docs/diploma

annotate thesis/tex/4-MasqmailsFuture.tex @ 259:5db159540ba8

moved some text
author meillo@marmaro.de
date Wed, 14 Jan 2009 12:28:06 +0100
parents 68ef2040912a
children b390fb627f10
rev   line source
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@256 8 ---
meillo@256 9
meillo@256 10 \textbf{Full featured or stripped down}
meillo@256 11
meillo@256 12 Here shows a misfit off: On the one hand does \masqmail\ want to be a \sendmail\ replacement. But on the other hand, is it not designed to be used like \sendmail. If \masqmail\ is inteded to replace other \MTA{}s, then one may replace another one with it. Hence it must be secure enough. It either needs the security features or must drop the unsecure funtionality. The second option, however, leads to being \emph{no} replacement for other \MTA{}s. It is a valid decision to not be a replacement for \sendmail\ or thelike, but this is a design decision---the change of a primary goal.
meillo@256 13
meillo@256 14 If \masqmail\ should be an \MTA\ to replace others, a switch to a better suited architecture that provides good security and extendability by design, seems required. But if \masqmail\ is wanted to cover some special jobs, not to replace common \MTA{}s, then its architecture depends on the special requirements of the specific job; \MTA\ architectures, like discussed by \person{Hafiz}, may be inadequate.
meillo@256 15
meillo@256 16 What future is to choose for \masqmail---one to be a full featured \MTA, or one to be a stipped down \MTA\ for special jobs?
meillo@256 17
meillo@256 18 The critical point to discuss upon is surely the listening on a port to accepte messages from outside via \NAME{SMTP} (herafter also refered to as the \NAME{SMTP}-in channel). This feature is required for an \MTA\ to be a \name{smart host}, to relay mail. But running as deamon and listening on a port requires much more security effort, because the program is put in direct contact with attackers and other bad guys.
meillo@256 19
meillo@256 20 \MTA{}s without \SMTP-in channels can not receive mail from arbitrary outside hosts. They are only invoked by local users. This lowers the security need a lot---however, security is a general goal and still required, but on a lower level. Unfortunately, as they do not receive mail anymore (except by local submission), they are just better \name{forwarders} that are able to send mail directly to the destination.
meillo@256 21
meillo@256 22 This is not what \masqmail\ was intended to be. Programs that cover this purpose are available; one is \name{msmtp}.
meillo@256 23
meillo@256 24 \masqmail\ shall be a complete \mta. It shall be able to replace ones like \sendmail.
meillo@256 25
meillo@256 26 ---
meillo@256 27
meillo@132 28
meillo@132 29
meillo@132 30
meillo@177 31 \section{Requirements}
meillo@219 32 \label{sec:mta-requirements}
meillo@146 33
meillo@185 34 This section identifies the requirements for a modern \masqmail. Most of them will apply to modern \MTA{}s in general.
meillo@185 35
meillo@218 36
meillo@185 37
meillo@219 38 \subsection{Functional requirements}
meillo@146 39
meillo@232 40 Functional requirements are about the function of the software. They define what the program can do and in what way.
meillo@232 41 %fixme: add ref
meillo@239 42 The requirements are named ``\NAME{RF}'' for ``requirement, functional''.
meillo@219 43
meillo@219 44
meillo@239 45 \paragraph{\RF1: Incoming and outgoing channels}
meillo@232 46 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \sendmail\ command, and mail received on a \NAME{TCP} port. Thus it is common to split the incoming channels into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view \cite{hafiz05}.
meillo@219 47
meillo@232 48 \SMTP\ is the primary mail transport protocol today, but with the increasing need for new protocols (see section \ref{sec:what-will-be-important}) in mind, support for more than just \SMTP\ is good to have. New protocols will show up, maybe multiple protocols need to be supported then. This leads to multiple remote channels, one for each supported protocol as it was done in other \MTA{}s. Best would be interfaces to add further protocols as modules.
meillo@219 49
meillo@219 50
meillo@232 51 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \path{uucp}), or delivered locally by appending to a mailbox. 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.
meillo@219 52
meillo@232 53 %todo: is the def of MTA: transfer between machines, or transfer between users?
meillo@232 54 Local mail delivery is a job that uses root privilege to be able to switch to any user in order to write to his mailbox. It is possible to deliver without being root privilege, but delivery to user's home folders is not generally possible then. Thus even the modular \MTA{}s \qmail\ and \postfix\ use root privilege for it. As mail delivery to local users is \emph{not} included in the basic job of an \MTA{} and introduces a lot of new complexity, why should the \MTA\ bother? In order to keep the system simple, reduce privilege, and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \NAME{MDA}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and the like. Hence passing the message, and the responsibility for it, over to an \NAME{MDA} seems to be best.
meillo@219 55
meillo@232 56 This means an outgoing connection that pipes mail into local commands is required. To other outgoing channels applies what was already said about incoming channels.
meillo@219 57
meillo@232 58 \begin{figure}
meillo@232 59 \begin{center}
meillo@232 60 \includegraphics[scale=0.75]{img/mta-channels.eps}
meillo@232 61 \end{center}
meillo@232 62 \caption{Incoming and outgoing channels required}
meillo@232 63 \label{fig:mta-channels}
meillo@232 64 \end{figure}
meillo@219 65
meillo@232 66 An overview on in and outgoing channels required for an \MTA, gives figure \ref{fig:mta-channels}.
meillo@219 67
meillo@239 68 %fixme: write about submission (port 587)
meillo@219 69
meillo@219 70
meillo@219 71
meillo@219 72
meillo@239 73 \paragraph{\RF2: Mail queuing}
meillo@232 74 Mail queuing removes the need to deliver instantly as a message is received. The queue provides fail-safe storage of mails until they are delivered. Mail queues are probably used in all \mta{}s, excluding the simple forwarders. The mail queue is essential for \masqmail, as \masqmail\ is used for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message. This may be after a reboot. Hence the mail queue must provide persistence.
meillo@219 75
meillo@219 76 The mail queue and the module(s) 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 an already sent mail to be sent again.
