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annotate thesis/tex/4-MasqmailsFuture.tex @ 189:79803ad327ca

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reworked general requirements
author meillo@marmaro.de
date Tue, 30 Dec 2008 13:22:21 +0100
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children 74a6cbdc7255
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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
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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@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}.
meillo@185 55 %fixme: refer to ch01 and ch02
meillo@146 56
meillo@146 57
meillo@177 58 \subsubsection*{Security}
meillo@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.
meillo@177 60
meillo@177 61
meillo@177 62 \subsubsection*{Reliability}
meillo@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.
meillo@177 64
meillo@189 65
meillo@189 66 \subsubsection*{Robustness}
meillo@189 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[page~18--21]{raymond03}
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@189 74 \subsubsection*{Maintainability}
meillo@189 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.
meillo@146 76
meillo@189 77
meillo@189 78 \subsubsection*{Testability}
meillo@189 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.
meillo@189 80
meillo@189 81
meillo@189 82 \subsubsection*{Performance}
meillo@189 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{kernighan99}
meillo@189 84
meillo@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.
meillo@146 86
meillo@146 87
meillo@189 88 \subsubsection*{Availability}
meillo@189 89 Availability is important for server programs. They must stay operational, even when bad guys run \name{denial of service} attacks.
meillo@146 90
meillo@146 91
meillo@189 92 \subsubsection*{Portability}
meillo@189 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}.
meillo@189 94
meillo@189 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.
meillo@189 96
meillo@189 97
meillo@189 98 \subsubsection*{Usability}
meillo@189 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.
meillo@189 100
meillo@189 101 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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meillo@185 106
meillo@185 107 \subsection{Functional requirements}
meillo@185 108
meillo@185 109 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}.
meillo@185 110
meillo@185 111 An \MTA\ therefore needs at least a mail receiving facility and a mail sending facility.
meillo@185 112
meillo@185 113
meillo@185 114
meillo@185 115 \subsubsection*{Incoming channels}
meillo@185 116
meillo@185 117 \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.
meillo@185 118
meillo@185 119
meillo@185 120 \subsubsection*{Outgoing channels}
meillo@185 121
meillo@185 122 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
meillo@185 123
meillo@185 124 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.
meillo@185 125
meillo@185 126 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.
meillo@185 127
meillo@185 128 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.
meillo@185 129
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meillo@185 131
meillo@185 132 \subsubsection*{Mail queue}
meillo@185 133
meillo@185 134 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.
meillo@185 135
meillo@185 136 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.
meillo@185 137
meillo@185 138 \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.)
meillo@185 139
meillo@185 140 \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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meillo@185 144
meillo@185 145 \subsubsection*{Header sanitizing}
meillo@185 146
meillo@185 147 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.
meillo@185 148
meillo@185 149 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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meillo@185 153
meillo@185 154 \subsubsection*{Aliasing}
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meillo@185 156 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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meillo@185 160
meillo@185 161 \subsubsection*{Choose route to use}
meillo@185 162
meillo@185 163 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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meillo@185 167
meillo@185 168 \subsubsection*{Authentication}
meillo@185 169
meillo@185 170 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.
meillo@185 171
meillo@185 172 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.
meillo@185 173
meillo@185 174 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:
meillo@185 175 \begin{itemize}
meillo@185 176 \item \SMTP-after-\NAME{POP}: Uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@185 177 \item \SMTP authentication: An extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@185 178 \item Certificates: They confirm the identity of someone.
meillo@185 179 \end{itemize}
meillo@185 180
meillo@185 181
meillo@185 182
meillo@185 183 \subsubsection*{Encryption}
meillo@185 184
meillo@185 185 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.
meillo@185 186
meillo@185 187 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}.
meillo@185 188
meillo@185 189 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.
meillo@185 190
meillo@185 191
meillo@185 192
meillo@185 193 \subsubsection*{Spam prevention}
meillo@185 194
meillo@185 195 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.
meillo@185 196
meillo@185 197 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.
meillo@185 198
meillo@185 199 Doing some basic checks during the \SMTP\ dialog seems to be a must.
meillo@185 200
meillo@185 201
meillo@185 202
meillo@185 203 \subsubsection*{Virus checking}
meillo@185 204
meillo@185 205 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.
meillo@185 206
meillo@185 207 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.
meillo@185 208
meillo@185 209 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.
meillo@185 210
meillo@185 211
meillo@185 212
meillo@185 213 \subsubsection*{Archiving}
meillo@185 214
meillo@185 215 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.
