docs/diploma

annotate thesis/tex/2-MailTransferAgents.tex @ 99:d24fdd3d5990

added lots of comments and annotations about what to do
author meillo@marmaro.de
date Sun, 16 Nov 2008 22:29:51 +0100
parents a6f8a93abd64
children 6e2eaf91e59f
rev   line source
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.
meillo@89 4
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.
meillo@89 6
meillo@99 7 \section{Advantages of \MTA{}s}
meillo@99 8
meillo@99 9 << why are they important? >> %FIXME
meillo@99 10
meillo@89 11
meillo@89 12 \section{Types of \MTA{}s}
meillo@89 13 ``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}.}
meillo@89 14
meillo@89 15 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.
meillo@89 16
meillo@89 17 Following are groups of \mta{}s that will \emph{not} be regarded further.
meillo@89 18
meillo@92 19 \subsection*{Relay-only \MTA{}s}
meillo@89 20 \label{subsec:relay-only}
meillo@89 21 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.
meillo@89 22
meillo@89 23 Most \MTA{}s can be configured to act as such a \name{forwarder}. But this is usually an additional functionality.
meillo@89 24
meillo@89 25 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.
meillo@89 26
meillo@89 27 Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
meillo@89 28
meillo@89 29
meillo@92 30 \subsection*{Groupware}
meillo@89 31 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.
meillo@89 32
meillo@89 33 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.
meillo@89 34
meillo@89 35 Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.
meillo@89 36
meillo@89 37
meillo@92 38 \subsection*{``Real'' \MTA{}s}
meillo@89 39 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''.
meillo@89 40
meillo@89 41 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?
meillo@89 42
meillo@89 43 This group is of importance in this document. The programs selected for the comparison are ``real \MTA{}s''.
meillo@89 44
meillo@89 45
meillo@89 46
meillo@92 47 \subsection*{Programs to sort out}
meillo@89 48
meillo@89 49 \name{Mail transfer agent}s can be segmented in various ways, apart from the classification above. Groups of programs wiproperties significantly different from \masqmail\ will be sorted out now.
meillo@89 50
meillo@92 51 \subsection*{Non-\emph{sendmail-compatible} \MTA{}s}
meillo@89 52 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.
meillo@89 53
meillo@89 54 Hence all \MTA{}s not having a \emph{sendmail-compatible} interface or not offering it as a compatibility addon, will not be covered here.
meillo@89 55
meillo@89 56 An Examples here is \name{Apache James}. %FIXME: check if correct
meillo@89 57
meillo@89 58
meillo@92 59 \subsection*{Non-free software}
meillo@89 60 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.
meillo@89 61
meillo@89 62 The comparison should be seen from \masqmail's point of view, so non-free software is out of the way.
meillo@89 63
meillo@89 64
meillo@89 65
meillo@89 66 \section{Popular \MTA{}s}
meillo@89 67 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.
meillo@89 68
meillo@89 69 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
meillo@89 70
meillo@89 71 \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}.
meillo@89 72
meillo@89 73 Other members are: \name{smail}, \name{zmailer}, \name{mmdf}, and more; they all are less important and rarely used.
meillo@89 74
meillo@89 75 Following is a small introduction to each of the five programs chosen for comparision.
meillo@89 76
meillo@92 77 \subsection*{\sendmail}
meillo@89 78 \label{sec:sendmail}
meillo@89 79 \sendmail\ is the most popular \mta. Since it was one of the first \MTA{}s and was shipped by many vendors of \unix\ systems.
meillo@89 80
meillo@89 81 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.
meillo@89 82
meillo@89 83 \sendmail\ is focused on transfering mails between different protocols and networks, this lead to a very flexible (though complex) configuration.
meillo@89 84
meillo@89 85 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.}.
meillo@89 86
meillo@89 87 Further development will go into the project \name{MeTA1} which succeeds \sendmail.
meillo@89 88
meillo@89 89 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}.
meillo@89 90
meillo@89 91
meillo@92 92 \subsection*{\name{qmail}}
meillo@89 93 \label{sec:qmail}
meillo@89 94 \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.
meillo@89 95
meillo@89 96 \name{qmail} first introduced may innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed.
meillo@89 97
meillo@89 98 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.
meillo@89 99
meillo@89 100 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@89 101
meillo@89 102
meillo@92 103 \subsection*{\name{postfix}}
meillo@89 104 \label{sec:postfix}
meillo@89 105 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.