meillo@219 77
meillo@219 78
meillo@219 79
meillo@219 80
meillo@239 81 \paragraph{\RF3: Header sanitizing}
meillo@219 82 Mail coming into the system often lacks important header lines. At least the required ones must be added by 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. Changing the locally known domain part of email addresses to globally known ones is an example. \masqmail\ needs to be able to rewrite the domain part dependent on the route used to send the message, to prevent messages to get classified as spam.
meillo@219 83
meillo@225 84 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 contain. Multiple recipients lead to multiple different envelopes, containing all the same mail message.
meillo@219 85
meillo@219 86
meillo@219 87
meillo@219 88
meillo@239 89 \paragraph{\RF4: Aliasing}
meillo@225 90 Email addresses can have aliases, thus they need to be expanded. Aliases can be of different kind: another local user, a remote user, a list containing local and 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 recipient's domain part may require a different route to be used.
meillo@219 91
meillo@219 92
meillo@219 93
meillo@219 94
meillo@239 95 \paragraph{\RF5: Selecting a route}
meillo@232 96 One key feature of \masqmail\ is its ability to send mail out over different routes. The online state defines the active route to be used. A specific route may not be suited for all messages, thus these messages are hold back until a suiting route is active. For more information on this concept see section \ref{sec:masqmail-routes}.
meillo@219 97
meillo@219 98
meillo@219 99
meillo@219 100
meillo@239 101 \paragraph{\RF6: Authentication}
meillo@232 102 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a 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. It may be also wanted to refuse all connections to the \MTA\ except ones from a specific set of hosts.
meillo@219 103
meillo@232 104 Several ways to restrict access are available. The most simple one is restriction by the \NAME{IP} address. No extra complexity is added this way, but the \NAME{IP} addresses have to be static or within known ranges. This approach is often used to allow relaying for local nets. The access check can be done by the \MTA\ or by a guard (e.g.\ \NAME{TCP} \name{Wrappers}) before. The main advantage here is the minimal setup and maintainence work needed. This kind of access restriction is important to be implemented.
meillo@219 105
meillo@232 106 This authentication based on \NAME{IP} addresses is impossible in situations where hosts with changing \NAME{IP} addresses, that are not part of a known subnet, need access. Then a authentication mechanism based on some \emph{secret} is required. Three common approaches exist:
meillo@219 107 \begin{itemize}
meillo@232 108 \item \SMTP-after-\NAME{POP}: Uses authentication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards. The variant \SMTP-after-\NAME{IMAP} exists too.
meillo@232 109 \item \SMTP\ authentication: An extension to \SMTP. It allows to request authentication before mail is accepted. Here no helper protocols are needed.
meillo@232 110 \item Certificates: The identity of a user or a host is confirmed by certificates that are signed by trusted authorities. Certificates are closely related to encryption, they do normally satisfy both needs: \NAME{SSL} tunnels encrypt the data transmission and allow to identify the remote user/host by his certificate.
meillo@219 111 \end{itemize}
meillo@232 112 At least one of the secret-based mechanisms should be supported.
meillo@219 113
meillo@219 114
meillo@219 115
meillo@239 116 \paragraph{\RF7: Encryption}
meillo@232 117 Electronic mail is vulnerable to sniffing attacks, because in generic \SMTP\ all data transfer is unencrypted. Unencrypted is the message's content, the email addresses in header and envelope, but also authentication dialogs that transfer plain text passwords (e.g.\ \NAME{PLAIN} and \NAME{LOGIN}). Hence encryption is important.
meillo@219 118
meillo@219 119 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@219 120
meillo@239 121 Using secure tunnels, that are provided by external applications, is prefered because the \MTA\ needs not to bother with encryption then. Outgoing \SMTP\ connections can get encrypted using a secure tunnel, created by an external application (like \name{openssl}). But incoming connections can not use external secure tunnels, because the remote \NAME{IP} address is hidden then; all connections appear to come from localhost instead. Figure \ref{fig:stunnel} depicts the situation of using an application like \name{stunnel} for incoming connections. The connection to port 25 comes from localhost, and that is the information the \MTA\ gets. Authentication based on \NAME{IP} addresses and many spam prevention methods are useless then.
meillo@219 122
meillo@232 123 \begin{figure}
meillo@232 124 \begin{center}
meillo@232 125 \includegraphics[scale=0.75]{img/stunnel.eps}
meillo@232 126 \end{center}
meillo@232 127 \caption{Using \name{stunnel} for incoming connections}
meillo@232 128 \label{fig:stunnel}
meillo@232 129 \end{figure}
meillo@219 130
meillo@239 131 To provide encrypted incoming channels, the \MTA\ could implement encryption and listen on a port that is dedicated to encrypted \SMTP\ (\NAME{SMTPS}). This approach would be possible, but it is deprecated in favor for \NAME{STARTTLS}. \RFC3207 ``\SMTP\ Service Extension for Secure \SMTP\ over Transport Layer Security'' shows this in not mentioning \NAME{SMTPS} on port 465. Also port 465 is not even reserved for \NAME{SMTPS} anymore \citeweb{iana:port-numbers}.
meillo@219 132
meillo@239 133 \NAME{STARTTLS}---defined in \RFC2487---is what \RFC3207 recommends to use for secure \SMTP. The connection then goes over port 25 (or the submission port 587), but gets encrypted as the \NAME{STARTTLS} keyword is issued.
meillo@232 134
meillo@239 135 \NAME{STARTTLS} encryption should be supported.
meillo@232 136
meillo@232 137
meillo@232 138
meillo@239 139 \paragraph{\RF8: Spam prevention}
meillo@219 140 Spam is a major threat nowadays, but it is a war that is hard to win. The goal is to provide state-of-the-art spam protection, but not more (see section \ref{sec:swot-analysis}).
meillo@219 141
meillo@239 142 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 themselves.
meillo@219 143
meillo@239 144 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 ways 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 \cite{cole07} \cite{levine08}), requesting special client behavior (\name{greylisting} \cite{harris03}, \name{hashcash} \cite{back02}), or statistical analysis (\name{bayesian filters} \cite{graham02}) are checks that may be used. Running more checks leads to better results, but takes more system resources and more time.
meillo@219 145
meillo@239 146 Doing some basic checks during the \SMTP\ dialog seems to be a must \cite[page~25]{eisentraut05}. They should best be included into the \MTA, because they need to be fast to avoid \SMTP\ dialog timeouts. Internal interfaces to specialized modules seem to be best.