meillo@185 216
meillo@185 217 \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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meillo@185 225
meillo@185 226 \section{Work to do}
meillo@185 227
meillo@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.
meillo@185 229
meillo@185 230
meillo@187 231
meillo@187 232 \subsubsection*{Fulfilled requirements}
meillo@187 233
meillo@187 234 \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.
meillo@187 235
meillo@187 236 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.
meillo@187 237
meillo@187 238 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.
meillo@187 239
meillo@187 240 \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.
meillo@187 241
meillo@187 242 There is currently no way of archiving every message going through \masqmail.
meillo@187 243
meillo@187 244
meillo@187 245 %fixme: write about non-functional requirements
meillo@189 246 Making \masqmail\ a \name{crash-only software}\cite{candea03} would be a step to make it more robust.
meillo@187 247
meillo@187 248
meillo@187 249 \subsubsection*{Missing parts}
meillo@187 250
meillo@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.
meillo@187 252
meillo@187 253 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.
meillo@187 254
meillo@187 255 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.
meillo@187 256
meillo@187 257 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.
meillo@187 258
meillo@187 259 %fixme: what about non-functional requirements?
meillo@185 260
meillo@185 261
meillo@185 262
meillo@185 263
meillo@185 264 \subsubsection*{Discussion on architecture}
meillo@146 265
meillo@187 266 %fixme: why is there a need for a new arch??
meillo@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.
meillo@187 268
meillo@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.
meillo@187 270
meillo@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
meillo@188 272 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
meillo@161 273
meillo@161 274 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.
meillo@161 275
meillo@161 276 \begin{figure}
meillo@161 277 \begin{center}
meillo@161 278 \input{input/masqmail-arch.tex}
meillo@161 279 \end{center}
meillo@188 280 \caption{Internal structure of \masqmail}
meillo@161 281 \label{fig:masqmail-arch}
meillo@161 282 \end{figure}
meillo@161 283
meillo@163 284 \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.
meillo@161 285
meillo@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}
meillo@161 287
meillo@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:
meillo@163 289 \begin{quote}
meillo@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}
meillo@163 291 \end{quote}
meillo@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.
meillo@161 293
meillo@188 294 Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
meillo@163 295 \begin{quote}
meillo@163 296 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.
meillo@163 297 %
meillo@163 298 %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.
meillo@163 299 \cite[page 55]{graff03}
meillo@163 300 \end{quote}
meillo@161 301
meillo@161 302
meillo@188 303 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.
meillo@161 304
meillo@188 305 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.
meillo@161 306
meillo@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):
meillo@188 308 {\small
meillo@188 309 \begin{verbatim}
meillo@188 310 Total Physical Source Lines of Code (SLOC) = 9,041
meillo@188 311 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22)
meillo@188 312 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05))
meillo@188 313 Schedule Estimate, Years (Months) = 0.70 (8.39)
meillo@188 314 (Basic COCOMO model, Months = 2.5 * (person-months**0.38))
meillo@188 315 Estimated Average Number of Developers (Effort/Schedule) = 2.89
meillo@188 316 Total Estimated Cost to Develop = $ 272,690
meillo@188 317 (average salary = $56,286/year, overhead = 2.40).
meillo@188 318 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler
meillo@188 319 \end{verbatim}
meillo@188 320 }
meillo@188 321 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.
meillo@161 322
meillo@188 323 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.
meillo@177 324
meillo@188 325 The further section describes a new modern design for \masqmail. A plan to rewrite \masqmail\ from scratch.
meillo@177 326
meillo@177 327
meillo@177 328
meillo@188 329 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@177 330
meillo@177 331
meillo@177 332
meillo@177 333
meillo@185 334 \section{A design from scratch}
meillo@177 335
meillo@185 336 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.
meillo@161 337
meillo@163 338
meillo@163 339
meillo@163 340
meillo@185 341 \subsection{Design decisions}
meillo@185 342
meillo@185 343 One major design idea of the design were:
meillo@185 344 \begin{itemize}
meillo@185 345 \item free the internal system from in and out channels
meillo@185 346 \item arbitrary protocol handlers have to be addable afterwards
meillo@185 347 \item a single facility for scanning (all mail goes through it)
meillo@185 348 \item concentrate on mail transfer
meillo@185 349 \end{itemize}
meillo@163 350
meillo@163 351
meillo@163 352 \subsubsection*{Incoming channels}
meillo@163 353
meillo@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.
meillo@163 355
meillo@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?
meillo@170 357 \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.
meillo@163 358
meillo@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.
