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author meillo@marmaro.de
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1 \chapter{\masqmail's present and future}
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3 This chapter \dots %fixme write text here
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8 \section{Existing code base}
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10 Here regarded is version 0.2.21 of \masqmail. This is the last version released by Oliver \person{Kurth}, and the basis for my thesis.
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13 \subsubsection*{The source code}
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15 \masqmail\ is written in the 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.
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17 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. It also offers handy data containers, easy-to-use implementations of data structures, and much more.
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19 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.
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21 \masqmail\ does not provide an interface to plug in modules with additional functionality. There exists no add-on or module system. The code is only separated by function to the various source files. Some functional parts can be included or excluded by defining symbols. Adding maildir support at compile time, means giving the option \verb+--enable-maildir+ to the \path{configure} call. This preserves the concerning code to get removed by the preprocessor. Unfortunately the \verb+#ifdef+s are scattered through all the source, leading to source code that is hard to read.
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22 %fixme: refer to ifdef-considered-harmful ?
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26 \subsubsection*{Features}
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28 \masqmail\ accepts mail on the command line and via \SMTP. Mail queueing and alias expansion is supported. \masqmail\ is able to deliver mail to local mailboxes (in \name{mbox} or \name{maildir} format) or pass it to a \name{mail delivery agent} (like \name{procmail}). Mail destinated to remote locations is sent using \SMTP\ or can be piped to commands, being gatesways to \NAME{UUCP} or \NAME{FAX} for example.
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30 Outgoing \SMTP\ connections feature \SMTP-\NAME{AUTH} and \SMTP-after-\NAME{POP} authentication, but incoming connections do not. Using wrappers for outgoing connections is supported. This offers a two way communication through a wrapper application like \name{openssl}.
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31 %todo: what about SSL/TLS encryption?
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33 \masqmail\ focuses on non-permanent online connections, thus a concept of online routes is used. One may configure any amount of routes to send mail. Each route can have criterias, like matching \texttt{From:} or \texttt{To:} headers, to determine if mail is allowed to be sent using it. Mail to destinations outside the local net gets queued until \masqmail\ is informed about the existance of a online connection.
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35 The \masqmail\ executable can be called under various names for sendmail-compatibility reasons. This is organized by symbolic links with different names pointing to the \masqmail\ executable. The \sendmail\ names are \path{/usr/lib/sendmail} and \path{/usr/sbin/sendmail} because many programs expect the \mta\ to be located there. Further more \sendmail\ supports calling it with a different name instead of supplying command line arguments. The best known of this shortcuts is \path{mailq}, which is equivilent to calling it with the argument \verb+-bq+. \masqmail\ recognizes the names \path{mailq}, \path{smtpd}, \path{mailrm}, \path{runq}, \path{rmail}, and \path{in.smtpd}. The first two are inspired by \sendmail. Not implemented is the name \path{newaliases} because \masqmail\ does not generate binary representations of the alias file.\footnote{A shell script located named \path{newaliases}, that invokes \texttt{masqmail -bi}, can provide the command to satisfy other software needing it.} \path{hoststat} and \path{purgestat} are missing for sendmail-compatibility.
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36 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
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37 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
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39 Additional to the \mta\ job, \masqmail\ also offers mail retrieval services with being a \NAME{POP3} client. It can fetch mail from different remote locations, dependent on the active online route.
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46 \section{Requirements}
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48 This section identifies the requirements for a modern \masqmail. Most of them will apply to modern \MTA{}s in general.
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52 \subsection{General requirements}
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54 Here follows a list of quality requirements for \masqmail, or other kinds of programs in similar environment and with similar jobs. These requirements specify the non-functional properties of the software, thus they are also called \name{non-functional requirements}. The list is based on \person{Hafiz} \cite[page~2]{hafiz05}, with insperation from \person{Spinellis} \cite[page~6]{spinellis06}.
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55 %fixme: refer to ch01 and ch02
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58 \subsubsection*{Security}
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59 \MTA{}s are critical points for computer security, as they are accessable from external networks. They must be secured with high effort. Properties like high priviledge level, work load influenced from extern, work on unsafe data, and demand for reliability, increase the need for security. \masqmail\ needs to be secure enough for its target field of operation.
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62 \subsubsection*{Reliability}
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63 Reliability is the second essential quality property for an \MTA. Mail for which the \MTA\ took responsibility must never get lost. The \MTA\ must not be \emph{the cause} of any mail loss, no matter what happens. Unreliable \mta{}s are of no value.
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66 \subsubsection*{Robustness}
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67 Being robust means handling errors properly. Small errors may get tolerated, large errors may kill a process. Killed processes should be automatically restarted and lead to a clean state again. Log messages should be written in every case. Robust software does not need a special environment, it creates the right environment itself. \person{Raymond}'s \name{Rule of Robustness} and his \name{Rule of Repair} are good descriptions.\cite[pages~18--21]{raymond03}
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70 \subsubsection*{Extendability}
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71 Modern needs like large messages demand for more efficient mail transport through the net. Aswell is a final solution needed to defeat the spam problem. New mail transport protocols seem to be the only good solutions for both problems. They also can improve reliability, authentication, and verification issues. \masqmail's architecture needs to be extendable, to allow new features to be added afterwards. For example new mail transfer protocols as they appear and are used.
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74 \subsubsection*{Maintainability}
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75 Maintaining software takes much time and effort. \person{Spinellis} guesses ``40\,\% to 70\,\% of the effort that goes into a software system is expended after the system is written first time.'' \cite[page~1]{spinellis03}. This work is called \emph{maintaining}. Hence making software good to maintain is effort that will easy 40 to 70\,\% of the work afterwards.
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78 \subsubsection*{Testability}
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79 Good testability make maintainance easier too, because functionality is directly verifiable when changes are done, thus removing the uncertainty. Modularized software makes testing easier, because parts can be testet without external influences. \person{Spinellis} sees testability as a subquality of maintainability.
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82 \subsubsection*{Performance}
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83 Also called ``efficiency''. Efficient software requires few time and few resources. The merge of communication hardware and its move from service providers to homes and to mobile devices, demand smaller and more resource-friendly software. The amount of mail will be lower, even if much more mail will be sent. More important will be the energy consumption and heat emission. These topics increased in relevance during the past years and they are expected to become more central.
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86 \subsubsection*{Availability}
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87 Availability is important for server programs. They must stay operational, even during \name{denial of service} attacks.
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90 \subsubsection*{Portability}
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91 Source code that compiles and runs on various operationg systems is called portable. Portability can be achieved by using standard features of the programming language and common libraries. Basic rules for portable code are defined by \person{Kerighan} and \person{Pike} \cite{kernighan99}. Portable code lets software spread faster.
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94 \subsubsection*{Usability}
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95 Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property very important from the user's point of view. Software with bad usability is rarely used, no matter how good it is. If roughly equivilent substitutes with better usability exist, the user will switch to one of them. Usability here means easy to set up and configure, too; users are also administrators here. Having \mta{}s on many home servers and clients, requires easy and standardized configuration. The common setups should be configurable with single actions by the user. Complex configuration should be possible, but focused must be the most common form of configuration: choosing one of several standard setups.