meillo@219 147
meillo@239 148 More detailed checks after the message is queued should be done using external scanners. Interfaces to invoke them need to be defined. (See also the remarks about \name{amavis} in the next section.)
meillo@219 149
meillo@219 150
meillo@219 151
meillo@239 152
meillo@239 153
meillo@239 154 \paragraph{\RF9: Virus checking}
meillo@225 155 Related to spam is malicious 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's 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 \MTA\ responsible for the recipient seems to be at a good position to do this work, so it is often done there.
meillo@219 156
meillo@219 157 In any way should malware checking be performed by external programs that may be invoked by the \mta. But using mail deliver agents, like \name{procmail}, are better suited locations to invoke content scanners.
meillo@219 158
meillo@219 159 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 nice to have, though.
meillo@219 160
meillo@219 161
meillo@219 162
meillo@239 163 \paragraph{\RF10: Archiving}
meillo@219 164 Mail archiving and auditability become more important as email establishes as technology for serious business communication. 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@219 165
meillo@225 166 \postfix\ for example has a \texttt{always\_bcc} feature, to send a copy of every outgoing mail to a definable recipient. At least this functionality should be given, although a more complete approach is preferable.
meillo@219 167
meillo@219 168
meillo@219 169
meillo@219 170
meillo@219 171
meillo@219 172 \subsection{Non-functional requirements}
meillo@219 173
meillo@225 174 Here follows a list of non-functional requirements for \masqmail. These requirements specify the quality properties of software. The list is based on \person{Hafiz} \cite[page~2]{hafiz05}, with inspiration from \person{Spinellis} \cite[page~6]{spinellis06} and \person{Kan} \cite{kan03}.
meillo@185 175 %fixme: refer to ch01 and ch02
meillo@239 176 These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''.
meillo@146 177
meillo@146 178
meillo@239 179 \paragraph{\RG1: Security}
meillo@239 180 \MTA{}s are critical points for computer security, as they are accessible from external networks. They must be secured with high effort. Properties like the need for high privilege level, from outside influenced work load, work on unsafe data, and demand for reliability, increase the need for security. This is best done by modularization, also called \name{compartementalization}, as described in section \ref{sec:discussion-mta-arch}. \masqmail\ needs to be secure enough for its target field of operation. \masqmail\ is targeted to workstations and private networks, with explicit warning to not use it on permanent online hosts \citeweb{masqmail:homepage2}. But as non-permanent online connections and trustable environments become rare, \masqmail's security should be so good, that it is usable with permanent online connections and in unsafe environments. For example should mails with bad content not break \masqmail.
meillo@177 181
meillo@177 182
meillo@259 183
meillo@239 184 \paragraph{\RG2: Reliability}
meillo@239 185 Reliability is the second essential quality property for an \MTA. Mail for which the \MTA\ took responsibility must never get lost while it is within the \MTA{}s responsibility. The \MTA\ must not be \emph{the cause} of any mail loss, no matter what happens. Unreliable \mta{}s are of no value. However, as the mail transport infrastructure are distributed systems, one of the communication partners or the transport medium may crash at any time during mail tranfer. Thus reliability is needed for mail transfer communication too.
meillo@177 186
meillo@239 187 The goal is to transfer exactly one copy of the message. \person{Tanenbaum} evaluates the situation and comes to the conclusion that ``in general, there is no way to arrange this.'' \cite[pages~377--379]{tanenbaum02}. Only strategies where now mail gets lost are acceptable; he identifies three of them, but one generates more duplicates than the others, so two strategies remain. (1) The client always reissues the transfer; the server first sends an acknowledgement, then handles the transfer. (2) The client reissues the transfer only if no acknowledgement was received; the server first handles the transfer and sends the acknowledgement afterwards. The first strategy does not need acknowledgements at all, however, it will lose mail if the second transfer fails too.
meillo@189 188
meillo@239 189 Hence, mail transfer between two processes must use the strategy: The client reissues if it receives no acknowledgement; the server first handles the message and then sends the acknowledgement. This strategy only leads to duplicates if a crash happens in the time between the message is fully transfered to the server and the acknowlegement is received by the client. No mail will get lost.
meillo@239 190
meillo@239 191
meillo@239 192 \paragraph{\RG3: Robustness}
meillo@219 193 Being robust means handling errors properly. Small errors may get corrected, large errors may kill a process. Killed processes should restarted automatically and lead to a clean state again. Log messages should be written in every case. Robust software does not need a special environment, it creates a friendly environment itself. \person{Raymond}'s \name{Rule of Robustness} and his \name{Rule of Repair} are good descriptions \cite[pages~18--21]{raymond03}.
meillo@177 194
meillo@177 195
meillo@239 196 \paragraph{\RG4: Extendability}
meillo@219 197 \masqmail's architecture needs to be extendable, to allow new features to be added afterwards. The reason for this need are changing requirements. New requirements appear, like more efficient mail transfer of large messages or a final solution for spam problem. Extendability is the ability of software to include new function with little work.
meillo@196 198
meillo@146 199
meillo@239 200 \paragraph{\RG5: Maintainability}
meillo@219 201 Maintaining software takes much time and effort. \person{Spinellis} guesses ``40\,\% to 70\,\% of the effort that goes into a software system is expended after the system is written first time.'' \cite[page~1]{spinellis03}. This work is called \emph{maintaining}. Hence making software good to maintain will ease work afterwards.
meillo@146 202
meillo@189 203
meillo@239 204 \paragraph{\RG6: Testability}
meillo@225 205 Good testability make maintenance easier too, because functionality is directly verifiable when changes are done, thus removing uncertainty. Modularized software makes testing easier, because parts can be tested without external influences. \person{Spinellis} sees testability as a sub-quality of maintainability.
meillo@189 206
meillo@189 207
meillo@239 208 \paragraph{\RG7: Performance}
meillo@196 209 Also called ``efficiency''. Efficient software requires few time and few resources. The merge of communication hardware and its move from service providers to homes and to mobile devices, demand smaller and more resource-friendly software. The amount of mail will be lower, even if much more mail will be sent. More important will be the energy consumption and heat emission. These topics increased in relevance during the past years and they are expected to become more central.
