Mercurial > docs > diploma
annotate thesis/tex/4-MasqmailsFuture.tex @ 193:ee687abf386c
a result is needed here
| author | meillo@marmaro.de |
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| date | Wed, 31 Dec 2008 13:57:31 +0100 |
| parents | 74a6cbdc7255 |
| children | b80438534651 |
| rev | line source |
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| 109 | 1 \chapter{\masqmail's present and future} |
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3 This chapter \dots %fixme write text here |
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| 137 | 8 \section{Existing code base} |
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| 142 | 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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| 137 | 26 \subsubsection*{Features} |
| 27 | |
| 142 | 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. |
| 132 | 29 |
| 142 | 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}. |
| 137 | 31 %todo: what about SSL/TLS encryption? |
| 132 | 32 |
| 142 | 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. |
| 34 | |
| 137 | 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. |
| 36 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd | |
| 37 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd | |
| 132 | 38 |
| 137 | 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. |
| 132 | 40 |
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| 177 | 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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| 177 | 51 |
| 52 \subsection{General requirements} | |
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| 189 | 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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| 137 | 57 |
| 177 | 58 \subsubsection*{Security} |
| 189 | 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 security needed. \masqmail\ needs to be secure enough for its target field of operation. |
| 177 | 60 |
| 61 | |
| 62 \subsubsection*{Reliability} | |
| 189 | 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 the cause of any mail loss, no matter what happens. Unreliable \mta{}s are of no value. |
| 64 | |
| 177 | 65 |
| 189 | 66 \subsubsection*{Robustness} |
| 191 | 67 Being robust means handling errors properly. Small errors may get tolerated, large errors may kill a process, to get restarted afterwards. 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} |
| 177 | 68 |
| 69 | |
| 70 \subsubsection*{Extendability} | |
| 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\ should be able to support new mail transfer protocols as they appear and are used. | |
| 72 %fixme: like old sendmail, but not too much like it | |
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| 189 | 74 \subsubsection*{Maintainability} |
| 191 | 75 Maintaining software takes much time and effort. \person{Spinellis} measures ``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 is maintaining work. Hence making software good to maintain is effort that will become invalueable afterwards. |
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| 189 | 77 |
| 78 \subsubsection*{Testability} | |
| 79 Good testability make maintainance easier, because functionality is directly verifiable when changes are done, thus removing the uncertaintay. Modularized software makes testing easier, because parts can be testet without external influences. | |
| 80 | |
| 81 | |
| 82 \subsubsection*{Performance} | |
| 191 | 83 Also called ``efficiency''. Software requiring few time and few resources is nice. But as performance improvements are in contrast to many other quality poperties (reliability, maintainability, usability, capability \cite[page~5]{kan03}), japardizing them 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} |
| 189 | 84 |
| 149 | 85 The merge of communication hardware and the move of email services from providers to homes, demands 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. \masqmail\ is not a program to be used on large servers, but to be used on small devices. Thus focusing on energy and heat, not on performance, is the direction to go. |
| 132 | 86 |
| 87 | |
| 189 | 88 \subsubsection*{Availability} |
| 89 Availability is important for server programs. They must stay operational, even when bad guys run \name{denial of service} attacks. | |
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| 189 | 92 \subsubsection*{Portability} |
| 93 Not to forget is portability. It can be accieved 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}. | |
| 94 | |
| 95 Focusing on \unix\ systems seems to be okay, but being portable between different flavors of them is important. Also should the \masqmail\ be independent of the underlying file system. | |
| 96 | |
| 97 | |
| 98 \subsubsection*{Usability} | |
| 99 Usability, not mentioned by \person{Hafiz} 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. | |
| 100 | |
| 101 Usability here means easy to set up and configure, too. 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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| 137 | 104 |
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107 \subsection{Functional requirements} |
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108 |
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109 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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111 An \MTA\ therefore needs at least a mail receiving facility and a mail sending facility. |
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115 \subsubsection*{Incoming channels} |
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117 \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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120 \subsubsection*{Outgoing channels} |
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122 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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124 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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126 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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128 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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132 \subsubsection*{Mail queue} |
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133 |
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134 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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135 |
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136 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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138 \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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140 \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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145 \subsubsection*{Header sanitizing} |
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147 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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148 |
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149 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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150 |
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151 |
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152 |
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153 |
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154 \subsubsection*{Aliasing} |
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155 |
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156 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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157 |
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158 |
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159 |
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160 |
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161 \subsubsection*{Choose route to use} |
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162 |
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163 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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164 |
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165 |
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166 |
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167 |
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168 \subsubsection*{Authentication} |
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169 |
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170 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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171 |
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172 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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173 |