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101 \subsection{Functional requirements}
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103 This section identifies the needed functionality for a modern \MTA. The basic job of a \mta\ is to tranport mail from a sender to a recipient. This is the definition of such kind of software and this is how \MTA{}s are generally seen \cite[page 19]{dent04} \cite[pages 3-5]{hafiz05}.
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105 An \MTA\ therefore needs at least a mail receiving facility and a mail sending facility.
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109 \subsubsection*{Incoming channels}
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111 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \sendmail\ command, and mail received via the \SMTP\ daemon. Thus it is common to split the incoming channel into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view. \SMTP\ is the primary mail transport protocol today, but with the increasing need for new protocols\ref{FIXME} in mind, support for more than just \SMTP\ is good to have. This leads to more than one remote channel.
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114 \subsubsection*{Outgoing channels}
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116 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
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118 Outgoing channels are similar for \qmail, \postfix, and \name{sendmail X}: All of them have a module to send mail using \SMTP, and one for writing into a local mailbox. Local mail delivery is a job that requires root priveledge to be able to switch to any user in order to write to his mailbox.
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120 As mail delivery to local users, is \emph{not} included in the basic job of an \MTA{}, why should it care about it? In order to keep the system simple and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \name{mail delivery agent}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and thelike. Hence handling the message and the responsiblity over to a \NAME{MDA}, like \name{procmail} or \name{maildrop}, seems to be the right way to go.
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122 This means an outgoing connection that pipes mail into local commands is required. Other outgoing channels, one for each supportet protocol, may be designed like it was done in other \MTA{}s.
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126 \subsubsection*{Mail queue}
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128 Additionally to the mail receiving and sending facilities, mail queues are a basic feature. A mail queue removes the need to deliver intantly as a message is received. They provide fail-safe storage of mails until they are delivered. Mail queues are probably used in all \mta{}s, excluding the simple forwarders. A mail queue is a essential requirement for \masqmail, as it is to be used for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message.
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129
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130 The mail queue and the module to manage it are the central part of the whole system. This demands especially for robustness and reliability, as a failure here can lead to loosing mail. An \MTA\ takes over responsibility for mail in accepting it, hence loosing mail messages is absolutely to avoid. This covers any kind of crash situation too. The worst thing acceptable to happen is a mail to be sent twice.
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131
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132 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has three of them: \name{incoming}, \name{active}, and \name{deferred}. (The \name{maildrop} queue is excluded, as it is only used for the \texttt{sendmail} command.)
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133
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134 \MTA\ setups that do external content scanning tend to require two separate queues. To use \sendmail\ in such setups requires two independent instances, with two separate queues, running. \exim\ can handle it with special \name{router} and \name{transport} rules, but the data flow gets complicated. Having two independent queues seems to be preferable.
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135
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136
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137
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138
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139 \subsubsection*{Header sanitizing}
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140
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141 Mail coming into the system often lacks important header lines. At least the required ones must be added from the \MTA. One example is the \texttt{Date:} header, another is the, not required but recommended \texttt{Message-ID:} header. Apart from adding missing headers, rewriting headers is important too, to change the locally known domain part of email addresses to globally known ones for example. \masqmail\ needs also the ability to rewrite the domain part dependent on the route used to send the message.
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142
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143 Generating the envelope is a related job. The envelope specifies the actual recipient of the mail, no matter what the \texttt{To:}, \texttt{Cc:}, and \texttt{Bcc:} headers tell. Multiple reciptients lead to multiple different envelopes, containing all the same mail message.
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144
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145
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146
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147
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148 \subsubsection*{Aliasing}
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149
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150 Email addresses can have aliases and need to be expanded. Aliases can be of different kind: different local user, remote user, list of local and/or remote users, or a command. Most important are the aliases in the \path{aliases} file, usually located at \path{/etc/aliases}. Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
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151
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152
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153
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154
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155 \subsubsection*{Choose route to use}
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156
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157 One key feature of \masqmail\ is its ability to send mail out in different ways. The decision is based on the current online state and whether a route may be used for a message or not. The online state can be retrieved in tree ways, explained in \ref{sec:fixme}. A route to send is found by checking every available route for being able to transfer the current message, until one matches.
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158
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159
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160
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161
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162 \subsubsection*{Authentication}
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163
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164 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a major source of spam. The solution is restricting relay\footnote{Relaying is passing mail, that is not from and not for the own system, through it.} access.
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165
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166 Several ways to restrict access are available. The most simple one is restrictiction relaying by the \NAME{IP} address. No extra complexity is added this way, but static \NAME{IP} addresses are needed.
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167
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168 If static access restriction is not possible, for example mail from locations with changing \NAME{IP} addresses, some kind of authentication mechanism is required. Three common kinds exist:
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169 \begin{itemize}
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170 \item \SMTP-after-\NAME{POP}: Uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
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171 \item \SMTP authentication: An extension to \SMTP. Authentication can be requested before mail is accepted.
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172 \item Certificates: They confirm the identity of someone.
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173 \end{itemize}
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174
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175
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176
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177 \subsubsection*{Encryption}
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178
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179 Electronic mail is very weak to sniffing attacks, because all data transfer is unencrypted. This concerns the message's content, as well as the email addresses in header and envelope, but also authentication dialogs that may transfer plain text passwords (\NAME{PLAIN} and \NAME{LOGIN} are examples). Thus encryption is wanted.
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180
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181 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used by any of them\citeweb{wikipedia:tls}.
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182
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183 Outgoing \SMTP\ connections can get encrypted using a secure tunnel, created by an external application. Incoming connections, can not use this technique because the remote \NAME{IP} address is hidden then; \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
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184
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185
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186
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187 \subsubsection*{Spam prevention}
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188
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189 Spam is a major threat nowadays, but it is a war in which the good guys tend to lose. Putting much effort in fighting spam results in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. The goal is to provide state-of-the-art spam protection, but not more (see section \ref{sec:swot-analysis}). As spam is not just a nuisance for end users, but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages, \MTA{}s need to protect themself.
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190
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191 Filtering spam can be done in two ways: Refusing spam during the \SMTP\ dialog or checking for spam after the mail was accepted and queued. Both have advantages and disadvantages, so modern \MTA{}s use them in combination. Spam is identified by the results of a set of checks. Static rules, querying databases (\NAME{DNS} blacklists), requesting special client behavior (\name{greylisting}, \name{hashcash}), or statistical analysis (\name{bogofilter}) are checks that may be used. Running more checks leads to better results, but takes more system resources and time.
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192
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193 Doing some basic checks during the \SMTP\ dialog seems to be a must.
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194
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195
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196
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197 \subsubsection*{Virus checking}
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198
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199 Related to spam is malicous content (short: \name{malware}) like viruses, worms, trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail body. \MTA{}s searching for malware is equal to real world's post offices opening letters to check if they contain something that could harm the recipient. This is not a mail transport job. But the last \MTA---the one responsible for the recipient---seems to be at a good position to do this work.