meillo@146 210
meillo@146 211
meillo@239 212 \paragraph{\RG8: Availability}
meillo@225 213 Availability is important for server programs. They must stay operational by blocking \name{denial of service} attacks and the like.
meillo@146 214
meillo@146 215
meillo@239 216 \paragraph{\RG9: Portability}
meillo@225 217 Source code that compiles and runs on various operation systems is called portable. Portability can be achieved by using standard features of the programming language and common libraries. Basic rules to achieve portable code are defined by \person{Kernighan} and \person{Pike} \cite{kernighan99}. Portable code lets software spread faster.
meillo@189 218
meillo@189 219
meillo@239 220 \paragraph{\RG10: Usability}
meillo@219 221 Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property very important from the user's point of view. Software with bad usability is rarely used, no matter how good it is. If substitutes with better usability exist, the user will switch to one of them. Here, usability includes setting up and configuring; and the term ``users'' includes administrators. Having \mta{}s on home servers and workstations requires easy and standardized configuration. The common setups should be configurable with little action by the user. Complex configuration should be possible, but focused must be the most common form of configuration: choosing one of several common setups.
meillo@185 222
meillo@185 223
meillo@185 224
meillo@185 225
meillo@242 226 \subsection{Thoughts about architecture}
meillo@239 227 \label{sec:discussion-mta-arch}
meillo@187 228
meillo@242 229 %todo: what's this section to do with requirements?
meillo@242 230
meillo@225 231 \masqmail's current architecture is monolithic like \sendmail's and \exim's. But more than the other two, is it one block of interweaved code. \exim\ has a highly structured code with many internal interfaces, a good example is the one for authentication ``modules''. %fixme: add ref
meillo@219 232 \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules.
meillo@188 233 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
meillo@161 234
meillo@239 235 Figure \ref{fig:masqmail-arch} is a call graph generated from \masqmail's source code, excluding logging functions. It gives a impression of how interweaved the internals are. There are no compartments existent.
meillo@219 236 %fixme: what is included, what not?
meillo@161 237
meillo@161 238 \begin{figure}
meillo@161 239 \begin{center}
meillo@219 240 \vspace*{2ex}
meillo@256 241 %\includegraphics[scale=0.75]{img/callgraph.eps}
meillo@256 242 \includegraphics[scale=0.75]{img/masqmail-3-omitlog5.eps}
meillo@161 243 \end{center}
meillo@200 244 \caption{Call graph of \masqmail\ to show its internal structure}
meillo@161 245 \label{fig:masqmail-arch}
meillo@161 246 \end{figure}
meillo@161 247
meillo@225 248 \sendmail\ improved its old architecture by adding the milter interface, to include further functionality by invoking external programs. \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} describes this evolution of \mta\ architecture very well \cite{hafiz05}.
meillo@161 249
meillo@239 250 Every one of these programs 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 for modular designs in order to keep the software simple. Simplicity is a key property for security. ``the essence of security engineering is to build systems that are as simple as possible.'' \cite[page 45]{graff03}.
meillo@161 251
meillo@219 252 \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@163 253 \begin{quote}
meillo@218 254 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.
meillo@218 255 \hfill\cite[page 64]{hafiz05}
meillo@163 256 \end{quote}
meillo@219 257 Equal does \person{Dent} see the situation for \postfix: ``The modular architecture of Postfix forms the basis for much of its security.'' \cite[page 7]{dent04}
meillo@200 258
meillo@200 259 Modularity is also needed to satisfy modern \MTA\ requirements, in providing a clear interface to add functionality without increasing the overall complexity much.
meillo@161 260
meillo@242 261 Modularity is no direct requirement, but a goal that has positive influence on important requirements like security, testability, extendability, maintainability, and not least simplicity. These quality properties then, on their part, make achieving the functional requirements easier.
meillo@239 262
meillo@242 263 Hence, aspiration for modularity, by compartmentalization, improves the overall quality and function of the software. It can be seen as an architectural requirement for a secure and modern \MTA.
meillo@239 264
meillo@239 265
meillo@239 266
meillo@239 267
meillo@239 268 \section{Fulfilled requirements}
meillo@239 269 \label{sec:fulfilled-requirements}
meillo@239 270
meillo@239 271 Here follows a description of how far the requirements are already fulfilled by \masqmail.
meillo@239 272
meillo@239 273
meillo@239 274 \paragraph{\RF1: In/out channels}
meillo@239 275 \masqmail's incoming and outgoing channels are the ones required for an \MTA{}s at the moment. They are depicted in figure \ref{fig:masqmail-in-out} on page \pageref{fig:masqmail-in-out}. This is all what is currently needed. But new protocols and mailing concepts are likely to appear (see section \ref{sec:electronic-mail}). \masqmail\ has no support for adding further protocols. Thus modifications at many places in the source are needed to add them though. Today, support for further protocols is not needed, so \masqmail\ is regarded to fulfill \RF1, but the probable future need should be kept in mind.
meillo@239 276
meillo@239 277 \paragraph{\RF2: Queueing}
meillo@239 278 One single mail queue is used in \masqmail; it satisfies all current requirements.
meillo@239 279
meillo@239 280 \paragraph{\RF3: Header sanitizing}
meillo@239 281 The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled.
meillo@239 282
meillo@239 283 \paragraph{\RF4: Aliasing}
meillo@239 284 Aliasing is done on delivery. All common kinds of aliases in the global aliases file are supported. \name{.forward} aliasing is not, but this is less common and seldom used.
meillo@239 285
meillo@239 286 \paragraph{\RF5: Select route}
meillo@239 287 Setting of the route to use is done on delivery. Headers can get rewritten a second time then. This part does provide all the functionality required.
meillo@239 288
meillo@239 289 \paragraph{\RF6: Authentication}
meillo@239 290 Static authentication, based on \NAME{IP} addresses, can be achieved with \person{Venema}'s \NAME{TCP} \name{Wrapper} \cite{venema92}, by editing the \path{hosts.allow} and \path{hosts.deny} files. This is only relevant to authenticate host that try to submit mail into the system. Dynamic (secret-based) \SMTP\ authentication is already supported in form of \NAME{SMTP-AUTH} and \SMTP-after-\NAME{POP}, but only for outgoing connections. For incoming connections, only address-based authentication is supported.
