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author meillo@marmaro.de
date Thu, 22 Jan 2009 12:15:47 +0100
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57 \subsubsection*{Other segmenting} 57 \subsubsection*{Other segmenting}
58 \name{Mail transfer agents} can also be split in other ways. 58 \name{Mail transfer agents} can also be split in other ways.
59 59
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. \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 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. \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
61 61
62 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? 62 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. 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, 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.
63 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
64 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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123 121
124 \subsubsection*{sendmail} 122 \subsubsection*{sendmail}
125 \label{sec:sendmail} 123 \label{sec:sendmail}
126 \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 124 \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 \citeweb{wikipedia:sendmail}.
127 125
128 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 126 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 \cite[page~5]{vixie01}.
129 127
130 \sendmail\ designed to transfer mails between different protocols and networks, this lead to a very flexible, though complex, configuration. 128 \sendmail\ designed to transfer mails between different protocols and networks, this lead to a very flexible, though complex, configuration.
131 129
132 It was first released with \NAME{BSD} 4.1c in 1983. 130 It was first released with \NAME{BSD} 4.1c in 1983.
133 %fixme: write about its importance and about sendmail-compat 131 %fixme: write about its importance and about sendmail-compat
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142 140
143 \subsubsection*{exim} 141 \subsubsection*{exim}
144 \label{sec:exim} 142 \label{sec:exim}
145 \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 143 \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 \cite{blanco05}.
146 144
147 \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 145 \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 to integrate spam and malware checkers are provided by design.
148 146
149 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007. 147 The program is \freesw, released under the \NAME{GPL}. The latest stable version is 4.69 from December 2007.
150 148
151 One finds \exim\ on its homepage \citeweb{exim:homepage}. The standard literature is \person{Hazel}'s \exim\ book \cite{hazel01}. 149 One finds \exim\ on its homepage \citeweb{exim:homepage}. The standard literature is \person{Hazel}'s \exim\ book \cite{hazel01}.
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155 \subsubsection*{qmail} 153 \subsubsection*{qmail}
156 \label{sec:qmail} 154 \label{sec:qmail}
157 \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 155 \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. His own words are: ``This is why I started writing qmail: I was sick of the security holes in sendmail and other \MTA{}s.'' \cite{qmail:homepage1}.
158 156
159 \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 157 \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\ \citeweb{wikipedia:qmail}.
160 158
161 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. 159 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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163 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}. 161 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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197 \end{table} 195 \end{table}
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199 197
200 \subsubsection*{Architecture} 198 \subsubsection*{Architecture}
201 199
202 Architecture is most important when comparing \MTA{}s. Many other properties of a program depend on its architecture. %fixme: add ref? 200 Architecture is most important when comparing \MTA{}s. Many other properties of a program depend on its architecture. \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.
203 \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.
204 201
205 Two different architecture types show off: monolithic and modular \mta{}s. 202 Two different architecture types show off: monolithic and modular \mta{}s.
206 203
207 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. 204 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.
208 205
232 229
233 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. 230 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.
234 231
235 \subsubsection*{Security} 232 \subsubsection*{Security}
236 233
237 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 234 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 configured to be secure today. But the modular architecture, used by \qmail\ and \postfix, is generally seen to be conceptually more secure, however. \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?
238 But the modular architecture, used by \qmail\ and \postfix, is generally seen to be conceptually more secure, however. %fixme: ref
239 \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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