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annotate thesis/tex/4-MasqmailsFuture.tex @ 188:afb72fb64962

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worked on discussion on architecture
author meillo@marmaro.de
date Mon, 29 Dec 2008 21:43:52 +0100
parents 7f4d97584a6f
children 79803ad327ca
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meillo@109 1 \chapter{\masqmail's present and future}
meillo@93 2
meillo@185 3 This chapter \dots %fixme write text here
meillo@185 4
meillo@185 5
meillo@185 6
meillo@185 7
meillo@137 8 \section{Existing code base}
meillo@185 9
meillo@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.
meillo@142 11
meillo@93 12
meillo@185 13 \subsubsection*{The source code}
meillo@185 14
meillo@185 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.
meillo@185 16
meillo@185 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.
meillo@185 18
meillo@185 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.
meillo@185 20
meillo@185 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.
meillo@185 22 %fixme: refer to ifdef-considered-harmful ?
meillo@185 23
meillo@185 24
meillo@185 25
meillo@137 26 \subsubsection*{Features}
meillo@93 27
meillo@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.
meillo@93 29
meillo@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}.
meillo@137 31 %todo: what about SSL/TLS encryption?
meillo@93 32
meillo@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.
meillo@142 34
meillo@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.
meillo@109 36 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@109 37 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@109 38
meillo@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.
meillo@109 40
meillo@137 41
meillo@132 42
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meillo@132 45
meillo@177 46 \section{Requirements}
meillo@146 47
meillo@185 48 This section identifies the requirements for a modern \masqmail. Most of them will apply to modern \MTA{}s in general.
meillo@185 49
meillo@185 50
meillo@177 51
meillo@177 52 \subsection{General requirements}
meillo@146 53
meillo@185 54 The following list of general requirements applies not only to \masqmail, but to all kinds of programs in similar environment doing similar jobs with the same intention. These requirements can be covered by the term ``quality''.
meillo@185 55 %fixme: add cites
meillo@185 56 %fixme: refer to ch01 and ch02
meillo@185 57 General requirements specify the non-functional properties of the software, thus they are also called \name{non-functional requirements}.
meillo@146 58
meillo@146 59
meillo@177 60 \subsubsection*{Security}
meillo@177 61 \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. Unsecure and unreliable \mta{}s are of no value. \masqmail\ needs to b e secure enough for its target field of operation.
meillo@177 62
meillo@177 63
meillo@177 64 \subsubsection*{Reliability}
meillo@177 65 << crash only software >>
meillo@177 66
meillo@177 67 << dont lose mail >>
meillo@177 68
meillo@177 69
meillo@177 70 \subsubsection*{Extendability}
meillo@177 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.
meillo@177 72 %fixme: like old sendmail, but not too much like it
meillo@146 73
meillo@146 74
meillo@146 75 \subsubsection*{Ressource friendly software}
meillo@149 76 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.
meillo@146 77
meillo@146 78
meillo@146 79 \subsubsection*{Easy configuration}
meillo@149 80 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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meillo@185 87
meillo@185 88 \subsection{Functional requirements}
meillo@185 89
meillo@185 90 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}.
meillo@185 91
meillo@185 92 An \MTA\ therefore needs at least a mail receiving facility and a mail sending facility.
meillo@185 93
meillo@185 94
meillo@185 95
meillo@185 96 \subsubsection*{Incoming channels}
meillo@185 97
meillo@185 98 \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.
meillo@185 99
meillo@185 100
meillo@185 101 \subsubsection*{Outgoing channels}
meillo@185 102
meillo@185 103 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
meillo@185 104
meillo@185 105 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.
meillo@185 106
meillo@185 107 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.
meillo@185 108
meillo@185 109 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.
meillo@185 110
meillo@185 111
meillo@185 112
meillo@185 113 \subsubsection*{Mail queue}
meillo@185 114
meillo@185 115 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.
meillo@185 116
meillo@185 117 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.
meillo@185 118
meillo@185 119 \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.)
meillo@185 120
meillo@185 121 \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.
meillo@185 122
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meillo@185 125
meillo@185 126 \subsubsection*{Header sanitizing}
meillo@185 127
meillo@185 128 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.
meillo@185 129
meillo@185 130 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.