meillo@239 291
meillo@239 292 \paragraph{\RF7: Encryption}
meillo@239 293 Similar is the situation for encryption which is also only available for outgoing channels; here a wrapper application like \name{openssl} is needed. This creates a secure tunnel to send mail trough, but state-of-the-art is using \NAME{STARTTLS}, which is not supported. For incoming channels, no encryption is available. The only possible setup to provide encryption of incoming channels is using an application like \name{stunnel} to translate between the secure connection to the remote host and the \MTA. Unfortunately, this suffers from the problem explained in section \ref{sec:FIXME} and figure \ref{fig:stunnel}. Anyway, this would still be no \NAME{STARTTLS} support.
meillo@239 294
meillo@239 295 \paragraph{\RF8: Spam handling}
meillo@239 296 \masqmail\ nowadays does not provide special support for spam filtering. Spam prevention by not accepting spam during the \SMTP\ dialog is not possible at all. Spam filtering is only possible by using two \masqmail\ instances with an external spam filter inbetween. The mail flow is from the receiving \MTA\ instance, which accepts mail, to the filter application that processes and possible modifies it, to the second \MTA\ which is responsible for further delivery of the mail. This is a concept that works in general. And it is a good concept in principle to separate work with clear interfaces. But the need of two instances of the same \MTA (each for only half of the job) with doubled setup, is more a work-around. Best is to have this data flow respected in the \MTA\ design, like in \postfix. But the more important part of spam handling, for sure, is done during the \SMTP\ dialog in completely refusing unwanted mail.
meillo@239 297
meillo@239 298 \paragraph{\RF9: Malware handling}
meillo@239 299 For malware handling applies nearly the same, except all checks are done after mail is accepted. So the possible setup is the same with the two \MTA\ instances and the filter inbetween. \masqmail\ does support such a setup, but not in a nice way.
meillo@239 300
meillo@239 301 \paragraph{\RF10: Archiving}
meillo@239 302 There is currently no way of archiving every message going through \masqmail.
meillo@239 303
meillo@239 304
meillo@239 305 %Non-functional requirements are not so easy to be marked as fulfilled or not. Instead they are discussed here.
meillo@239 306
meillo@239 307 \paragraph{\RG1: Security}
meillo@259 308 \masqmail's current security is bad. However, it seems acceptable for using \masqmail\ on workstations and private networks, if the environment is trustable and \masqmail\ is protected against remote attackers. In environments where untrusted components or persons have access to \masqmail, its security is too low.
meillo@259 309 Its author states it ``is not designed to'' such usage \citeweb{masqmail:homepage2}. This is a clear indicator for being careful. Issues like high memory consumption, low performance, and denial-of-service attacks---things not regarded by design---may cause serious problems. In any way, is a security report missing that confirms \masqmail's security level.
meillo@259 310
meillo@239 311
meillo@239 312 \paragraph{\RG2: Reliability}
meillo@239 313 Similar is its reliability not good enough. Situations where only one part of sent message was removed from the queue, and the other part remained as garbage, showed off \citeweb{debian:bug245882}. Problems with large mail and small bandwidth were also reported \citeweb{debian:bug216226}. Fortunately, lost email was no big problem yet, but \person{Kurth} warns:
meillo@163 314 \begin{quote}
meillo@239 315 There may still be serious bugs in [masqmail], so mail might get lost. But in the nearly two years of its existence so far there was only one time a bug which caused mail retrieved via pop3 to be lost in rare circumstances.
meillo@239 316 \hfill\citeweb{masqmail:homepage2}
meillo@163 317 \end{quote}
meillo@239 318 In summary: Current reliability needs to be improved.
meillo@239 319 %fixme: state machine
meillo@161 320
meillo@239 321 \paragraph{\RG3: Robustness}
meillo@239 322 The logging behavior of \masqmail\ is good, although it does not cover all problem situations. For example, if the queue directory is world writeable by accident (or as action of an intruder), any user can remove messages from the queue or replace them with own ones. \masqmail\ does not even write a debug message in this case. The origin of this problem, however, is \masqmail's trust in its environment.
meillo@239 323
meillo@239 324 \paragraph{\RG4: Extendability}
meillo@239 325 \masqmail's extendability is very poor. This is a general problem of monolithic software, but can thus be provided with high effort. \exim\ is an example for good extendability in a monolithic program.
meillo@239 326
meillo@239 327 \paragraph{\RG5: Maintainability}
meillo@239 328 The maintainability of \masqmail\ is equivalent to other software of similar kind. Missing modularity and therefore more complexity makes the maintainer's work harder. In summary is \masqmail's maintainability bearable, like in average Free Software projects.
meillo@239 329
meillo@239 330 \paragraph{\RG6: Testability}
meillo@239 331 The testability suffers from missing modularity. Testing program parts is hard to do. Nevertheless, it is done by compiling parts of the source to special test programs.
meillo@239 332
meillo@239 333 \paragraph{\RG7: Performance}
meillo@239 334 The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal.
meillo@239 335
meillo@239 336 \paragraph{\RG8: Availability}
meillo@239 337 This applies equal to availability. Hence no further work needs to be done her.
meillo@239 338
meillo@239 339 \paragraph{\RG9: Portability}
meillo@239 340 The code's portability is good with view on \unix-like operation systems. At least \name{Debian}, \name{Red Hat}, \NAME{SUSE}, \name{Slackware}, \name{Free}\NAME{BSD}, \name{Open}\NAME{BSD}, and \name{Net}\NAME{BSD} are reported to be able to compile and run \masqmail\ \citeweb{masqmail:homepage2}. Special requirements for the underlying file system are not known. Therefore, the portability is already good.
meillo@239 341
meillo@239 342 \paragraph{\RG10: Usability}
meillo@239 343 The usability, from the administrator's point of view, is very good. \masqmail\ was developed to suite a specific, limited job---its configuration does perfect match. The user's view does not reach to the \MTA, as it is hidden behind the \name{mail user agent}.
meillo@239 344
meillo@239 345
meillo@239 346
meillo@242 347 \paragraph{Modularity}
meillo@242 348 Modularity---the important architectural goal---is currently not existent in \masqmail's code. The whole source is interweaved.