meillo@89 106
meillo@89 107 Today \name{postfix} is taken by many \unix systems and \gnulinux distributions as default \MTA.
meillo@89 108
meillo@89 109 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.
meillo@89 110
meillo@89 111 Additional information is available on the program's homepage \citeweb{postfix:homepage}, on \citeweb{jdebp} and \citeweb{wikipedia:postfix}.
meillo@89 112
meillo@89 113
meillo@92 114 \subsection*{\name{exim}}
meillo@89 115 \label{sec:exim}
meillo@89 116 \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.
meillo@89 117
meillo@89 118 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.
meillo@89 119
meillo@89 120 \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.
meillo@89 121
meillo@89 122 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
meillo@89 123
meillo@89 124 One finds \name{exim} on its homepage \citeweb{exim:homepage}. More information about it can be retrieved from \citeweb{wikipedia:exim} and \citeweb{jdebp}.
meillo@89 125
meillo@89 126
meillo@92 127 \subsection*{\masqmail}
meillo@89 128 \label{sec:masqmail}
meillo@89 129 The \masqmail\ program was written by Oliver Kurth, starting in 1999. His aim was to create a small \mta\ which is especially focused on computers with dial-up connections to the internet. \masqmail\ is easy configurable for situations which are rarely solveable with the common \MTA{}s.
meillo@89 130
meillo@89 131 \masqmail\ queues mail for destinations outside the local network if no connection to the internet is online. If the machine goes online, this mail is sent. Mail to local machines is sent immediately.
meillo@89 132
meillo@89 133 While the other \MTA{}s are more general purpose \MTA{}s, \masqmail\ aims on special situations only. Nevertheless can it handle ordinary mail transfers too.
meillo@89 134
meillo@89 135 \masqmail\ is released under the \GPL, which makes it \freesw. The latest stable version is 0.2.21 from November 2005.
meillo@89 136
meillo@89 137 The program's new homepage \citeweb{masqmail:homepage} provides further information about this \MTA.
meillo@89 138
meillo@89 139
meillo@89 140
meillo@89 141
meillo@89 142 \section{Comparison of \MTA{}s}
meillo@89 143
meillo@89 144 % http://shearer.org/MTA_Comparison
meillo@89 145 % http://www.geocities.com/mailsoftware42/
meillo@89 146 % http://fanf.livejournal.com/50917.html
meillo@89 147 % http://archives.neohapsis.com/archives/postfix/2006-07/1762.html
meillo@89 148 % http://www.oreillynet.com/lpt/a/6849
meillo@89 149 % http://www.mailradar.com/mailstat/
meillo@89 150
meillo@89 151 \subsection{First release}
meillo@89 152 sendmail: 1983
meillo@89 153
meillo@89 154 postfix: 1999
meillo@89 155
meillo@89 156 qmail: 1996 (first beta 0.70), 1997 (first general 1.0)
meillo@89 157
meillo@89 158 exim: 1995
meillo@89 159
meillo@89 160 masqmail: 1999
meillo@89 161
meillo@89 162 exchange: 1993
meillo@89 163
meillo@89 164
meillo@89 165 \subsection{Lines of code (with sloccount on debian packages)}
meillo@89 166 sendmail: 93k
meillo@89 167
meillo@89 168 postfix: 92k
meillo@89 169
meillo@89 170 qmail: 18k
meillo@89 171
meillo@89 172 exim: 54k
meillo@89 173
meillo@89 174 masqmail: 14k
meillo@89 175
meillo@89 176 exchange: (no source available)
meillo@89 177
meillo@89 178
meillo@89 179 \subsection{Architecture}
meillo@89 180 sendmail: monolitic
meillo@89 181
meillo@89 182 postfix: modular
meillo@89 183
meillo@89 184 qmail: modular
meillo@89 185
meillo@89 186 exim: monolitic
meillo@89 187
meillo@89 188 masqmail: monolitic
meillo@89 189
meillo@92 190 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@92 191
meillo@92 192
meillo@89 193 exchange: (unknown)
meillo@89 194
meillo@89 195
meillo@89 196 \subsection{Design goals}
meillo@89 197 sendmail: flexibility
meillo@89 198
meillo@89 199 postfix: performance and security
meillo@89 200
meillo@89 201 qmail: security
meillo@89 202
meillo@89 203 exim: general, flexible \& extensive facilities for checking
meillo@89 204
meillo@89 205 masqmail: for non-permanent internet connection
meillo@89 206
meillo@89 207 exchange: groupware
meillo@89 208
meillo@89 209
meillo@89 210 \subsection{Market share (by Bernstein in 2001)}
meillo@89 211 sendmail: 42\%
meillo@89 212
meillo@89 213 postfix: 1.6\%
meillo@89 214