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200
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201 In any way should malware checking be done by external programs that may be invoked by the \mta. But using mail deliver and processing agents, like \name{procmail}, a better suited locations to invoke content scanners.
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202
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203 A popular email filter framework is \name{amavis} which integrates various spam and virus scanners. The common setup includes a receiving \MTA\ which sends it to \name{amavis} using \SMTP, \name{amavis} processes the mail and sends it then to a second \MTA\ that does the outgoing transfer. Having interfaces to such scanners is, for sure, good to have.
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204
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205
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206
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207 \subsubsection*{Archiving}
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208
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209 Mail archiving and auditability become more important as electronic mail becomes more important. The ability to archive verbatim copies of every mail coming into and every mail going out of the system, with relation between them, appears to be a goal to achieve.
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210
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211 \postfix\ for example has a \texttt{always\_bcc} feature, to send a copy of every mail to a definable reciptient. At least this funtionality should be given, although a more complete approach is preferable.
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212
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213
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214
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215
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217
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218
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219
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220 \section{Work to do}
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221
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222 After \masqmail's features were presented in section \ref{sec:fixme} and the requirements for modern \mta{}s were identified in section \ref{sec:fixme}, here the differences between them are shown.
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223
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224
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225
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226 \subsubsection*{Fulfilled requirements}
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227
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228 \masqmail's incoming and outgoing channels are the common ones: the \texttt{sendmail} command and \SMTP\ for incoming mail; local delivery, piping to commands, and \SMTP\ for outgoing mail. Support for other protocols is not available. To add it, modifications at many places in the source are needed.
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229
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230 One single mail queue is used in \masqmail. The envelope and mail headers are generated when the mail is put into the queue. Aliasing is done on delivery, after the route to be used was determined. Headers may be rewritten then. These parts do all provide the functionality required.
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231
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232 Static authentication, based on \NAME{IP} addresses, can be set up using the \path{hosts.allow} and \path{hosts.deny} files. Dynamic authentication is supported in form of \NAME{SMTP-AUTH} and \SMTP-after-\NAME{POP}, but only for outgoing connections. The same for encryption which is also only available for outgoing \SMTP\ connections; here a wrapper application like \name{openssl} needs to be used. Support for authentication and encryption of incoming connections is completely missing, but a basic requirement for all secure emailing.
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233
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234 \masqmail\ does not provide special support for spam filter or content checking. But it is possible to invoke external filter applications by running two independent instances of \masqmail, connected by the filter application. The receiving \MTA\ instance accepts mail and pushes it into the filter. The filter application receives mail, processes it, possible modifies it, and pushes it over to a second \MTA\ instance. The second \MTA\ is responsible for further delivery of the mail. Appendix \ref{app:FIXME} shows configuration files to create such a setup. This is a concept that works in general. However, real spam \emph{prevention}---to not accept spam mail at all---or good filter interfaces are not available, but are nessesary for using \masqmail\ in an unsafe environment.
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235
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236 There is currently no way of archiving every message going through \masqmail.
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237
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238
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239 Non-functional requirements are not so easy to be marked as fulfilled or not. Instead they are discussed here.
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240
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241 \masqmail\ needs to be ``secure enough'', but what is ``secure enough''? This depends on its target field. Currently \masqmail\ is targeted to workstations and private networks, with explicit warning for use on permanent online hosts \citeweb{masqmail:homepage2}. \masqmail's current security is bad. (For instance does a long time known attack against \sendmail, described by \person{Sill} \cite[page~4]{sill02}, still outwit \masqmail). The security, however, seems acceptable for use on workstations and private networks, if the environment is trusted. In environments where untrusted components or persons have access to \masqmail, its security is too low.
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242
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243 Similar for its reliability. It has been reported that \masqmail\ has not sent mail under some circumstances \citeweb{FIXME}. %fixme
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244 The author also noticed problems with where only one part of the queued message was removed, the other part remained as garbage. Fortunately, reports abou lost mail are not known. Current reliability is not good enough.
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245
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246 The logging behavior of \masqmail\ is good, but does not cover everything. Also it trusts its environment. For example, if the queue directory is world writeable by accident (or as action of an intruder), any user can remove messages from the queue or replace them with own ones. \masqmail\ does not even write a debug message in this case.
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247
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248 \masqmail's extendability is very poor. This is a general problem of monolithic software, but can thus be provided with high effort. \exim\ is an example for good extendability for a monolithic program.
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249
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250 The maintainability of \masqmail\ appears to be equivilent to other software of similar kind. Missing modularity and therefore more complexity makes the maintainer's work harder. In summary is \masqmail's maintainability bearable, and in view of its missing modularity good. The testability suffers from the same. Testing program parts is hard, but done by compiling source parts to special test programs.
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251
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252 The performance---effenciency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal. As well is its availability. Hence no further work needs to be done her.
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253
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254 The code's protability is good with view on \unix-like operation systems. At least \name{Debian}, \name{Redhat}, \NAME{SUSE}, \name{slackware}, \name{Free}\NAME{BSD}, \name{Open}\NAME{BSD}, and \name{Net}\NAME{BSD} are reported to be able to compile and run \masqmail\ \citeweb{masqmail:homepage2}. About special requirements for the underlying file system is not heard of. The portability is already good.
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255
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256 The usability, from the administator's point of view, is very good. \masqmail\ was developed to suite a specific, limited job, its configuration does perfect match. The user's view does not reach to the \MTA, as it is hidden behind the \name{mail user agent}.
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257
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258
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259
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260 \subsubsection*{Missing parts}
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261
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262 Support for other protocols than \SMTP\ seems not to be nessesary at the moment. Adding such support will need lots of work in all parts of \masqmail, hence delaying it until the support is needed appears to be the best solution.
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263
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264 Authentication of incoming \SMTP\ connections is definately needed and should be added soon. The same applies to encryption of incomming connections. These two features are essential for restricting relaying and providing privacy.
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265
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266 As authentication can be a guard against spam, filter facilities have lower priority. But basic spam filtering and interfaces for external tools should be implemented in future. Content checking, if really nessesary, should be left over to the \NAME{MDA}, to deal with it in local delivery.
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267
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268 Archiving again is prefered to be implemented soon. It does not require much work, but enables all kinds of statistical analysis. Also it is a requirement for companies to archive their mail communication.
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269
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270 Non-functional requirements need improvements too.
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271
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272 \masqmail's security is bad and limits it thus to a field of operation that shrinks. Security becomes more important and networking and interaction increases. Save and trusted environment become rare. Improving security is an important thing to do in future.
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273
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274 Reliability is also to improve. It is a key quality property for an \MTA, and not good enough in \masqmail. Also is the program lacking robustness. Checking the environment and reporting bad characteristics is wanted. Especially improving robustness inrelation to the queue is favorable; applying ideas of \name{crash-only software}\cite{candea03} will be a good step.
