@128 138 Further development will go into the project \name{MeTA1} (the former name was \name{sendmail X}) which succeeds \sendmail.
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meillo@217 140 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and in the, so called, ``Bat Book'' \cite{costales97}.
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meillo@117 143
meillo@120 144 \subsubsection*{exim}
meillo@117 145 \label{sec:exim}
meillo@248 146 \exim\ was started in 1995 by \person{Philip Hazel} at the \name{University of Cambridge}. It is a fork of \name{smail-3}, and inherited a monolithic architecture similar to \sendmail's. But having no separation of the individual components of the system did not hurt. Its security is quite good. %fixme: ref
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meillo@217 148 \exim\ is highly configurable, especially in the field of mail policies. This makes it easy to specify how mail is routed through the system and who is allowed to send email to whom. Also interfaces for integration of virus and spam checkers are provided by design. %fixme: ref
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meillo@117 150 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
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meillo@217 152 One finds \exim\ on its homepage \citeweb{exim:homepage}. The standard literature is \person{Hazel}'s \exim\ book \cite{hazel01}.
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meillo@120 156 \subsubsection*{qmail}
meillo@89 157 \label{sec:qmail}
meillo@248 158 \qmail\ is seen by its community as ``a modern SMTP server which makes sendmail obsolete'' \citeweb{qmail:homepage2}. It was written by \person{Daniel~J.\ Bernstein} starting in 1995. His primary goal was to create a secure \MTA\ to replace the popular, but vulnerable, \sendmail. %fixme: ref
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meillo@223 160 \qmail\ first introduced many innovative concepts in \mta\ design. The most obvious contrast to \sendmail\ and \exim\ is its modular design. But \qmail\ was not the first modular \MTA. \NAME{MMDF}, which predates even \sendmail, was modular too. Regardless of \NAME{MMDF}'s modular architecture, \qmail\ is generally seen as the first security-aware \MTA. %fixme:ref
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meillo@225 162 The latest release of \qmail\ is version 1.03 from July 1998. In November 2007, afterwards, \qmail's source was put into the \name{public domain}. This makes it Free Software.
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meillo@223 164 Because of \person{Bernstein}'s inactivity though changing requirements since 1998, ``[a] motley krewe of qmail contributors (see the README) has put together a netqmail-1.06 distribution of qmail. It is derived from Daniel Bernstein's qmail-1.03 plus bug fixes, a few feature enhancements, and some documentation.'' \citeweb{netqmail:homepage}.
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meillo@248 166 \qmail's homepages are \citeweb{qmail:homepage1} and \citeweb{qmail:homepage2}. The best book about \qmail, from \person{Bernstein}'s view, is \person{Dave Sill}'s handbook \cite{sill02}. His free available guide ``Life with qmail'' is another valuable source \cite{lifewithqmail}.
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meillo@117 169
meillo@120 170 \subsubsection*{postfix}
meillo@89 171 \label{sec:postfix}
meillo@248 172 The \postfix\ project started in 1999 at \name{IBM research}, then called \name{VMailer} or \name{IBM Secure Mailer}. \person{Wietse Venema}'s program ``attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different.''\citeweb{postfix:homepage} In fact, \postfix\ was mainly designed after qmail's architecture to gain security. But in contrast to \qmail\ it aims much more on being fast and full-featured.
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meillo@132 174 Today \postfix\ is taken by many \unix\ systems and \gnulinux\ distributions as default \MTA.
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meillo@312 176 The latest stable version is numbered 2.5.6 from December 2008. \postfix\ is covered by the \name{IBM Public License 1.0} which is a \freesw\ license.
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meillo@217 178 Additional information can be retrieved from the program's homepage \citeweb{postfix:homepage}. \person{Dent}'s \postfix\ book \cite{dent04} claims to be ``the definitive guide'', and it is.
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meillo@120 185 \section{Comparison of MTAs}
meillo@308 186 \label{sec:mta-comparison}
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meillo@312 188 This section does not try to provide a throughout \MTA\ comparison, because this is already done by others. Remarkable comparisons are the one by \person{Dan Shearer} \cite{shearer06} and a discussion on the mailing list \name{plug@lists.q-linux.com} \cite{plug:mtas}. Tabular overviews may be found at \citeweb{mailsoftware42}, \citeweb{wikipedia:comparison-of-mail-servers}, and \cite[section 1.9]{lifewithqmail}.
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meillo@316 190 Here provided is an overview important properties of the four previously introduced \MTA{}s. The data comes from the above stated sources and is collected in table \ref{tab:mta-comparison}\footnote{The lines of code were measured with \person{David~A.\ Wheeler}'s \name{sloccount} \citeweb{sloccount}.}.
meillo@126 191
meillo@117 192 \begin{table}
meillo@217 193 % FIXME: improve table data!!!
meillo@126 194 \begin{center}
meillo@271 195 \input{tbl/mta-comparison.tbl}
meillo@126 196 \end{center}
meillo@312 197 \caption{Comparison of \MTA{}s}
meillo@126 198 \label{tab:mta-comparison}
meillo@117 199 \end{table}
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meillo@89 201
meillo@201 202 \subsubsection*{Architecture}
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meillo@132 204 Architecture is most important when comparing \MTA{}s. Many other properties of a program depend on its architecture. %fixme: add ref?
meillo@313 205 \person{Munawar Hafiz} \cite{hafiz05} discusses in detail on \MTA\ architecture, comparing \sendmail, \qmail, \postfix, and \name{sendmail X}. \person{Jonathan de Boyne Pollard}'s \MTA\ review \cite{jdebp} is a source too.
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meillo@132 207 Two different architecture types show off: monolithic and modular \mta{}s.