meillo@163 360
meillo@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.
meillo@163 362
meillo@163 363
meillo@163 364 \subsubsection*{Outgoing channels}
meillo@163 365
meillo@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.
meillo@170 367
meillo@163 368 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.
meillo@163 369
meillo@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.
meillo@163 371
meillo@170 372 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.
meillo@170 373
meillo@170 374
meillo@170 375
meillo@170 376 \subsubsection*{Mail queue}
meillo@170 377
meillo@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.
meillo@170 379
meillo@170 380 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.
meillo@170 381
meillo@170 382 \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.)
meillo@170 383
meillo@170 384 \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}.
meillo@163 385
meillo@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}.
meillo@182 387 %fixme: check, cite, and think about
meillo@182 388
meillo@163 389
meillo@165 390
meillo@165 391 \subsubsection*{Sanitize mail}
meillo@165 392
meillo@170 393 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.
meillo@165 394
meillo@170 395 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.
meillo@170 396
meillo@170 397 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.
meillo@170 398
meillo@170 399
meillo@170 400
meillo@165 401 \subsubsection*{Aliasing}
meillo@165 402
meillo@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:
meillo@170 404 \begin{itemize}
meillo@170 405 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@170 406 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@170 407 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@170 408 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@170 409 \end{itemize}
meillo@170 410 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@165 411
meillo@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.
meillo@163 413
meillo@163 414
meillo@163 415
meillo@185 416 \subsubsection*{Choose route to use}
meillo@185 417
meillo@185 418 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.
meillo@185 419
meillo@185 420 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).
meillo@185 421
meillo@185 422 \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.
meillo@185 423 %fixme: see hafiz05 page 57: maybe put the rewriting into the sending module (like smx, exim, courier) (problem with archiving of all outgoing mail?)
meillo@185 424
meillo@185 425
meillo@185 426
meillo@163 427 \subsubsection*{Authentication}
meillo@163 428
meillo@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.
meillo@163 430
meillo@170 431 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}.
meillo@170 432
meillo@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:
meillo@170 434 \begin{enumerate}
meillo@185 435 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@185 436 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@185 437 \item Certificates: confirm the identity of someone.
meillo@170 438 \end{enumerate}
meillo@170 439
meillo@170 440
meillo@163 441
meillo@163 442 \subsubsection*{Encryption}
meillo@163 443
meillo@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.
meillo@165 445
meillo@172 446 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}.
meillo@165 447
meillo@172 448 \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.
meillo@165 449
meillo@172 450 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.
meillo@165 451
meillo@172 452 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@165 453
meillo@172 454 \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.
meillo@165 455
meillo@165 456
meillo@165 457
meillo@165 458
meillo@163 459
meillo@163 460 \subsubsection*{Spam prevention}
meillo@163 461
meillo@177 462 ---
meillo@177 463 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.
meillo@177 464 ---
meillo@177 465
meillo@173 466 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.
meillo@163 467
meillo@173 468 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.
meillo@163 469
meillo@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.
meillo@163 471
meillo@173 472 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.
meillo@165 473
meillo@173 474 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.'')
meillo@165 475
meillo@173 476 \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.
meillo@165 477
meillo@163 478
meillo@163 479
meillo@163 480
meillo@163 481 \subsubsection*{Virus checking}
meillo@163 482
meillo@173 483 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.
meillo@163 484
meillo@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.
meillo@163 486
meillo@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.
meillo@163 488
meillo@177 489 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.
meillo@165 490
meillo@165 491
meillo@177 492 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@177 493 %\begin{verbatim}
meillo@177 494 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@177 495 %| |
meillo@177 496 %+----------------------------+
meillo@177 497 %\end{verbatim}
meillo@177 498 %
meillo@177 499 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@177 500 %
meillo@177 501 %MailScanner:
meillo@177 502 %incoming queue --> MailScanner --> outgoing queue
meillo@177 503 %
meillo@177 504 %postfix: with one instance possible, exim and sendmail need two instances running
meillo@165 505
meillo@163 506
meillo@173 507 %message body <-> envelope, header
meillo@173 508 %
meillo@173 509 %anti-virus: clamav
meillo@173 510 %postfix: via amavis
meillo@173 511 %exim: via content-scanning-feature called from acl
meillo@173 512 %sendmail: with milter
meillo@173 513 %procmail
meillo@173 514 %
meillo@173 515 %virus scanner work on file level
meillo@173 516 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@173 517 %if the mail is okay, it goes via smtp to a second mta
meillo@173 518
meillo@173 519 %what amavis recognizes:
meillo@173 520 %- invalid headers
meillo@173 521 %- banned files
meillo@173 522 %- viruses
meillo@173 523 %- spam (using spam assassin)
meillo@173 524 %
meillo@173 525 %mimedefang: uses milter interface with sendmail
meillo@173 526
meillo@163 527
meillo@163 528
meillo@163 529 \subsubsection*{Archiving}
meillo@163 530
meillo@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.
