docs/diploma

annotate thesis/tex/3-MailTransferAgents.tex @ 119:73fe291f79e6

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author meillo@marmaro.de
date Wed, 03 Dec 2008 23:27:44 +0100
parents 098bd0876934
children 3b5e6ffd7b27
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meillo@89 1 \chapter{Mail transfer agents}
meillo@89 2
meillo@117 3 After having analyzed the market for electronic mail and trends for it in the last chapter, this chapter takes a look at \mta{}s, the intelligent most important parts of the email infrastructure. \MTA{}s will be grouped by similarities, and 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 a comparison of these programs in several disciplines will be done.
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meillo@89 7
meillo@89 8 \section{Types of \MTA{}s}
meillo@117 9 ``Mail transfer agent'' is a term covering a variety of programs. One thing is common to them: they transfer email from one machine to another.
meillo@89 10
meillo@117 11 This is how Bryan Costales defines a \mta\ in \cite{costales97}:
meillo@117 12 \begin{quote}
meillo@117 13 A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
meillo@117 14 \end{quote}
meillo@117 15 \name{The Free Dictionary} is a bit more concrete on the term: \citeweb{website:thefreedictionary}
meillo@117 16 \begin{quote}
meillo@117 17 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 MTAs (routing) for delivery to remote recipients.
meillo@117 18 \end{quote}
meillo@89 19
meillo@117 20 Common to all \MTA{}s is the transfer of mail to other machines; 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.
meillo@89 21
meillo@117 22 Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside.
meillo@117 23
meillo@117 24
meillo@117 25 \subsubsection*{Relay-only \MTA{}s}
meillo@89 26 \label{subsec:relay-only}
meillo@89 27 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 29 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 31 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 33 Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
meillo@89 34
meillo@89 35
meillo@117 36 \subsubsection*{Groupware}
meillo@89 37 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 39 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 41 Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.
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meillo@89 43
meillo@117 44 \subsubsection*{``Real'' \MTA{}s}
meillo@117 45 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''---programs that transfer mail between hosts.
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meillo@117 47 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}). They cover everything in between the other two groups. %FIXME: are postfix and qmail good examples?
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meillo@117 49 This group is of importance in this document. All programs selected for the comparison in the following section are ``real \MTA{}s''. \masqmail\ is one too.
meillo@89 50
meillo@89 51
meillo@117 52 \subsubsection*{Other segmenting}
meillo@117 53 \name{Mail transfer agents} can also be splitted in other ways.
meillo@117 54 \begin{itemize}
meillo@117 55 \item
meillo@117 56 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. Hence being sendmail-compatible is a major property of a \mta. %todo: how many MTAs are sendmail-compatible?
meillo@117 57 \MTA{}s not having a \emph{sendmail-compatible} interface or not offering it as a compatibility addon, will not be covered here. One example for such a program is \name{Apache James}. %FIXME: check if correct
meillo@89 58
meillo@117 59 \item
meillo@117 60 Another separation can be done between \freesw\ programs and proprietary software. 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@117 61 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, %fixme: is this the right target field? see chap02
meillo@119 62 need to be compared against other projects of similar shape. The document should be seen from \masqmail's point of view---an \MTA\ for a \unix\ system on home servers, workstations, or maybe embedded platforms---so non-free software is out of the way.
meillo@117 63 \end{itemize}
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meillo@89 68 \section{Popular \MTA{}s}
meillo@89 69
meillo@117 70 %todo: include market share analyses here
meillo@89 71
meillo@117 72 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 here. These are \emph{sendmail-compatible} ``smart'' \freesw\ \MTA{}s that focus on mail transfer.
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meillo@117 74 For the comparision, five programs are taken: \sendmail, \name{exim}, \name{qmail}, \name{postfix}, 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@117 76 \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}. %fixme: need this sentence?
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meillo@89 78 Other members are: \name{smail}, \name{zmailer}, \name{mmdf}, and more; they all are less important and rarely used.
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meillo@117 80 Following is a small introduction to each of the five programs chosen for comparision, except \masqmail\ which already was intoduced in chapter \ref{chap:introduction}.
meillo@89 81
meillo@117 82
meillo@117 83
meillo@117 84 \subsubsection*{\sendmail}
meillo@89 85 \label{sec:sendmail}
meillo@117 86 \sendmail\ is the most popular \mta, since it was one of the first and was shipped as default \MTA{}s by many vendors of \unix\ systems. %fixme: ref
meillo@89 87
meillo@117 88 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. %fixme: ref
meillo@89 89
meillo@89 90 \sendmail\ is focused on transfering mails between different protocols and networks, this lead to a very flexible (though complex) configuration.
meillo@89 91
meillo@89 92 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 94 Further development will go into the project \name{MeTA1} which succeeds \sendmail.
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meillo@89 96 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}.