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275
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276 Extendability, maintainability, and testability do all lack from the monolithic architecture and are nearly impossible to improve without changing the programs structure. These properties can hardly be retrofitted into software. But extendability might become important in the future, and the other two make all further work on the software easier.
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277
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278 Performance is a nice-to-have property, but as performance improvements are in contrast to many other quality poperties (reliability, maintainability, usability, capability \cite[page~5]{kan03}), jeopardizing these to gain some more performance should not be done. \person{Kernighan} and \person{Pike} state clear: ``[T]he first principle of optimization is \emph{don't}.''\cite[page~165]{kernighan99}. \masqmail\ is not a program to be used on large servers, but on small devices. Thus important for \masqmail\ is focusing on energy and heat, maybe also system resources, not on performance.
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279
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280 Focusing on being portable amoung the various flavors of \unix\ systems seems to be okay, because these systems are the ones \MTA{}s run on usually. Problems with non-\unix platforms are primary expected to come from file systems lacking required features. But no special care should be taken here.
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281
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282 Configuration could be eased more, by providing configuration generators to be able to run \masqmail\ right ``out of the box'' with only running one of several configuration scripts for common setups. This would make \masqmail\ better usable by people not technical educated.
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286
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288
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289 \section{Discussion on architecture}
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290
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291 %fixme: why is there a need for a new arch??
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292 Adding authentication and encryption support is limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast do support for new protocols or mail processing interfaces to external programs require a lot of effort. Changes in many parts of the source code are required. It is no good idea to implement large retro-fitted features in a software that is critical in security and reliability, like \masqmail. Worse if these features need changes in the program's structure, like adding mail scanning interfaces would do.
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293
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294 If such features are needed, it is best do redesign the program's structure and rebuild it. A program's structure is primary its architecture. Which is probably the most influencing design decision, and has the greatest impact on the program's future capabilities. The architecture defines what the program can do, how it can be used. If the architecture does not fit the requirements, develpement reached a dead end \dots\ further work will make everything worse. The only good solution is to change the architecture, which, sadley but most likely, means a redesign from scratch.
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295
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296 \masqmail's current artitecture is monolitic like \sendmail's and \exim's. But more than the other two, is it one block of interweaved code. \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules. \exim\ has a highly structured code with many internal interfaces, a good example is the one for authentication ``modules''. %fixme: add ref
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297 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
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298
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299 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.
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300
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301 \begin{figure}
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302 \begin{center}
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303 \input{input/masqmail-arch.tex}
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304 \end{center}
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305 \caption{Internal structure of \masqmail}
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306 \label{fig:masqmail-arch}
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307 \end{figure}
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308
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309 \sendmail\ improved its old architecture, for example by adding the milter interface. \exim\ was designed and is carefully maintained with a modular-like code structure in mind. \qmail\ started from scratch with a ``security-first'' approach, \postfix\ improved on it, and \name{sendmail X}/\name{MeTA1} tries to adopt the best of \qmail\ and \postfix, to completely replace the old \sendmail\ architecture. \person{Hafiz} \cite{hafiz05}. describes this evolution of \mta\ architecture very well.
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311 Every one of the popular \MTA{}s is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and future requirements like---probably---the use of new mail transport protocols demand modular designs for keeping the software simple. Simplicity is a key property for security. ``[T]he essence of security engenieering is to build systems that are as simple as possible.''\cite[page 45]{graff03}
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313 \person{Hafiz} agrees: ``The goal of making software secure can be better achieved by making the design simple and easier to understand and verify.''\cite[page 64]{hafiz05} He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:
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314 \begin{quote}
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315 A perfect example is the contrast between the feature envy early \sendmail\ architecture implemented as one process and the simple, modular architecture of \qmail. The security of \qmail\ comes from its compartmentalized simple processes that perform one task only and are therefore testable for security. \cite[page 64]{hafiz05}
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316 \end{quote}
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317 As well does \person{Dent}: ``The modular architecture of Postfix forms the basis for much of its security.''\cite[page 7]{dent04} Modularity is also needed to satisfy modern \MTA\ requirements, in providing a clear interface to add functionality without increasing the overall complexity much.
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318
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319 Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
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320 \begin{quote}
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321 Good design is the sword and shield of the security-conscious developer. Sound design defends your application from subversion or misuse, protecting your network and the information on it from internal and external attacks alike. It also provides a safe foundation for future extensions and maintainance of the software.
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322 %
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323 %Bad design makes life easier for attackers and harder for the good guys, especially if it contributes to a false sends of security while obscuring pertinent failings.
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324 \cite[page 55]{graff03}
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325 \end{quote}
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326
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327
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328 All this leads to a rewrite of \masqmail, using a modern, modular architecture, \emph{if} further features need to be added, ones that require changes in \masqmail's structure. As well is a rewrite needed, if \masqmail\ should become a modern \MTA, with good quality properties.
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329
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330 But redesigning and rewriting a software from scratch is hard. It takes time to design a new architecture, which must prove it is secure and reliable. And much time and work is needed to implement the design, test it, fix bugs, and so on. Thus the gain of a new design must overweight the effort needed to spend.
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331
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332 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
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333 {\small
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334 \begin{verbatim}
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335 Total Physical Source Lines of Code (SLOC) = 9,041
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336 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22)
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337 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05))
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338 Schedule Estimate, Years (Months) = 0.70 (8.39)
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339 (Basic COCOMO model, Months = 2.5 * (person-months**0.38))
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340 Estimated Average Number of Developers (Effort/Schedule) = 2.89
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341 Total Estimated Cost to Develop = $ 272,690
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342 (average salary = $56,286/year, overhead = 2.40).
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343 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler
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344 \end{verbatim}
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345 }
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346 The development cost is not relevant for a \freesw\ project of volunteer developers, but the time needed is. About 24 man-months are estimated. The current code base was written almost completely by Oliver \person{Kurth} within four years, in his spare time. This means he needed around twice as much time.
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347
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348 Given the asumptions that an equal amount of code is to be produced, a third of existing code can be reused plus concepts and experience, and development speed is like \person{Kurth}'s. Then it would take about two years to have a redesigned new \masqmail\ with the same features. Less time could be needed if a simpler architecture allows faster develpement, better testing, and less bugs.
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349
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350 The further section describes a new modern design for \masqmail. A plan to rewrite \masqmail\ from scratch.
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351
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353
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354 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
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363
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364 \section{Result}
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365
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366 Directions to go
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367
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368 Now how could \masqmail\ be like in, say, five years?
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369
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370 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.
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371
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372
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373
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374
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375 1) fix the current version
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376
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377
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378 2) create a new one
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379 But how is the effort of this complete rewrite compared to what is gained afterwards?
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380 << would one create it at all? >>
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381
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382
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383 pro---contra
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384
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385
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387
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388 << short term goals --- long term goals >>
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389
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390 do it like sendmail: first do the most needed stuff on the old design to make it still usable. Then design a new version from scratch, for the future.