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174 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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175 \begin{itemize} |
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176 \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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177 \item \SMTP authentication: An extension to \SMTP. Authentication can be requested before mail is accepted. |
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178 \item Certificates: They confirm the identity of someone. |
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179 \end{itemize} |
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180 |
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181 |
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183 \subsubsection*{Encryption} |
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184 |
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185 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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186 |
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187 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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188 |
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189 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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190 |
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191 |
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192 |
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193 \subsubsection*{Spam prevention} |
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194 |
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195 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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196 |
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197 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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198 |
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199 Doing some basic checks during the \SMTP\ dialog seems to be a must. |
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200 |
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201 |
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202 |
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203 \subsubsection*{Virus checking} |
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204 |
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205 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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206 |
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207 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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208 |
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209 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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210 |
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211 |
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212 |
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213 \subsubsection*{Archiving} |
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214 |
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215 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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216 |
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217 \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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218 |
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221 |
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225 |
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226 \section{Work to do} |
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227 |
| 187 | 228 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. |
| 229 | |
| 230 | |
| 231 | |
| 232 \subsubsection*{Fulfilled requirements} | |
| 233 | |
| 234 \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. | |
| 235 | |
| 236 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. | |
| 237 | |
| 238 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. | |
| 239 | |
| 240 \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. | |
| 241 | |
| 242 There is currently no way of archiving every message going through \masqmail. | |
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243 |
| 187 | 244 |
| 245 %fixme: write about non-functional requirements | |
| 189 | 246 Making \masqmail\ a \name{crash-only software}\cite{candea03} would be a step to make it more robust. |
| 187 | 247 |
| 248 | |
| 249 \subsubsection*{Missing parts} | |
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250 |
| 187 | 251 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. |
| 252 | |
| 253 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. | |
| 254 | |
| 255 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. | |
| 256 | |
| 257 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. | |
| 258 | |
| 259 %fixme: what about non-functional requirements? | |
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263 |
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264 \subsubsection*{Discussion on architecture} |
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265 |
| 187 | 266 %fixme: why is there a need for a new arch?? |
| 188 | 267 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. |
| 187 | 268 |
| 188 | 269 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. |
| 161 | 270 |
| 188 | 271 \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 |
| 272 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block. | |
| 161 | 273 |
| 274 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure. | |
| 275 | |
| 276 \begin{figure} | |
| 277 \begin{center} | |
| 278 \input{input/masqmail-arch.tex} | |
| 279 \end{center} | |
| 188 | 280 \caption{Internal structure of \masqmail} |
| 161 | 281 \label{fig:masqmail-arch} |
| 282 \end{figure} | |
| 283 | |
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284 \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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285 |
| 188 | 286 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} |
| 161 | 287 |
| 188 | 288 \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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289 \begin{quote} |
| 188 | 290 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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291 \end{quote} |
| 188 | 292 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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293 |
| 188 | 294 Security comes from good design, as \person{Graff} and \person{van Wyk} explain: |
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295 \begin{quote} |
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296 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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297 % |
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298 %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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299 \cite[page 55]{graff03} |
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300 \end{quote} |
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301 |
| 188 | 302 |
| 303 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. | |
| 304 | |
| 305 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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306 |
| 188 | 307 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code): |
| 308 {\small | |
| 309 \begin{verbatim} | |
| 310 Total Physical Source Lines of Code (SLOC) = 9,041 | |
| 311 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22) | |
| 312 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05)) | |
| 313 Schedule Estimate, Years (Months) = 0.70 (8.39) | |
| 314 (Basic COCOMO model, Months = 2.5 * (person-months**0.38)) | |
| 315 Estimated Average Number of Developers (Effort/Schedule) = 2.89 | |
| 316 Total Estimated Cost to Develop = $ 272,690 | |
| 317 (average salary = $56,286/year, overhead = 2.40). | |
| 318 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler | |
| 319 \end{verbatim} | |
| 320 } | |
| 321 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. | |
| 322 | |
| 323 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. | |
| 324 | |
| 325 The further section describes a new modern design for \masqmail. A plan to rewrite \masqmail\ from scratch. | |
| 161 | 326 |
| 327 | |
| 328 | |
| 188 | 329 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05} |
| 177 | 330 |
| 331 | |
| 332 | |
| 333 | |
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334 \section{A design from scratch} |
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335 |
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336 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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337 |
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338 |
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339 |
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340 |
