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annotate thesis/tex/2-MailTransferAgents.tex @ 112:7de7bbebd147

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author meillo@marmaro.de
date Wed, 26 Nov 2008 10:42:57 +0100
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meillo@89 1 \chapter{Mail transfer agents}
meillo@89 2
meillo@89 3 After having read about the history of electronic mail and the basics of \mta{}s in the last chapter, this chapter introduces a group of \mta{}s. Among them, the already mentioned \sendmail. The selected group will be delimited against other groups of \MTA{}s, which are described as well.
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meillo@89 5 The chosen programs will be presented to the reader in a short overview and with the most important facts. The next chapter will show a comparison of these programs in several disciplines.
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meillo@89 7
meillo@89 8 \section{Types of \MTA{}s}
meillo@89 9 ``Mail transfer agent'' is a term covering a variety of programs. One thing is common to them: they transfer email from one \emph{thing} to another. These \emph{things} can be hosts, meaning independent machines, or protocols like \NAME{SMTP} and \NAME{UUCP}, between which mail is transfered.\footnote{\sendmail{}'s initial purpose was moving mail between \NAME{UUCP}, \NAME{SMTP}, and \name{Berknet}.}
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meillo@89 11 Beside this common property, \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@89 13 Following are groups of \mta{}s that will \emph{not} be regarded further.
meillo@89 14
meillo@92 15 \subsection*{Relay-only \MTA{}s}
meillo@89 16 \label{subsec:relay-only}
meillo@89 17 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.
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meillo@89 19 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@89 21 One would use such a program 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 a \name{relay-only} \MTA, while there is one \name{mail server} that acts as a \name{smart host}. The ``dumb'' clients send mail to this one \name{mail server} which does all the work.
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meillo@89 23 Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
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meillo@89 25
meillo@92 26 \subsection*{Groupware}
meillo@89 27 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 actual funktionality. Modules for mail transfer, file storage, calendars, resource management, instant messaging, etc., are commonly available.
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meillo@89 29 One would use one of these program suites if the main work to do is not mail transfer, but providing integrated communication facilities and team working support for a group of people. The most common scenario are companies. They have \name{groupware} running to provide adequate services for their teams to work efficently. But one may use \name{groupware} on the home server for his family members also.
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meillo@89 31 Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.
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meillo@89 33
meillo@92 34 \subsection*{``Real'' \MTA{}s}
meillo@89 35 There is a third type of \mta{}s in between the minimalistic \name{relay-only} \MTA{}s and the bloated \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''.
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meillo@89 37 Common to them is their focus on transfering email, while being able to act as \name{smart host}. Their variety ranges from ones mostly restricted to mail transfer (\name{qmail}) to others already having interfaces for adding further mail processing modules (\name{postfix})---thus everything in between the other two groups. %FIXME: are postfix and qmail good examples?
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meillo@89 39 This group is of importance in this document. The programs selected for the comparison are ``real \MTA{}s''.
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meillo@89 42
meillo@92 43 \subsection*{Non-\emph{sendmail-compatible} \MTA{}s}
meillo@89 44 Due to \sendmail's significance---described in section \ref{sec:sendmail}---compatiblity interfaces for \sendmail\ are of importance for \unix\ \MTA{}s. Being not \emph{sendmail-compatible} does not need to matter for some fields of action, but makes the program ineligible for serving as a general purpose \MTA\ on \unix\ systems.
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meillo@89 46 Hence all \MTA{}s not having a \emph{sendmail-compatible} interface or not offering it as a compatibility addon, will not be covered here.
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meillo@89 48 An Examples here is \name{Apache James}. %FIXME: check if correct
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meillo@89 50
meillo@92 51 \subsection*{Non-free software}
meillo@89 52 Only programs being \freesw\ are regarded, because comparing \freesw\ with proprietary or commercial software is not what typical users of programs like \masqmail\ do. Comparison with those 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, need to be compared against other projects of similar shape.
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meillo@89 54 The comparison should be seen from \masqmail's point of view, so non-free software is out of the way.
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meillo@89 57
meillo@89 58 \section{Popular \MTA{}s}
meillo@89 59 The programs remaining are \emph{sendmail-compatible} ``smart'' \MTA{}s that focus on mail transfer and are \freesw. One would not use a program for a job it is not suited for. Therefor only \mta{}s that are mostly similar to \masqmail\ are regarded.