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391
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392 << which parts to take out and do within the thesis >>
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400
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401
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402
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403
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404
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405 \chapter{A design from scratch}
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406
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407 The last sections identified the jobs that need to be done by a modern \MTA; problems and prefered choices were mentioned too. Now the various jobs are assigned to modules, of which an architecture is created. It is inpired by existing ones and driven by the identified jobs and requirements.
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408
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409
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410
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411
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412 \subsection{Design decisions}
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413
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414 One major design idea of the design were:
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415 \begin{itemize}
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416 \item free the internal system from in and out channels
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417 \item arbitrary protocol handlers have to be addable afterwards
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418 \item a single facility for scanning (all mail goes through it)
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419 \item concentrate on mail transfer
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420 \end{itemize}
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421
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422
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423 \subsubsection*{Incoming channels}
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424
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425 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \sendmail\ command, and mail received via the \SMTP\ daemon. It is therefor common to split the incoming channel into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view.
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426
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427 In contrast is \name{sendmail X}: Its locally submitted messages go to the \SMTP\ daemon, which is the only connection towards the mail queue. %fixme: is it a smtp dialog? or a second door?
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428 \person{fanf} proposes a similar approach. He wants the \texttt{sendmail} command to be a simple \SMTP\ client that contacts the \SMTP\ daemon of the \MTA\ like it is done by connections from remote. The advantage here is one single module where all \SMTP\ dialog with submitters is done. Hence one single point to accept or refuse incoming mail. Additionally does the module to put mail into the queue not need to be \name{setuid} or \name{setgid} because it is only invoked from the \SMTP\ daemon. The \MTA's architecture would become simpler and common tasks are not duplicated in modules that do similar jobs.
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429
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430 But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP\ being the main mail transfer protocol. To \qmail\ and \postfix\ new modules to support other ways of message receival may be added without change of other parts of the system. Also is it better to have more independent modules if each one is simpler then.
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431
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432 With the increasing need for new protocols in mind, it seems better to have single modules for each incoming channel, although this leads to duplicated acceptance checks.
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433
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434
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435 \subsubsection*{Outgoing channels}
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436
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437 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
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438
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439 Outgoing channels are similar for \qmail, \postfix, and \name{sendmail X}: All of them have a module to send mail using \SMTP, and one for writing into a local mailbox. Local mail delivery is a job that requires root priveledge to be able to switch to any user in order to write to his mailbox. Modular \MTA{}s do not need \name{setuid root}, but the local delivery process (or its parent) needs to run as root.
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440
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441 As mail delivery to local users, is \emph{not} included in the basic job of an \MTA{}, why should it care about it? In order to keep the system simple and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \name{mail delivery agent}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and thelike. Hence handling the message and the responsiblity over to a \NAME{MDA}, like \name{procmail} or \name{maildrop}, seems to be the right way to go.
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442
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443 This means an outgoing connection that pipes mail into local commands is required. Other outgoing channels, one for each supportet protocol, may be designed like it was done in other \MTA{}s.
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444
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445
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446
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447 \subsubsection*{Mail queue}
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448
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449 Mail queues are probably used in all \mta{}s, excluding the simple forwarders. A mail queue is a essential requirement for \masqmail, as it is to be used for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message.
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450
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451 The mail queue and the module to manage it are the central part of the whole system. This demands especially for robustness and reliability, as a failure here can lead to loosing mail. An \MTA\ takes over responsibility for mail in accepting it, hence loosing mail messages is absolutely to avoid. This covers any kind of crash situation too. The worst thing acceptable to happen is a mail to be sent twice.
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452
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453 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has three of them: \name{incoming}, \name{active}, and \name{deferred}. (The \name{maildrop} queue is excluded, as it is only used for the \texttt{sendmail} command.)
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454
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455 \MTA\ setups that include content scanning tend to require two separate queues. To use \sendmail\ in such setups requires two independent instances, with two separate queues, running. \exim\ can handle it with special \name{router} and \name{transport} rules, but the data flow gets complicated. Hence an idea is to use two queues, \name{incoming} and \name{active} in \postfix's terminology, with the content scanning within the move from \name{incoming} to \name{active}.
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456
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457 \sendmail, \exim, \qmail, and \masqmail\ all use at least two files to store one message in the queue: one file contains the message body, another the envelope and header information. The one containing the mail body is not modified at all. \postfix\ takes a different approach in storing queued messages in an internal format within one file. \person{Finch} takes yet another different approach in suggesting to store the whole queue in one single file with pointers to separating positions \cite{finchFIXME}.
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458 %fixme: check, cite, and think about
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459
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460
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461
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462 \subsubsection*{Sanitize mail}
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463
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464 Mail coming into the system often lacks important header lines. At least the required ones must be added from the \MTA. A good example is the \texttt{Message-Id:} header.
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465
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466 In \postfix, this is done by the \name{cleanup} module, which invokes \name{rewrite}. The position in the message flow is after coming from one of the several incoming channels and before the message is stored into the \name{incoming} queue. Modules that handle incoming channels may also add headers, for example the \texttt{From:} and \texttt{Date:} headers. \name{cleanup}, however, does a complete check to make the mail header complete and valid.
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467
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468 Apart from deciding where to sanitize the mail header, is the question where to generate the envelope. The envelope specifies the actual recipient of the mail, no matter what the \texttt{To:}, \texttt{Cc:}, and \texttt{Bcc:} headers tell. Multiple reciptients lead to multiple different envelopes, containing all the same mail message.
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469
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470
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471
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472 \subsubsection*{Aliasing}
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473
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474 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
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475 \begin{itemize}
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476 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
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477 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
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478 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
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479 \item a command (e.g.\ ``\texttt{bob: |foo}'')
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480 \end{itemize}
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481 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
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482
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483 Aliasing is often handled in expanding the alias and reinjecting the mail into the system. Unfortunately, the mail is processed twice then; additionally does the system have to handle more mail this way. If it is wanted to check the new recipient address for acceptance and do all processing again, then reinjecting it is the best choice.
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484
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485
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486
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487 \subsubsection*{Choose route to use}
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488
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489 One key feature of \masqmail\ is its ability to send mail out in different ways. The decision is based on the current online state and whether a route may be used for a message or not. The online state can be retrieved in tree ways, explained in \ref{sec:fixme}. A route to send is found by checking every available route for being able to transfer the current message, until one matches.
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490
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491 This functionality should be implemented in the module that is responsible to invoke one of the outgoing channel modules (for example the one for \SMTP\ or the pipe module).
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492
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493 \masqmail\ can rewrite the envelope's from address and the \texttt{From:} header, dependent on the outgoing route to use. This rewrite must be done \emph{after} it is clear which route a mail will take, of course, so this may be not the module where other header editing is done.
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494 %fixme: see hafiz05 page 57: maybe put the rewriting into the sending module (like smx, exim, courier) (problem with archiving of all outgoing mail?)
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495
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496
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497
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498 \subsubsection*{Authentication}
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499
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500 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a major source of spam. The solution is restricting relay\footnote{Relaying is passing mail, that is not from and not for the own system, through it.} access.