meillo@185 131
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meillo@185 134
meillo@185 135 \subsubsection*{Aliasing}
meillo@185 136
meillo@185 137 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.
meillo@185 138
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meillo@185 141
meillo@185 142 \subsubsection*{Choose route to use}
meillo@185 143
meillo@185 144 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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meillo@185 148
meillo@185 149 \subsubsection*{Authentication}
meillo@185 150
meillo@185 151 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.
meillo@185 152
meillo@185 153 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.
meillo@185 154
meillo@185 155 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:
meillo@185 156 \begin{itemize}
meillo@185 157 \item \SMTP-after-\NAME{POP}: Uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@185 158 \item \SMTP authentication: An extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@185 159 \item Certificates: They confirm the identity of someone.
meillo@185 160 \end{itemize}
meillo@185 161
meillo@185 162
meillo@185 163
meillo@185 164 \subsubsection*{Encryption}
meillo@185 165
meillo@185 166 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.
meillo@185 167
meillo@185 168 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}.
meillo@185 169
meillo@185 170 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.
meillo@185 171
meillo@185 172
meillo@185 173
meillo@185 174 \subsubsection*{Spam prevention}
meillo@185 175
meillo@185 176 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.
meillo@185 177
meillo@185 178 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.
meillo@185 179
meillo@185 180 Doing some basic checks during the \SMTP\ dialog seems to be a must.
meillo@185 181
meillo@185 182
meillo@185 183
meillo@185 184 \subsubsection*{Virus checking}
meillo@185 185
meillo@185 186 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.
meillo@185 187
meillo@185 188 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.
meillo@185 189
meillo@185 190 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.
meillo@185 191
meillo@185 192
meillo@185 193
meillo@185 194 \subsubsection*{Archiving}
meillo@185 195
meillo@185 196 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.
meillo@185 197
meillo@185 198 \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.
meillo@185 199
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meillo@185 205
meillo@185 206
meillo@185 207 \section{Work to do}
meillo@185 208
meillo@187 209 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.
meillo@185 210
meillo@185 211
meillo@187 212
meillo@187 213 \subsubsection*{Fulfilled requirements}
meillo@187 214
meillo@187 215 \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.
meillo@187 216
meillo@187 217 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.
meillo@187 218
meillo@187 219 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.
meillo@187 220
meillo@187 221 \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.
meillo@187 222
meillo@187 223 There is currently no way of archiving every message going through \masqmail.
meillo@187 224
meillo@187 225
meillo@187 226 %fixme: write about non-functional requirements
meillo@187 227
meillo@187 228
meillo@187 229
meillo@187 230 \subsubsection*{Missing parts}
meillo@187 231
meillo@187 232 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.
meillo@187 233
meillo@187 234 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.
meillo@187 235
meillo@187 236 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.
meillo@187 237
meillo@187 238 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.
meillo@187 239
meillo@187 240 %fixme: what about non-functional requirements?
meillo@185 241
meillo@185 242
meillo@185 243
meillo@185 244
meillo@185 245 \subsubsection*{Discussion on architecture}
meillo@146 246
meillo@187 247 %fixme: why is there a need for a new arch??
meillo@188 248 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.
meillo@187 249
meillo@188 250 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.
meillo@187 251
meillo@188 252 \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
meillo@188 253 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
meillo@161 254
meillo@161 255 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.
meillo@161 256
meillo@161 257 \begin{figure}
meillo@161 258 \begin{center}
meillo@161 259 \input{input/masqmail-arch.tex}
meillo@161 260 \end{center}
meillo@188 261 \caption{Internal structure of \masqmail}
meillo@161 262 \label{fig:masqmail-arch}
meillo@161 263 \end{figure}
meillo@161 264
meillo@163 265 \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.
meillo@161 266
meillo@188 267 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}
meillo@161 268
meillo@188 269 \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:
meillo@163 270 \begin{quote}
meillo@188 271 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}
meillo@163 272 \end{quote}
meillo@188 273 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.
meillo@161 274
meillo@188 275 Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
meillo@163 276 \begin{quote}
meillo@163 277 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.
meillo@163 278 %
meillo@163 279 %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.