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meillo@313 209 Monolithic \MTA{}s are \sendmail, \name{smail}, \exim, and \masqmail. They all consist of one single \emph{setuid root}\footnote{\emph{setuid root} lets a program run with the rights of its owner, here root. This is considered to be a security risk. Thus it it should be avoided if possible.} binary which does all the work.
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meillo@313 211 Modular \MTA{}s are \NAME{MMDF}, \qmail, \postfix, and \name{MeTA1}. They consist of several programs, each doing a part of the overall job. The different programs run with the least permissions the need, and \emph{setuid root} can be avoided completely.
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meillo@313 213 The architecture does not directly define the program's security, but ``[t]he goal of making a software secure can be better achieved by making the design simple and easier to understand and verify'' \cite[chapter 6]{hafiz05}. \exim, though being monolithic, has a fairly clean security record. But it is very hard to keep the security up, as the program growth. \person{Wietse Venema} (the author of \postfix) says, it was the architecture that enabled \postfix\ to grow without running into security problems. \cite[page 13]{venema:postfix-growth}
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meillo@217 215 The modular design, with each sub-program doing one part of the overall job, conforms to the \name{Unix Philosophy}. The Unix Philosophy \cite{gancarz95} demands ``small is beautiful'' and ``make each program do one thing well''. Monolithic \MTA{}s fail here.
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meillo@132 217 Today modular \mta\ architectures are the state-of-the-art.
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meillo@217 220 \subsubsection*{Spam checking and content processing}
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meillo@313 222 Spam and malware increased during the last years. Today it is important for an \MTA\ to be able to provide checking for bad mail. This can be done by implementing functionality into the \MTA, or by invoking external programs to do this job.
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meillo@313 224 \sendmail\ invented \name{milter} which is the common abbreviation for the \name{sendmail mail filter} \NAME{API}. It is used to interface external programs of various kind. \postfix\ adopted the \name{milter} interface, but is also able to easily include scanning modules into its modular structure. \qmail\ is pretty old and did not evolve with the changing market situation. Anyhow, its modular structure enables external scanners to be included into \qmail. \exim\ has the advantage that is was designed with the goal to provide extensive scanning facilities. It is therefore very good suited to scan itself or invoke external scanners.
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meillo@313 226
meillo@313 227 \subsubsection*{Provider independence}
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meillo@217 229 In chapter \ref{chap:market-analysis}, it was tried to figure out trends and future requirements for \MTA{}s. The four programs are compared on these (possible) future requirements now.
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meillo@313 231 The first trend was provider independence, requiring easy configuration. \postfix\ seems to do best here. It used primary two configuration files (\path{master.cf} and \path{main.cf}) which are easy to manage. \sendmail\ appears to have a bad position. Its configuration file \path{sendmail.cf} is cryptic and very complex (it has legendary Turing-completeness) thus it needs simplification wrappers around it to provide easier configuration. They exist in form of the \name{m4} macros that generate a \path{sendmail.cf} file. But adjusting the generated result by hand appears to be necessary for non-trivial configurations. \qmail's configuration files are simple, but the whole system is complex to set up; it requires various system users and is hardly usable without applying several patches to add functionality that is required nowadays. \name{netqmail} is the community effort to help in the latter point. \exim\ has only one single configuration file (\path{exim.conf}), but it suffers most from its flexibility---like \sendmail. Flexibility and easy configuration are almost always contrary goals.
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meillo@313 233 \subsubsection*{Performance}
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meillo@313 235 As second trend, the decreasing necessity for high performance was identified. This goes along with the move of \MTA{}s from service providers to home servers. \postfix\ focuses much on performance, this might not be an important point in the future. Of course there still will be the need for high performance \MTA{}s, but a growing share of the market will not require high performance. Energy and space efficiency is related to performance; it is a similar goal in a different direction. Optimization, be it for performance or other efficiencies, is often in contrast to simplicity and clarity, which effect security. Optimizing does in most times decrease the simplicity and clarity. Simple \mta{}s not aiming for high performance are what is needed in future. The simple design of \qmail (\qmail\ is still fast) seems to be a good example.
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meillo@313 237 \subsubsection*{Security}
meillo@217 238
meillo@217 239 The third trend---even more security awareness---is addressed by each of the four programs. It seems as if all widely used \mta{}s provide good security nowadays. Even \sendmail\ can be considered secure today. %fixme:ref
meillo@217 240 But the modular architecture, used by \qmail\ and \postfix, is generally seen to be conceptually more secure, however. %fixme: ref
meillo@132 241 \sendmail's creators have started \name{MeTA1}, a modular \MTA\ merging the best of \qmail\ and \postfix, to replace the old \sendmail. It will be interesting to watch \exim's future---will it become modular too?
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meillo@287 248 \section{Summary}
meillo@193 249
meillo@313 250 This chapter first took an overview over the field of \MTA{}s. Three major types of \MTA{}s were identified: Relay-only \MTA{}s (also called forwarders), groupware, and the ``real \MTA{}s''. \masqmail\ belongs to the last group, it is additionally sendmail-compatible and Free Software.
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meillo@313 252 Next a look at the market shares of \MTA{}s was taken and it was seen that four \MTA{}s, that are similar to \masqmail, have high importance: \sendmail, \postfix, \qmail, and \exim. Their combined share is between one third and the half of the market. The rest is split between proprietary \MTA{}s, unknown software behind mail security layers, and a rest of really small market shares.
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meillo@313 254 Each one of these four major Free Software \MTA{}s was presented afterwards and at the end, these programs were compared on some selected properties.
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meillo@313 256 Now, the reader should have a general knowledge about the four important \MTA{}s. Further chapters will refer frequently to them.
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meillo@132 259 %todo: my own poll (?)
meillo@117 260