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meillo@117 99
meillo@117 100 \subsubsection*{\name{exim}}
meillo@117 101 \label{sec:exim}
meillo@117 102 \name{exim} was started in 1995 by Philip Hazel at the \name{University of Cambridge}. It is forked of \name{smail-3}, and inherited the monolitic architecture, similar to \sendmail's. 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. %fixme: ref
meillo@117 103
meillo@117 104 \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. %fixme: ref
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meillo@117 106 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
meillo@117 107
meillo@117 108 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@117 109
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meillo@117 111
meillo@117 112 \subsubsection*{\name{qmail}}
meillo@89 113 \label{sec:qmail}
meillo@117 114 \name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''.%fixme: ref
meillo@117 115 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. %fixme: ref
meillo@89 116
meillo@117 117 \name{qmail} first introduced many innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed. %fixme:ref
meillo@117 118 %fixme: what about mmdf?
meillo@89 119
meillo@89 120 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@89 121
meillo@89 122 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@89 123
meillo@89 124
meillo@117 125
meillo@117 126 \subsubsection*{\name{postfix}}
meillo@89 127 \label{sec:postfix}
meillo@89 128 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@119 130 Today \name{postfix} is taken by many \unix\ systems and \gnulinux\ distributions as default \MTA.
meillo@89 131
meillo@89 132 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@89 133
meillo@89 134 Additional information is available on the program's homepage \citeweb{postfix:homepage}, on \citeweb{jdebp} and \citeweb{wikipedia:postfix}.
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meillo@89 141 \section{Comparison of \MTA{}s}
meillo@89 142
meillo@117 143 << general fact in table \ref{tab:mta-comparison} >>
meillo@89 144
meillo@117 145 \begin{table}
meillo@117 146 \begin{tabular}[hbt]{| p{0.13\textwidth} || p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} |}
meillo@117 147 \hline
meillo@89 148
meillo@117 149 & sendmail & exim & qmail & postfix & masqmail \\
meillo@117 150 \hline \hline
meillo@117 151 First release & 1983 & 1995 & 1996 & 1999 & 1999 \\
meillo@117 152 \hline
meillo@117 153 Lines of code (with sloccount on debian packages)& 93k & 54k & 18k & 92k & 14k \\
meillo@117 154 \hline
meillo@117 155 Architecture & monolitic & monolitic & modular & modular & monolitic \\
meillo@117 156 \hline
meillo@117 157 Design goals & flexibility & general, flexible \& extensive facilities for checking & security & performance and security & for non-permanent internet connection \\
meillo@117 158 \hline
meillo@117 159 Market share (by Bernstein in 2001) & 42\% & 1.6\% & 17\% & 1.6\% & (unknown) \\
meillo@117 160 \hline
meillo@89 161
meillo@117 162 \end{tabular}
meillo@117 163 \caption{Comparison of MTAs}
meillo@117 164 \label{tab:mta-comparison}
meillo@117 165 \end{table}
meillo@89 166
meillo@89 167
meillo@117 168 \subsection{about market share}
meillo@89 169
meillo@117 170 \subsection{About architecture}
meillo@89 171
meillo@117 172 \subsection{Security comparision}
meillo@89 173
meillo@89 174
meillo@117 175 \url{http://shearer.org/MTA_Comparison}
meillo@89 176
meillo@117 177 \url{http://www.geocities.com/mailsoftware42/}
meillo@89 178
meillo@117 179 \url{http://fanf.livejournal.com/50917.html}
meillo@89 180
meillo@117 181 \url{http://archives.neohapsis.com/archives/postfix/2006-07/1762.html}
meillo@89 182
meillo@117 183 \url{http://www.oreillynet.com/lpt/a/6849}
meillo@89 184
meillo@117 185 \url{http://www.mailradar.com/mailstat/}
meillo@89 186
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meillo@89 191
meillo@101 192 << complexity >>
meillo@89 193
meillo@101 194 << security >>
meillo@89 195
meillo@101 196 << simplicity of configuration and administration >>
meillo@89 197
meillo@101 198 << flexibility of configuration and administration >>
meillo@89 199
meillo@101 200 << code size >>
meillo@89 201
meillo@101 202 << code quality >>
meillo@89 203
meillo@101 204 << documentation (amount and quality) >>
meillo@89 205
meillo@101 206 << community (amount and quality) >>
meillo@89 207
meillo@101 208 << used it myself >>
meillo@89 209
meillo@101 210 << had problems with it >>
meillo@89 211
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meillo@89 214
meillo@99 215 << quality criteria >> %FIXME
meillo@99 216
meillo@99 217 << standards of any kind >> %FIXME
meillo@99 218
meillo@99 219 << how to compare? >> %FIXME
meillo@99 220
meillo@99 221 << (bewertungsmatrix) objectivity >> %FIXME
meillo@99 222
meillo@99 223 << how many criterias for ``good''? >> %FIXME
meillo@99 224
meillo@93 225
meillo@93 226
meillo@117 227 << from the practice of programming: are the names good? check the significant number of characters. (intern: 31char, extern: 6char caseless; ProgC p.184) >>
meillo@93 228
meillo@89 229
meillo@117 230 ---
meillo@89 231
meillo@117 232 But for example delivery of mail to local users is \emph{not} what \mta{}s should care about, although most \MTA\ are able to deliver mail, and many do. (\name{mail delivery agents}, like \name{procmail} and \name{maildrop}, are the right programs for this job.)
meillo@117 233
meillo@117 234
meillo@117 235 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@117 236
meillo@117 237
meillo@117 238 ---
meillo@117 239
meillo@117 240
meillo@117 241 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.