meillo@242 349
meillo@242 350
meillo@242 351
meillo@239 352
meillo@239 353
meillo@239 354
meillo@239 355 \section{Work to do}
meillo@239 356
meillo@239 357 After the requirements for modern \mta{}s were identified in section \ref{sec:mta-requirements} and \masqmail's features were set against them in section \ref{sec:fulfilled-requirements}, here the the work that is left to do is identified. Table \ref{tab:requirements} lists all requirements with importance and the work needed to achieve them. The attention a work task should receive---the focus---depends on its importance and the amount of work it includes.
meillo@239 358
meillo@239 359 \begin{table}
meillo@239 360 \begin{center}
meillo@239 361 \input{input/requirements.tex}
meillo@239 362 \end{center}
meillo@239 363 \caption{Importance of and pending work for requirements}
meillo@242 364 \label{tab:requirements}
meillo@239 365 \end{table}
meillo@239 366
meillo@239 367 The importance is ranked from `-{}-' (not important) to `++' (very important). The pending work is ranked from `-{}-' (nothing) to `++' (very much). Large work tasks with high importance need to receive much attention, they are in focus. In contrast should small low importance work receive few attention. Here the attention/focus a task should get is calculated by summing up the importance and the pending work with equal weight. Normally, tasks with high focus are the ones of high priority and should be done first.
meillo@239 368
meillo@239 369 The functional requirements that receive highest attention are \RF6: authentication, \RF7: encryption, and \RF8: spam handling. Of the non-functional requirements, \RG1: security, \RG2: reliability, and \RG4: Extendability, rank highest.
meillo@239 370
meillo@241 371 These tasks are presented in more detail now. They are sorted in the suggested order to work on them..
meillo@239 372
meillo@239 373
meillo@239 374 \subsubsection*{\TODO1: Encryption (\RF7)}
meillo@241 375 Encryption is chosen first, as it is essential to providing privacy. Encryption by using \NAME{STARTTLS} is definitely needed and should be added soon. Without support for it, encrypted email transfer is hardly possible.
meillo@239 376
meillo@241 377 \subsubsection*{\TODO2: Authentication (\RF6)}
meillo@241 378 Authentication of incoming \SMTP\ connections also needed and should be added soon. It is important for restricting access to prevent relaying. For workstations and local networks, it has only medium importance and address-based authentication is sufficient in most times. But secret-based authentication is mandatory to receive mail from the internet.
meillo@239 379
meillo@239 380 \subsubsection*{\TODO3: Security (\RG1)}
meillo@241 381 \masqmail's security is bad, thus the program is forced into a limited field of operation. The field of operation even shrinks, as security becomes more important and networking and interaction increases. Save and trusted environment become rare.
meillo@239 382
meillo@241 383 Compartementalization, ref secure coding, postfix ...
meillo@241 384
meillo@241 385 Improving security is an important thing to do. Especially, \masqmail's security should be tested throughout to get a definitive view how good it really is and where the weak spots are.
meillo@239 386
meillo@239 387 \subsubsection*{\TODO4: Reliability (\RG2)}
meillo@241 388 Reliability is also to improve. It is a key quality property for an \MTA, and not good enough in \masqmail. Reliability is strong related to the queue, thus improvements there are favorable. Applying ideas of \name{crash-only software} \cite{candea03} will be a good step. \person{Candea} and \person{Fox} see in killing the process the best way to stop a running program. Doing so inevitably demands for good reliability of the queue, and the startup inevitably demands for good recovery. The critical situations for reliability are nothing special anymore, they are common. Hence they are regulary tested and will definately work.
meillo@242 389 % persistence, database
meillo@239 390
meillo@241 391 \subsubsection*{\TODO5: Spam handling (\RF8)}
meillo@241 392 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.
meillo@239 393
meillo@241 394
meillo@241 395 \subsubsection*{\TODO6: Extendability (\RG4)}
meillo@241 396 Extendability does suffer from the monolithic architecture and is nearly impossible to improve without changing the programs structure. This property can hardly be retrofitted into software. Extendability is expected become important in the future as new protocols need to be supported.
meillo@241 397
meillo@254 398 \masqmail\ lacks 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 at compile time. Adding maildir support, 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 a code that is hard to read.
meillo@254 399 %fixme: refer to ifdef-considered-harmful ?
meillo@254 400
meillo@239 401
meillo@239 402
meillo@239 403
meillo@239 404 \subsubsection*{Further \NAME{TODO}s}
meillo@239 405
meillo@239 406 Support for other protocols than \SMTP\ seems not to be necessary at the moment. Adding such support will need lots of work in many parts of \masqmail. Hence delaying this work until the support becomes mandatory, appears to be the best strategy. This way work can be saved if some protocols never become popular.
meillo@239 407
meillo@239 408 Archiving again is preferred to be implemented soon. It does not require much work, but enables all kinds of statistical analysis.
meillo@239 409
meillo@239 410 Performance is a property that is nice to have. But as performance improvements are in contrast to many other quality properties (reliability, maintainability, usability, capability \cite[page~5]{kan03}), jeopardizing these to gain some more performance should not be done. \person{Kernighan} and \person{Pike} state clear: ``[T]he first principle of optimization is \emph{don't}.''\cite[page~165]{kernighan99}. \masqmail\ is not a program to be used on large servers, but on small devices. Thus important for \masqmail\ could be energy and heat saving, maybe also system resources, but not performance. Anyway, simplicity and clearness are of higher value.
meillo@239 411
meillo@239 412 Portability among the various flavors of \unix\ systems is a goal, because these systems are the ones \MTA{}s run on usually. Portability problems with non-\unix\ platforms are primary expected to come from file systems lacking required features. But no special care should be taken here.
meillo@242 413 % unix fs on windows
meillo@239 414
meillo@239 415 Configuration could be eased more, by providing configuration generators to be able to use \masqmail\ right ``out of the box'' after running one of several configuration scripts for common setups. This would improve \masqmail's usability for not technical educated people.
meillo@242 416 % masqmail as portable app?
meillo@239 417
meillo@239 418
meillo@239 419
meillo@239 420
meillo@239 421
meillo@239 422
meillo@239 423 \section{Ways for further development}
meillo@239 424
meillo@241 425 Futher development of software can always go three different ways:
meillo@241 426 \begin{enumerate}
meillo@241 427 \item[S1:] Improve the current code base.