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341 \subsection{Design decisions} |
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342 |
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343 One major design idea of the design were: |
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344 \begin{itemize} |
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345 \item free the internal system from in and out channels |
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346 \item arbitrary protocol handlers have to be addable afterwards |
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347 \item a single facility for scanning (all mail goes through it) |
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348 \item concentrate on mail transfer |
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349 \end{itemize} |
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350 |
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351 |
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352 \subsubsection*{Incoming channels} |
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353 |
| 170 | 354 \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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355 |
| 170 | 356 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? |
| 357 \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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358 |
| 170 | 359 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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360 |
| 170 | 361 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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362 |
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363 |
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364 \subsubsection*{Outgoing channels} |
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365 |
| 170 | 366 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox. |
| 367 | |
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368 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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369 |
| 170 | 370 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. |
| 371 | |
| 372 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. | |
| 373 | |
| 374 | |
| 375 | |
| 376 \subsubsection*{Mail queue} | |
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377 |
| 170 | 378 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. |
| 379 | |
| 380 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. | |
| 381 | |
| 382 \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.) | |
| 383 | |
| 384 \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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385 |
| 182 | 386 \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}. |
| 387 %fixme: check, cite, and think about | |
| 388 | |
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389 |
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390 |
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391 \subsubsection*{Sanitize mail} |
| 170 | 392 |
| 393 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. | |
| 394 | |
| 395 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. | |
| 396 | |
| 397 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. | |
| 398 | |
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399 |
| 170 | 400 |
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401 \subsubsection*{Aliasing} |
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402 |
| 170 | 403 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be: |
| 404 \begin{itemize} | |
| 405 \item a different local user (e.g.\ ``\texttt{bob: alice}'') | |
| 406 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'') | |
| 407 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'') | |
| 408 \item a command (e.g.\ ``\texttt{bob: |foo}'') | |
| 409 \end{itemize} | |
| 410 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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411 |
| 172 | 412 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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413 |
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414 |
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415 |
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416 \subsubsection*{Choose route to use} |
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417 |
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418 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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419 |
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420 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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421 |
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422 \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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423 %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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424 |
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425 |
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426 |
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427 \subsubsection*{Authentication} |
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428 |
| 170 | 429 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. |
| 430 | |
| 431 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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432 |
| 170 | 433 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: |
| 434 \begin{enumerate} | |
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435 \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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436 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted. |
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437 \item Certificates: confirm the identity of someone. |
| 170 | 438 \end{enumerate} |
| 439 | |
| 440 | |
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441 |
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442 \subsubsection*{Encryption} |
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443 |
| 172 | 444 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. |
| 445 | |
| 446 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}. | |
| 447 | |
| 448 \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. | |
| 449 | |
| 450 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. | |
| 451 | |
| 452 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement. | |
| 453 | |
| 454 \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. | |
| 455 | |
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456 |
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457 |
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458 |
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459 |
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460 \subsubsection*{Spam prevention} |
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461 |
| 177 | 462 --- |
| 463 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. | |
| 464 --- | |
| 465 | |
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466 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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467 |
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468 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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469 |
| 177 | 470 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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471 |
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472 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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473 |
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474 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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476 \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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477 |
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478 |
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479 |
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480 |
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481 \subsubsection*{Virus checking} |
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482 |
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483 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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484 |
| 177 | 485 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. |
| 161 | 486 |
| 177 | 487 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. |
| 488 | |
| 489 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. | |
| 161 | 490 |
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491 |
| 177 | 492 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner |
| 493 %\begin{verbatim} | |
| 494 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient | |
| 495 %| | | |