meillo@163 280 \cite[page 55]{graff03}
meillo@163 281 \end{quote}
meillo@161 282
meillo@161 283
meillo@188 284 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.
meillo@161 285
meillo@188 286 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.
meillo@161 287
meillo@188 288 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
meillo@188 289 {\small
meillo@188 290 \begin{verbatim}
meillo@188 291 Total Physical Source Lines of Code (SLOC) = 9,041
meillo@188 292 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22)
meillo@188 293 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05))
meillo@188 294 Schedule Estimate, Years (Months) = 0.70 (8.39)
meillo@188 295 (Basic COCOMO model, Months = 2.5 * (person-months**0.38))
meillo@188 296 Estimated Average Number of Developers (Effort/Schedule) = 2.89
meillo@188 297 Total Estimated Cost to Develop = $ 272,690
meillo@188 298 (average salary = $56,286/year, overhead = 2.40).
meillo@188 299 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler
meillo@188 300 \end{verbatim}
meillo@188 301 }
meillo@188 302 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.
meillo@161 303
meillo@188 304 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.
meillo@177 305
meillo@188 306 The further section describes a new modern design for \masqmail. A plan to rewrite \masqmail\ from scratch.
meillo@177 307
meillo@177 308
meillo@177 309
meillo@188 310 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@177 311
meillo@177 312
meillo@177 313
meillo@177 314
meillo@185 315 \section{A design from scratch}
meillo@177 316
meillo@185 317 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.
meillo@161 318
meillo@163 319
meillo@163 320
meillo@163 321
meillo@185 322 \subsection{Design decisions}
meillo@185 323
meillo@185 324 One major design idea of the design were:
meillo@185 325 \begin{itemize}
meillo@185 326 \item free the internal system from in and out channels
meillo@185 327 \item arbitrary protocol handlers have to be addable afterwards
meillo@185 328 \item a single facility for scanning (all mail goes through it)
meillo@185 329 \item concentrate on mail transfer
meillo@185 330 \end{itemize}
meillo@163 331
meillo@163 332
meillo@163 333 \subsubsection*{Incoming channels}
meillo@163 334
meillo@170 335 \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.
meillo@163 336
meillo@170 337 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?
meillo@170 338 \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.
meillo@163 339
meillo@170 340 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.
meillo@163 341
meillo@170 342 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.
meillo@163 343
meillo@163 344
meillo@163 345 \subsubsection*{Outgoing channels}
meillo@163 346
meillo@170 347 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
meillo@170 348
meillo@163 349 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.
meillo@163 350
meillo@170 351 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.
meillo@163 352
meillo@170 353 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.
meillo@170 354
meillo@170 355
meillo@170 356
meillo@170 357 \subsubsection*{Mail queue}
meillo@170 358
meillo@170 359 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.
meillo@170 360
meillo@170 361 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.
meillo@170 362
meillo@170 363 \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.)
meillo@170 364
meillo@170 365 \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}.
meillo@163 366
meillo@182 367 \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}.
meillo@182 368 %fixme: check, cite, and think about
meillo@182 369
meillo@163 370
meillo@165 371
meillo@165 372 \subsubsection*{Sanitize mail}
meillo@165 373
meillo@170 374 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.
meillo@165 375
meillo@170 376 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.
meillo@170 377
meillo@170 378 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.
meillo@170 379
meillo@170 380
meillo@170 381
meillo@165 382 \subsubsection*{Aliasing}
meillo@165 383
meillo@170 384 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
meillo@170 385 \begin{itemize}
meillo@170 386 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@170 387 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@170 388 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@170 389 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@170 390 \end{itemize}
meillo@170 391 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@165 392
meillo@172 393 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.
meillo@163 394
meillo@163 395
meillo@163 396
meillo@185 397 \subsubsection*{Choose route to use}
meillo@185 398
meillo@185 399 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.
meillo@185 400
meillo@185 401 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).
meillo@185 402
meillo@185 403 \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.
meillo@185 404 %fixme: see hafiz05 page 57: maybe put the rewriting into the sending module (like smx, exim, courier) (problem with archiving of all outgoing mail?)
meillo@185 405
meillo@185 406
meillo@185 407
meillo@163 408 \subsubsection*{Authentication}
meillo@163 409
meillo@170 410 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.