meillo@241 428 \item[S2:] Add wrappers or interposition filters.
meillo@241 429 \item[S3:] Redesign the software from scratch and rebuild it.
meillo@241 430 \end{enumerate}
meillo@239 431
meillo@241 432 The first two strategies base on the available source code, and can be applied in combination. The third strategy abandonnes the old code and starts over again. Wrappers and interposition filters would then be outright included into the new architecture.
meillo@239 433
meillo@241 434
meillo@241 435 The requirements are now regarded, each on its own. Each one is linked to the development strategy that is prefered to reach the specific requirement. Some requirements may be well achievable by using different strategies, so they are linked to all of them. The order of the requirements in the list depend on their level of focus. This linking of strategies to the requirements is shown in table \ref{tab:strategies}.
meillo@241 436
meillo@241 437 \begin{table}
meillo@241 438 \begin{center}
meillo@241 439 \input{input/strategies.tex}
meillo@241 440 \end{center}
meillo@241 441 \caption{Development strategies and their suitability for requirements}
meillo@241 442 \label{tab:strategies}
meillo@241 443 \end{table}
meillo@241 444
meillo@241 445 Implementing \TODO1 encryption and \TODO2 authentication, for example, are limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast does adding support for new protocols or mail processing interfaces to external programs (\TODO5) require a lot of effort. Changes in many parts of the source code are required. It is a bad idea to implement large retro-fitted features into software that is critical about security and reliability, like \MTA{}s. Worse if these features need changes in the program's structure, like adding mail scanning interfaces (\TODO5) would do.
meillo@239 446
meillo@239 447 If such large features are needed, it is best to redesign the program's structure and rebuild it. A program's structure is primary its architecture. Which is the most influencing design decision, and has the greatest impact on the program's future capabilities. The architecture defines what the program can do, and how it can be used. If the architecture does not fit to the requirements, development will reach a dead end \dots\ further work then will make everything worse. The only good solution is to change the architecture, which, sadly but most likely, means a redesign from scratch.
meillo@239 448
meillo@241 449 Quality properties, like security (\TODO3) and reliability (\TODO3), as well as extendability (\TODO6) and maintainability, can hardly be added afterwards---if at all. Only structural changes will improve them. Hence, if security, reliability, extendability (to add support for future mail transfer protocols), or maintainability shall be improved, a redesign of \masqmail\ is the only sane way to go.
meillo@241 450
meillo@241 451
meillo@241 452
meillo@241 453 Next, the best strategy for further development needs to be discovered. The focus points of the requirements, for which a strategy is prefered, are summed up to obtain a score for the strategies. Herefor only positive focus points are regarded, with each plus symbol counting one. (Respecting negative focus points also leads to a similar result.)
meillo@241 454
meillo@241 455 S1: Improve current code, gets a score of 9 points. S2: Wrappers and interposition filters, has a score of 7 points. S3: New design, scores on top with 17 points. As S1 and S2 may be used in combination, a combined score is important to calculate. The combination has in total 13 points, but it is still beaten by S3.
meillo@241 456
meillo@241 457 This leads to the conclusion, that S3 (A new design) is the best strategy for further development, from this point of view.
meillo@241 458
meillo@241 459
meillo@241 460 \subsubsection*{S3: A new design from scratch}
meillo@241 461
meillo@241 462 However, a redesign and rewrite of software from scratch is hard. It takes time to design a new architecture, which then must prove it is secure and reliable. As well is much time and work needed to implement the design, test it, fix bugs, and so on. If flaws in the design appear during prototype implementation, it is necessary to start again. Thus the gain of a new design must overweight the effort needed.
meillo@241 463
meillo@241 464 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
meillo@241 465 {\small
meillo@241 466 \begin{verbatim}
meillo@241 467 Total Physical Source Lines of Code (SLOC) = 9,041
meillo@241 468 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22)
meillo@241 469 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05))
meillo@241 470 Schedule Estimate, Years (Months) = 0.70 (8.39)
meillo@241 471 (Basic COCOMO model, Months = 2.5 * (person-months**0.38))
meillo@241 472 Estimated Average Number of Developers (Effort/Schedule) = 2.89
meillo@241 473 Total Estimated Cost to Develop = $ 272,690
meillo@241 474 (average salary = $56,286/year, overhead = 2.40).
meillo@241 475 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler
meillo@241 476 \end{verbatim}
meillo@241 477 }
meillo@248 478 The development cost is not relevant for a \freesw\ project with volunteer developers, but the development time is. About 24 man-months are estimated. The current code base was written almost completely by \person{Oliver Kurth} within four years, in his spare time. This means he needed around twice as much time. Of course, he programmed as a volunteer developer, not as employee with eight work-hours per day.
meillo@241 479
meillo@241 480 Given the assumptions that (1) an equal amount of code needs to be produced for a new \masqmail, (2) a third of existing code can be reused plus concepts and knowledge, and (3) development speed is like \person{Kurth}'s. Then it would take between two and three years to have a redesigned new \masqmail\ with the same features that \masqmail\ now has. Less time would be needed if a simpler architecture allows faster development, better testing, and less bugs.
meillo@241 481
meillo@241 482
meillo@241 483
meillo@254 484 ---
meillo@254 485
meillo@254 486 on venema: ``there'll be no second postfix'':
meillo@254 487
meillo@254 488 Yes there will be one, when postfix becomes obsolete, and this day will come as requirements change. See sendmail for example: it had nearly the whole market ... and now it's dying. sendmail once also thought it would be the number 1, forever.
meillo@254 489
meillo@254 490 cf. Tanenbaum vs. Linux: It was too early. Linux' time has not ended, but it will some day.
meillo@254 491
meillo@254 492 anyway, masqmail is not intended to become a second postfix. It's more inteded to become a second qmail, but with a differend target field.
meillo@254 493
meillo@241 494
meillo@241 495 ---
meillo@241 496
meillo@241 497
meillo@241 498 Remarkable is the distribution of the score points between functional and non-functional requirements. S1 (Improve current code) gets most points from functional requirements. Thus it is the best strategy to improve them. S3 (New design), in contrast, scores high for non-functional requirements. Thus it is best chosen to improve the software's quality. S2 (Wrappers and interposition filters) is balanced.