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meillo@89 61 For the comparision, five programs are taken. These are: \sendmail, \name{qmail}, \name{postfix}, \name{exim}, and \masqmail. The four alternatives to \masqmail\ are the most important representatives of the regarded group. % FIXME: add ref that affirm that
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meillo@89 63 \name{courier-mta} is also a member of this group, being even closer to \name{groupware} than \name{postfix}. It is excluded here, because the \NAME{IMAP} and webmail parts of the mail server suite are more in focus than its \MTA. Common mail server setups even bundle \name{courier-imap} with \name{postfix}.
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meillo@89 65 Other members are: \name{smail}, \name{zmailer}, \name{mmdf}, and more; they all are less important and rarely used.
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meillo@89 67 Following is a small introduction to each of the five programs chosen for comparision.
meillo@89 68
meillo@92 69 \subsection*{\sendmail}
meillo@89 70 \label{sec:sendmail}
meillo@89 71 \sendmail\ is the most popular \mta. Since it was one of the first \MTA{}s and was shipped by many vendors of \unix\ systems.
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meillo@89 73 The program was written by Eric Allman as the successor of his program \name{delivermail}. \sendmail\ was first released with \NAME{BSD} 4.1c in 1983. 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.
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meillo@89 75 \sendmail\ is focused on transfering mails between different protocols and networks, this lead to a very flexible (though complex) configuration.
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meillo@89 77 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 of \name{Sendmail, Inc.}.
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meillo@89 79 Further development will go into the project \name{MeTA1} which succeeds \sendmail.
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meillo@89 81 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}.
meillo@89 82
meillo@89 83
meillo@92 84 \subsection*{\name{qmail}}
meillo@89 85 \label{sec:qmail}
meillo@89 86 \name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''. It was written by Daniel~J.\ Bernstein starting in 1995. His primary goal was to create a secure \MTA\ to replace the popular, but vulnerable, \sendmail.
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meillo@89 88 \name{qmail} first introduced may innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed.
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meillo@89 90 Since November 2007, \name{qmail} is released in the \name{public domain} which makes it \freesw. The latest release is 1.03 from July 1998.
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meillo@89 92 The programs homepages are \citeweb{qmail:homepage1} and \citeweb{qmail:homepage2}. Further information about \name{qmail} is available on \citeweb{lifewithqmail}, \citeweb{wikipedia:qmail} and \citeweb{jdebp}.
meillo@89 93
meillo@89 94
meillo@92 95 \subsection*{\name{postfix}}
meillo@89 96 \label{sec:postfix}
meillo@89 97 The \name{postfix} project was started in 1999 at \name{IBM research}, then called \name{VMailer} or \name{IBM Secure Mailer}. 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, \name{postfix} was mainly designed after qmail's architecture to gain security. But in contrast to \name{qmail} it aims much more on being fast and full-featured.
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meillo@89 99 Today \name{postfix} is taken by many \unix systems and \gnulinux distributions as default \MTA.
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meillo@89 101 The latest stable version is numbered 2.5.5 from August 2008. \name{postfix} is covered by the \name{IBM Public License 1.0} which is a \freesw\ license.
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meillo@89 103 Additional information is available on the program's homepage \citeweb{postfix:homepage}, on \citeweb{jdebp} and \citeweb{wikipedia:postfix}.
meillo@89 104
meillo@89 105
meillo@92 106 \subsection*{\name{exim}}
meillo@89 107 \label{sec:exim}
meillo@89 108 \name{exim} was started in 1995 by Philip Hazel at the \name{University of Cambridge}. Its age is about the same as \name{qmail}'s, but the architecture is totally different.
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meillo@89 110 While \name{qmail} took a completely new approach, \name{exim} forked of \name{smail-3}, and therefor is monolitic like that and like \sendmail. But having no separation of the individual components of the system, like \name{qmail} and \name{postfix} have, did not hurt. Its security is comparably good.
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meillo@89 112 \name{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 check programs are provided by design.
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meillo@89 114 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
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meillo@89 116 One finds \name{exim} on its homepage \citeweb{exim:homepage}. More information about it can be retrieved from \citeweb{wikipedia:exim} and \citeweb{jdebp}.