meillo@163 411
meillo@170 412 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}.
meillo@170 413
meillo@170 414 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:
meillo@170 415 \begin{enumerate}
meillo@185 416 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@185 417 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@185 418 \item Certificates: confirm the identity of someone.
meillo@170 419 \end{enumerate}
meillo@170 420
meillo@170 421
meillo@163 422
meillo@163 423 \subsubsection*{Encryption}
meillo@163 424
meillo@172 425 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.
meillo@165 426
meillo@172 427 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}.
meillo@165 428
meillo@172 429 \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.
meillo@165 430
meillo@172 431 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.
meillo@165 432
meillo@172 433 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@165 434
meillo@172 435 \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.
meillo@165 436
meillo@165 437
meillo@165 438
meillo@165 439
meillo@163 440
meillo@163 441 \subsubsection*{Spam prevention}
meillo@163 442
meillo@177 443 ---
meillo@177 444 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.
meillo@177 445 ---
meillo@177 446
meillo@173 447 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.
meillo@163 448
meillo@173 449 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.
meillo@163 450
meillo@177 451 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.
meillo@163 452
meillo@173 453 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.
meillo@165 454
meillo@173 455 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.'')
meillo@165 456
meillo@173 457 \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.
meillo@165 458
meillo@163 459
meillo@163 460
meillo@163 461
meillo@163 462 \subsubsection*{Virus checking}
meillo@163 463
meillo@173 464 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.
meillo@163 465
meillo@177 466 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.
meillo@163 467
meillo@177 468 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.
meillo@163 469
meillo@177 470 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.
meillo@165 471
meillo@165 472
meillo@177 473 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@177 474 %\begin{verbatim}
meillo@177 475 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@177 476 %| |
meillo@177 477 %+----------------------------+
meillo@177 478 %\end{verbatim}
meillo@177 479 %
meillo@177 480 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@177 481 %
meillo@177 482 %MailScanner:
meillo@177 483 %incoming queue --> MailScanner --> outgoing queue
meillo@177 484 %
meillo@177 485 %postfix: with one instance possible, exim and sendmail need two instances running
meillo@165 486
meillo@163 487
meillo@173 488 %message body <-> envelope, header
meillo@173 489 %
meillo@173 490 %anti-virus: clamav
meillo@173 491 %postfix: via amavis
meillo@173 492 %exim: via content-scanning-feature called from acl
meillo@173 493 %sendmail: with milter
meillo@173 494 %procmail
meillo@173 495 %
meillo@173 496 %virus scanner work on file level
meillo@173 497 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@173 498 %if the mail is okay, it goes via smtp to a second mta
meillo@173 499
meillo@173 500 %what amavis recognizes:
meillo@173 501 %- invalid headers
meillo@173 502 %- banned files
meillo@173 503 %- viruses
meillo@173 504 %- spam (using spam assassin)
meillo@173 505 %
meillo@173 506 %mimedefang: uses milter interface with sendmail
meillo@173 507
meillo@163 508
meillo@163 509
meillo@163 510 \subsubsection*{Archiving}
meillo@163 511
meillo@177 512 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.
meillo@163 513
meillo@177 514 \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.
meillo@163 515
meillo@163 516
meillo@163 517
meillo@161 518
meillo@161 519
meillo@185 520 \subsection{The resulting architecture}
meillo@161 521
meillo@178 522 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.
meillo@161 523
meillo@177 524 \begin{figure}
meillo@177 525 \begin{center}
meillo@177 526 \input{input/masqmail-arch-new.tex}
meillo@177 527 \end{center}
meillo@177 528 \caption{A new designed architecture for \masqmail}
meillo@177 529 \label{fig:masqmail-arch-new}
meillo@177 530 \end{figure}
meillo@161 531
meillo@178 532 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.
meillo@178 533
meillo@178 534 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.
meillo@178 535 %fixme: do i need all this ``quesses''??
meillo@161 536
meillo@161 537
meillo@185 538 \subsubsection*{Modules and queues}
meillo@161 539
meillo@178 540 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.
meillo@161 541
meillo@161 542
meillo@178 543 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.
meillo@182 544 %fixme: should be no daemon
meillo@178 545
meillo@178 546
meillo@178 547 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.