meillo@163 532
meillo@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.
meillo@163 534
meillo@163 535
meillo@163 536
meillo@161 537
meillo@161 538
meillo@185 539 \subsection{The resulting architecture}
meillo@161 540
meillo@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.
meillo@161 542
meillo@177 543 \begin{figure}
meillo@177 544 \begin{center}
meillo@177 545 \input{input/masqmail-arch-new.tex}
meillo@177 546 \end{center}
meillo@177 547 \caption{A new designed architecture for \masqmail}
meillo@177 548 \label{fig:masqmail-arch-new}
meillo@177 549 \end{figure}
meillo@161 550
meillo@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.
meillo@178 552
meillo@178 553 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.
meillo@178 554 %fixme: do i need all this ``quesses''??
meillo@161 555
meillo@161 556
meillo@185 557 \subsubsection*{Modules and queues}
meillo@161 558
meillo@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.
meillo@161 560
meillo@161 561
meillo@178 562 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.
meillo@182 563 %fixme: should be no daemon
meillo@178 564
meillo@178 565
meillo@178 566 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.
meillo@178 567
meillo@178 568
meillo@178 569 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.
meillo@178 570
meillo@178 571
meillo@178 572 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended.
meillo@178 573
meillo@178 574
meillo@178 575 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
meillo@178 576
meillo@178 577 \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.
meillo@178 578
meillo@178 579 \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
meillo@178 580 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
meillo@132 581
meillo@132 582
meillo@137 583
meillo@185 584 \subsubsection*{Inter-module communication}
meillo@180 585
meillo@180 586 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.
meillo@180 587
meillo@180 588 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.
meillo@180 589
meillo@180 590 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.
meillo@180 591
meillo@180 592 \begin{figure}
meillo@180 593 \begin{center}
meillo@180 594 \input{input/ipc-protocol.tex}
meillo@180 595 \end{center}
meillo@180 596 \caption{State diagram of the protocol used for \NAME{IPC}}
meillo@180 597 \label{fig:ipc-protocol}
meillo@180 598 \end{figure}
meillo@180 599
meillo@180 600 % timing
meillo@180 601 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.
meillo@182 602 %fixme: split between header and data
meillo@180 603
meillo@180 604 % semantics
meillo@180 605 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.
meillo@180 606
meillo@180 607 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
meillo@182 608 %fixme: split between header and data
meillo@180 609
meillo@180 610 % syntax
meillo@180 611 Data transfer is done sending plain text data. %fixme: utf8 ?
meillo@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?
meillo@182 613 %fixme: split between header and data
meillo@180 614
meillo@180 615 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
meillo@137 616
meillo@149 617
meillo@149 618
meillo@185 619 \subsubsection*{Spool file format}
meillo@177 620
meillo@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}.
meillo@180 622
meillo@182 623 \begin{figure}
meillo@182 624 \begin{center}
meillo@182 625 \input{input/queue-data-flow.tex}
meillo@182 626 \end{center}
meillo@182 627 \caption{Data flow of messages in the queue}
meillo@182 628 \label{fig:queue-data-flow}
meillo@182 629 \end{figure}
meillo@180 630
meillo@182 631 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.
meillo@180 632
meillo@182 633 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.
meillo@182 634
meillo@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.
meillo@182 636
meillo@182 637 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.
meillo@182 638
meillo@182 639
meillo@185 640 \begin{tabular}[hbt]{ l l }
meillo@185 641
meillo@185 642 \mbox{ queue-in:} & \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 - & - & - \\
meillo@185 649 \hline
meillo@185 650 0600 & - & - \\
meillo@185 651 \hline
meillo@185 652 0600 & - & 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{scanning:} & \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 & 0600 & 0600 \\
meillo@185 670 \hline
meillo@185 671 0700 & 0700 & 0600 \\
meillo@185 672 \hline
meillo@185 673 - & 0700 & 0600 \\
meillo@185 674 \hline
meillo@185 675 \end{tabular}
meillo@185 676 } \\
meillo@185 677
meillo@185 678 \quad & \\
meillo@185 679
meillo@185 680 \mbox{queue-out:} & \mbox{
meillo@185 681 \begin{tabular}[hbt]{| c | c | c |}
meillo@185 682 \hline
meillo@185 683 incoming & outgoing & pool \\
meillo@185 684 \hline
meillo@185 685 \hline
meillo@185 686 - & 0700 & 0600 \\
meillo@185 687 \hline
meillo@185 688 - & 0700 & - \\
meillo@185 689 \hline
meillo@185 690 - & - & - \\
meillo@185 691 \hline
meillo@185 692 \end{tabular}
meillo@185 693 } \\
meillo@185 694
meillo@185 695 \end{tabular}
meillo@182 696
meillo@182 697 A sample header file.