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meillo@89 122
meillo@89 123 \section{Comparison of \MTA{}s}
meillo@89 124
meillo@89 125 % http://shearer.org/MTA_Comparison
meillo@89 126 % http://www.geocities.com/mailsoftware42/
meillo@89 127 % http://fanf.livejournal.com/50917.html
meillo@89 128 % http://archives.neohapsis.com/archives/postfix/2006-07/1762.html
meillo@89 129 % http://www.oreillynet.com/lpt/a/6849
meillo@89 130 % http://www.mailradar.com/mailstat/
meillo@89 131
meillo@89 132 \subsection{First release}
meillo@89 133 sendmail: 1983
meillo@89 134
meillo@89 135 postfix: 1999
meillo@89 136
meillo@89 137 qmail: 1996 (first beta 0.70), 1997 (first general 1.0)
meillo@89 138
meillo@89 139 exim: 1995
meillo@89 140
meillo@89 141 masqmail: 1999
meillo@89 142
meillo@89 143 exchange: 1993
meillo@89 144
meillo@89 145
meillo@89 146 \subsection{Lines of code (with sloccount on debian packages)}
meillo@89 147 sendmail: 93k
meillo@89 148
meillo@89 149 postfix: 92k
meillo@89 150
meillo@89 151 qmail: 18k
meillo@89 152
meillo@89 153 exim: 54k
meillo@89 154
meillo@89 155 masqmail: 14k
meillo@89 156
meillo@89 157 exchange: (no source available)
meillo@89 158
meillo@89 159
meillo@89 160 \subsection{Architecture}
meillo@89 161 sendmail: monolitic
meillo@89 162
meillo@89 163 postfix: modular
meillo@89 164
meillo@89 165 qmail: modular
meillo@89 166
meillo@89 167 exim: monolitic
meillo@89 168
meillo@89 169 masqmail: monolitic
meillo@89 170
meillo@92 171 Like its anchestor \sendmail, \masqmail\ is a monolitic program. It consists of only one \emph{setuid root}\footnote{Runs as user root, no matter which user invoked it.}\index{setuid root} binary file, named \path{masqmail}. All functionality is included in it; of course some more comes from dynamic libraries linked.
meillo@92 172
meillo@92 173
meillo@89 174 exchange: (unknown)
meillo@89 175
meillo@89 176
meillo@89 177 \subsection{Design goals}
meillo@89 178 sendmail: flexibility
meillo@89 179
meillo@89 180 postfix: performance and security
meillo@89 181
meillo@89 182 qmail: security
meillo@89 183
meillo@89 184 exim: general, flexible \& extensive facilities for checking
meillo@89 185
meillo@89 186 masqmail: for non-permanent internet connection
meillo@89 187
meillo@89 188 exchange: groupware
meillo@89 189
meillo@89 190
meillo@89 191 \subsection{Market share (by Bernstein in 2001)}
meillo@89 192 sendmail: 42\%
meillo@89 193
meillo@89 194 postfix: 1.6\%
meillo@89 195
meillo@89 196 qmail: 17\%
meillo@89 197
meillo@89 198 exim: 1.6\%
meillo@89 199
meillo@89 200 masqmail: (unknown)
meillo@89 201
meillo@89 202 exchange: 18\%
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meillo@89 206
meillo@101 207 << complexity >>
meillo@89 208
meillo@101 209 << security >>
meillo@89 210
meillo@101 211 << simplicity of configuration and administration >>
meillo@89 212
meillo@101 213 << flexibility of configuration and administration >>
meillo@89 214
meillo@101 215 << code size >>
meillo@89 216
meillo@101 217 << code quality >>
meillo@89 218
meillo@101 219 << documentation (amount and quality) >>
meillo@89 220
meillo@101 221 << community (amount and quality) >>
meillo@89 222
meillo@101 223 << used it myself >>
meillo@89 224
meillo@101 225 << had problems with it >>
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meillo@99 230 << quality criteria >> %FIXME
meillo@99 231
meillo@99 232 << standards of any kind >> %FIXME
meillo@99 233
meillo@99 234 << how to compare? >> %FIXME
meillo@99 235
meillo@99 236 << (bewertungsmatrix) objectivity >> %FIXME
meillo@99 237
meillo@99 238 << how many criterias for ``good''? >> %FIXME
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meillo@93 240
meillo@93 241
meillo@101 242 % from the practice of programming
meillo@101 243 % names: are they good?
meillo@101 244 % check the significant number of characters. (intern: 31char, extern: 6char caseless; ProgC p.184)
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