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501
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502 Several ways to restrict access are available. The most simple one is restrictiction by the \NAME{IP} address. No extra complexity is added this way, but static \NAME{IP} addresses are mandatory. This kind of restriction may be enabled using the operating system's \path{hosts.allow} and \path{hosts.deny} files. To allow only connections to port 25 from localhost or the local network \texttt{192.168.100.0/24} insert the line ``\texttt{25: ALL}'' into \path{hosts.deny} and ``\texttt{25: 127.0.0.1, 192.168.100.}'' into \path{hosts.allow}.
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503
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504 If static access restriction is not possible, for example if mail from locations with changing \NAME{IP} addresses wants to be accepted, some kind of authentication mechanism is required. Three common kinds exist:
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505 \begin{enumerate}
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506 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
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507 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
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508 \item Certificates: confirm the identity of someone.
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509 \end{enumerate}
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510
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511
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512
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513 \subsubsection*{Encryption}
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514
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515 Electronic mail is very weak to sniffing attacks, because all data transfer is unencrypted. This concerns the message's content, as well as the email addresses in header and envelope, but also authentication dialogs that may transfer plain text passwords (\NAME{PLAIN} and \NAME{LOGIN} are examples). Adding encryption is therefor wanted.
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516
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517 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used by any of them\citeweb{wikipedia:tls}.
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518
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519 \TLS\ allows to create secure tunnels through which arbitrary programs can communicate. Hence one can add secure communication afterwards to programs without changing them. \name{OpenSSL} for example---a free implementation---allows traffic to be piped into a command; a secure tunnel is created and the traffic is forwarded through it. Or a secure tunnel can be set up between a local and a remote port; this tunnel can then be used by any application.
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520
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521 The \NAME{POP} protocol, for example, is good suited for such tunneling, but \SMTP\ is is not generally. Outgoing \SMTP\ client connections can be tunneled without problem---\masqmail\ already provides a configure option called \texttt{wrapper} to do so. Tunneling incomming connections to a server leads to problems with \SMTP. As data comes encrypted through the tunnel to the receiving host and gets then decrypted and forwarded on local to the port the application listens on. From the \MTA's view, this makes all connections appear to come from localhost, unfortunately. Figure \ref{fig:stunnel} depicts the data flow.
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522
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523 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
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524
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525 \masqmail\ is already able to encrypt outgoing connections, but encryption of incoming connections, using \NAME{STARTTLS} should be implemented. This only affects the \SMTP\ server module.
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526
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527
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528
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529
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530
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531 \subsubsection*{Spam prevention}
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532
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533 ---
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534 Spam is a major threat nowadays and the goal is to reduce it to a bearable level (see section \ref{sec:swot-analysis}). Spam fighting is a war are where the good guys tend to lose. Putting too much effort there will result in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. Hence \masqmail\ should be able to provide state-of-the-art spam protection, but not more.
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535 ---
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536
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537 Spam is a major threat to email, as described in section \ref{sec:swot-analysis}. The two main problems are forgable sender addresses and that it is cheap to send hundreds of thousands of messages. Hence, spam senders can operate in disguise and have minimal cost.
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538
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539 As spam is not just a nuisance for end users, but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages, \MTA{}s need to protect themself. Two approaches are used.
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540
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541 First refusing spam during the \SMTP\ dialog. This is the way it was meant by the designers of the \SMTP\ protocol. They thought checking the sender and reciptient mail addresses would be enough, but as they are forgable it is not. More and more complex checks need to be done. Checking needs time, but \SMTP\ dialogs time out if it takes too long. Thus only limited time can be used, during the \SMTP\ dialog, for checking if a message seems to be spam. The advantage is that acceptance of bad messages can be simply refused---no responsibility for the message is takes and no further system load is added. See \RFC2505 (especially section 1.5) for detail.
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543 Second checking for spam after the mail was accepted and queued. Here more processing time can be invested, so more detailed checks can be done. But, as responsibility for messages was taken by accepting them, it is no choice to simply delete spam mail. Checks for spam do not lead to sure results, they just indicate the possibility the message is unwanted mail. \person{Eisentraut} indicates actions to take after a message is recognized as probably spam \cite[pages 18--20]{eisentraut05}. The only acceptable one, for mail the \MTA\ is responsible for, is adding further or rewriting existent header lines. Thus all further work on the message is the same as for non-spam messages.
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544
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545 Modern \MTA{}s use both techniques in combination. Checks during the \SMTP\ dialog tend to be implemented in the \mta\ to make it fast; checks after the message was queued are often done using external programs (\name{spamassassin} is a well known one). \person{Eisentraut} sees the checks during the \SMTP\ dialog to be essentiell: ``Ganz ohne Analyse während der SMTP-Phase kommt sowieso kein MTA aus, und es ist eine Frage der Einschätzung, wie weit man diese Phase belasten möchte.''\cite[page 25]{eisentraut05} (translated: ``No \MTA\ can go without analysis during the \SMTP\ dialog, anyway, and it is a question of estimation how much to stress this period.'')
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546
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547 \NAME{DNS} blacklists (short: \NAME{DNSBL}) and \name{greylisting} are checks to be done before accepting the message. Invoking \name{spamassassin}, to add headers containing the estimated spam probability, is best to be invoked after the message is queued.
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548
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549
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550
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551
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552 \subsubsection*{Virus checking}
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553
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554 Related to spam is malicous content (short: \name{malware}) like viruses, worms, trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail body. The same situation in the real world is post offices opening letters to check if they contain something that could harm the recipient. This is not a mail transport concern. Apart of not being the right program to do the job, the \MTA\---the one which is responsible for the recipient---is at a good position to do this work.
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555
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556 In any way should malware checking be done by external programs that may be invoked by the \mta. But using mail deliver and processing agents, like \name{procmail}, seem to be better suited locations to invoke content scanners.
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557
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558 A popular email filter framework is \name{amavis} which integrates various spam and virus scanners. The common setup includes a receiving \MTA\ which sends it to \name{amavis} using \SMTP, \name{amavis} processes the mail and sends it then to a second \MTA\ that does the outgoing transfer. \postfix\ and \exim\ can be configured so that one instance can work as both, the \MTA\ for incoming and outgoing transfer. A setup with \sendmail\ needs two separate instances running. It must be quarateed that all mail flows through the scanner.
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559
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560 A future \masqmail\ would do good to have a single point, where all traffic flows through, that is able to invoke external programs to do mail processing of any kind.