meillo@180 698 \begin{verbatim}
meillo@182 699 1LGtYh-0ut-00 (backup copy of the file name)
meillo@182 700 MF:<meillo@dream> (envelope: sender)
meillo@182 701 RT: <user@example.org> (envelope: recipient)
meillo@182 702 PR:local (meta info: protocol)
meillo@182 703 ID:meillo (meta info: id/user/ip)
meillo@182 704 DS: 18 (meta info: size)
meillo@182 705 TR: 1230462707 (meta info: timestamp)
meillo@182 706 (following: headers)
meillo@182 707 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
meillo@182 708 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
meillo@182 709 HD:To: user@example.org
meillo@182 710 HD:Subject: test mail
meillo@182 711 HD:From: <meillo@dream>
meillo@182 712 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
meillo@182 713 HD:Message-ID: <1LGtYh-0ut-00@dream>
meillo@180 714 \end{verbatim}
meillo@180 715
meillo@180 716
meillo@177 717
meillo@182 718
meillo@185 719 \subsubsection*{Rights and permission}
meillo@177 720
meillo@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.
meillo@182 722
meillo@182 723 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.
meillo@182 724
meillo@182 725 \begin{table}
meillo@182 726 \begin{center}
meillo@182 727 \input{input/new-masqmail-permissions.tex}
meillo@182 728 \end{center}
meillo@182 729 \caption{Ownership and permissions of the modules}
meillo@182 730 \label{tab:new-masqmail-permission}
meillo@182 731 \end{table}
meillo@182 732
meillo@182 733 \begin{figure}
meillo@182 734 \begin{center}
meillo@182 735 \input{input/new-masqmail-queue.tex}
meillo@182 736 \end{center}
meillo@182 737 \caption{Ownership and permissions of the queue}
meillo@182 738 \label{fig:new-masqmail-queue}
meillo@182 739 \end{figure}
meillo@182 740
meillo@182 741
meillo@182 742
meillo@182 743
meillo@182 744
meillo@180 745 setuid/setgid or not?
meillo@149 746
meillo@180 747 what can crash if an attacker succeeds?
meillo@180 748
meillo@180 749 where to drop privelege?
meillo@180 750
meillo@180 751 how is which process invoked?
meillo@180 752
meillo@180 753 master process? needed, or wanted?
meillo@180 754
meillo@180 755 which are the daemon processes?
meillo@149 756
meillo@149 757
meillo@149 758
meillo@149 759
meillo@149 760
meillo@93 761
meillo@93 762
meillo@185 763 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@185 764 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@185 765 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@185 766 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@185 767 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@185 768 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@185 769 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@185 770 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@185 771 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@185 772
meillo@185 773
meillo@185 774
meillo@185 775
meillo@185 776
meillo@185 777
meillo@185 778
meillo@185 779
meillo@99 780
meillo@93 781
meillo@93 782
meillo@187 783 \section{Result}
meillo@187 784
meillo@187 785 Directions to go
meillo@161 786
meillo@185 787 Now how could \masqmail\ be like in, say, five years?
meillo@185 788
meillo@161 789 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.
meillo@161 790
meillo@93 791
meillo@146 792
meillo@146 793
meillo@185 794 1) fix the current version
meillo@163 795
meillo@185 796
meillo@185 797 2) create a new one
meillo@163 798 But how is the effort of this complete rewrite compared to what is gained afterwards?
meillo@163 799 << would one create it at all? >>
meillo@163 800
meillo@163 801
meillo@185 802 pro---contra
meillo@146 803
meillo@146 804
meillo@185 805
meillo@93 806
meillo@146 807 << short term goals --- long term goals >>
meillo@146 808
meillo@163 809 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.
meillo@163 810
meillo@140 811 << which parts to take out and do within the thesis >>
meillo@93 812
meillo@167 813