meillo@89 215 qmail: 17\%
meillo@89 216
meillo@89 217 exim: 1.6\%
meillo@89 218
meillo@89 219 masqmail: (unknown)
meillo@89 220
meillo@89 221 exchange: 18\%
meillo@89 222
meillo@89 223
meillo@89 224
meillo@89 225
meillo@99 226 \subsection{complexity}
meillo@89 227
meillo@99 228 \subsection{security}
meillo@89 229
meillo@99 230 \subsection{simplicity of configuration and administration}
meillo@89 231
meillo@99 232 \subsection{flexibility of configuration and administration}
meillo@89 233
meillo@99 234 \subsection{code size}
meillo@89 235
meillo@99 236 \subsection{code quality}
meillo@89 237
meillo@99 238 \subsection{documentation (amount and quality)}
meillo@89 239
meillo@99 240 \subsection{community (amount and quality)}
meillo@89 241
meillo@99 242 \subsection{used it myself}
meillo@89 243
meillo@99 244 \subsection{ had problems with it}
meillo@89 245
meillo@89 246
meillo@89 247
meillo@89 248
meillo@99 249 << quality criteria >> %FIXME
meillo@99 250
meillo@99 251 << standards of any kind >> %FIXME
meillo@99 252
meillo@99 253 << how to compare? >> %FIXME
meillo@99 254
meillo@99 255 << (bewertungsmatrix) objectivity >> %FIXME
meillo@99 256
meillo@99 257 << how many criterias for ``good''? >> %FIXME
meillo@99 258
meillo@93 259
meillo@93 260
meillo@93 261
meillo@89 262
meillo@89 263
meillo@92 264 \subsubsection*{masqmail stuff}
meillo@89 265
meillo@92 266 The \masqmail\ executable can be called under various names for \name{sendmail-compatibility} reasons. This is commonly organized by creating symbolic links with with different names to the \masqmail\ executable. These are \path{/usr/lib/sendmail} and \path{/usr/sbin/sendmail} because many programs expect a \mta\ to be located there. Further more \sendmail\ provides shortcuts by calling it with a different name instead of supplying command line arguments. The best known of it is \path{mailq}, which is equivilent to calling the \MTA\ with the argument \verb+-bq+. \masqmail\ reacts to the names \path{mailq}, \path{smtpd}, \path{mailrm}, \path{runq}, \path{rmail}, and \path{in.smtpd}. The last four are an addition to \sendmail. Not implemented is the name \path{newaliases} because it is not relevant to \masqmail. To provide the command nonetheless, one may write a shell script located at \path{/usr/bin/newaliases}, that simply invokes \verb+masqmail -bi+.
meillo@89 267
meillo@92 268 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@92 269 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@89 270
meillo@92 271 \masqmail\ is written in the \NAME{C} programming language. The program, as of version 0.2.21, consists of 34 source code and eight header files, containing about 9,000 lines of code\footnote{Measured with \name{sloccount} by David A.\ Wheeler.}. Additionally, it includes a \name{base64} implementation (about 300 lines) and \name{md5} code (about 150 lines). For systems that do not provide \name{libident}, this library is distributed as well (circa 600 lines); an available shared library however has higher precedence in linking.
meillo@89 272
meillo@92 273 The only mandatory dependency is \name{glib}---a cross-platform software utility library, originated in the \NAME{GTK+} project. It provides safer replacements for many standard library functions. (The unsafe \verb+sprintf()+ is one example.) Also it offers handy data containers, easy-to-use implementations of data structures, and much more.
meillo@89 274
meillo@92 275 With \masqmail\ comes the small tool \path{mservdetect}; it helps setting up a configuration that uses the \name{mserver} system to detect the online state. Two other binaries get compiled for testing purposes: \path{readtest} and \path{smtpsend}. All three programms use \masqmail\ source code; they only add a file with a \verb+main()+ function each.
meillo@89 276
meillo@92 277 \masqmail\ does not provide an interface for modules with additional functionality. There exists no add-on or module system. But the code is separated by function to the various source files, and some functional parts can be included or excluded by defining symbols. This means adding some argument (like \verb+--enable-maildir+) to the \verb+configure+ call. Thus the concerning code gets not removed by the preprocessor.