| 496 %+----------------------------+ | |
| 497 %\end{verbatim} | |
| 498 % | |
| 499 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running. | |
| 500 % | |
| 501 %MailScanner: | |
| 502 %incoming queue --> MailScanner --> outgoing queue | |
| 503 % | |
| 504 %postfix: with one instance possible, exim and sendmail need two instances running | |
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505 |
| 161 | 506 |
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507 %message body <-> envelope, header |
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508 % |
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509 %anti-virus: clamav |
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510 %postfix: via amavis |
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511 %exim: via content-scanning-feature called from acl |
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512 %sendmail: with milter |
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513 %procmail |
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514 % |
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515 %virus scanner work on file level |
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516 %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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517 %if the mail is okay, it goes via smtp to a second mta |
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518 |
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519 %what amavis recognizes: |
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520 %- invalid headers |
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521 %- banned files |
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522 %- viruses |
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523 %- spam (using spam assassin) |
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524 % |
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525 %mimedefang: uses milter interface with sendmail |
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526 |
| 161 | 527 |
| 528 | |
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529 \subsubsection*{Archiving} |
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530 |
| 177 | 531 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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532 |
| 177 | 533 \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. |
| 161 | 534 |
| 535 | |
| 536 | |
| 537 | |
| 177 | 538 |
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539 \subsection{The resulting architecture} |
| 177 | 540 |
| 178 | 541 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. |
| 177 | 542 |
| 543 \begin{figure} | |
| 544 \begin{center} | |
| 545 \input{input/masqmail-arch-new.tex} | |
| 546 \end{center} | |
| 547 \caption{A new designed architecture for \masqmail} | |
| 548 \label{fig:masqmail-arch-new} | |
| 549 \end{figure} | |
| 161 | 550 |
| 178 | 551 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. |
| 552 | |
| 553 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. | |
| 554 %fixme: do i need all this ``quesses''?? | |
| 161 | 555 |
| 149 | 556 |
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557 \subsubsection*{Modules and queues} |
| 177 | 558 |
| 178 | 559 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. |
| 560 | |
| 561 | |
| 562 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. | |
| 182 | 563 %fixme: should be no daemon |
| 178 | 564 |
| 565 | |
| 566 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. | |
| 567 | |
| 568 | |
| 569 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. | |
| 177 | 570 |
| 178 | 571 |
| 572 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended. | |
| 573 | |
| 177 | 574 |
| 178 | 575 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form. |
| 576 | |
| 577 \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. | |
| 578 | |
| 579 \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 | |
| 580 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module. | |
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581 |
| 132 | 582 |
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583 |
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584 \subsubsection*{Inter-module communication} |
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585 |
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586 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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587 |
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588 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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589 |
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590 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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591 |
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592 \begin{figure} |
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593 \begin{center} |
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594 \input{input/ipc-protocol.tex} |
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595 \end{center} |
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596 \caption{State diagram of the protocol used for \NAME{IPC}} |
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597 \label{fig:ipc-protocol} |
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598 \end{figure} |
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599 |
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600 % timing |
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601 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. |
| 182 | 602 %fixme: split between header and data |
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603 |
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604 % semantics |
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605 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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606 |
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607 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 |
| 182 | 608 %fixme: split between header and data |
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609 |
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610 % syntax |
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611 Data transfer is done sending plain text data. %fixme: utf8 ? |
| 182 | 612 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? |
| 613 %fixme: split between header and data | |
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614 |
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615 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol. |
| 177 | 616 |
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617 |
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618 |
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619 \subsubsection*{Spool file format} |
| 177 | 620 |
| 182 | 621 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}. |
| 622 | |
| 623 \begin{figure} | |
| 624 \begin{center} | |
| 625 \input{input/queue-data-flow.tex} | |
| 626 \end{center} | |
| 627 \caption{Data flow of messages in the queue} | |
| 628 \label{fig:queue-data-flow} | |
| 629 \end{figure} | |
| 630 | |
| 631 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. | |
| 632 | |
| 633 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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634 |
| 182 | 635 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. |
| 636 | |
| 637 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. | |
| 638 | |
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639 |
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640 \begin{tabular}[hbt]{ l l } |
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641 |
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642 \mbox{ queue-in:} & \mbox{ |
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643 \begin{tabular}[hbt]{| c | c | c |} |
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644 \hline |
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645 incoming & outgoing & pool \\ |
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646 \hline |
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647 \hline |
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648 - & - & - \\ |
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649 \hline |
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650 0600 & - & - \\ |
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651 \hline |
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652 0600 & - & 0600 \\ |
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653 \hline |
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654 0700 & - & 0600 \\ |
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655 \hline |
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656 \end{tabular} |
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657 } \\ |
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658 |
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659 \quad & \\ |