meillo@178 548
meillo@178 549
meillo@178 550 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.
meillo@178 551
meillo@178 552
meillo@178 553 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended.
meillo@178 554
meillo@178 555
meillo@178 556 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
meillo@178 557
meillo@178 558 \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.
meillo@178 559
meillo@178 560 \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
meillo@178 561 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
meillo@132 562
meillo@132 563
meillo@137 564
meillo@185 565 \subsubsection*{Inter-module communication}
meillo@180 566
meillo@180 567 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.
meillo@180 568
meillo@180 569 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.
meillo@180 570
meillo@180 571 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.
meillo@180 572
meillo@180 573 \begin{figure}
meillo@180 574 \begin{center}
meillo@180 575 \input{input/ipc-protocol.tex}
meillo@180 576 \end{center}
meillo@180 577 \caption{State diagram of the protocol used for \NAME{IPC}}
meillo@180 578 \label{fig:ipc-protocol}
meillo@180 579 \end{figure}
meillo@180 580
meillo@180 581 % timing
meillo@180 582 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.
meillo@182 583 %fixme: split between header and data
meillo@180 584
meillo@180 585 % semantics
meillo@180 586 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.
meillo@180 587
meillo@180 588 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
meillo@182 589 %fixme: split between header and data
meillo@180 590
meillo@180 591 % syntax
meillo@180 592 Data transfer is done sending plain text data. %fixme: utf8 ?
meillo@182 593 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?
meillo@182 594 %fixme: split between header and data
meillo@180 595
meillo@180 596 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
meillo@137 597
meillo@149 598
meillo@149 599
meillo@185 600 \subsubsection*{Spool file format}
meillo@177 601
meillo@182 602 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}.
meillo@180 603
meillo@182 604 \begin{figure}
meillo@182 605 \begin{center}
meillo@182 606 \input{input/queue-data-flow.tex}
meillo@182 607 \end{center}
meillo@182 608 \caption{Data flow of messages in the queue}
meillo@182 609 \label{fig:queue-data-flow}
meillo@182 610 \end{figure}
meillo@180 611
meillo@182 612 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.
meillo@180 613
meillo@182 614 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.
meillo@182 615
meillo@182 616 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.
meillo@182 617
meillo@182 618 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.
meillo@182 619
meillo@182 620
meillo@185 621 \begin{tabular}[hbt]{ l l }
meillo@185 622
meillo@185 623 \mbox{ queue-in:} & \mbox{
meillo@185 624 \begin{tabular}[hbt]{| c | c | c |}
meillo@185 625 \hline
meillo@185 626 incoming & outgoing & pool \\
meillo@185 627 \hline
meillo@185 628 \hline
meillo@185 629 - & - & - \\
meillo@185 630 \hline
meillo@185 631 0600 & - & - \\
meillo@185 632 \hline
meillo@185 633 0600 & - & 0600 \\
meillo@185 634 \hline
meillo@185 635 0700 & - & 0600 \\
meillo@185 636 \hline
meillo@185 637 \end{tabular}
meillo@185 638 } \\
meillo@185 639
meillo@185 640 \quad & \\
meillo@185 641
meillo@185 642 \mbox{scanning:} & \mbox{
meillo@185 643 \begin{tabular}[hbt]{| c | c | c |}
meillo@185 644 \hline
meillo@185 645 incoming & outgoing & pool \\
meillo@185 646 \hline
meillo@185 647 \hline
meillo@185 648 0700 & - & 0600 \\
meillo@185 649 \hline
meillo@185 650 0700 & 0600 & 0600 \\
meillo@185 651 \hline
meillo@185 652 0700 & 0700 & 0600 \\
meillo@185 653 \hline
meillo@185 654 - & 0700 & 0600 \\
meillo@185 655 \hline
meillo@185 656 \end{tabular}
meillo@185 657 } \\
meillo@185 658
meillo@185 659 \quad & \\
meillo@185 660
meillo@185 661 \mbox{queue-out:} & \mbox{
meillo@185 662 \begin{tabular}[hbt]{| c | c | c |}
meillo@185 663 \hline
meillo@185 664 incoming & outgoing & pool \\
meillo@185 665 \hline
meillo@185 666 \hline
meillo@185 667 - & 0700 & 0600 \\
meillo@185 668 \hline
meillo@185 669 - & 0700 & - \\
meillo@185 670 \hline
meillo@185 671 - & - & - \\
meillo@185 672 \hline
meillo@185 673 \end{tabular}
meillo@185 674 } \\
meillo@185 675
meillo@185 676 \end{tabular}
meillo@182 677
meillo@182 678 A sample header file.