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561
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562
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563 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
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564 %\begin{verbatim}
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565 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
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566 %| |
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567 %+----------------------------+
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568 %\end{verbatim}
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569 %
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570 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
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571 %
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572 %MailScanner:
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573 %incoming queue --> MailScanner --> outgoing queue
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574 %
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575 %postfix: with one instance possible, exim and sendmail need two instances running
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576
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577
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578 %message body <-> envelope, header
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579 %
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580 %anti-virus: clamav
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581 %postfix: via amavis
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582 %exim: via content-scanning-feature called from acl
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583 %sendmail: with milter
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584 %procmail
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585 %
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586 %virus scanner work on file level
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587 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
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588 %if the mail is okay, it goes via smtp to a second mta
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589
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590 %what amavis recognizes:
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591 %- invalid headers
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592 %- banned files
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593 %- viruses
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594 %- spam (using spam assassin)
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595 %
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596 %mimedefang: uses milter interface with sendmail
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597
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598
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599
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600 \subsubsection*{Archiving}
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601
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602 Mail archiving and auditability become more important as electronic mail becomes more important. Ability to archive verbatim copies of every mail coming into and every mail going out of the system, with relation between them, appears to be a goal to achieve.
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603
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604 \postfix\ for example has a \texttt{always\_bcc} feature, to send a copy of every mail to a definable reciptient. At least this funtionality should be given, although a more complete approach is preferable.
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605
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606
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607
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608
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609
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610 \subsection{The resulting architecture}
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611
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612 The result is a symetric design, featuring the following parts: Any number of handlers for incoming connections to receive mail and pass it to the module that stores it into the incoming queue. A central scanning module take mail from the incoming queue, processes it in various ways and puts it afterwards into the outgoing queue. Another module takes it out there and passes it to a matching transport module that transfers it to the destination. In other words, three main modules (queue-in, scanning, queue-out) are connected by the two queues (incoming, outgoing); on each end are more modules to receive and send mail---for each protocol one. Figure \ref{fig:masqmail-arch-new} depicts the new designed architecture.
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613
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614 \begin{figure}
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615 \begin{center}
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616 \input{input/masqmail-arch-new.tex}
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617 \end{center}
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618 \caption{A new designed architecture for \masqmail}
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619 \label{fig:masqmail-arch-new}
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620 \end{figure}
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621
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622 This architecture is heavily influenced by the ones of \qmail\ and \postfix. Both have different incoming channels that merge in the module that puts mail into the queue; central is the queue (or more of them); and one module takes mail from the queue and passes it to one of the outgoing channels. Mail processing, in any way, is build in in a more explicit way than done in the other two. It is more similar to the \NAME{AR} module of \name{sendmail X}, which is the central point for spam checking.
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623
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624 Special regard was put on addable support for further mail transfer protocols. This appears to be most similar to \qmail, which was designed to handle multiple protocols.
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625 %fixme: do i need all this ``quesses''??
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626
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628 \subsubsection*{Modules and queues}
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629
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630 The new architecture consists of several modules and two queues. They are defined in more detail now, and the jobs, identified above, are assigned to them. First the three main modules, then the queues, and afterwards the modules for incoming and outgoing transfer.
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631
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632
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633 The \name{queue-in} module creates new spool files in the \name{incoming} queue for incoming messages. It is a process running in background, waiting for connections from one of the receiver modules. When one of them requests for a new spool file, the \name{queue-in} module opens one and returns a positive result. The receiver module then sends the envelope and message, which is written into the spool file by \name{queue-in}. If all went well, another positive result is returend.
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634 %fixme: should be no daemon
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635
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636
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637 The \name{scanning} module is the central part of the system. It takes spooled messages from the \name{incoming} queue, works on them, and writes them to the \name{outgoing} queue afterwards (the message is then removed from the \name{incoming} queue, of course). The main job is the processing done on the message. Headers are fixed and missing ones are added if necessary, aliasing is done, and external processing of any kind is triggered. The \name{scanning} module can run in background and look for new mail in regular intvals or signals may be sent to it by \name{queue-in}. Alternatively it can be called by \name{cron}, for example, to do single runs.
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638
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639
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640 The \name{queue-out} module takes messages from the \name{outgoing} queue, queries information about the online connection, and then selects matching routes, creates envelopes for each recipient and passes the messages to the correct transport module. Successfully transfered messages are removed from the \name{outgoing} queue. This module includes some tasks specific to \masqmail.
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641
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642
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643 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended.
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644
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645
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646 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
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647
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648 \name{Receiver modules} are the communication interface between outside senders and the \name{queue-in} module. Each protocol needs a corresponding \name{receiver module} to be supported. Most popular are the \name{sendmail} module (which is a command to be called from the local host) and the \name{smtpd} module (which listens on port 25). Other modules to support other protocols may be added as needed.
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649
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650 \name{Transport modules}, on the oppersite side of the system, are the modules to send outgoing mail; they are the interface between \name{queue-out} and remote hosts or local commands for further processing. The most popular ones are the \name{smtp} module (which acts as the \SMTP\ client) and the \name{pipe} module (to interface gateways to other systems or networks, like fax or uucp). A module for local delivery is not included, as it is in most other \MTA{}s; the reasons are described in FIXME.%fixme
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651 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
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652
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653
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655 \subsubsection*{Inter-module communication}
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656
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657 Communication between modules is required to exchange data and status information. It is also called ``Inter-process communication'' (short: \NAME{IPC}), as modules are programs being part of a larger system, and processes are generally seen as programs in execution.
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658
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659 The connections between \name{queue-in} and \name{scanning}, aswell as between \name{scanning} and \name{queue-out} is provided by the queues, only sending signals to trigger instant runs may be useful. Communication between receiving and transport modules and the outside world are done using the specific protocol they do handle.
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660
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661 Left is only communication between the receiver modules and \name{queue-in}, and between \name{queue-out} and the transport modules. Data is exchanged done using \unix\ pipes and a simple protocol is used.
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662
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663 \begin{figure}
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664 \begin{center}
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665 \input{input/ipc-protocol.tex}
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666 \end{center}
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667 \caption{State diagram of the protocol used for \NAME{IPC}}
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668 \label{fig:ipc-protocol}
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669 \end{figure}
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670
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671 % timing
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672 One dialog consists of the four phases: connection attempt, acceptance reply, data transfer, success reply. The order is always the same. The connection attempt and data transfer are sent by the client process; replies are sent by the server process.
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673 %fixme: split between header and data
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674
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675 % semantics
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676 The connection attempt is simply opening the connection. This starts the dialog. A positive reply by the server leads to the data transfer, but a negative reply refuses the connection and resets both client and server to the state before the connection attempt. If the connection attempt was accepted, the client sends the data ending with a terminator sequence. When this terminator appears, the server process knows the complete data was transfered. The server process takes responsibility of the data in sending a positive success reply. A negative success reply resets both client and server to the state before the connection attempt.
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677
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678 The data transfered needs to be of specific format. Used is the same format in which messages are spooled in the mail queues. See the following section for details. %fixme: check if it is the following section
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679 %fixme: split between header and data
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680
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681 % syntax
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682 Data transfer is done sending plain text data. %fixme: utf8 ?
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683 The terminator sequence used to indicate the end of the data transfer is a single dot on a line on its own. Line separators are the combination of \name{Carriage Return} and \name{Line Feed}, as it is used in various Internet protocols like \SMTP. Replys are one-digit numbers with \texttt{0} meaning success and any other number (\texttt{1}--\texttt{9}) indicate failure. %fixme: What are the octal values?