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660 |
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661 \mbox{scanning:} & \mbox{ |
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662 \begin{tabular}[hbt]{| c | c | c |} |
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663 \hline |
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664 incoming & outgoing & pool \\ |
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665 \hline |
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666 \hline |
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667 0700 & - & 0600 \\ |
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668 \hline |
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669 0700 & 0600 & 0600 \\ |
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670 \hline |
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671 0700 & 0700 & 0600 \\ |
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672 \hline |
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673 - & 0700 & 0600 \\ |
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674 \hline |
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675 \end{tabular} |
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676 } \\ |
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677 |
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678 \quad & \\ |
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679 |
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680 \mbox{queue-out:} & \mbox{ |
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681 \begin{tabular}[hbt]{| c | c | c |} |
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682 \hline |
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683 incoming & outgoing & pool \\ |
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684 \hline |
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685 \hline |
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686 - & 0700 & 0600 \\ |
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687 \hline |
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688 - & 0700 & - \\ |
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689 \hline |
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690 - & - & - \\ |
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691 \hline |
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692 \end{tabular} |
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693 } \\ |
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694 |
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695 \end{tabular} |
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696 |
| 182 | 697 A sample header file. |
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698 \begin{verbatim} |
| 182 | 699 1LGtYh-0ut-00 (backup copy of the file name) |
| 700 MF:<meillo@dream> (envelope: sender) | |
| 701 RT: <user@example.org> (envelope: recipient) | |
| 702 PR:local (meta info: protocol) | |
| 703 ID:meillo (meta info: id/user/ip) | |
| 704 DS: 18 (meta info: size) | |
| 705 TR: 1230462707 (meta info: timestamp) | |
| 706 (following: headers) | |
| 707 HD:Received: from meillo by dream with local (masqmail 0.2.21) id | |
| 708 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100 | |
| 709 HD:To: user@example.org | |
| 710 HD:Subject: test mail | |
| 711 HD:From: <meillo@dream> | |
| 712 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100 | |
| 713 HD:Message-ID: <1LGtYh-0ut-00@dream> | |
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714 \end{verbatim} |
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715 |
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716 |
| 177 | 717 |
| 182 | 718 |
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719 \subsubsection*{Rights and permission} |
| 109 | 720 |
| 182 | 721 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. |
| 722 | |
| 723 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. | |
| 724 | |
| 725 \begin{table} | |
| 726 \begin{center} | |
| 727 \input{input/new-masqmail-permissions.tex} | |
| 728 \end{center} | |
| 729 \caption{Ownership and permissions of the modules} | |
| 730 \label{tab:new-masqmail-permission} | |
| 731 \end{table} | |
| 732 | |
| 733 \begin{figure} | |
| 734 \begin{center} | |
| 735 \input{input/new-masqmail-queue.tex} | |
| 736 \end{center} | |
| 737 \caption{Ownership and permissions of the queue} | |
| 738 \label{fig:new-masqmail-queue} | |
| 739 \end{figure} | |
| 740 | |
| 741 | |
| 742 | |
| 743 | |
| 744 | |
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745 setuid/setgid or not? |
| 149 | 746 |
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747 what can crash if an attacker succeeds? |
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748 |
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749 where to drop privelege? |
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750 |
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751 how is which process invoked? |
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752 |
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753 master process? needed, or wanted? |
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754 |
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755 which are the daemon processes? |
| 149 | 756 |
| 757 | |
|
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758 |
| 149 | 759 |
| 760 | |
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761 |
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762 |
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763 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command |
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764 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security |
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765 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery |
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766 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format |
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767 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics |
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768 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues |
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769 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues |
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770 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification |
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771 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning |
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772 |
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773 |
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774 |
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775 |
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776 |
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777 |
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778 |
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779 |
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| 187 | 783 \section{Result} |
| 784 | |
| 785 Directions to go | |
| 161 | 786 |
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787 Now how could \masqmail\ be like in, say, five years? |
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| 161 | 789 This section discusses about what shapes \masqmail\ could have---which directions the development could go to. |
| 790 | |
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791 |
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793 |
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794 1) fix the current version |
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795 |
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796 |
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797 2) create a new one |
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798 But how is the effort of this complete rewrite compared to what is gained afterwards? |
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799 << would one create it at all? >> |
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800 |
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801 |
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802 pro---contra |
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803 |
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804 |
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805 |
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806 |
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807 << short term goals --- long term goals >> |
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808 |
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809 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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| 140 | 811 << which parts to take out and do within the thesis >> |
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812 |
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813 |