meillo@180 679 \begin{verbatim}
meillo@182 680 1LGtYh-0ut-00 (backup copy of the file name)
meillo@182 681 MF:<meillo@dream> (envelope: sender)
meillo@182 682 RT: <user@example.org> (envelope: recipient)
meillo@182 683 PR:local (meta info: protocol)
meillo@182 684 ID:meillo (meta info: id/user/ip)
meillo@182 685 DS: 18 (meta info: size)
meillo@182 686 TR: 1230462707 (meta info: timestamp)
meillo@182 687 (following: headers)
meillo@182 688 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
meillo@182 689 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
meillo@182 690 HD:To: user@example.org
meillo@182 691 HD:Subject: test mail
meillo@182 692 HD:From: <meillo@dream>
meillo@182 693 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
meillo@182 694 HD:Message-ID: <1LGtYh-0ut-00@dream>
meillo@180 695 \end{verbatim}
meillo@180 696
meillo@180 697
meillo@177 698
meillo@182 699
meillo@185 700 \subsubsection*{Rights and permission}
meillo@177 701
meillo@182 702 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.
meillo@182 703
meillo@182 704 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.
meillo@182 705
meillo@182 706 \begin{table}
meillo@182 707 \begin{center}
meillo@182 708 \input{input/new-masqmail-permissions.tex}
meillo@182 709 \end{center}
meillo@182 710 \caption{Ownership and permissions of the modules}
meillo@182 711 \label{tab:new-masqmail-permission}
meillo@182 712 \end{table}
meillo@182 713
meillo@182 714 \begin{figure}
meillo@182 715 \begin{center}
meillo@182 716 \input{input/new-masqmail-queue.tex}
meillo@182 717 \end{center}
meillo@182 718 \caption{Ownership and permissions of the queue}
meillo@182 719 \label{fig:new-masqmail-queue}
meillo@182 720 \end{figure}
meillo@182 721
meillo@182 722
meillo@182 723
meillo@182 724
meillo@182 725
meillo@180 726 setuid/setgid or not?
meillo@149 727
meillo@180 728 what can crash if an attacker succeeds?
meillo@180 729
meillo@180 730 where to drop privelege?
meillo@180 731
meillo@180 732 how is which process invoked?
meillo@180 733
meillo@180 734 master process? needed, or wanted?
meillo@180 735
meillo@180 736 which are the daemon processes?
meillo@149 737
meillo@149 738
meillo@149 739
meillo@149 740
meillo@149 741
meillo@93 742
meillo@93 743
meillo@185 744 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@185 745 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@185 746 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@185 747 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@185 748 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@185 749 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@185 750 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@185 751 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@185 752 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@185 753
meillo@185 754
meillo@185 755
meillo@185 756
meillo@185 757
meillo@185 758
meillo@185 759
meillo@185 760
meillo@99 761
meillo@93 762
meillo@93 763
meillo@187 764 \section{Result}
meillo@187 765
meillo@187 766 Directions to go
meillo@161 767
meillo@185 768 Now how could \masqmail\ be like in, say, five years?
meillo@185 769
meillo@161 770 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.
meillo@161 771
meillo@93 772
meillo@146 773
meillo@146 774
meillo@185 775 1) fix the current version
meillo@163 776
meillo@185 777
meillo@185 778 2) create a new one
meillo@163 779 But how is the effort of this complete rewrite compared to what is gained afterwards?
meillo@163 780 << would one create it at all? >>
meillo@163 781
meillo@163 782
meillo@185 783 pro---contra
meillo@146 784
meillo@146 785
meillo@185 786
meillo@93 787
meillo@146 788 << short term goals --- long term goals >>
meillo@146 789
meillo@163 790 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.
meillo@163 791
meillo@140 792 << which parts to take out and do within the thesis >>
meillo@93 793
meillo@167 794