meillo@241 499
meillo@241 500
meillo@241 501
meillo@241 502 %\subsubsection*{The need for structural changes}
meillo@239 503
meillo@239 504 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@239 505
meillo@241 506 ---
meillo@239 507
meillo@239 508
meillo@241 509
meillo@241 510
meillo@241 511
meillo@241 512
meillo@241 513
meillo@241 514 \subsubsection*{A redesign from scratch}
meillo@241 515
meillo@241 516 Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
meillo@241 517 \begin{quote}
meillo@241 518 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 maintenance of the software.
meillo@241 519 %
meillo@241 520 %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@241 521 \hfill\cite[page 55]{graff03}
meillo@241 522 \end{quote}
meillo@241 523
meillo@241 524 All this leads to the wish of a rewrite of \masqmail, using a modern, modular architecture, \emph{if} further features need to be added---features that require changes in \masqmail's structure. But a rewrite is also mandatory, if \masqmail\ should become a modern \MTA, with good quality properties.
meillo@241 525
meillo@241 526
meillo@254 527 \subsubsection*{Further reasons for a new design}
meillo@241 528
meillo@254 529 impressing simplicity of qmail: only about 1000 SLOC per file (= about one module). It's obvious what it does. cf. suckless.org
meillo@241 530
meillo@241 531
meillo@254 532 do not try to safe obsolete stuff. This will not work (see sendmail).
meillo@241 533
meillo@254 534 It is often done in commercial software, when it's about making money. Free software with volunteer programmers in contrast care about good software..
meillo@254 535
meillo@254 536 If the design is bad, one should never hesitate to abandonne obsolete stuff and build it from scratch. (cf. makefiles and tab).
meillo@254 537
meillo@254 538 But making a cut is hard, as it is still ``good enough''.
meillo@254 539
meillo@254 540
meillo@254 541 ---
meillo@254 542
meillo@254 543 repair strategies is only useful in the short time view and for hard times. but if the future is bright, one must invest. here it means redesigning to build up a more modern product. cf. ch02: the future is bright!
meillo@254 544
meillo@254 545 Masqmail should have been redesigned in 2002 or so, when the old design was still quite suitable ... it already delayed too long.
meillo@254 546
meillo@254 547 Clinging to much to existing code will be no help, it is an indicator for fear. Having the courage to through bad code away to make it better, shows the view forward.
meillo@254 548
meillo@254 549 ---
meillo@254 550
meillo@254 551 repairing leaves a worse feeling. Free Software ``sells'' if it has a good userbase. Although qmail is somehow outdated and its author has released no new version since about 10 years, qmail has a very strong userbase and community.
meillo@254 552
meillo@254 553 Good design, concepts and philosophy gives users good feelings and faith for the software. They become interested in using it and to contribute.
meillo@254 554
meillo@254 555
meillo@254 556
meillo@254 557 The goal is good software. The wish to do good work is the motivation volunteers have. Work plans that lead to a good product will motivate volunteers to help with it. Hence more helpers may make the 2,5 man years for the new design, even become less absolute time than, few helping people that try to improve the existing code.
meillo@241 558
meillo@241 559
meillo@241 560
meillo@241 561 \section{Result}
meillo@241 562
meillo@239 563 The most needed features---authentication and encryption---can be added to the current code base with changes in only few parts of the source. These changes should be made soon. Archiving of mail is another feature to add then. More complete logging coverage, reporting of unsafe environment, and fixing high risk security flaws are quality improvements to do. All this work should be done on basis of the current code.
meillo@239 564
meillo@239 565 All other work depends on how the plans for \masqmail's future look like.
meillo@239 566
meillo@239 567 What shall \masqmail\ be like, in, for instance, five years?
meillo@239 568
meillo@239 569 Two ways of further development come to mind.
meillo@239 570
meillo@239 571 First, stick to the old architecture and try to add features as possible. This approach needs less effort to be spent, because a working code is already present. Further development is only adding small increments to a exiting code base. But the further development goes, the larger is the work needed to add more functionality, and the more bugs will appear, caused by the increasing complexity. Quality of the software will decrease, because lacking of clear internal structure encourages further work to be quick fixes rather than good solutions.
meillo@239 572
meillo@239 573 Second, the way of designing \masqmail\ from scratch and rebuilding it. A lot of time and work is required to do this. Additionally, a new design from scratch introduces new risks: Is the design really better? Was thought of everything? Will there come problems not foreseeable now? Starting from scratch also means a step back. Against these disadvantages stands the gain from the new design: Further development will be easier and probably faster, overall quality will be better and easier to keep up, and dead ends for further development are better avoidable.
meillo@239 574
meillo@239 575 Essentially, the decision for one of the ways depends on the question whether \masqmail\ should remain what it is, then the first option seems to be the right one to choose. Or whether \masqmail\ should become a modern \mta\ which is able to expand to include new functionality, then the second option is to choose.
meillo@239 576
meillo@239 577 Security, extendability, and the other quality properties appear to have also crucial importance in this decision. If they are required for future versions of \masqmail, then a new design is a must.
meillo@239 578
meillo@239 579 \person{Graff} and \person{van Wyk} describe the situation well: ``[I]n today's world, your software is likely to have to operate in a very hostile security environment.'' \cite{graff03}. An old-fashioned \mta\ depends, for sure, on a dieing branch, called \name{trusted environments}. And nothing other than a fresh and better design will help to survive.
meillo@239 580
meillo@239 581
meillo@239 582
meillo@196 583
meillo@219 584 The suggested further development plan for \masqmail\ is:
meillo@219 585 \begin{enumerate}
meillo@225 586 \item The short time goal: Add the most needed features, being authentication and encryption, to the current code base. \item The long time goal: Design a new architecture that satisfies the requirements identified, especially the quality requirements. The implementation of this design shall then, after being usable and throughout tested, supersede the old \masqmail.
meillo@219 587 \end{enumerate}
meillo@196 588
meillo@219 589 This plan is similar to the change from \sendmail\ to \name{sendmail X}/\name{MeTA1}, except the \sendmail\ change was much too late.
meillo@196 590
meillo@219 591 The following chapter is about the work on the current code base, to reach the short time goals. The chapter afterwards then introduces a new, modern design for future versions of \masqmail.
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meillo@219 594 %The plan is to 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.