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684 %fixme: split between header and data
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685
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686 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
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687
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688
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689
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690 \subsubsection*{Spool file format}
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691
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692 The spool file format is basically the same as the one in current \masqmail: one file for the message body, the other for envelope and header information. The data file is stored in a separate data pool. It is written by \name{queue-in}, \name{scanning} can read it if necessary, \name{queue-out} reads it to generate the outgoing message, and deletes it after successful transfer. The header file (including the envelope) is written into the \name{incoming} queue. The \name{scanning} modules reads it, processes it, and writes a modified copy into the \name{outgoing} queue; the file in \name{incoming} is deleted then. \name{queue-out} finally takes the header file from \name{outgoing} to generate the resulting message. This data flow is shown in figure \ref{fig:queue-data-flow}.
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693
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694 \begin{figure}
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695 \begin{center}
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696 \input{input/queue-data-flow.tex}
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697 \end{center}
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698 \caption{Data flow of messages in the queue}
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699 \label{fig:queue-data-flow}
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700 \end{figure}
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701
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702 The queue consists of three directories within the queue path. Two, named \name{incoming} and \name{outgoing}, for storing the header files; one, called \name{pool}, to store the message bodies. The files being part of one message share the same unique name. The header files internal structure can be the same as the one of current \masqmail.
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703
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704 Messages in queues are a header file in \name{incoming} or \name{outgoing} and a data file in \name{pool}. The header file owner's executable bit indicates if the file is ready for further processing: the module that writes the file into the queue sets the bit as last action. Modules that read from the queue can process messages with the bit set.
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705
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706 No spool files are modified after they are written to disk. Modifications to header files can be made by the \name{scanning} module in the ``move'' from \name{incoming} to \name{outgoing}---it is a create and remove, actually. Further rewriting can happen in \name{queue-out}, as well without altering the file.
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707
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708 Data files do not change at all within the system. They are written in default local plain text format. Required translation is done in the receiver and transport modules.
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709
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710
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711 \begin{tabular}[hbt]{ l l }
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712
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713 \mbox{ queue-in:} & \mbox{
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714 \begin{tabular}[hbt]{| c | c | c |}
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715 \hline
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716 incoming & outgoing & pool \\
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717 \hline
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718 \hline
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719 - & - & - \\
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720 \hline
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721 0600 & - & - \\
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722 \hline
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723 0600 & - & 0600 \\
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724 \hline
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725 0700 & - & 0600 \\
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726 \hline
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727 \end{tabular}
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728 } \\
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729
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730 \quad & \\
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731
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732 \mbox{scanning:} & \mbox{
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733 \begin{tabular}[hbt]{| c | c | c |}
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734 \hline
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735 incoming & outgoing & pool \\
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736 \hline
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737 \hline
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738 0700 & - & 0600 \\
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739 \hline
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740 0700 & 0600 & 0600 \\
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741 \hline
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742 0700 & 0700 & 0600 \\
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743 \hline
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744 - & 0700 & 0600 \\
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745 \hline
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746 \end{tabular}
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747 } \\
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748
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749 \quad & \\
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750
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751 \mbox{queue-out:} & \mbox{
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752 \begin{tabular}[hbt]{| c | c | c |}
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753 \hline
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754 incoming & outgoing & pool \\
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755 \hline
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756 \hline
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757 - & 0700 & 0600 \\
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758 \hline
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759 - & 0700 & - \\
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760 \hline
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761 - & - & - \\
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762 \hline
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763 \end{tabular}
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764 } \\
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765
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766 \end{tabular}
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767
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768 A sample header file.
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769 \begin{verbatim}
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770 1LGtYh-0ut-00 (backup copy of the file name)
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771 MF:<meillo@dream> (envelope: sender)
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772 RT: <user@example.org> (envelope: recipient)
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773 PR:local (meta info: protocol)
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774 ID:meillo (meta info: id/user/ip)
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775 DS: 18 (meta info: size)
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776 TR: 1230462707 (meta info: timestamp)
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777 (following: headers)
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778 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
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779 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
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780 HD:To: user@example.org
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781 HD:Subject: test mail
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782 HD:From: <meillo@dream>
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783 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
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784 HD:Message-ID: <1LGtYh-0ut-00@dream>
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785 \end{verbatim}
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786
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787
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788
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789
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790 \subsubsection*{Rights and permission}
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791
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792 The user set required for \qmail\ seems to be too complex. One special user, like \postfix\ uses, is more appropriate. \name{root} privilege and \name{setuid} permission is avoided as much as possible.
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793
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794 Table \ref{tab:new-masqmail-permissions} shows the suggested ownership and permissions of the modules. Figure \ref{fig:new-masqmail-queue} shows the permissions and ownership used for the queue.
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795
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796 \begin{table}
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797 \begin{center}
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798 \input{input/new-masqmail-permissions.tex}
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799 \end{center}
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800 \caption{Ownership and permissions of the modules}
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801 \label{tab:new-masqmail-permission}
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802 \end{table}
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803
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804 \begin{figure}
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805 \begin{center}
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806 \input{input/new-masqmail-queue.tex}
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
807 \end{center}
e35c7962d84f wrote about spool file and permissions
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parents: 180
diff changeset
808 \caption{Ownership and permissions of the queue}
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
809 \label{fig:new-masqmail-queue}
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
810 \end{figure}
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
811
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
812
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
813
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
814
e35c7962d84f wrote about spool file and permissions
meillo@marmaro.de
parents: 180
diff changeset
815
180
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
816 setuid/setgid or not?
149
ccf0de1ae337 new content and rework
meillo@marmaro.de
parents: 146
diff changeset
817
180
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
818 what can crash if an attacker succeeds?
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
819
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
820 where to drop privelege?
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
821
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
822 how is which process invoked?
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
823
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
824 master process? needed, or wanted?
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
825
d75bd7a72d8b wrote about IPC, drawed picture of new arch
meillo@marmaro.de
parents: 178
diff changeset
826 which are the daemon processes?
149
ccf0de1ae337 new content and rework
meillo@marmaro.de
parents: 146
diff changeset
827
ccf0de1ae337 new content and rework
meillo@marmaro.de
parents: 146
diff changeset
828
163
5681a18270b5 new content about architecture; some restructuring
meillo@marmaro.de
parents: 161
diff changeset
829
149
ccf0de1ae337 new content and rework
meillo@marmaro.de
parents: 146
diff changeset
830
ccf0de1ae337 new content and rework
meillo@marmaro.de
parents: 146
diff changeset
831
93
a6f8a93abd64 new chapter 4 (split ch5); added lots of annotations
meillo@marmaro.de
parents:
diff changeset
832
a6f8a93abd64 new chapter 4 (split ch5); added lots of annotations
meillo@marmaro.de
parents:
diff changeset
833
185
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
834 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
835 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
836 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
837 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
838 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
839 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
840 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
841 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
842 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
843
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
844
1611abd5443b restructuring and new content (a large commit)
meillo@marmaro.de
parents: 182
diff changeset
845